Plastics Fatigue



Paper Title: Significance of Fatigue Testing Parameters in Plastics versus MetalsAuthor’s Name: Mehrdad Zoroufi Organization: Element Materials Technology, New Berlin, Wisconsin, USA Fatigue behavior of plastics has seen a growing attention in the past few years in automotive, aerospace, medical, and other leading industries. New testing techniques and standards are being developed to address this rapidly increasing industrial and scientific need. On the other hand, metal fatigue, its various aspects, testing techniques and scientific details have been practiced and documented significantly in the past few decades. Plastics are affected differently by fatigue test parameters than metals, and this fact necessitates specific approaches that, in many cases, are different from those of metals. In fatigue testing, plastics have shown more sensitivity to many parameters including geometry, loading mode, stress or strain amplitude, mean stress, stress or strain rate, R-ratio, frequency, moisture and temperature. The effect of many of these parameters, like strain rate, frequency and temperature are more pronounced than metals. This study intends to address the distinctions between plastics and metals with respect to fatigue testing parameters. Common applicable fatigue test standards are compared, the significance of each standard is elaborated, and differences are magnified. Fatigue crack initiation characteristics including stress-based and strain-based methods as well as fatigue crack propagation methods and standards are discussed. A number of experimental results are also presented to further distinguish the distinctions with respect to frequency. Outline Introduction - Plastics and Fatigue Testing Effective Parameters in Fatigue Behavior of Plastics Standards in Fatigue Testing of Metals versus Plastics Case Study – Frequency Effects on Fatigue Life of a Polymer Conclusions References depending on the thermal conductivity of the material. force. frequency and type of loading directly affect the hysteresis loss. Several factors such as temperature susceptibility even at room temperature. and large differences between tensile and compressive strengths. For instance. Parameters like stress level. under cyclic loading. a set of material tests are to be completed by an analysis and life prediction. stress concentration. the effects of significant parameters that influence fatigue behavior of plastics should be assessed. The experimental fatigue study of plastics should consist of a multi step procedure. thermal fatigue failure that involves thermal softening (yielding) and mechanical fatigue failure that is due to the conventional fatigue crack initiation and propagation under cyclic loads [2]. displacement or strain. chain scission. as well as a (limited) number of full scale part or component test. waveform. a proper test control method should be selected. i. depending on force (stress) level and frequency. In general. Plastics. In general. Increasing mean stress results in increasing or decreasing crack propagation rate in plastics depending on their failure mechanism. While ABS undergoes shear yielding. temperature increases associated with testing. plastics that are more susceptible to crazing.Plastics and Fatigue Testing Experimental fatigue investigation of engineering plastics or polymers is a newer subject of researches compared to metals. Third. especially metals. frequency. while the portion of the loop for high-impact polystyrene (HIPS) in tension is much larger than the compressive portion. provide significant confidence to the engineer with regard to the combined material testing and analysis he already performed. or cross-link rupture. . Mean stress is a significant parameter affecting fatigue life of plastics. HIPS fails under crazing that requires a tensile component of stress. though in many cases complicated and costly. experience an increase in crack propagation rate with increasing mean stress. If a part or component test is aimed. part of the strain energy generated in the material converts into heat and. some plastics with a molecular structure susceptible to hysteretic loss or capable of structural reorganization during crack propagation exhibit decreased crack propagation rate with increasing mean stress. it has been shown [3] that a relatively symmetric loop could be obtained under fully reversed cyclic loading of acrylonitrilebutadienestyrene (ABS). craze development. have low thermal conductivity that makes them more susceptible to this phenomenon. Two main mechanisms have been observed in fatigue failure of many plastics. They are to be distinguished from that of other categories of materials. environment. The shape of hysteresis loop differs in various plastics. Many of the established test methods for metals were adopted in plastics fatigue testing. first.e. These may include (but not limited to) mean stress. On the other hand. a consistent laboratory condition should be maintained for all of the tests that a comparison is targeted. increases the body temperature. These full-scale tests. Effective Parameters in Fatigue Behavior of Plastics Temperature is one of the most influential factors in plastics fatigue and the temperature increase due to hysteresis loss plays an important role in this regard. in general.Introduction . This is attributed to the difference in failure mechanism of the material. due to the differences in microstructure. Second. large inelastic deformation and cyclic softening are among factors to be considered for plastics fatigue testing [1]. and thermal effects due to cycling. proximity of test temperature and melting point. fatigue crack growth. frequencies ranging from 1 to 200 Hz have only a small effect on S-N behavior for most structural metals [6]. Fatigue strength is generally unaffected for most metallic materials in the regime of 0. decrease the crack propagation rate in amorphous polymers [4]. Compressive residual stresses generated at the crack tip as a result of unloading following a tensile overload. more pronounced in ductile polymers. which are mostly due to viscoelastic structure of these materials. Standards in Fatigue Testing of Metals versus Plastics ASTM methods or practices for fatigue testing of metals have been developed and are widely used in industry and academia. also. The methods and practices available for strain controlled fatigue. although a number of those developed for metals can be extended and used in plastics fatigue testing. Crack tip blunting could be categorized as thermal and plastic blunting mechanisms. fatigue of plastics involves the effects of a number of parameters that either are not significant in metals. . as well as their usage and scope. Fretting fatigue testing of plastics has not been addressed directly yet. thermomechanical fatigue. In addition. Test methods for force controlled fatigue testing of plastics have been developed. Increase in residual stresses. Crack closure. a number of ASTM guidelines are developed for analysis of the test data. If heating and corrosion effects are negligible or controlled. Test methods for plastics are newer and limited. which reduces the stress intensity factor range and retards the fatigue crack propagation [3]. increasing frequency and loading/straining rate have detrimental effect on fatigue life of most plastics. Plastic crack tip blunting occurs as the crack propagates and the crack surfaces move with respect to each other. crack closure and blunting are some of the influential mechanisms. induces compressive residual stresses at the crack.01 to 100 Hz [5]. Table 1 list the major ASTM standards or guidelines related to fatigue testing. or affect the fatigue life with a different mechanism. Pronounced sensitivity to strain rate was reported for a number of polymers [7].Tensile and compressive overloads could result in increased or decreased fatigue crack propagation and life depending on the structure of the polymer or the propagation mechanism. hysteretic effects. Thermal crack tip blunting occurs due to energy dissipation and local heating. frequency and force/strain rate. On the other hand. However. These may include temperature. creep fatigue and creep fatigue crack growth testing can be used for plastics. reducing the crack propagation rate. not full scale components Uniform axial gage section or hourglass specimens (round or flat) Inelastic strain (mechanical or thermal) is significant compared to elastic strain Applicable to failure in relatively low cycles (<105 cycles). not full scale components As a system response. Round bar uniform gage section specimens subjected to uniaxial loading. although it could be used for higher cycles as well Important for components that undergo overloads Cyclic hardening or softening can be determined Strain rate. instead of a material response. metallic materials Specific ASTM Standards for Fatigue Testing of Plastics D7791 Standard Test Method for Force controlled. relatively elastic Frequency: 1-25 Hz.Table 1 – Applicable ASTM standards for fatigue testing Practice/ Method Title Usage Number Applicable ASTM Standards for Fatigue Testing E466 Standard Practice for Force controlled. nominally homogeneous materials Force controlled. less than 5 Hz recommended Uniform gage section specimens subjected to uniaxial loading. line. Uniaxial Fatigue Properties of plastics Plastics D7774 Standard Test Method for Flexural Fatigue Properties of Plastics Force controlled. Conducting Force Controlled metals Constant Amplitude Axial Fatigue Tests of Metallic Materials E606 Standard Test Method for Strain controlled. less than 5 Hz recommended Three or four point bending systems are used. depends on both the material of the fatigue specimen and fretting pad. plastics Comments Strains are predominantly elastic Air and room temperature Covers testing of unnotched and notched specimens. not full scale components Originally specified for metals. not full scale components Constant-amplitude load-controlled tests with controlled loading/unloading rates or hold-times at the maximum load. no material limitation E2368 Standard Practice for Strain Controlled Thermomechanical Fatigue Testing E2714 Standard Test Method for Creep-Fatigue Testing E2760 Standard Test Method for Creep-Fatigue Crack Growth Testing Strain and temperature controlled. or both Applicable to temperatures and hold-times for which the magnitudes of time-dependent inelastic strains at the crack tip are significant in comparison to the time-independent inelastic strains Compact tension specimens Two types of creep-ductile (dominant creep strains) and creep brittle (creep strain is less than elastic strains at crack tip) Uniform gage section specimens subjected to uniaxial loading. nominally homogeneous materials E2789 Standard Guide for Fretting Fatigue Testing Fretting loading (controlled forces and displacements). and the method by which the loading and displacement are imposed Point. Strain-Controlled Fatigue homogeneous Testing materials E647 Standard Test Method for Measurement of Fatigue Crack Growth Rates Force controlled. relaxation and creep effects can be derived Uniform axial gage section or hourglass specimens (round or flat). not full scale components Specimens cut from sheets/plate or molded Density measurement needed 3 specimens at each of the four stress or strain level to be tested Specimen may need conditioning Temperature and media rather than room temperature and air not addressed. relatively elastic Fatigue tests in tension. not intended for full scale components . compression fatigue tests only for rigid plastics Frequency: 1-25 Hz. the geometry of contact between the two. no material limitation Force or strain controlled. tubular specimens are preferred Fatigue testing at strain rates or with cycles involving sufficiently long hold times to be responsible for the cyclic deformation response and cycles to crack formation to be affected by creep (and oxidation). specimens should be large enough to ensure predominantly elastic loading and avoid buckling No limitation on specimen type Residual stresses and crack closure are not addressed In-phase and out of phase independent control of strain and temperature Uniform axial gage section specimens. has also been applied to polymer No material limitation. Fully reversed testing in flexure Rigid and semi rigid plastics Stress and strain levels below proportional limit. not full scale components Specimen geometries similar to those of ASTM E606. or area contact setups Uniaxial loading Stress and strain levels below proportional limit. inevitabaly. A power law curve fit based on the least squares method was implemented to draw the S-N (stress versus life) lines. The tests were performed at two to seven force levels (depending on the frequency of test). The scatter of data increases with the increase in frequency. the body temperature of the part should be controlled or contained in order to gain longer lives. Photographs of typical failed specimens at different frequencies are provided in Figure 3. No specimen temperature control was implemented during the tests due to the requirements defined for the testing program. As the frequency increases. The tests were ran at frequencies of 1. In addition. A sinusoidal waveform was used. if the body temperature is contained in the real application. especially in plastics with their molecular and structural specifications. results in an increase in body temperature. On the other hand. 2. It was also observed that the area at the vicinity of final fracture section that experienced color change becomes smaller with the increase in frequency. Plots of maximum stress versus the logarithm of the number of cycles to failure (Nf) at the four frequencies tested are provided in Figure 1. Uniform cross section specimens according to ASTM D638 [9] were molded. as the fracture mode changes from mostly brittle with limited deformation at 1 Hz to mostly ductile with large deformation at 10 Hz. If an application involves plastics parts under relatively higher frequency cyclic loading. fatigue life decreases more than an order of magnitude as the frequency increases from 1 to 5 Hz. possibly indicating the interference of other parameters like manufacturing parameters and surface finish in the material fatigue strength. while other parameters such as manufacturing parameters could also play a significant role in fatigue strength of the specimen tested at higher frequencies. and R (minimum force/maximum force) ranging from 0.200 psi maximum stress. uniaxial force controlled tests were performed on a polypropylene material based on ASTM D7791 [8]. while at higher frequencies is localized at the weakest section of the gage length. at shorter lives.048. as one the influential parameters on fatigue life of plastics. at a minimum force of 10 pounds. but no stable force amplitudes could be maintained. A servo hydraulic load frame and matching controller was used for the tests.Case Study – Frequency Effects on Fatigue Life of a Polymer To investigate the effects of frequency. For instance. the frequency effect is more pronounced than at longer lives. measures should be . Higher frequencies were attempted. the material does not basically have enough time to respond to the cyclic loading condition. 5 and 10 Hz. due to its fluctuating nature. affects fatigue life by easing material flow and increasing ductility. The frequency effect in plastics is in many ways attached to temperature. This could also be attributed to the viscoelastic nature of the plastics and the fact that at higher frequencies. Similarly. The increased plastic deformation with the increase in frequency could be clearly observed.033 to 0. Comparison of the S-N curves shows that significant decrease in fatigue strength occurs with the increase in frequency. this temperature effect becomes more pronounced and. This indicates that the deformation at lower frequencies is spread over the gage length. machined and conditioned. for a fair comparison of fatigue strength at different frequencies. The maximum forces were selected not to exceed the specimen yield force of 330 pounds. at 4. the strain versus life data and the r-N curves are plotted in Figure 2. It could be concluded that material’s fatigue strength plays the main role in specimen’s resistance to failure at lower frequencies. maximum force ranging from 210 to 310 pounds. Fatigue loading. 2010.” in ASM Handbook. March 1992.exercised to maintain specimen body temperature within a specified range at different frequencies. 3. Skibo.R. 2. 2000. “Fatigue Crack Growth in Polymers Subjected to Fully Compressive Cyclic Loads. “Fatigue and Durability of Structural Materials. A. 8. 15.R. Issue 6.S. Fatemi. 1988. residual stresses and crack closure. The case study exercised in this study shows significant decrease in fatigue strength with the increase in frequency in the polypropylene material tested.Fatigue investigation of plastics should be distinguished from that of metals to include parameters such as temperature. and H. Vol. Issue 4. and S. 5. Wiley Interscience. “Standard Test Method for Tensile Properties of Plastics. Vol. “Metal Fatigue in Engineering. ASM International. Pruitt.ASTM E466-07.O.” ASM International. Mechanical Testing and Evaluation. ASM International.L. development of new test methods. Hertzberg. “Frequency Sensitivity of Fatigue Processes in Polymeric Solids.S. 2006.A. 4. 27.Standards need to be reviewed and new standards need to be developed to address the parameters specific to plastics. 8.R. Fuchs.I. . April 1975.” in Engineering Plastics. Vol. Manson. Halford. Hermann. “Fatigue Testing and Behavior of Plastics.Moet and H.” Journal of Materials Science.” ASTM International. 2010. and M.” ASTM International.” Polymer Engineering & Science.ASTM D638-10. 2007. 9.A. measures should be taken to control the body temperature of the specimen or part during fatigue testing.” 2nd Edition. J. References 1.” ASTM International. Vol. “Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. Pruitt.L. Manson and G. Stephens. overloads.W. Stephens.Due to entanglement of temperature and frequnecy in fatigue crack initiation and propagation of plastics. Conclusions 1. pages 252–260.ASTM D7791-10. 2. 4. pp 16081616. and applying new limitations. “Fatigue Failure. frequency. hysteritic loop.R. The experimental procedures should include the effects of these parameters as well as the simulation of real life full scale component tests. 6. 2.Frequency showed to be a major player in fatigue life of plastics. Suresh. 3. R. This may involve design of new specimens. Aglan.A. R. “Standard Test Method for Uniaxial Fatigue Properties of Plastics. 2000. 7. Engineered Materials Handbook. .Figure 1 – Maximum stress versus life for different frequencies. .Figure 2 – Maximum strain versus life for different frequencies. . and (d) 10 Hz. (b) 2 Hz. (c) 5 Hz.(a) (b) (c) (d) Figure 3 – Photographs of typical failed specimens after fatigue tests at (a) 1 Hz.
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