Chicago Comprehensive Guide to Bolt Modeling in Ansys 15 0

An overview of methods formodelling bolts in ANSYS V15 Dragana Jandric ANSYS Inc Technical Support Engineer 1 2014 PresentedMay at28,the 2014 ANSYS Regional Conference Chicago May 23, 2014 © 2011 ANSYS, Inc. Outline • Introduction • Bolt Modeling - Bolt Modeling Method 1 – 3D bolt representation - Bolt Modeling Method 2 – 3D bolt representation - Bolt Modeling Method 3 – bolt thread contact - Bolt Modeling Method 4 – bolt thread contact - Bolt Modeling Method 5 – screw joint - Bolt Modeling Method 6 – line body representation - Bolt Modeling Method 7 – line body representation - Bolt Modeling Method 8 – beam connection • Result Comparison • Summary 2 © 2011 ANSYS, Inc. May 28, 2014 Introduction • Bolted connections are common to many industries • Basic requirements of a bolted connections are: – Bolt should transfer the load realistically across the connecting elements – Bolt must have adequate strength – Joint must remain intact – Connection must have adequate fatigue and fraction life • Bolted analysis is no different than any other FEA calculation • This presentation shows different approaches of modeling bolted connection using full 3D models to beam models • It also explores examples and best practices and explains enhancements in ANSYS v15 3 © 2011 ANSYS, Inc. May 28, 2014 May 28. 2014 . a simple eight bolt flange is taken. In the following slides following aspects will be considered: – Geometry – Meshing – Contact – Pre-tension loading – Post processing 6 © 2011 ANSYS.Bolt modeling To demonstrate different ways to model bolts. Inc. High computational time 7 © 2011 ANSYS.Bolt modeling Bolt can be modeled as: .APDL post-processing . 2014 Line Body + Easy to set up + Low computational time + Some post-processing tools .Beam connection Solid Body + Most accurate + All contact details available + Easy post-processing .No contact detail .No stress detail in flange .Solid body .No stress detail in flange Beam Connection + Easy to set up + Low computational time + No geometry required .Line body . May 28.Creating line bodies .Mesh refinement .Geometry preparation .No contact detail . Inc. May 28.Model and analysis considerations Approach to modelling the bolts usually involve making engineering decisions about the following: • Prepare geometry – Bolt and flange • Mesh – Minimum DOF for best representation – Consider contact areas for load transfer/stress – Hex / tet • Three step analysis: – Step 1: preload by load or adjustment – Step 2: fix the pretension. 2014 . Inc. release any temporary restraining boundary conditions – Step 3: Apply in-service loads 8 © 2011 ANSYS. May 28. Inc. each has a different method of modelling the bolt • Upper / lower flanges are multi-body.Overview of model • Eight sectors. linear analysis – 500N pre-load to all bolts 9 © 2011 ANSYS. sweep-able parts • All contacts are asymmetric & bonded • Analysis settings: – Upper / lower flanges fixed at pipe OD – 2 step (load/lock ). 2014 . check element quality 10 © 2011 ANSYS. Inc. May 28. 2014 .Bolt model 1: Key features of this approach: • No/very little geometry preparation • Full thread on bolt and nut • Good geometric representation of stiffness of bolt/nut will be captured if mesh is dense enough • Contact areas give accurate representation of bolt head and nut contact area to flange • Most cases will produce a tetrahedral mesh. Inc. May 28. 2014 .Bolt model 2: Key features of this approach: • Some geometry preparation. threads removed on bolt and nut • Care should be taken not to alter bolt shank stiffness as this will affect bolt deflection and load transfer in the system during pre-tension and in-service loading • Contact areas give accurate representation of bolt head and nut contact area to flange • Load between bolt and nut is transferred via bonded contact. • Most cases will produce a tetrahedral mesh. check element quality 11 © 2011 ANSYS. May 28.Bolt model 3: Key features of this approach: • Geometry same as bolt model 2 • New V15 bolt thread contact applied (recommended 4 elements span 1 thread width) • Contact sizing option to increase number of elements in thread area • Contact results show helical load transfer at threads 12 © 2011 ANSYS. Inc. 2014 . May 28. Inc. New V15 bolt thread contact applied (recommended 4 elements span 1 thread width) © 2011 ANSYS. respecting size of contact area under bolt head/nut and bolt shank diameter – Decompose to sweep-able bodies – Multi-body back together – Prepare 1 fastener and use pattern to replace others Can take quite a few mesh controls to get a good quality mesh Bodies are modified to mesh them with hex mesh. 2014 .Bolt model 4: Key features of this approach: • Significant amount of geometry preparation on • • • 13 bolt and nut – De-feature. Inc._jid. 2014 .1.pitch.Bolt model 5: Key features of this approach: • Geometry and mesh same as bolt 4 • Bolt thread contact replaced with a cylindrical joint • APDL commands to redefine joint as a screw joint keyo. May 28._jid.(pas/2/pi) 14 © 2011 ANSYS.screw._wbjoint pi=acos(-1) secjoin.17 sectype..joint.12 pas=1 secjoin. MPC couple U-Rot inside pinball. note: for edge contacts WB automatically extends spider out 1 element for load transfer 15 © 2011 ANSYS. model size significantly reduced • Contact.Bolt model 6: Key features of this approach: • Geometry preparation – Bolt/nut geometry replaced with a line body • Line body meshed as beam elements. May 28. Inc. 2014 . end of bolt to cylindrical edge of bolt hole. end of bolt to flange cylindrical face. May 28. 2014 . model size significantly reduced • Contact. MPC couple U-Rot inside pinball 16 © 2011 ANSYS. Inc.Bolt model 7: Key features of this approach: • Geometry preparation – Bolt/nut geometry replaced with a line body – Upper/lower flanges have been split and mutli-bodied back together to give a contact area to attach the beam ends to • Line body meshed as beam elements. May 28.Bolt model 8: Key features of this approach: • No bolt/nut geometry • Use “Body-Body > Beam” – Single beam188 element between mobile/reference geometry – Scope to edge or surface of bolt holes on flanges – Radius of beam = bolt shank diameter • Recommend use of named selections (flange edge/surface geometry) and object generator to copy a master “body-body beam” • This method cannot use a bolt pre-tension load directly. need to apply load via APDL inistate commands 17 © 2011 ANSYS. 2014 . Inc. etc.Overview of workflow Module “B” is the original 8 bolt flange. mesh sizing.e. 2014 . Inc. May 28. i. 18 © 2011 ANSYS. this can be duplicated to investigate bolt modelling further. frictional contact. Inc. May 28. 2014 .Bolt Pretension How to apply bolt pretension • Insert Bolt Pretension load • Select geometry to apply load to – Solid body > select body or face – Line body > select edge • Define load – Load / lock / magnitude etc 19 © 2011 ANSYS. .bolt_stress alls 21 © 2011 ANSYS.ename.real.5 ! bolt shank radius mm bolt_load = 500 ! bolt pretension load N bolt_area = (22/7)*(bolt_rad*bolt_rad) bolt_stress = 1.r.Bolt Pretension How to apply bolt pretension to a “body-body beam” Identify beams for loading purposes: beam_bolt_id = _bid • Initial stress to beam188 element is applied using command snippet bolt_rad = 2...-2 inistate.stre inistate.define.dtyp... Inc. 2014 ! select all beam elements in model ! select bolts defined as beams only ! select nodes on beam bolt element ! enter solution to define bolt load ! select element coordinate system ! set to initial stress definition ! define bolt stress ! reselect all entities .csys. May 28..beam_bolt_id nsle /solu inistate.5*bolt_load/bolt_area esel.s.set.188 esel.set. Inc. beam) along the shank of the bolt Why … because ANSYS “bolt-pretension” load splits the bolt shank and connects the resulting faces (solid) / vertices( beam) to a pilot node. tet.Bolt Pretension A word on meshing … ensure there is at least 2 elements (hex. May 28. 2014 . the load is then applied via the pilot nodes 22 © 2011 ANSYS. 2014 . Inc. May 28.consistent regardless of how bolt has been modelled 23 © 2011 ANSYS.Results comparison Flange deflection . biggest difference is with beam connector where spider extends out 1 element depth Bolt head 24 © 2011 ANSYS.Results comparison Stress in flange . Inc. May 28. 2014 Nut .some differences between “line” and “area” contacts. Results comparison Stress in bolt shank: • Solid body bolts > scope stress to bolt body – Results fairly consistent regardless of method used to model bolt • Line body bolts > Post process using “Beam Tool” or “User Result > Beamdirect” • Body-body beam connector > APDL commands to post process 25 © 2011 ANSYS. May 28. Inc. 2014 . 7 MPa 26 © 2011 ANSYS. Inc.Results comparison Stress in bolt shank: • Line body bolts > Post process using “Beam Tool” or “User Result > Beamdirect” – 25.5 MPa vs Solid 25. May 28. 2014 .4 to 26. 14 /title.smisc.773. May 28. 1.31 ETABLE.4 MPa – Line body 25. 2014 set.37623044565048 /DIST.last esel.9 N • Bolt shank stress = 25. Inc..Results comparison Stress in bolt shank: • Body-body beam connector > APDL commands to post process • Axial bolt force = 498.7 Mpa 27 © 2011 ANSYS.ax2 ! Direct Stress Axial ETABLE.s.beam_bolt_id !Length unit for the following data is MM /FOC. 1.5452039539814 .png PLLS.smisc. 136.1 ETABLE. 1. 5.383469365249 . Axial Force Diagram /SHOW.46091721952 /VIEW.Direct Stress Axial /SHOW.171. 62.113.645190993093 /ANG.613482745931 .sdir1.36 /title. -623.ax2.sdir2 .smisc.type.-13.5 MPa – Solid 25.sdir1.png PLLS.558237213941 ETABLE.sdir2.9820904842815 .ax1.smisc.ax1.4 MPa • Bolt shank stress comparison – Beam connector 25.4 to 26. 1. 744 0.428 Nut 47 0.0876 © 2011 ANSYS.00402 Bolt 5 26.00669 Bolt 6 25.Results comparison EQV [MPa] 28 Beam Axial Stress [MPa] Deformation [mm] Bolt 1 26. 2014 . Inc.485 Bolt 7 25.00459 Bolt 3 26.0071 Bolt 2 26.265 0.485 Bolt 8 25.171 0.0876 Flange 47 0.390 0. May 28.00459 Bolt 4 26.736 0. and much faster True Thread Simulation 29 © 2011 ANSYS.75 hrs 69K 11. Inc.65 hrs 69K MPC Method MPC Method 1.1 M . 2014 Bolt Section Method Elements True Thread Model Wall Time 115 hrs Bolt Section Method 12.Bolt Thread Modeling – Enhancements in v15 • Analyze Bolt Threads without physically modeling the threads • Threads defined as a Contact geometry correction • Accuracy closer to true thread modeling than bonded MPC method. May 28. • Define thread parameters in contact details window – Mean Pitch diameter – Thread Pitch distance – Thread angle – Starting/ending orientation points (Program Controlled defaults to top and bottom of scoped cylindrical bolt body center).Bolt Thread Modeling – Enhancements in v15 • Build conventional surface to surface asymmetric contact between cylindrical faces. 2014 . May 28. Inc. – In MAPDL. use SECTYPE and SECDATA commands 30 © 2011 ANSYS. May 28.Bolt Thread Modeling – Enhancements in v15 • With sufficient mesh refinement. stress profiles match very closely 31 © 2011 ANSYS. Inc. 2014 . 2014 • No stress detail in flange . Inc.Summary Beam Connector + - • Easy to setup • Easy to setup • No geometry required for bolt • Low computation time • Low computation time • Good simplification of bolt/flange stiffness • • Solid body • Most accurate/realistic representation of joint • Stresses available for all parts depending on how modelled • All contact details available. May 28. depending on how modelled • Post-processing tools available Good simplification of bolt/flange stiffness Some post-processing tools available • No contact detail between fastener and flange • Requires line bodies in model • Some geometry preparation liklely to be required • No stress detail in flange • No contact detail between fastener and flange • Mesh controls will be required • Need to know correct initial stress to achieve required pretension • Large model / high computational time • 32 Line body APDL post-processing © 2011 ANSYS. Submodeling can often be used to develop more detailed bolt modeling 33 © 2011 ANSYS.Summary • This presentation showed different methods how to model bolt pretension. May 28. Inc. Traditionally bolts are one of the difficult connection to model mainly because of difficulties in including a preload produced by installation torque of tightening the bolt • Modeling and representation of threads in the analysis adds complexity • There are other scenarios that could be included in the finite element model of a bolted connection. 2014 .