2001 Rev.0 - Magnetic Particle Testing Level 1 & 2 Combined - Note Book

MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOKNASA-MT-2001 REV.0 TABLE OF CONTENTS CHAPTER 1 – QUALIFICATION, CERTIFICATION AND AUTHORISATION .............................. 2 CHAPTER 2 – BASIC PRINCIPLES .......................................................................................... 5 CHAPTER 3 – MAGNETIC PROPERTIES .............................................................................. 14 CHAPTER 4 – CURRENT TYPES........................................................................................... 25 CHAPTER 5 – MAGNETIZING METHODS............................................................................ 27 CHAPTER 6 – DEMAGNETISATION .................................................................................... 46 CHAPTER 7 – DETECTION MEDIUM................................................................................... 48 CHAPTER 8 – VIEWING CONDITIONS ................................................................................ 56 CHAPTER 9 – MAGNETIC FIELD INDICATORS .................................................................... 61 CHAPTER 10 – PERFORMANCE CHECKS ............................................................................ 65 CHAPTER 11 – INTERPRETATION VS. EVALUATION .......................................................... 68 PAGE 1 OF 70 MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 CHAPTER 1 – QUALIFICATION, CERTIFICATION AND AUTHORISATION SNT-TC-1A & ISO 9712 It is important that the technician be qualified and certified in the NDT method before the technique is used and the test results evaluated. The American Society for Nondestructive Testing recommends the use of their document Recommended Practice No SNT-TC-1A. The International Standards Organisation requires the use of their Specification, namely ISO 9712. These documents provides the employer with the necessary guidelines to properly qualify and certify the NDT technician in all methods. To comply with these documents, the employer must establish a written practice which describes in detail how the technician will be trained, examined and certified. These documents specifies the initial number of hours of classroom instruction and months or hours of experience necessary to be certified as an NDT testing technician. The main difference between these documents are that: SNT-TC-1A requires Company (Employer) Certification, and ISO 9712 requires Certification by a Body such as PCN or CSWIP. PAGE 2 OF 70 MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 LEVELS OF QUALIFICATION Level 1 An individual certified to Level 1 has demonstrated competence to carry out NDT according to written instructions and under the supervision of Level 2 or Level 3 personnel. Within the scope of the competence defined on the certificate, Level 1 personnel may be authorized by the employer to perform the following in accordance with NDT instructions: a) b) c) d) set up NDT equipment; perform the tests; record and classify the results of the tests according to written criteria; report the results. Level 1 certified personnel shall neither be responsible for the choice of test method or technique to be used, nor for the evaluation of test results. Level 2 An individual certified to Level 2 has demonstrated competence to perform NDT according to NDT procedures. Within the scope of the competence defined on the certificate, Level 2 personnel may be authorized by the employer to: a) b) c) d) e) f) g) h) i) select the NDT technique for the testing method to be used; define the limitations of application of the testing method; translate NDT codes, standards, specifications, and procedures into NDT instructions adapted to the actual working conditions; set up and verify equipment settings; perform and supervise tests; interpret and evaluate results according to applicable standards, codes, specifications or procedures; carry out and supervise all tasks at or below Level 2; provide guidance for personnel at or below Level 2; report the results of NDT. Level 3 An individual certified to Level 3 has demonstrated competence to perform and direct NDT operations for which he is certified. Level 3 personnel have demonstrated: a) b) c) the competence to evaluate and interpret results in terms of existing standards, codes, and specifications; sufficient practical knowledge of applicable materials, fabrication, process, and product technology to select NDT methods, establish NDT techniques, and assist in establishing acceptance criteria where none are otherwise available; a general familiarity with other NDT methods. PAGE 3 OF 70       Practical 1: Visible. 1 Hour. and procedures. Complete a Written Instruction. A minimum of 70% must be scored on each segment of the exam with an aggregate of 80% in order to pass.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. (10 points) 1 Hour. Fill out a Test Report. 5 Open Code Book Questions. wet. provide guidance for NDT personnel at all levels. establish. Level 3 personnel may be authorized to: a) b) assume full responsibility for a test facility or examination centre and staff. designate the particular test methods. at NASA will comprise of the following: General examination: Closed book.  Fill out a Test Report. wet.  1 Hour. continuous method with AC Yoke on welded sample.  2 Hours. PAGE 4 OF 70 . 40 Multi-choice questions. carry out and supervise all tasks at all levels. codes. and NDT instructions to be used. interpret standards. procedures. and validate NDT instructions and procedures.0 Within the scope of the competence defined on the certificate.  Complete a Technique sheet.    Practical examination:  Pre-Test Calibrations:  Complete a Calibration Procedure as allocated by examiner. specifications. 3 Hours. review for editorial and technical correctness. Practical 2:  Fluorescent.    Specific examination: 20 Multi-choice questions. c) d) e) f) EXAMINATION BREAKDOWN The ‘end of Course’ examination (SNT-TC-1A). continuous method with AC Yoke on welded sample. cables and other devices. then fine particles of ferromagnetic powder. These characteristics make MPI one of the most widely utilized non-destructive testing methods. The method is used to inspect a variety of product forms including castings. Clean area using a wire brush if required.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. When the paint is dry. 6. iron filings) to detect flaws in components. A basic sequence of operations for the examination of a weld using MPI with a permanent magnet and black ink is shown below: 1. power generation. 3. straddle the magnet over the weld at 90˚ to the weld axis. MPI is fast and relatively easy to apply. prods. A leakage field has a north and South Pole on either side of it. To look for transverse weld discontinuities. There are many ways to apply a magnetic field. Interpret the area. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective. Some examples of industries that use magnetic particle inspection are the structural steel. Apply the magnetic particles. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines. and part surface preparation is not as critical as it is for some other NDT methods. Evaluate in accordance with the relevant specification.0 CHAPTER 2 – BASIC PRINCIPLES INTRODUCTION Magnetic Particle Testing (Inspection) (MT or MPI) is a Non-Destructive Testing method used for defect detection. Any discontinuity in the test area which cuts across the magnetic field creates a leakage field. cobalt. or some of their alloys. 7.e. coils. e. and weldments. and therefore will attract the ferromagnetic particles in great numbers. petrochemical. 5. 4. by the use of permanent magnets. turn magnet approximately 90˚ and re-apply the ink. electromagnetic yokes. MPI uses magnetic fields and small magnetic particles (i. BASIC PRINCIPLES A magnetic field is introduces into a specimen to be tested. 8. Look for indication with their length perpendicular to the weld axis. automotive. PAGE 5 OF 70 .g. Apply a thin layer of white contrast paint. or ferromagnetic particles in a liquid suspension. Evaluate in accordance with the relevant specification. Look for indications with their length lying along the same axis as the weld. Interpret the area. nickel. and aerospace industries. forgings. 2. are applied to the area being tested. and Faraday discovered that all matter including liquids and gasses were affected by magnetism. magnetic particle inspection was quickly replacing the oil-and-whiting method (an early form of the liquid penetrant inspection) as the method of choice by the railroad industry to inspect steam engine boilers.0 HISTORY Magnetism is the ability of matter to attract other matter to itself. He noticed that the metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fine ferromagnetic powder to the parts caused a build-up of powder over flaws and formed a visible indication. William Hoke realized that magnetic particles (coloured metal shavings) could be used with magnetism as a means of locating defects. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. Later on Bergmann. wheels. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. but only a few responded to a noticeable extent. In the early 1920’s. The earliest known use of magnetism to inspect an object took place as early as 1868. In the early 1930’s. The image shows a 1928 Electro-Magnetic Steel Testing Device (MPI) made by the Equipment and Engineering Company Ltd. (ECO) of Strand. the MT inspection method is used extensively to check for flaws in a large variety of manufactured materials and components. axles. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Today. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. Becquerel. England. PAGE 6 OF 70 . This was a form of Non-Destructive Testing but the term was not commonly used until some time after World War I.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. and tracks. This discovery was brought to his attention in the machine shop. Care is required to avoid local heating and burning of highly finished parts or surfaces at the points of electrical contact. as does every method. It is the responsibility of personnel conducting or checking tests to ensure that the test procedures are adequately performed. Disclosing the nature of discontinuities without impairing the material. It will detect cracks filled with foreign material. Separating acceptable and unacceptable material in accordance with predetermined standards. the operator must be aware of.0 TEST PROCEDURES Approved procedures for magnetic particle testing are formulated from analysis of the test specimen. Some of these are the following:     It will work only on ferromagnetic materials. and that the test objective is accomplished. Some of these are the following:     It is the best and most reliable method available for finding surface cracks. It is not in all cases reliable for locating discontinuities which lie wholly below the surface. review of its past history. especially very fine and shallow ones. It will work well through thin coatings of paint. TEST OBJECTIVE The objective of magnetic particle testing is to ensure maximum reliability by providing a means of:    Obtaining a visual image of an indication related to a discontinuity below the surface or at the surface of a material. PAGE 7 OF 70 . or other nonmagnetic coverings such as plating. Procedures found incorrect or inadequate must be brought to the attention of responsible supervision for correction.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. ADVANTAGES The magnetic particle method has a number of outstanding advantages within its field of usefulness that is. it has. and take into account by observing the precautions which they dictate. on ferromagnetic materials. These. DISADVANTAGES Although the method has many desirable and attractive advantages. certain limitations. Exceedingly heavy currents are sometimes required for the testing of very large castings and forgings. experience and information available concerning discontinuities in like or similar articles. No elaborate pre-cleaning is ordinarily necessary. Contact Pads Replaceable metal pad. This is accomplished by passing the current directly through the article or through a conductor which passes into or through a hole in the article. through which the magnetizing current is drawn. Black light filter A filter that transmits black light while suppressing the transmission of visible light and harmful ultraviolet radiation. for the purpose of creating a circular field in the ring or tube. Continuous Method An inspection method in which ample amounts of magnetic particles are applied. Contact Head The electrode. Circular Magnetization A method of inducing a magnetic field in an article so that the magnetic lines of force take the form of concentric rings about the axis of the current. This light has a wavelength of 3200 to 4000 angstrom units (A). Coil Shot A pulse of magnetizing current passed through a coil surrounding an article for the purpose of longitudinal magnetization. which the magnetic flux must cross. The circular method is applicable for the detection of discontinuities with axes approximately parallel to the axis of the current through the article. during the time the magnetizing current is applied. PAGE 8 OF 70 . Alternating Current Electric current periodically reversing in polarity or direction of flow. Circular Field See “Field. This is between visible light and ultraviolet light. Central Conductor An electrical conductor that is passed through the opening in a ring or tube. or are present on the piece. peaking at 3650 A. Bath The suspension of iron oxide particles in a liquid vehicle (light oil or water). Carbon Steel Steel which does not contain significant amounts of alloying elements other than carbon and manganese. or around the hole. usually of copper braid.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. circular Magnetic. the space is referred to as an air gap. Carrier Fluid The fluid in which fluorescent and non-fluorescent magnetic particles are suspended to facilitate their application in the wet method. fixed to the magnetic particle testing unit. Black light Radiant energy in the near ultraviolet range. A measure of the magnetizing or demagnetizing strength of the coil. Ampere The unit of electrical current. Ampere Turns The product of the number of turns in a coil and the number of amperes flowing through it. Cracks produce small air gaps on the surface of an article. or any hole in an article.” Coercive Force The reverse magnetizing force necessary to remove residual magnetism in demagnetizing an article. on the spectrum. One ampere is the current that flows through a conductor having a resistance of one ohm at a potential of one volt. placed on contact heads to give good electrical contact thereby preventing damage to the article under test.0 TERMINOLOGY Air Gap When a magnetic circuit contains a small gap. or near. etc. PAGE 9 OF 70 . The current may be alternating. But if the article being magnetized is irregular in shape. Field. and at which they lose their residual magnetism: approximately 1200 to 1600°F (649 to 871°C) for many metals. etc. Bipolar Longitudinal magnetic field within an article that creates two poles. indications of subsurface defects. are not necessarily defects as they may not affect the serviceability of the part in which they exist. Dry Method Magnetic particle inspection in which the particles employed are in the dry powder form. Due to the action of the chemicals in eating away the surface. Current Induction Method A method of magnetization in which a circulating current is induced in a ring-shaped component by fluctuating magnetic field. half-wave rectified alternating.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. seams. discontinuities. for example. Discontinuity An interruption (cracks. For example – forging flow lines. Note that all cracks. laps. Electromagnet A magnet created by inserting a suitable metal core within. Diffuse Indications Indications that are not clearly defined as. various surface or subsurface conditions are exposed or exaggerated and made visible to the eye. Ferromagnetic A term applied to materials which can be magnetized and strongly attracted by a magnetic field. or direct. Distorted Field The direction of a magnetic field in a symmetrical object will be substantially uniform if produced by a uniformly applied magnetizing force. Direct Current An electric current which flows steadily in one direction. and defects. A fault in any material or part which is detrimental to its serviceability. porosity. A discontinuity may or may not affect the usefulness of the article. inclusions. Demagnetization The reduction in the degree of residual magnetism in ferromagnetic materials to an acceptable level. forging laps. a magnetizing field formed by passing electric current through a coil of insulated wire.) in the normal physical structure of configuration of an article. seams. Dry Powder (Dry Method) Finely divided ferromagnetic particles suitably selected and prepared for magnetic particle inspection by the dry method. Defect A discontinuity that interferes with the usefulness of an article or exceeds acceptability limits established by applicable specifications. Etching The process of exposing subsurface conditions of metal articles by removal of the outside surface through the use of chemical agents. Current Flow Method A method of circular magnetization by passing a currant through an article via prods or contact heads. Curie Point The temperature at which ferromagnetic materials can no longer be magnetized by outside forces..0 Core That part of the magnetic circuit which is within the electrical winding. the field is distorted and does not follow a straight path or have a uniform distribution. Furring Build-up. Flux Lines Imaginary magnetic lines used as a means of explaining the behavior of magnetic fields. Circular Magnetic Generally. Magnetic Leakage The magnetic field that leaves or enters the surface of an article at a magnetic pole. Magnetic The space within and surrounding a magnetized article.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. Their conception is based on the pattern of lines produced when iron filings are sprinkled over a piece of paper laid over a permanent magnet. Flux Density This is the flux-per-unit area through an element which cuts the unit area at right angles to the direction of the flux. Flux density is usually designated by the letter B and its unit is the gauss. Numerically. one gauss is one line of flux per square centimeter of area and is designated by the letter “B”. or a conductor carrying current. Heads The clamping contacts on a stationary magnetizing unit. the unit is a single line of force called the Maxwell designated by the Greek letter Phi(π).0 Fields. Residual Magnetic The field that remains in magnetizable material after the magnetizing force has been removed. PAGE 10 OF 70 . Also called “lines of force”. Flux Penetration. Field. of magnetic particles due to excessive magnetization of the article under examination resulting in a furry appearance … also referred to as “Fur” or “Grass”. Flux Leakage Magnetic lines of force which leave and enter an article at poles on the surface. Gauss The unit of flux density. Magnetic The depth to which a magnetic flux is present in an article. Field. Fluorescence The emission of visible radiation by a substance as the result of. Field. Field. and only during. Resultant Magnetic. in which the magnetic force is present. the absorption of black light radiation. Longitudinal Magnetic A magnetic field where in the flux lines traverse the component in a direction essentially parallel with the axis of the magnetizing coil or to a line connecting the two poles at the magnetizing yoke. Vector See Field. or bristling. Fluorescent Magnetic Particle Inspection The magnetic particle inspection process employing a finely divided fluorescent ferromagnetic Inspection medium that fluoresces when activated by black light of 3200 to 4000 Angstroms. Flash Magnetization Magnetization by current flow of very brief duration. Field Resultant Magnetic A magnetic field that is the result of two magnetic forces impressed upon the same area of a magnetizable object at the same time…sometimes called a “vector field”. Field. the magnetic field in and surrounding any electrical conductor or article resulting from a current being passed through the conductor or article or from prods. Intercepts of the loop with the B and H axes and the points of maximum and minimum magnetizing force define important magnetic characteristics of the material. Horseshoe Magnet A bar magnet. Magnet. Longitudinal Field See “Field. Usually the term applies to a permanent magnet. Inductance The magnetism produced in a ferromagnetic body by some outside magnetizing force. Inspection The process of examining and checking materials and articles for possible defects or for deviation from established standards.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.” Longitudinal Magnetization The process of inducting a magnetic field into the article such that the magnetic lines of force extending through the article are approximately parallel to the axis of the magnetizing coil or to a line connecting the two poles when yokes (electromagnets) are used. plotted as a function of magnetizing force. Permanent A highly retentive metal that has been strongly magnetized for example. Lines of Force. and positive direction sequentially.0 Head Shot A short pulse of magnetizing current passed through an article or a central conductor while clamped between the head contacts of a stationary magnetizing unit for the purpose of circularly magnetizing the article. bent into the shape of a horseshoe so that the two poles are adjacent. the curve forms a characteristic S-shaped loop. Leakage Field The magnetic field forced out into the air by the distortion of the field within an article. B. The magnetism is not the result of passing current through the article. Indication Any magnetically held magnetic particle pattern on the surface of an article being tested. Hysteresis Loop A curve showing the flux density. PAGE 11 OF 70 . the alloy Alnico. Longitudinal Magnetic. See Flux Lines. 2) The phenomenon exhibited by a magnetic system wherein its state is influenced by its previous magnetic history. Magnetic Field Strength The measured intensity of a magnetic field at a point always external to the magnet or conductor usually expressed in Oersted. Interpretation The determining of the cause and significance of indications of discontinuities from the standpoint of whether they are detrimental defects or false or non-relevant indications. magnetic. Magnetic Field See “Field. H. Hysteresis 1) The lagging of the magnetic effect when the magnetic force acting upon a ferromagnetic body is changed.” Magnetic Field Meter An instrument designed to detect and/or measure the flux density and polarity of magnetic fields. As the magnetizing force is increased to the saturation point in both the positive. negative. 3) The ratio of flux density produced to magnetizing force. is a main concern as only ferromagnetic materials can be strongly magnetized. Paramagnetic Materials which are slightly attracted by a magnetic field. Permeability 1) The ease with which a material can become magnetized. manganese. Pole The area on a magnetized article from which the magnetic field is leaving or returning to the article. The subdivision of paramagnetic. The reluctance of the material determines the magnitude of the flux produced by a given magnetic force. Examples are chromium. Prods Hand-held electrodes attached to cables used to transmit the magnetizing current from the source to the article under inspection. Magnetizing Force This is the total force tending to set up a magnetic flux by a magnetizing current.0 Magnetic Material Some materials are attracted by a magnet while others are repelled. or may be false or non-relevant indications. These materials are known as paramagnetic materials.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. Magnetic Particle Inspection Indications The accumulation of ferromagnetic particles that may be either true indications of discontinuities. called ferromagnetic. It is usually designated by the letter H and its unit is the Oersted. PAGE 12 OF 70 .another piece of ferromagnetic material that is magnetized to a different value. ferromagnetic particles in paste form used in preparing wet suspensions. Reluctance is analogous to the resistance. and aluminum. Magnetizing Current The flow of either alternating. Reluctance The opposition of a magnetic material to the establishment of magnetic flux. Rectified Alternating Current Alternating current which has been converted into direct current. whereas materials which repel are known as diamagnetic materials. Magnetic Writing A form of non-relevant indications caused when the surface of a magnetized part comes in contact with . 2) The ratio between field strength produced and the magnetizing force (B/H). Oersted A unit of field strength which produces magnetic induction and is designated by the letter “H”. Magnetic Particle Inspection A nondestructive inspection method for locating discontinuities in ferromagnetic materials. rectified alternating or direct current used to induce magnetism into the article being inspected. Paste (Slurry) Finely divided. It utilizes flux leakage that forms magnetic poles to attract finely divided magnetic particles which mark the discontinuity. Non-relevant Indication A magnetic particle indication due to a leakage magnetic field which is not caused by an actual discontinuity in the magnetized material. in an electric circuit. but by some other condition which does not affect the usefulness of the article (such as a change of section). From the definition of magnetism it follows that magnetic materials are those that are attracted by magnetism. ” Retentivity The ability of a material to retain a portion of the magnetic force induced in it after the magnetizing force has been removed.” Residual Magnetism The amount of magnetism that a magnetic material retains after the magnetizing force is removed … also called “residual field. by means of an external electromagnet shaped like a yoke. defects oriented in different directions in an article.76 lx (lux) 1 °C (? °F . Resultant Magnetic. CONVERSION TABLE 1m 1 000 mm 1 000 000 µm 1 Bar 14. Solenoid (Coil) An electric conductor formed into a coil. around which is wound a coil carrying the magnetizing current.0 Residual Field See “Field. Yoke Magnetization A longitudinal magnetic field induced in an article.” Residual Method A procedure in which the indicating material is applied after the magnetizing force has been discontinued. Resultant Field See “Field. Vector Field See “Field. sometimes with adjustable pole pieces.” Wet Method The inspection method employing ferromagnetic particles suspended in a liquid (oil or water) as a vehicle. Suspension The correct term applied to the liquid bath in which is suspended the ferromagnetic particles used in the wet magnetic particle inspection method. Resultant Magnetic. either solid or laminated. Subsurface Discontinuity Any discontinuity which does not open onto the surface of the article in which it exists.5 Psi 100 000 Pascal 3650 Å 365 nm 10 W/m² 1 000 µw/cm² 1 fc (foot-candle) 10. Sensitivity The capacity of degree of responsiveness to magnetic particle inspection. or in an area of an article. often wrapped around a central core of highly permeable material. Test Piece An article containing known artificial or natural defects used for checking the efficiency of magnetic particle flaw detection processes. Yoke A “U” or “C” shaped piece of highly permeable magnetic material. Saturation The point in the magnetization of a magnetizable article at which an increase in the magnetizing force produces no increase in the magnetic field within the article.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. more accurately.32) x 5/9 1 000 000 000 nm PAGE 13 OF 70 . Swinging Field Magnetization Magnetic fields induced in two different directions alternately and quickly to detect. Residual Magnetic. The domains can be aligned by bringing them within an existing magnetic field. If the domains are aligned in a common direction. A vector field is formed which is the resultant direction and strength of the two imposed fields. PAGE 14 OF 70 . but if the poles are alike then there is a repulsion. then the material will be magnetised and the material itself will have its own north and South Pole. each domain has its own north and South Pole. When two magnetizing fields are imposed simultaneously in the same part. If two pieces of magnetised material are placed with their dissimilar poles end to end there is an attraction. When these domains are randomly positioned. the object is not magnetized in two directions at the same time. therefore: like poles repel. The poles of magnetised materials have an inherent attraction/ repulsion effect. the material is unmagnetized. and C equals the resultant magnetizing force. then the material is said to be permanently magnetised. If the domains remain aligned when they are removed from the influence of the magnetic field. unlike poles attract.0 CHAPTER 3 – MAGNETIC PROPERTIES MAGNETISM All materials consist of atoms and molecules which may or may not have a permanent magnetic influence depending on the electron configuration within the material. Atoms in magnetic materials group together in regions called magnetic domains. This is illustrated in below where A is the first magnetizing force.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. B is the second force. He suggested properties for these lines of force. Faraday used this hypotheses to account for the repulsion of like poles. since the lines of force stretch from one pole to another.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. he could account for the attraction of unlike poles. the tension of the lines of force must increase as they shorten. The area where the exit Poles are concentrated is called the magnet's North Pole and the area where the entrance poles are concentrated is called the magnet's South Pole. like pieces of stretched elastic. But. It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet. the lines of force also tend to curve outwards. seeming to suggest that they repel each other. PAGE 15 OF 70 . which he imagined as spreading out from all magnetic poles into the surrounding space. since the repulsive force between two like poles increases as the poles approach.0 LINES OF FORCE Faraday used the concept of lines of force to explain what happens in the space between two magnets. By assuming the lines were in tension. With like poles. and from the South Pole to the North Pole within the magnet. i. PAGE 16 OF 70 . Their density decreases with increasing distance from the poles. They do not cross one another. the number of lines of force in a unit area decreases. (Repel each other laterally) They seek paths of least magnetic resistance. that is: from the north pole to South Pole external to the magnet.0 The properties of magnetic lines of force are as follows:       They form closed loops between north and south poles.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.e. They are considered to have direction. They are in a constant state of tension. cobalt. (µ < 1) Paramagnetic materials These are very weakly attracted by a magnetic field and include oxygen and most metals including austenitic stainless steel.0 MATERIAL PROPERTIES The degree to which materials are capable of being influenced by a magnetic field varies greatly from material to material. nickel and many of their alloys. Diamagnetic materials These are. titanium and most non-metals. They also exhibit permanent magnetism and can themselves be magnetized. lithium and tantalum. magnesium. ±240 or more) PAGE 17 OF 70 . (µ ≥ 1) Ferromagnetic materials These are strongly attracted by a magnetic field and include iron. repelled by a magnetic field and include copper. however.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. to a very slight degree. (µ > 1 . molybdenum. they fall into three specific categories defined by their behaviour in the magnetic field. MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 PERMEABILITY (µ) For magnetic particle inspection, the only materials of interest are those which are ferromagnetic. Within this group, some materials are more easily magnetised than others, that is to say, more permeable. To permeate means to spread through. In this context it refers to the ease by which the magnetic lines of force are spread through the material. Soft iron and low carbon steel have a high permeability, i.e. they are easy to magnetise. Hard iron and high carbon steel have a low permeability, i.e. they are difficult to magnetise. An alternative description favoured in the USA would be, the ability to concentrate magnetic fields and it is shown on the 'slope' of the B/H curve which varies continuously. Permeability (µ) may be calculated by dividing the flux density (B) achieved by the magnetising force applied (H). µ= The permeability of a material may be given a value based on a ratio when compared with free space. These values vary depending on alloy composition, heat treatment and any working applied. RELUCTANCE (R) Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. Reluctance is the reciprocal of permeability, i.e. R= RETENTIVITY When a magnetising force is removed from a ferromagnetic material the amount of magnetism remaining will vary between materials and depends upon the permeability of the material. The remaining magnetism is termed residual magnetism and the material is said to have retentivity or retained magnetism. If a material has high permeability it is very difficult to magnetise, that is to say it has high magnetic reluctance, but once magnetisation has been achieved then it does not give up the magnetic force easily, therefore it has high retentivity. High permeability = Low retentivity Low permeability = High retentivity PAGE 18 OF 70 MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 RESIDUAL MAGNETISM Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. The equivalent term residual magnetization is generally used in engineering applications. In transformers, electric motors and generators a large residual magnetization is not desirable as it is an unwanted contamination, for example a magnetization remaining in an electromagnet after the current in the coil is turned off. Where it is unwanted, it can be removed by demagnetisation. COERCIVE FORCE Coerce means to forcibly control; in this context it relates to the reversed magnetising force which is necessary to remove remnant or residual magnetism for demagnetisation of a part. To summarise: PERMEABILITY RELUCTANCE RESIDUAL MAGNETISM RETENTIVITY COERCIVE FORCE This refers to the ease with which a magnetic flux is established in the article being inspected. This is the opposition of a magnetic material to the establishment of a magnetic flux. A material with a high permeability will have a low reluctance. This refers to the amount of magnetism retained after the magnetizing force is removed. Refers to the ability of the material to retain a certain amount of residual magnetism. Refers to the reverse magnetizing force necessary to remove the residual magnetism from the part. MAGNETIC FLUX (ɸ) AND MAGNETIC FLUX DENSITY (B) When a specimen is magnetised, lines of force or flux exist within the specimen the stronger the magnetising force applied, the greater the amount of flux produced. The magnetising force may be applied by using a permanent magnet or electrically operated magnetic flow apparatus, or by passing an electric current through the specimen. Magnetic flux is measured in Webers (Wb). 1 Wb = 108 lines of force The number of lines of force (or flux) passing transversely through a given cross-sectional area is known as the flux density (B) Flux density (B) = Where: ɸ = flux A = Area ɸ PAGE 19 OF 70 MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 Flux density is measured in Tesla (T) 1 Wb/m2 = 1 Tesla (T). The old (cgs) unit for flux density which is still widely encountered is the Gauss: 1 Gauss = 1 line of force/cm2. 10,000 (104) Gauss = 1 Tesla 1 Gauss = 0.1 mT MAGNETIC FIELD STRENGTH (H) The magnetic field strength or magnetising force is that which is needed to induce a flux in a magnetic circuit and is measured in amperes per metre (A/m), or in old (cgs) units, the Oersted. 1 Oersted =79.58 Amperes per metre. MAGNETIC HYSTERESIS A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). It is often referred to as the B-H loop. An example hysteresis loop is shown below. The loop is generated by measuring the magnetic flux of a ferromagnetic material while the magnetizing force is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point "a" almost all of the magnetic domains are aligned and an PAGE 20 OF 70 MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. this is 50 times per second. As the magnetizing force is increased in the negative direction. When H is reduced to zero. in the U. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. a complete hysteresis loop is produced with each cycle of current. where the flux has been reduced to zero.) As the magnetizing force is reversed. This is referred to as the point of retentivity on the graph and indicates the Remanence or level of residual magnetism in the material. the curve moves to point "c".) The force required to remove the residual magnetism from the material is called the coercive force or coercivity of the material. for making magnetic particles or for magnetising purposes. When AC is used for magnetising a specimen. The gradient of the loop also gives information regarding the usefulness of materials for use in magnetising apparatus." It will have a level of residual magnetism equal to that achieved in the other direction." At this point. (Some of the magnetic domains remain aligned but some have lost their alignment. Reducing H to zero brings the curve to point "e. For example. The curve will take a different path from point "f" back to the saturation point where it with complete the loop. Increasing H back in the positive direction will return B to zero. the material will again become magnetically saturated but in the opposite direction (point "d"). The material has reached the point of magnetic saturation.0 additional increase in the magnetizing force will produce very little increase in magnetic flux. it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero. PAGE 21 OF 70 . (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero. the curve will move from point "a" to point "b. This is called the point of coercivity on the curve.K. a material which exhibits a steep gradient will attain a high flux density when using a low magnetising force. A material which exhibits a hysteresis loop with a narrow appearance will have low retentivity and therefore may be useful for making magnetic particles.* A material which exhibits a hysteresis loop with a wide appearance will have high retentivity and therefore may be useful for making permanent magnets. Magnetic particle inspection relies on a leakage of flux occurring within this circuit. Magnetic particle inspection used for the detection of surface breaking discontinuities and only in ferromagnetic materials. i. known as the skin effect. MPI test equipment using alternating current as an output produces a high density magnetic flux at the surface of the test component. PAGE 22 OF 70 . This would accumulate at the area of the leakage field and give an indication of the defect's existence. therefore in order to examine a specimen completely. they bend around these defects taking the path of least resistance. because the magnetic field penetrants much further into the test specimen in comparison with MPI test methods which use alternating current. the lines of force must be applied in different directions. However. produces a far stronger flux leakage field on the surface breaking. this may be caused by a break or discontinuity in the material. therefore any break or discontinuity causing a flux leakage will. This is because the magnetic fields induced are concentrated at the surface of the components. flux leakages may also be caused by changes in metallurgy.e. compared to permanent magnets or direct current test equipment. because of the magnetic poles. Where the flux re-enters the circuit a South Pole is created.0 FLUX LEAKAGE When a magnetic field is created within a ferromagnetic within a ferromagnetic specimen. Opposite poles attract. or near surface discontinuities. Any linear discontinuities running parallel with the flux or small non-linear discontinuities. these discontinuities therefore remain undetected. This phenomenon.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. sub-surface discontinuities may be detected if using permanent magnets or electrical systems using direct or rectified current. It is unlikely that any form of MPI would be used to detect discontinuities deeper than 2 mm or 3 mm below the surface. but will usually remain effective down to about 45˚ of this axis (BS EN ISO 17638: 2009 and BS EN ISO 9934: 2001 quotes ± 60˚). For a discontinuity to be detected by MPI it must interrupt the lines of force. Because it is a change in magnetic permeability that causes a leakage field. Below that it is unlikely that the discontinuity will be found. lines of magnetic flux are developed and flow through and around the material completing a circuit. attract a ferromagnetic material such as iron powder. MPI is most effective in detecting discontinuities with their major axis at 90˚ to the lines of force. equiaxed defects do not break the lines of force. Where the flux leaves the circuit a North Pole is created. the conductivity of the test material and its permeability. would normally be located by other methods of NDT.g. caused by rough/ uneven surfaces or changes in permeability. ELECTROMAGNETISM There is a fundamental relationship between electricity and magnetism. Because of this problem.0 The depth of flux penetration is governed by the wave frequency of the alternating current. e. in a plane perpendicular to the direction of travel of the electric charge. the movement of an electric charge will create a magnetic force field around it. assuming the detection of sub-surface defects is a requirement. If any of these variables increase. PAGE 23 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. sub-surface. or body defects. the depth of penetration will decrease. It is difficult to try and interpret very weak and diffused MPI indications which could be from sources other than defects. the direction and orientation of which are given by the right hand rule if we assume the current flow.0 Electrons that are moving in a current carrying conductor set up a magnetic field. PAGE 24 OF 70 . The magnetic field is essentially uniform down the length of the coil when it is wound tighter. Please be aware that the field outside the coil is weak and is not suitable for magnetizing ferromagnetic materials. The concentrated magnetic field inside a coil is very useful in magnetizing ferromagnetic materials for inspection using the magnetic particle testing method. A loosely wound coil is illustrated below to show the interaction of the magnetic field. The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil. by convention. a magnetic field develops that flows through the center of the loop or coil along its longitudinal axis and circles back around the outside of the loop or coil.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. is opposite to electron flow. Current Flow Theory = Right Hand Rule = + to – Electron Flow Theory = Left Hand Rule = – to + When a current carrying conductor is formed into a loop or several loops to form a coil. or the left hand rule if we assume the direction of electron flow. straight coil of wire is called a solenoid and can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil. A long. the reversing AC can be converted to a one directional current. However. Current from single phase 110 volts. current is said to flow from the positive to the negative terminal. it is convenient to make use of it for magnetic particle inspection. the skin effect limits the use of AC since many inspection applications call for the detection of subsurface defects. the magnetic field produced by DC generally penetrates the entire cross-section of the component.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. This phenomenon is known as the "skin effect" and occurs because the changing magnetic field generates eddy currents in the test object. As previously mentioned. thus reducing the net magnetic flux below the surface. AC can be converted to current that is very much like DC through the process of rectification. are used when generating an electric field in a component. Since AC is readily available in most facilities. the field produced using alternating current is concentrated in a thin layer at the surface of the component. Therefore. The type of current used can have an effect on the inspection results. In actuality. to three phase 440 volts. In the United States. so the types of currents commonly used will be briefly reviewed. the magnetic field will be limited to narrow region at the surface of the component. A battery is the most common source of direct current. Rectified Alternating Current Clearly. Luckily. DC is very desirable when inspecting for subsurface defects because DC generates a magnetic field that penetrates deeper into the material.0 CHAPTER 4 – CURRENT TYPES As seen in the previous pages. With the use of rectifiers. the electrons flow in the opposite direction. The eddy currents produce a magnetic field that opposes the primary field. the convenient access to AC. drives its use beyond surface flaw inspections. PAGE 25 OF 70 . Alternating current and direct current are the two basic types of current commonly used. it is recommended that AC be used only when the inspection is limited to surface defects. However. Conversely. In ferromagnetic materials. Direct Current Direct current (DC) flows continuously in one direction at a constant voltage. when AC is used to induce a magnetic field in ferromagnetic materials. Alternating Current Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second. The three commonly used types of rectified current are described below. 60 cycle current is the commercial norm but 50 cycle current is common in many countries. electric current is often used to establish the magnetic field in components during magnetic particle inspection. Current flow is often modified to provide the appropriate field within the part. PAGE 26 OF 70 . No current flows during the time when the reverse cycle is blocked out. HWAC is most often used to power electromagnetic yokes. The reverse half of each cycle is blocked out so that a one directional. This type of electrical current is also highly desirable for magnetic particle testing because when it is rectified and filtered.0 Half Wave Rectified Alternating Current (HWAC) When single phase alternating current is passed through a rectifier. the amperage is half of the unaltered AC. This added mobility is especially important when using dry particles. Since half of the current is blocked out. This produces a pulsating DC with no interval between the pulses. providing the inspector with the advantages of each current form. The pulsation of the HWAC helps magnetic particle indications form by vibrating the particles and giving them added mobility. Filtering is usually performed to soften the sharp polarity switching in the rectified current. Stationary magnetic particle equipment wired with three phase AC will usually have the ability to magnetize with AC or DC (three phase full wave rectified). This type of current is often referred to as half wave DC or pulsating DC.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. the depth of the subsurface magnetic field is improved. current is allowed to flow in only one direction. Full Wave Rectified Alternating Current (FWAC) (Single Phase) Full wave rectification inverts the negative current to positive current rather than blocking it out. Three Phase Full Wave Rectified Alternating Current Three phase current is often used to power industrial equipment because it has more favorable power transmission and line loading characteristics. The HWAC repeats at same rate as the unrectified current (60 hertz typical). the resulting current very closely resembles direct current. While particle mobility is not as good as half-wave AC due to the reduction in pulsation. pulsating current is produced. The current rises from zero to a maximum and then returns to zero. The pulsation is reported to significantly improve inspection sensitivity. This magnetization technique is often referred to as a "coil shot. a longitudinal magnetic field can be established in the component. whatever apparatus is used to magnetise. especially for site inspections. When multiple items are being tested by the residual method. This method is generally only used on materials with a high retentivity. These methods is by far the most widely used today. LONGITUDINAL MAGNETISATION When the length of a component is several times larger than its diameter.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. care must be taken to ensure that the components do not come into contact with each other before the detecting media is applied otherwise a phenomena know as magnetic writing will occur. RESIDUAL MAGNETISATION METHOD The residual method is a testing method where the magnetic ink or powder is applied and the test area is viewed after the magnetising force has been removed. The test is performed using the residual magnetism left in the sample.0 CHAPTER 5 – MAGNETIZING METHODS CONTINUOUS MAGNETISATION METHOD The continuous method is a testing method by which the magnetic ink or powder is applied to prior to and during energisation and the test area is viewed whilst the magnetising force is applied. This is always considered to be the most sensitive method. PAGE 27 OF 70 . due to the fact that the induced magnetic field is always the strongest whilst the magnetising force is being applied." It can be accomplished by placing a part in a fixed coil or wrapping the part with flexible cable. The component is often placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. Another method of longitudinally magnetising a part or rather a section of the part is by using Permanent magnets or Electromagnetic Yokes. The lifting power is the ability of the magnet to lift a piece of ferromagnetic material by attraction alone. this provides an ideal configuration for magnetic particle inspection. Some permanent magnets may have adjustable arms. When not in use a permanent magnet should have a keeper placed across the poles to prevent loss of magnetism. The field strength can vary considerably. But.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. others may have rollers attached to the poles. the magnetic lines of force will be concentrated in the gap between the magnetic poles.0 PERMANENT MAGNETS Permanent magnets are so called because they are able to maintain a magnetic field in the surrounding space. If the bar was formed into a closed loop. If a defect was present in the loop. or by its lifting power. The degree of magnetisation in permanent magnets is determined by the amount of pull required to lift the magnet clear of the work piece. if a bar magnet is simply formed into a U shape. whilst leaving the other pole attracted. depending on the flux density in the magnet and its shape. The pull off force is the force that has to be applied to one pole to break its attraction to the surface. PAGE 28 OF 70 . The simplest form of penetrant magnet is a bar magnet. Permanent magnets provide magnetic flow only in the specimen and produce a longitudinal magnetic field between the poles. a flux leakage would still occur. which is basically a piece of ferromagnetic material with a magnetic pole at each end. Certain specifications will state the minimum requirements for the strengths of permanent magnets. then the magnetic field would be fully contained within a closed circuit and no external field would exist. the rollers are set to keep the magnet just clear of the surface and enable it to be moved over the work piece with relative ease. Neither bar magnets or ring (looped) magnets have any use in MPI.  Have to be pulled from the test surface. Magnetism is induced into the yoke by encircling it with a coil through which a current is passed. which is laminated to reduce induction caused by eddy current flow (associated only with alternating current) this also helps to prevent the yoke becoming permanently magnetized.  No control over field strength (unless adjustable arms are used).MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.  They cling to vertical and overhead surfaces.  Low flux density unless rare earth magnets used.  Only small areas examined in each position.  Magnetic particles attracted to poles.only certain electromagnets allow for this. The yoke is made from highly permeable. low retentive steel. 2.  Inexpensive. producing a longitudinal magnetic field.0 Advantages of permanent magnets include:  No power supply required.  Very hard.  Limited application on awkward shapes. ELECTROMAGNETIC YOKES Electromagnetic yokes or electromagnets require a source of electrical energy which may be AC or DC.  No damage to the test piece from arcing. The test method used is sometimes referred to as the magnetic flow or magnetic flux path method.  Relatively lightweight (easily portable). By adjusting the current (amperage) flowing through the yoke .  Not recommended to be used in conjunction with flux indicators.  Toxic material when machined. Disadvantages include:  Deterioration with wear/ abuse.most electromagnets allow for this but not at all. By varying the distance between the poles . PAGE 29 OF 70 . the strength of the field produced can be varied in one of two ways: 1.  Both hands free after the magnet is placed onto the surface.  Keeper required when not in use.  DC yokes are not recommended to be used in conjunction with flux indicators. but Yokes is usually accepted as a surface technique only.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.  Leaves only one hand free.0 Certain specifications will state the minimum requirements for the lifting power of Yokes. The magnet will have a much greater pull on DC but the flux will be less at the surface of the component being tested.  Controllable field strength (not in all cases). PAGE 30 OF 70 . however the depth of the field within the test piece will depend upon the type of current used to induce magnetism. sub-surface defects will be more easily located using DC. The area of inspection for electromagnets is a rectangular area between the poles of the magnet(s). Disadvantages include:  Power supply required.  Only small areas can be examined at each magnet location.  No damage done to test piece. Surface discontinuities will be more readily found using AC. Electromagnets may operate direct from the mains supply of 240V but are available at 110V (battery packs are now available at 12/ 24/ 36V for more flexibility) when required for site use.  Can be switched on/off as required.  Relatively lightweight. The field produced from an electromagnetic yoke is longitudinal.  AC yokes can be used to demagnetise. travelling from pole to pole as with permanent magnets. Advantages of electromagnetic yokes include:  AC or rectified DC operation. 0 Typical Permanent Magnet and Yoke placement when testing welds.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. PAGE 31 OF 70 . NB. These requirements will be specified by the Procedure Code used. high amperage current is passed. good for detecting circumferential discontinuities in shafts or the bores of tubes. it should ideally be positioned close to one side of the coil and rotated to obtain the best results. The magnetic field passing through the centre if the coil . PAGE 32 OF 70 . When using any form of coil the field strength is determined by the current flowing in the circuit and by the number of turns in that coil.g.creates longitudinal magnetization and is therefore used to detect discontinuities which lie transverse to the components major axis. For practical purposes only defects which lie within the confines of the coil should be interpreted although the field will extend for 100 to 225 mm beyond either end. If the specimen being tested has a small diameter in relation to the inside diameter if the coil. through which a low voltage. thereby obtaining ampere/ turns.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The strongest magnetic field is on the inside edge of the coil. e. When using any of the current flow or threaded bar methods.typically three or five turns .0 COIL TECHNIQUE RIGID (FIXED) COIL This technique consists of placing the specimen inside a coil of tubular or solid construction. the fields strength is largely determined by the current (amperes) flowing in the circuit.  Expensive equipment.  No poles to attract magnetic particles.  Large areas inspected with each set-up. Disadvantages include:  Cumbersome long heavy cables required. Defects lying parallel to the cable will be the most readily detected.  AC energised equipment may be used for demagnetisation operations. this restricts the production of an adequate test area with a sufficient constant magnetic field. on. HWDC or FWDC.0 FLEXIBLE CABLES When a single conductor is used. Advantages of using a Coil:  Slightly Subsurface discontinuities may be found when using DC. the magnetic field reduces rapidly at increasing distance from the conductor. If the current is made to flow in the same direction through conductors spaced some distance apart. Current values to be used shall be specified by the Procedure Code. A current passed through the cable will then induce a magnetic field into the test piece.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. a relatively constant field is produced.  Heavy transformers required for large amperages. Values shall be calculated considering the Length/Diameter Ratio of the part and the number of turns in the coil.  Field strength can be altered.  Longer setting up times.  Predictable field strengths. PAGE 33 OF 70 . Flexible cable techniques can be used on a considerable variety of component shapes. On complex shapes the position and method in which the cable is wound may have to be found by experimentation to ensure an adequate field in all areas. or around the specimen. Configurations used are normally obtained with a heavy insulated flexible cable which is placed through.     The field strength varies from zero at the center of the component to a maximum at the surface.0 CIRCULAR MAGNETISATION As discussed previously. The following statements can be made about the distribution and intensity of the magnetic field.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The external field is exactly the same for the two materials provided the current level and conductor radius are the same. a magnetic field forms in and around the conductor. the field strength is dependent on the current strength. This is due to the permeability of the magnetic material. When the conductor is a magnetic material. It can be seen that in a nonmagnetic conductor carrying DC. the magnetic field strength is graphed versus distance from the center of the conductor. PAGE 34 OF 70 . In the images below. when current is passed through a solid conductor. (However. and if magnetic. The field strength outside the conductor decreases with distance from the conductor. the location on the B-H curve. a larger conductor is capable of carrying more current. the field strength within the conductor is much greater than it is in the nonmagnetic conductor. Inside the conductor. The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant.) The field strength outside the conductor is directly proportional to the current strength. the internal field strength rises from zero at the center to a maximum value at the surface of the conductor. The external field strength decrease with distance from the surface of the conductor. magnetic permeability of the material. The external field strength decreases with distance from the surface of the conductor. In a hollow circular conductor there is no magnetic field in the void area. This is known as the "skin effect. the field is concentrated in a thin layer near the surface of the conductor. It should be remembered that with AC the field is constantly varying in strength and direction. the field strength within the conductor is much greater than it was in the nonmagnetic conductor due to the permeability of the magnetic material.0 When the conductor is carrying alternating current. PAGE 35 OF 70 . However. The external field decreases with increasing distance from the surface as it does with DC. the internal magnetic field strength rises from zero at the center to a maximum at the surface. when the conductor is a magnetic material. The external field is exactly the same for the two materials provided the current level and conductor radius are the same." The skin effect is evident in the field strength versus distance graph for a magnetic conductor shown to the right. As with a solid conductor.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The magnetic field is zero at the inside wall surface and rises until it reaches a maximum at the outside wall surface. As can be seen in the field distribution image to the right. the field strength at the inside surface of hollow conductor is very low when a circular magnetic field was established by direct magnetization. As can be learned from these three field distribution images. it may be detectable. when current is passed through a nonmagnetic central conductor (copper bar). the direct method of magnetization is not recommended when inspecting the inside diameter wall of a hollow component for shallow defects. the magnetic field produced on the inside diameter surface of a magnetic tube is much greater and the field is still strong enough for defect detection on the OD surface.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. Therefore. a much better method of magnetizing hollow components for inspection of the ID and OD surfaces is with the use of a central conductor. However. the skin effect concentrates the magnetic field at the outside diameter of the component. PAGE 36 OF 70 . so if the defect has significant depth. The field strength increases rapidly as one moves out (into the material) from the ID.0 When AC is passed through a hollow circular conductor. and on the same axis.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. as the defects being sought. PAGE 37 OF 70 . Zinc shall not be used and copper or copper-tipped prods shall be used only in applications where complete assurance can be given that metallurgical damage will not occur. if used as prod material. Certain metals including copper and zinc (including galvanised prod tips) may. Arcing or excessive heating shall be regarded as a defect requiring a verdict of acceptability. Prods induce a circular magnetic field within the specimen using current values typically in the region of 1000 amps. Current requirements shall be specified by the Procedure Code and is based on the thickness of the part and the distance between the prods. correct positioning is essential to ensure that all possible defects are located. Ideally the prods should be in a line parallel to. If further testing is required on such affected areas. but only in well ventilated conditions. burning or arcing. The cleanliness of both prod contact faces and the component shall be such to ensure good electrical contact. To prevent this possible damage. contaminate and cause metallurgical damage to the component if arcing occurs. Note: Lead contact pads may be used. likewise. the current should be switched off before lifting the prods. Precautions when using current flow in respect to prods/clamps shall be taken to prevent excessive heating. because they may generate harmful vapour which may cause headaches and/or dizziness. Since the lines of force radiate from prods. the prod contact tips and the test surface must be kept clear of any contamination and the current must not be switched on until firm contact has been established. ideally they shall be made of steel or aluminium. For this reason and the fact that perfect contact is difficult to achieve with prods.0 PROD TECHNIQUE With this technique the current is introduced into the item under test by using electrical contacts known as prods. at this current level arcing can occur between the electrodes and the test surface causing damage. Prods shall have a minimum dimension of 10 mm and shall have as large a contact area as possible. it shall be carried out using a different technique. on/off control.  Low voltage output.  AC energised equipment may be used for demagnetisation operations. HWDC. DC.  Relatively fast coverage of area under test.  Classed as a two person operation.  Can be used in confined spaces.  No poles to attract magnetic particles.  Contacts and small test items can overheat.  May leave residual field which interfere with next prod positions.  Heavy transformer required.0 Advantages of the prod technique include:  AC.  Careful positioning and spacing of prods required.  Possible to switch on without creating a field. PAGE 38 OF 70 . Disadvantage include:  Risk of creating arc strikes (forming localised hard spots which may contain cracking). and FWDC equipment available.  Variable field strength.  Expensive equipment.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. on a test piece of 25 mm diameter. therefore the technique is used for detecting defects parallel to. The current flow is normally obtained from a transformer DC. using DC or rectified DC will reveal discontinuities typically up to 2 mm to 3 mm below the surface. by applying contacts to the ends of a test piece and passing a high amperage. low voltage current through it. i. if two test pieces of differing diameters using the same current. depending on the component's shape and dimensions. using AC will reveal only surface discontinuities. the magnetic field would be stronger in the smaller diameter test piece. although. the same current value would be used whether it was 10 cm long or 1 m long.0 CURRENT FLOW TECHNIQUE (DIRECT MAGNETISATION) (HEAD SHOT OR CLAMPS) Current flow techniques produce a circular magnetic field by passing a current through the test piece. Specifications usually stipulates the current requirements. Because the current values are dependant only on the test piece perimeter.g.g. longitudinal defects in bar. With portable equipment. i. e. concentric rings of magnetic lines of force radiate at 90˚ (perpendicular) to the current flow.e. This sets up a circular field in the ferromagnetic material in a direction at 90˚ to the current flow. of current flow. therefore. bar or tube. PAGE 39 OF 70 .e. i. and up to 45˚. the component is firmly clamped between contact heads. it may be preferable to use an alternative method.e. In fixed installations. The choice of power supply depends on the test requirements. electrical contact is made by the use of prods and/or clamps. Copper gauze is usually placed between the contracts and the test piece to increase the contract area and reduce the possibility of burning. Irregular shaped items may also be tested by contact heads.e. the length of the test piece is of no importance. FWDC or 3 PHASE FWDC. An ammeter is usually incorporated in the equipment to indicate the amount if current flowing through the work piece. bench equipment. The output voltage of current flow equipment is so low that there is no risk of electrical shock to the operator from the equipment's specimen contact points or test specimen.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. e. i. thus not recommended. HWDC. Current flow can also be achieved in regularly shaped items.  Possible to switch on without creating a field.  Contacts and small test items can overheat.  Low voltage output.  Relatively fast coverage of area under test.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 Advantages of the Head Shot technique include:  AC.  May leave residual fields.  Variable field strength. Disadvantage include:  Risk of creating arc strikes (forming localised hard spots which may contain cracking). HWDC.  No poles to attract magnetic particles.  AC energised equipment may be used for demagnetisation operations.  Heavy transformer required.  Expensive equipment. and FWDC equipment available. PAGE 40 OF 70 . on/off control. DC. this technique is not widely encountered.g. Current requirements is based on the outside diameter of the part. longitudinal pipe defects. This sets up a circular field in the surrounding ferromagnetic material in a direction at 90˚ to the current flow. especially because numerous samples may be tested at the same time.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. although the conductor need not always be central. or aperture. and a current passed through it.typically made of brass. lengths of pipe may also be examined by this method. providing these limitations are met. On site work. when practical. then a conductor . PAGE 41 OF 70 . and up to 45˚. but could not be modified by using a flexible cable instead of rigid conductor. Conductor may be located centrally to the specimen. copper or aluminium . The threaded bar technique is ideal for the testing of ring like specimens. e. When using a bar that is not covered with insulating material.is threaded through the bore. similarly to when using the headshot and will be specified by the Procedure Code. two conductors may be used on larger diameter test pieces. Sometimes known as the central conductor method.0 CENTRAL CONDUCTOR TECHNIQUE (INDUCTION METHOD) Induction MPI methods do not necessarily require any contact between the magnetising apparatus and the test specimen. of the current flow. A hollow part can be examined for discontinuities on the inside diameter of the part as well as on the outside. therefore the technique is used for detecting defects parallel to. Alternatively. The object being examined must be of hollow section and access must be available to both ends. care should be taken to ensure that components in contact with the bar cannot touch any part of the magnetic equipment at earth potential. but on larger diameters the conductor is often placed to one side to ensure sufficient flux strengths and the test piece rotated to allow for surface inspection. DC.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.  May leave residual fields.  Relatively fast coverage of area under test. PAGE 42 OF 70 .  Expensive equipment.  Low voltage output.  AC energised equipment may be used for demagnetisation operations. HWDC. on/off control. Disadvantages include:  Heavy transformer required.0 Advantages of the Central Conductor technique include:  AC.  No poles to attract magnetic particles.  Variable field strength.  No current through part. and FWDC equipment available. 0 MULTIDIRECTIONAL TECHNIQUE For this technique. magnetization is accomplished by high amperage power packs operating as many as three circuits that are energized one at a time in rapid succession. The effect of these rapidly alternating magnetizing currents is to produce an overall magnetization of the part in multiple directions.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. PAGE 43 OF 70 . Circular or longitudinal magnetic fields may be generated in any combination using the various techniques. longitudinal magnetization can be produced by wrapping the cable into the coil. Prods and cables are usually used with the mobile equipment.0 Whichever technique or combination of techniques are used to produce the magnetic flux in the part. However. or connecting to a central conductor. maximum sensitivity will be to linear discontinuities oriented perpendicular to the lines of flux. For optimum effectiveness in detecting all types of discontinuities. Typical portable equipment operates on 220 volts AC with an output of between 500 and 3 000 amperes. As with mobile equipment. One or more of the following techniques may be used: LONGITUDINAL MAGNETISATION CIRCULAR MAGNETISATION PERMANENT MAGNET PRODS AC YOKE HEAD SHOT DC YOKE CLAMPS RIGID COIL BAR CENTRAL CONDUCTOR CABLE WRAP COIL CABLE CENTRAL CONDUCTOR MULTI DIRECTIONAL YOKE OR BENCH UNIT TYPES OF POWER SUPPLIES PORTABLE EQUIPMENT Portable equipment is lighter and less expensive than the other types of magnetic particle testing equipment. with the lines of flux during one examination being approximately perpendicular to the lines of flux during the other. The cables used on mobile equipment vary from 5 meters to 30 meters. MOBILE EQUIPMENT Typical mobile equipment usually operates on 220 / 380 volts AC and will produce about 8 000 amperes. wrapping into a coil. the cables can be used for prods. Shorter cables will permit the maximum current output.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. These units usually have a choice of either AC or HWDC. each area is to be examined at least twice. PAGE 44 OF 70 . Mobile equipment will usually produce both AC and HWDC magnetizing current. It is also possible to use a central conductor clamped between the two cables to produce circular magnetization. Either the visible or fluorescent particles can be used. In some units. The tail stock can be moved and locked into place to accommodate parts of various lengths. or full wave DC. A pump and hose system is used to apply the particle solution to the components being inspected. A circular magnetic field is produced with direct magnetization.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 STATIONARY EQUIPMENT Stationary magnetic particle inspection equipment is designed for use in laboratory or production environment. To assist the operator in clamping the parts. The most common stationary system is the wet horizontal (bench) unit. Some of the systems offer a variety of options in electrical current used for magnetizing the component. half wave DC. which uses the coil and decaying AC. a demagnetization feature is built in. the contact on the headstock can be moved pneumatically via a foot switch. Most units also have a movable coil that can be moved into place so the indirect magnetization can be used to produce a longitudinal magnetic field. The units have head and tail stocks (similar to a lathe) with electrical contact that the part can be clamped between. The operator has the option to use AC. The wet magnetic particle solution is collected and held in a tank. Wet horizontal units are designed to allow for batch inspections of a variety of components. Most coils have five turns and can be obtained in a variety of sizes. PAGE 45 OF 70 . it is usually necessary to demagnetize the component. it will go through a reverse transformation and will contain no residual magnetic field. Remanent magnetic fields can:  Affect machining by causing cuttings to cling to a component. Any method of demagnetization will combine one of the methods to reduce the magnetizing field with one of the methods to reverse the magnetizing field.  Move the part away from the coil / yoke. REVERSING THE MAGNETIC FIELD  Reversing the part in the magnetic field. The Curie temperature for a low carbon steel is 770 oC or 1390oF.  Move the coil / yoke away from the part. Each time the magnetizing field is reduced and reversed.  Create a condition known as "arc blow" in the welding process. it will become austenitic and loses its magnetic properties.  Interfere with electronic equipment such as a compass. This random orientation of the magnetic domains can be achieved most effectively by heating the material above its curie temperature. Removal of a field may be accomplished in several ways.  Reversing the current through the coil.0 CHAPTER 6 – DEMAGNETISATION After conducting a magnetic particle inspection. When steel is heated above its curie temperature.  Cause abrasive particles to cling to bearing or faying surfaces and increase wear.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. REDUCING THE MAGNETIC FIELD  Reduce the magnet current. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the Earth's magnetic field. PAGE 46 OF 70 . the residual field is reduced. When it is cooled back down. Arc blow may cause the weld arc to wonder or filler metal to be repelled from the weld.  Reversing the coil (turn the coil 180°). so another method that returns the material to a nearly unmagnetized state is commonly used. A field meter is often used to verify that the residual flux has been removed from a component. Also. This can be accomplished by pulling a component out and away from a coil with AC passing through it. Industry standards usually require that the magnetic flux be reduced to less than 3 gauss after completing a magnetic particle inspection. The same can also be accomplished using an electromagnetic yoke with AC selected. Subjecting the component to a reversing and decreasing magnetic field will return the dipoles to a nearly random orientation throughout the material. PAGE 47 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. many stationary magnetic particle inspection units come with a demagnetization feature that slowly reduces the AC in a coil in which the component is placed.0 It is often inconvenient to heat a material above its curie temperature to demagnetize it. The Procedure Code will specify limits and different procedures as required. Additionally. Dry magnetic particle products are produced to include a range of particle sizes. the particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to defects. The size of the magnetic particles is also very important. therefore. dry testing particles cannot be made exclusively of the fine particles. Particles are available in a dry mix or a wet solution. The metal used for the particles has high magnetic permeability and low retentivity. gray.0 CHAPTER 7 – DETECTION MEDIUM As mentioned previously. Also. PAGE 48 OF 70 . and are about three times smaller in diameter and more than 20 times lighter than the coarse particles (0. windy conditions can reduce the sensitivity of an inspection. The fine particles are around 0. such as flaws. A pigment (somewhat like paint) is bonded to their surfaces to give the particles colour.002 inch) in size. small particles easily adhere to surface contamination. Particles start out as tiny milled (a machining process) pieces of iron or iron oxide. High magnetic permeability is important because it makes the particles attract easily to small magnetic leakage fields from discontinuities. DRY MAGNETIC PARTICLES Dry magnetic particles can typically be purchased in red. such as remnant dirt or moisture. It should also be recognized that finer particles will be more easily blown away by the wind. Low retentivity is important because the particles themselves never become strongly magnetized so they do not stick to each other or the surface of the part. and get trapped in surface roughness features. However.05 mm (0. yellow and several other colours so that a high level of contrast between the particles and the part being inspected can be achieved. black. This make them more sensitive to the leakage fields from very small discontinuities.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. reclaiming the dry particles is not recommended because the small particles are less likely to be recaptured and the "once used" mix will result in less sensitive inspections. Coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature.15 mm). 0001 mm. and are easier to clean from the part. the oxide particles are present mostly in clusters that settle out of suspension much faster than the individual particles. Water-based carriers form quicker indications. give off no petrochemical fumes. PAGE 49 OF 70 . The particles used with the wet method are smaller in size than those used in the dry method for the reasons mentioned above. due to their slight residual magnetism. research has shown that if dry powder consists only of long. However. Wet method magnetic particles products differ from dry powder products in a number of ways. The mix of globular and elongated particles result in a dry powder that flows well and maintains good sensitivity. Water-based solutions are usually formulated with a corrosion inhibitor to offer some corrosion protection. Therefore. This makes it possible to see and measure the concentration of the particles for process control purposes. Particles that fluoresce green-yellow are most common to take advantage of the peak colour sensitivity of the eye but other fluorescent colours are also available. slender particles tend align themselves along the lines of magnetic force. Most non-fluorescent particles are ferromagnetic iron oxides.01 mm and smaller and the synthetic iron oxides have particle diameters around 0.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. globular particles are added that are shorter. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Wet particles are also a mix of long slender and globular particles. Most dry particle mixes have particles with L/D ratios between one and two. are generally less expensive. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or eliminated. The carrier solutions can be water or oil-based. WET MAGNETIC PARTICLES Magnetic particles are also supplied in a wet suspension such as water or oil. slender particles. The particles are typically 0.0 The particle shape is also important. One way is that both visible and fluorescent particles are available. the application process would be less than desirable. This very small size is a result of the process used to form the particles and is not particularly desirable. However. However. as the particles are almost too fine to settle out of suspension. which are either black or brown in colour. present little or no fire hazard. Elongated particles come from the dispenser in clumps and lack the ability to flow freely and form the desired "cloud" of particles floating on the component. Long. oil-based carrier solutions offer superior corrosion and hydrogen embrittlement protection to those materials that are prone to attack by these mechanisms. The wet method also makes it easy to apply the particles uniformly to a relatively large area. MAGNETIC RUBBER The magnetic rubber technique was developed for detecting very fine cracks and is capable of revealing finer cracks than other magnetic techniques. Also. PAGE 50 OF 70 . Petroleum based carriers are primarily used in systems where maintaining the proper particle concentration is a concern. The trade-off. of course. such as the threads on the inside diameter of holes. is that inspection times are much longer and high cost. so anti-foaming agents must be added. low fire hazard. Modern solvent carriers are specifically designed with properties that have flash points above 93°C and keep nocuous vapours low.0 Suspension liquids used in the wet magnetic particle inspection method can be either a well refined light petroleum distillate or water containing additives. petroleum based carriers might be a better choice for a system that gets only occasional use or when regularly adjusting the carrier volume is undesirable. the technique can be used to examine difficult to reach areas. and the ability to form indications quicker than solvent-based carriers. The wetting agents create foaming as the solution is moved about. Therefore.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The petroleum based carriers require less maintenance because they evaporate at a slower rate than the water-based carriers. Additionally. where the moulded plugs can be removed and examined under ideal conditions and magnification if desired. water-based carriers are used more because of low cost. since water promotes corrosion in ferrous materials. However. Petroleum-based liquids are the most desirable carriers because they provided good wetting of the surface of metallic parts. corrosion inhibitors are usually added as well. Water-based carriers must contain wetting agents to disrupt surface films of oil that may exist on the part and to aid in the dispersion of magnetic particles in the carrier. The magnetic particles migrate to the leakage field caused by a discontinuity. coils or central conductors. The rubber cast is examined for evidence of discontinuities. direct current. The moulding can be retained as a permanent record of the inspection.0 The techniques uses a liquid (uncured) rubber containing suspended magnetic particles. all relevant health and safety data applicable to the supplied goods. A dam of modelling clay is often used to contain the compound in the region of interest. Do not use in confined spaces without masks or adequate ventilation. which appear as dark lines on the surface of the moulding. HEALTH AND SAFETY CONSIDERATIONS The supplier of MPI consumables is obliged to make available to the purchaser. The rubber cast is removed from the part. Use protective clothing. which is maintained while the rubber sets (active field). See also the COSHH Regulations in Unit MT11. Specific health and safety considerations are: Flammability: Asthmatic: Skin hazard: Read container labels for flash points. prods. PAGE 51 OF 70 . clamps. The direct current yoke is the most common magnetization source for magnetic rubber inspection. Alternating. or permanent magnets may be used to draw the particles to the leakage fields. discontinuity indications remain in place on the rubber. As the rubber cures. The system may include yokes. The user also has an obligation to comply with the health and safety requirements. Magnetic rubber methods requires similar magnetizing systems used for dry method magnetic particle tests. Inspections can be performed using either an applied magnetic field. The rubber compound is applied to the area to be inspected on a magnetized component.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The rubber is allowed to completely set. The rubber conforms to the surface contours and provides a reverse replica of the surface. which takes from 10 to 30 minutes. or the residual field from magnetization of the component prior to pouring the compound. MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 PAGE 52 OF 70 . 0 PAGE 53 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. 0 PAGE 54 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. 0 PAGE 55 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. VISIBLE METHOD Magnetic particle inspections that use visible particles can be conducted using natural or artificial lighting. lighting is a very important element of the inspection process. the use of artificial lighting is recommended. The light intensity usually required is 100 foot-candles or 1 000 Lux on the surface being inspected. To improve the uniformity of lighting from one inspection to the next. the lighting requirements are different for an inspection conducted using visible particles than they are for an inspection conducted using fluorescent particles. When using natural lighting. Inspector must constantly stay aware of the lighting conditions and make adjustments when needed. Artificial lighting should be white whenever possible and white flood or halogen lamps are most commonly used.0 CHAPTER 8 – VIEWING CONDITIONS ELECTROMAGNETIC SPECTRUM Magnetic particle inspection predominately relies on visual inspection to detect any indications that form.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. Therefore. It is advisable to choose a white light wattage that will provide sufficient light. but avoid excessive reflected light that could distract from the inspection. it is important to keep in mind that daylight varies from hour to hour. PAGE 56 OF 70 . Obviously. In fact. at start-up of the inspection cycle. The required check should be performed when a new bulb is installed. a bulb that is near the end of its operating life will often have an intensity of only 25% of its original output. or other foreign material can reduce the intensity or light by as much as 50%. it is important to keep white light to a minimum as it will significantly reduce the inspector’s ability to detect fluorescent indications. A bulb that produces acceptable intensity at 120 volts will produce significantly less at 110 volts.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The filter should be checked visually and cleaned as necessary before warming-up the light. Black light intensity will also be affected by voltage variations.0 FLUORESCENT METHOD When performing a magnetic particle inspection using fluorescent particles. For UV lights used in component evaluations. PAGE 57 OF 70 . the normally accepted intensity is 1000 µWatts/cm2 on the surface of the part. Black lights should never be used with a cracked filter as the output of white light and harmful black light will be increased. so it is important to provide constant voltage to the light. Light levels of less than 20 Lux are required by most procedures and some procedures require it to be less than 5 Lux at the inspection surface. if a change in intensity is noticed. The cleanliness of the filter should also be checked regularly since a coating of solvent carrier. oil. Standards and procedures require verification of lens condition and light intensity. Regularly checking the intensity of UV lights is very important because bulbs lose intensity over time. These days most of these limitations are not applicable due to the fact that LED Torches produces instant UV-A light at full potential without any filters. When performing a fluorescent magnetic particle inspection. Some specifications require that a white light intensity measurement be made at 380 mm from a UV light source to verify that the white light is being removed by the filter. the condition of the ultraviolet light and the ambient white light must be monitored. or every eight hours if in continuous use. PAGE 58 OF 70 . The output of a UV bulb spans a wide range of wavelengths. The short wavelengths of 312 to 334 nm are produced in low levels. it should not be used because harmful effects such as skin burns and eye damage can occur. A peak wavelength of 365 nm is produced at a very high intensity. The desired wavelength range for use in NDT is obtained by filtering the ultraviolet light generated by the light bulb. which is the safest to work with. This wavelength range is used because it is in the UV-A range. UV-C (100 nm to 280 nm) is even more dangerous to living cells and is used to kill bacteria in industrial and medical settings. The filter allows only radiation in the range of 320 to 400 nm and a little visible dark purple to pass. however. UV-B will do an effective job of causing substances to fluoresce.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. This wavelength of radiation is found in the arc created during the welding process.0 The desired wavelength range for use in non-destructive testing is between 350 nm and 380 nm with a peak wavelength at about 365 nm. yellow (622nm) and orange (677 nm) are also usually produced. green-yellow (546 nm). Wavelengths in the visible violet range (405 nm to 435 nm). A unit should be checked to make sure its calibration is current before taking any light readings. Some radiometers have the ability to measure both white and UV light.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. When an external circuit is connected to the cell. but is mostly available as separate units. The sensing area should always be kept clean and free of materials that could reduce or obstruct light reaching the sensor. an electrical current is produced. Radiometers are relatively unstable instruments and readings often change considerably over time. PAGE 59 OF 70 . Nowadays there are more sophisticated digital Light meters available. light intensity measurements where made using a radiometer. This current is linear with respect to incident light.0 LIGHT MEASUREMENT Historically. while others require a separate sensor for each measurement. some types incorporate UV-A and LUX in one device. they must be calibrated regularly. Therefore. Light striking a silicon photodiode detector causes a charge to build up between internal layers. They should be calibrated at least every six months. A radiometer is an instrument that translates light energy into an electrical current. it is annoying and interferes with vision while it exists. UV light will be reflected from surfaces just as white light will. one should be careful not to look directly into lights and to hold spot lights to avoid reflection. the fluid that fills the eye fluoresces. Special filtered glasses may be worn by the inspector to remove all UV light from reaching the eyes but allowing yellow-green light from fluorescent indications to pass. PAGE 60 OF 70 . When working around ultraviolet lights. This condition is called ocular fluorescence. To prevent injury. Technicians should never wear darkened or photo chromatic glasses as these glasses also filter or block light from fluorescent indications. so it is advisable to consider placement of lights to avoid this condition. and while it is considered harmless. When ultraviolet light enters the human eye.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 SAFETY UV-A lamps operate with wavelengths between 315-400 nm. shorter wavelengths than this can cause injuries to the eyes. a filter should be used which cuts out wavelengths below 315 nm. multiple longitudinal and circular magnetization or circular magnetization in multiple directions.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. and location as those expected in the test component. When the orientation of a defect is not well established. If the magnetizing current is excessively high when performing a wet fluorescent particle inspection. particles can be attracted to the surface of the part and not be allowed to migrate to the flux leakage fields of defects. When performing a dry particle inspection. It is difficult to detect discontinuities that intersect the magnetic field at an angle less than 45°. this may require longitudinal magnetization in two or more directions. Depending on the geometry of the component. Castrol Strips. If the fields are not balanced. FIELD STRENGTH The applied magnetic field must have sufficient strength to produce a satisfactory indication. When the field strength is excessive. Most specifications call for a field strength of 30 to 60 gauss at the surface when the magnetizing force is applied. Berthold Spoon or ASME Pie Gauge can sometimes be used. a vector field will be produced that may not detect some defects.0 CHAPTER 9 – MAGNETIC FIELD INDICATORS FIELD DIRECTION Determining the direction of the field is important when conducting a magnetic particle inspection because the defect must produce a significant disturbance in the magnetic field to produce an indication. Determining strength and direction of the fields is especially critical when inspecting with a multidirectional machine. Furring is when magnetic particles build up at the magnetic poles of a part. Adequate field strength may be determined by:   Performing an inspection on a standard specimen that is similar to the test component and has known or artificial defects of the same type. PAGE 61 OF 70 . an excessive longitudinal magnetic field will cause furring. Using a gauss meter with a Hall Effect probe to measure the peak values of the tangent field at the surface of the part in the region of interest. size. but not so strong that it produces non-relevant indications or limits particle mobility. components should be magnetized in a minimum of two directions at approximately right angles to each other. the magnetic field is forced out of the part before reaching the end of the component and the poles along its length attract particles and cause high background levels. The silver finish strips will show higher field strengths and are used in aerospace industries or areas that require a more critical examination. if the field is strong enough.0 PORTABLE MAGNETIC FIELD STRENGTH METERS (MAGNETOMETERS) To measure residual magnetism. SHIM TYPE/FOIL STRIPS (TRADE NAME CASTROL STRIPS/ELY STRIPS) These are small brass or silver finish strips containing artificial defects (3 slots) in Mu metal. The vane aligns itself with the magnetic field. manganese and iron. calibrated magnetic field strength meters of the type which contains a soft iron vane are commonly used. The brass finish strips are commonly used in general engineering applications and indicate low field strengths. three linear indications will show on the strip. they should be rotated on the specimen surface to define field direction. may cause damage to the instrument. Neither of these instruments should be brought into close contact with any strong magnetic fields as this.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. the strength and direction of the field is measured by the meter calibrated in gauss or Tesla's. In either case. similar non-calibrated instruments exist. Mu metal is a highly permeable material containing nickel. PAGE 62 OF 70 . the separating lines between the sections forming artificial defects. To determine the direction of the magnetic field the indicator should be rotated until one of the lines is perfectly visible. One side also has a chromium coating applied.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. this has a greater sensitivity than the opposite side which shows surface breaking defects. THE BERTHOLD PENETRAMETER (BERTHOLD SPOON) This field indicator contains two artificial defects at 90˚ to one another. There is a sensitivity adjustment consisting of a cover plate which can be turned to one of four setting which vary the distance of the cover plate and detecting media from the artificial defects. PAGE 63 OF 70 . When the indicator is placed on a magnetised component the magnetic field passes through the indicator. at this point the direction of the field is at 90˚ to the line. the greater gap the higher the field strength necessary to render the lines visible.0 THE ASME FIELD INDICATOR This consists of eight sections of mild steel brazed together. They are used in a manner similar to the ASME indicator. The separating lines between the sections become visible when the detecting media is applied. if this is the case. NOTE Magnetic field strength meters and portable flux indicators should not be used in conjunction with permanent magnets or with DC electromagnets for determining adequate flux density. as their name suggest. PAGE 64 OF 70 . The Hall Effect meter is calibrated with a series of standard magnets which provide known values of field strength within the working range of the instrument. Many terminologies are encountered when referring to types of magnetic field indicators. the magnitude of this voltage is directly proportional to the magnetic flux density within the probe. A flux indicator may be used with these magnets but only to verify the suitability or correct application of an ink or powder. The relative permeability of the semi-conducting crystal is very close to one. the field will not be detected. or axial fields depending on the probe design. when the probe is placed in a magnetic field.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. normal. Manufacturers supply a zero-field chamber which is required to set the zero. Circular fields may be contained completely within a specimen. however they will measure flux in the air not just on the surface of the test piece. An electric current is passed through a small semi-conducting crystal contained within a probe.0 HALL EFFECT METERS These instruments are usually used in laboratory type environments to measure tangential. The voltage generated by the probe is measured by an electronic circuit and indicated on a meter marked in units of field strength. which can be confusing. Hall effect meters use. the value of which depends upon the shape of the waveform and upon the measuring principle employed by the instrument. There are also alternative methods other than those listed below to check for residual fields. the Hall effect principle. A voltage is generated across the crystal. If the peak value of a time varying field is required. measuring the value with an advantage of not requiring probe movement to gain a reading. so the voltage generated is proportional to the magnetic field strength at the point where the probe is placed. the meter reading is multiplied by a conversion factor. d) Note the applied amperage and continue to apply the current progressively until the second and third holes are noted.) can obviously have an effect on the sensitivity of an inspection. CALIBRATION OF LIGHT METERS AND THERMOMETERS All Light Meters and Thermometers shall be calibrated every six months to 1 year. Note the amperage setting as the holes become visible. c) Slowly introduce a current and apply ink until the first hole (closest to the external surface) is visible. Therefore.0 CHAPTER 10 – PERFORMANCE CHECKS CALIBRATION OF ELECTRICAL SYSTEMS Changes in the performance of the electrical system of a magnetic particle inspection unit (POWER SUPPLY. etc. COIL SHOT. the following test should be carried out for each wave form available for use: a) Ensure the test piece is thoroughly demagnetised and pre-cleaned satisfactorily. by the manufacturer or an authorised laboratory. PROD UNIT. b) Position the test piece (see diagram below) between the head and tailstock of the bench equipment. when a malfunction is suspected. this test block shall be used in the same way than with the current flow technique. MAGNETIC FLOW PERFORMANCE CHECK For the Coil Technique. or every six months to twelve months as required by the Specification. BENCH UNIT. the electrical system (Ammeter and Timer) must be checked when the equipment is new. CURRENT FLOW PERFORMANCE CHECK When current flow techniques are used on bench units. PAGE 65 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. as stipulated by the Specification. 5 kg at the maximum pole spacing that will be used. A weight need only be verified again if damaged in a manner that could have caused potential loss of material. Each direct current or permanent magnetic yoke shall have a lifting power of at least 18 kg at the maximum pole spacing that will be used. Each alternating current electromagnetic yoke shall have a lifting power of at least 4. the magnetizing power of electromagnetic yokes shall have been checked within the past year. The magnetizing power of permanent magnetic yokes shall be checked daily prior to use. YOKE LIFTING TEST     Prior to use. Each weight shall be weighed with a scale from a reputable manufacturer and stencilled with the applicable nominal weight prior to first use.0 ALTERNATIVE TEST BLOCKS Alternative test blocks may be used to compare different batches of Inks as well as different equipment to establish sensitivity.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. The magnetizing power of all yokes shall be checked whenever the yoke has been damaged or repaired. PAGE 66 OF 70 . Glasses or lenses worn by examiners shall not be photosensitive.2 to 2.1 to 0. (Less than 20 Lux) Examiners shall be in a darkened area for at least 5 min prior to performing examinations to enable their eyes to adapt to dark viewing. and at the completion of the examination or series of examinations.4 ml in a 100-ml bath sample and from 1. whenever the light’s power source is interrupted or changed. Reflectors and filters should be checked and. Cracked or broken filters shall be replaced immediately. the required settling volume is from 0. if necessary. UV-LIGHT CALIBRATION TEST      It shall be performed in a darkened area. the suspension should be run through the recirculating system for at least 30 min to ensure thorough mixing of all particles which could have settled on the sump screen and along the sides or bottom of the tank. The black light intensity shall be measured with a black light meter prior to use. Before sampling.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.0 SETTLING TEST • • • This is accomplished through the use of an ASTM Test Method D 96 pear-shaped centrifuge tube. Black lights shall achieve a minimum of 1000 μW/cm2 on the surface of the part being examined throughout the examination. cleaned prior to use. Take a 100-ml portion of the suspension from the hose or nozzle. Concentration checks shall be made at least every eight hours. The volume settling out at the bottom of the tube is indicative of the particle concentration in the bath. demagnetize and allow it to settle for approximately 60 min with petroleum distillate suspensions or 30 min with water-based suspensions before reading. PAGE 67 OF 70 . For fluorescent particles.4 ml per 100 ml of vehicle for non-fluorescent particles unless otherwise specified by the particle manufacturer. e. LINEAR INDICATION: L > 3W ROUND INDICATION: L ≤ 3W SPECIFICATIONS Design engineers predetermine the acceptance criteria. grinding grooves.an interruption in the normal physical structure or configuration of a part. rejectable or needs rework.particles accumulated and held at a site by a leakage field. i. Indication . hair. CLASSIFICATION First determine whether the indication is round or linear (three times as long as wide). Non-Relevant or Relevant. dirt. EVALUATION DEFINITIONS Discontinuity . scale. INTERPRETATION To decide what condition caused the indication. NON-RELEVANT INDICATIONS (Caused by Discontinuities or may be a design feature) Caused by design features such as rivets. All relevant indications MUST be evaluated according to Acceptance standards. electrical interference (UT / ECT). thick background coating (MT).0 CHAPTER 11 – INTERPRETATION VS.5mm (ASME VIII) or an indication that is supposed to be there (part of manufacturing process). False. film marks (RT). fingerprints.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. weld curves or indications smaller than 1. RELEVANT INDICATIONS (Caused by Discontinuities – bad for part) Caused by discontinuities and can affect the service life of a part. PAGE 68 OF 70 . FALSE INDICATIONS (Not caused by Discontinuities) Can be caused by too high amperage (MT). rust (MT / PT) and does not necessarily break the surface continuity (MT / PT / UT / ECT /RT). False indications SHALL be eliminated and the part re-tested. EVALUATION To decide whether the indication is acceptable. Standards are written in clear specifications and must be adhered to at all times. lint. m) name. The test report shall then contain the test results. and in particular the magnetising levels. A common method of recording indications is to reproduce indications on a scaled diagram. g) magnetising technique. Adequate reporting is essential for the transmission of relevant and correct information after the test. tangential fields strengths. b) work location. including (as appropriate) indicated current values. h) detecting media used and contrast paint if used. PRESERVATION OF INDICATIONS Prior to the recording of indications. A separate diagram showing magnetising techniques should ideally be included where it is not obvious which technique has been applied. e) reference to the written test procedure and the technique sheets used. 5. A typical report would require the following: a) name of the company. Indications should be drawn with references to a datum on the test piece. before or after final machining). 3. 4. i) surface preparation. coil dimensions etc. 2. j) viewing conditions. l) date of test. Methods of recording indications are: 1. d) stage of test (e.. are as near as possible to the level recommended for the technique. Photographs Sketches Clear sticky tape Magnetic rubber Clear Lacquer PAGE 69 OF 70 . It is essential that a common datum be established on both the work piece and the record and that care be taken not to disturb the indications. c) description and identity of the part tested. k) method of recording or marking of indications. Any test report should include the information required by the relevant specification for the work being performed. contact or pole spacing. Mark out indications on test object so they can be repaired or reworked. waveform. it is essential to ensure that the test conditions.g. use two diagrams if necessary. including a detailed description of the indications and a statement as to whether they meet the acceptance criteria. before or after heat treatment. qualification and signature of the person performing the test. f) description of equipment used. The diagram(s) should not be overloaded with too much information.0 REPORTING Measure each relevant indication and fill out a detailed report.MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV. 0 SUMMARY OF DISCONTINUITIES PAGE 70 OF 70 .MAGNETIC PARTICLE TESTING LEVEL 1 & 2 COMBINED – NOTE BOOK NASA-MT-2001 REV.