Pre-tensioning techniques in LS-DYNA implicit. Anders Jonsson with contributions from Nils Karjan, Thomas Borrvall and Anders Jernberg

Transcription

1 Pre-tensioning techniques in LS-DYNA implicit Anders Jonsson with contributions from Nils Karjan, Thomas Borrvall and Anders Jernberg

2 Pre-tensioning techniques in implicit LS-DYNA Implicit set up the basics Bolt modelling in general See For an extensive description of bolts modelling, see Karajan, N., et al., Modeling bolts in LS-DYNA using explicit and implicit time integration, 15 th Int. LS-DYNA Users Conf. ( Bolt pre-tensioning Solid bolts Beam bolts Procedures for applying the pre-tensioning Some remarks on bending stiffness Contact issues Press-fit in contacts MORTAR contact INTERFERENCE contact Initial stresses in simulation of processes Spring back Summary and conclusions 3

3 Basic implicit set-up in LS-DYNA LS-DYNA is a versatile multi-physics solver. Many different analysis types are possible. But many settings are left to the user... Control cards, telling LS-DYNA how to solve the task Element formulations (many different formulations available) Contact definitions (many different formulations and options to chose from) Material models (over 250 to chose from) Based on the Guideline for implicit analyses, this may (in many cases) be greatly simplified. The objective of the Guideline is to reduce the time it takes to make these choices, so that the time can be spent on solving the actual task. The Guideline for implicit analyses is available for download from The Appendix P of the LS-DYNA keyword manual (R9.0 and later) also provides some recommendations, background and motivation to implicit control card settings. 5

4 Implicit set-up in LS-DYNA Control cards Identify analysis type and select appropriate control card include file. In many cases, *CONTROL_TERMINATION is the only required additional control card. Use an include file structure! Then the control card include files of the Guideline package can be used directly. Material data 7

5 Pre-processing GUI dedicated for implicit set up is available in LS-PrePost from V4.6 Including convenient support for Pre-stressing 10

6 BOLT PRE-TENSIONING 11

7 Bolt pre-tensioning - general Main keywords in LS-DYNA *INITIAL_AXIAL_FORCE_BEAM for pre-tension of beam-style bolts. Applies a force. Requires a spotweld beam (elform 9) with *MAT_SPOTWELD. To obtain an elastic beam bolt, specify a high yield stress. *INITIAL_STRESS_SECTON for solid element bolts. Applies a stress. Works for the common solid elements ±1, ±2, 16 Both keywords require a load curve to apply the pre-tensioning. It is recommended to let the curve end when the initialization is completed. For example, for pre-tensioning in implicit: *DEFINE_CURVE_TITLE Bolt pre-tensioning force 100, 0.,0. 1.,75. Pre-tensioning techniques in LS-DYNA-implicit

8 Bolt pre-tensioning Load curves In order to end the pre-tensioning, let the load curve end Should a constant force be required, let the load curve continue on a constant level *DEFINE_CURVE_TITLE Bolt pre-tensioning force 100, 0.,0. 1.,75. Pre-tensioning techniques in LS-DYNA-implicit

9 Bolt pre-tensioning Beam bolts Initialization of a normal force as pre-tension in the spot weld beam *INITIAL_AXIAL_FORCE_BEAM for pre-tension of beam-style bolts. Applies a force. NOTE! Spotweld beam (elform 9) with *MAT_SPOTWELD must be used. Make sure that all DOFs of the beam end nodes are connected! Avoid release conditions on CNRB:s! If *CONSTRAINED_INTERPOLATION is used, set DDOF = *INITIAL_AXIAL_FORCE_BEAM $# bsid lcid scale kbend *DEFINE_CURVE $# lcid sidr sfa sfo offa offo dattyp lcint 100 &dtprestr &BltForce $# a1 o BltForce force dtprestr end curve after pre-tension! time bsid: beam set ID containing the spot weld beams to be pre-tensioned dtprestr: parameter defining the initialization time of the pre-tension BltForce: parameter defining the pre-tension force Bending stiffness of bolt during initialization kbend=0: no bending stiffness kbend=1: beam has bending stiffness (starting with R10) Pre-tensioning techniques in LS-DYNA-implicit 14

10 Bolt pre-tensioning Solid bolts Initialization of a normal stress in a cross section of the solid elements *INITIAL_STRESS_SECTION $# issid csid lcid psid vid izshear *DEFINE_CURVE $# lcid sidr sfa sfo offa offo dattyp lcint 100 &dtprestr &BltStrss $# a1 o *DATABASE_CROSS_SECTION_PLANE_ID $# csid title 100 Cross Section Bolt $# psid xct yct zct xch ych zch radius $# xhev yhev zhev lenl lenm id itype BltStrss stress σ normal vector cross section with radius end curve after pre-tension! time psid: part set ID containing the solid elements to be pre-stressed {x,y,z}ct {x,y,z}ch: head and tail coordinate of normal vector of the cross section BltStress: the (normal) stress to be applied (provide reasonable value!) izshear: flag to activate shear stiffness during pre-stressing phase dtprestr Pre-tensioning techniques in LS-DYNA-implicit 15

11 Bolt pre-tensioning Solid bolts izshear: Allow shear stresses to develop during the pre-stressing phase Yields more realistic distribution of the normal stresses Normal stress distribution in the bolt at equilibrium using LS-DYNA implicit (in coming versions) izshear=0: yields homogeneous normal stress of 0.38 GPa izshear=2: yields inhomogeneous normal stresses averaging 0.38 GPa over the cross section izshear=0 izshear=1 Revised for implicit in current development versions (in coming versions) For explicit analysis this will be available as izshear=2 For implicit izshear=1 and izshear=2 are synonymous Pre-tensioning techniques in LS-DYNA-implicit 16

12 Bolt pre-tensioning Solid bolts Avoid pentahedron elements in the shank Normal stress distribution might be disturbed Cause convergence problems during implicit simulations with some LS-DYNA releases (R9.2, R10) Allow for large enough elements to account for initial contraction Elements with a pre-stress application shrink until equilibrium is reached Head and nut need to travel far enough to be in contact with the sheets hexahedrons only Best to account for this such that the deformed configuration leads to a nice mesh hexas & pentahedrons before pre-stress initialization after initialization at equilibrium Pre-tensioning techniques in LS-DYNA-implicit 17

13 Bolt pre-tensioning - recommendations If a beam bolt consists of many beam elements, apply *INITIAL_AXIAL_FORCE_BEAM to one beam element per bolt. Set KBEND = 1 to account also for the bending stiffness of the beam Add *DATABASE_HISTORY_BEAM_SET for the beam elements which are being pre-tensioned. Use an individual *DATABASE_CROSS_SECTION_PLANE for each *INITIAL_STRESS_SECTON for solid element bolts. By this, cross section forces and moments can be evaluated for each bolt individually from secforc. Avoid pentas in the bolt shank for solid bolts! IGS = 1 on *CONTROL_IMPLICIT_GENERAL (active in User s guide control cards) normally aid convergence for pre-tensioning of solid bolts. Beware that if the elements are initially (very) small and/or the structure surrounding the bolt is weak, there is a risk of ending up with Error termination due to Negative volume in solid element. The remedy is often simply to mesh the pre-tension section a bit coarser. 18

14 Possible procedures for applying the pre-tensioning in implicit Implicit static, in physical time, for example from t = 0 to t = 1. In case the assembly has initial rigid-body modes, implicit dynamics can be applied initially Using dynamic relaxation (DR) Implicit, static or dynamic Explicit 19

15 Pre-tensioning using implicit statics Implicit static, in physical time, for example from t = 0 to t = 1. Pre-tensioning is applied just like normal load application. Don t use a too small first time step size. Statics should work well for beam bolts connected to the structure by constrained nodal rigid bodies. Make sure that all DOFs of the beam nodes are connected! Avoid release conditions on CNRB:s! If *CONSTRAINED_INTERPOLATION is used, set DDOF = If the assembly only is connected by the (solid) bolts and contacts, the model will have rigid body modes at t = 0. LS-DYNA will print negative eigenvalue warnings. *** Warning (IMP+124) 11 negative eigenvalues detected Still, the pre-tensioning may converge. 20

16 Pre-tensioning using implicit dynamics Initial rigid body modes will be taken care of by the dynamics. Note that when dynamics is active, time means physical time. This means that the time to ramp up the pre-tensioning becomes important. Dynamic effects can then be ramped down, as the contacts establish, and the rest of the analysis can be performed as static. *CONTROL_IMPLICIT_DYNAMICS $ IMASS GAMMA BETA TDYBIR TDYDTH TDYBUR IRATE Settings for quasi-statics 21

17 Pre-tensioning using implicit dynamic relaxation (DR) The analysis is split in two stages, and bolt pre-tensioning takes place before t = 0. Implicit dynamic relaxation can be static and/or dynamic. Set SIDR = 1 on the pre-tension load curve to activate DR Output for 3D-plotting of states from the dynamic relaxation can be specified by *DATABASE_BINARY_D3DRLF. Note! Interval is cycles, not time! It is possible to specify separate implicit settings for the DR *CONTROL_IMPLICIT_AUTO_DYN *CONTROL_IMPLICIT_DYNAMICS_DYN *CONTROL_IMPLICIT_SOLUTION_DYN 22

18 Pre-tensioning using implicit dynamic relaxation (DR) Set SIDR = 1 on the pre-tension load curve Also other load curves that are to be active during DR should have SIDR = 1 for example to specify variation of max time step *DEFINE_CURVE_TITLE Bolt pre-tension 600 MPa $ LCID SIDR ,.0 1.,.6 *CONTROL_DYNAMIC_RELAXATION Pre-tensioning is ramped up $ NRCYCK DRTOL DRFCTR DRTERM TSSFDR IRELAL EDTTL IDRFLG *CONTROL_IMPLICIT_AUTO_DYN $ IAUTO ITEOPT ITEWIN DTMIN DTMAX DTEXP *CONTROL_IMPLICIT_DYNAMICS_DYN $ IMASS GAMMA BETA TDYBIR TDYDTH TDYBUR IRATE Pre-tensioning techniques in LS-DYNA-implicit

19 Pre-tensioning using explicit dynamic relaxation (DR) Dynamic relaxation analysis is split in two stages. The bolt pre-tensioning takes place before t = 0 using explicit dynamics. Can be very efficient for structures with many loose parts. Add contact damping to aid convergence (VDC = 20-40) of explicit DR. Set SIDR = 1 on the pre-tension load curve and IDRFLG = 1 to activate explicit DR. *CONTROL_TIMESTEP is also recommended. Use moderate mass scaling (< %) by dt2ms < 0. *DEFINE_CURVE_TITLE Bolt pre-tension 600 MPa $ LCID SIDR ,.6 1.,.6 *CONTROL_TIMESTEP Pre-tensioning is constant $ TDINT TSSFAC ISDO TSLIMT DT2MS E-04 *CONTROL_DYNAMIC_RELAXATION $ NRCYCK DRTOL DRFCTR DRTERM TSSFDR IRELAL EDTTL IDRFLG Pre-tensioning techniques in LS-DYNA-implicit

20 Procedures for applying the pre-tensioning in implicit Performance overview for the pipe example. Run on 4 cores, mpp-ls-dyna R10.1 double precision Option Time (relative) Comment Keyword file Dynamic Static implicit 1 No negative eigenvalues pretens002.key Implicit DR 0.97 No negative eigenvalues pretens004.key Explicit DR 1.19 No gain in this case pretens003.key Implicit static 1.26 Negative eigenvalues during first time step pretens001b.key Pre-tensioning techniques in LS-DYNA-implicit

21 Bolt pre-tensioning bending stiffness By default, LS-DYNA applies the pre-tensioning only in the axial direction. Stress gradients are not considered. 27

22 Bolt pre-tensioning bending stiffness By default, LS-DYNA applies the pre-tensioning only in the axial direction. Stress gradients are not considered. kbend = 0 MAT_24 in the bolt Pre-tensioning techniques in LS-DYNA-implicit

23 Bolt pre-tensioning bending stiffness For *INITIAL_AXIAL_FORCE_BEAM this is resolved from R10. Set the bending stiffness flag KBEND = 1 to account also for the bending stiffness of the beam during initialization. *INITIAL_AXIAL_FORCE_BEAM $# bsid lcid scale kbend For*INITIAL_STRESS_SECTON the bending issue is resolved from R9.1 for *MAT_ELASTIC. So one workaround is to specify a slice of elastic material where the pre-tensioning takes place. In coming LS-DYNA versions, the option IZSHEAR = 2 will prescribe the average stress over the cross section of each bolt. Then also the bending stiffness can be accounted for. *INITIAL_STRESS_SECTION $ isid csid lcid psid vid izshear Pre-tensioning techniques in LS-DYNA-implicit

24 Bolt pre-tensioning bending stiffness For beam bolts, this is resolved in R10. For solid bolts, add a slice of elastic material where the pre-tensioning takes place. KBEND = 1, rev MAT_24 in the bolt, but one layer of elastic material added Pre-tensioning techniques in LS-DYNA-implicit

25 Bolt pre-tensioning bending stiffness For beam bolts, this is resolved in R10. In coming versions, use IZSHEAR = 2 for solid bolts. KBEND = 1, rev MAT_24 in the bolt, IZSHEAR = 2 Pre-tensioning techniques in LS-DYNA-implicit

26 Possible procedures for applying the pre-tensioning in implicit Larger example. Pretension of 9 (5 M20, 4 M14) bolts in 2 directions, using explicit Dynamic Relaxation. Time for pre-tensioning: 12.5 minutes on 4 cores. 36

27 PRESS-FIT 37

28 Press fit in contacts It is strongly recommended to use *CONTACT_..._MORTAR_ID for implicit analyses. Mortar contact can also handle press-fit, using IGNORE = 3 ( small penetrations) or 4 (finite penetrations), and MPAR1 to specify time to resolve the initial penetrations, and MPAR2 to specify search distance for penetrating nodes. The solid contact thickness (PENMAX from R10) may require adjustment The contact surface is adjusted at t = 0 so that no initial penetrations exist The contact surface is then moved out using a linear ramp, to coincide with the meshed surface at t = MPAR1 It is possible to parametrize the grip by contact surface augmentation. For solids, this is most conveniently done in R10, where the parameter SLDTHK (optional card B of the contact definition) can be used to directly specify the thickness increase. 39

29 Contacts Press-fit Mortar example for large penetration / press-fit *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE_MORTAR_ID $# cid title 1interference $# ssid msid sstyp mstyp sboxid mboxid spr mpr $# fs fd dc vc vdc penchk bt dt $# sfs sfm sst mst sfst sfmt fsf vsf MPAR1 4is the time when the press-fit should be resolved $# soft sofscl lcidab maxpar sbopt depth bsort frcfrq $# penmax thkopt shlthk snlog isym i2d3d sldthk sldstf $# igap ignodprfac/mpadtstif/mpar2 unused unused flangl cid_rcf Set IGNORE = 4 for finite interference MPAR2 is the search distance 40

30 Contacts Press-fit Mortar example for large penetration / press-fit 41

31 Contacts Press-fit Mortar example for large penetration / press-fit 42

32 Contacts Press-fit Mortar example for large penetration / press-fit 43

33 Press fit in contacts In some cases for press-fit (often finite penetrations, IGNORE = 4) analyses using the Mortar contact may run into (convergence) problems. An option to try out is then *CONTACT_SURFACE_TO_SURFACE_INTERFERENCE_ID $# cid title 1014Bellow - locking ring $# ssid msid sstyp mstyp sboxid mboxid spr mpr $# fs fd dc vc vdc penchk bt dt 0.6 $# sfs sfm sst mst sfst sfmt fsf vsf Curve for scaling penalty stiffness. Start at (0., 0.) end at (t, 1.) $# lcid1 lcid $# soft sofscl lcidab maxpar sbopt depth bsort frcfrq $# penmax thkopt shlthk snlog isym i2d3d sldthk sldstf 1.0 $# igap ignodprfac/mpadtstif/mpar2 unused unused flangl cid_rcf 2 Set IGAP = 2 to disable sticky contact Pre-tensioning techniques in LS-DYNA-implicit

34 Press fit in contacts In some cases for press-fit (often finite penetrations, IGNORE = 4) analyses using the Mortar contact may run into (convergence) problems. An option to try out is then *CONTACT_SURFACE_TO_SURFACE_INTERFERENCE_ID A curve - starting at (0., 0.) - is given for scaling the penalty stiffness up to 1. May need some iterations to find the right curve. Start with linear ramp This contact has the sticky option on by default. Set IGAP = 2 to disable. Pre-tensioning techniques in LS-DYNA-implicit

35 Contacts Press-fit In some cases for press-fit (often finite penetrations, IGNORE = 4) analyses using the Mortar contact may run into (convergence) problems. An option to try out is then *CONTACT_SURFACE_TO_SURFACE_INTERFERENCE_ID 46

36 Press fit in contacts *CONTACT_..._MORTAR_ID Developed for implicit No sticky option, it is always off Contact surface is moved away until no initial penetrations exist, then moved back to the original position Cannot be used with Dynamic Relaxation (in current versions) *CONTACT_.._INTERFERENCE_ID Developed for explicit Separate option to resolve interference in Dynamic Relaxation Sees all penetrations. Contact stiffness is ramped up to resolve them. This contact has the sticky option on by default. Set IGAP = 2 to disable. Pre-tensioning techniques in LS-DYNA-implicit

37 Contacts Press-fit It is possible to parametrize the grip by contact surface augmentation. For solids, this is most conveniently done in R10, where the parameter SLDTHK (optional card B of the contact definition) can be used to directly specify the thickness increase. Note that both master and slave side will be augmented! Contact surface augmentation in R10 Pre-tensioning techniques in LS-DYNA-implicit

38 STRESS INITIALIZATION 49

39 Stress initialization This is the classical application of implicit: Springback after forming Use *INTERFACE_SPRINGBACK_LSDYNA to request dynain file ASCII keyword format or LSDA Binary format LS-DYNA will output the deformed geometry and residual stresses. Keywords: *NODE, *ELEMENT_..., *INITIAL_STRESS_... Note: Hyperelastic materials, like rubbers (MAT_77, MAT_181 ) and foams, are initialized by *INITIAL_FOAM_REFERENCE_GEOMETRY May be useful for multi-step simulations, assembly in several steps May require re-pretension of bolts to obtain correct pre-tension, due to loss of information regarding contact status Contact status will be included in the dynain file in coming versions 50

40 Stress initialization example Bolt pre-tensioning followed by loading dynain Stage 1: Bolt pre-tension *INITIAL_STRESS_SECTION (ramp) *INTERFACE_SPRINGBACK_LSDYNA Stage 2: Loading *INITIAL_STRESS_SECTION (constant) *LOAD_NODE_SET 51

41 Stress initialization example Bolt pre-tensioning followed by loading Stress history comparison: dynain w. solid lines, all-in-one with dashed 52

42 Stress initialization example Bolt pre-tensioning followed by loading dynain Stresses after pre-tension Stresses after re-pre-tension and standstill (1 s) 53

43 Summary Main keywords in LS-DYNA for bolts pre-tensioning *INITIAL_AXIAL_FORCE_BEAM for pre-tension of beam-style bolts. Applies a force. Requires a spotweld beam (elform 9) with *MAT_SPOTWELD. To obtain an elastic beam bolt, specify a high yield stress. *INITIAL_STRESS_SECTON for solid element bolts. Applies a stress. Works for the common solid elements ±1, ±2, 16 For press-fit in contacts, use MORTAR contact with IGNORE = 3 ( small penetrations) or 4 ( finite penetrations ) *CONTACT_...INTERFERENCE may be an option in some cases for finite penetrations Stress initialization can be used in a staged simulation process, using dynain files (*INTERFACE_SPRINGABACK_LSDYNA) 54

44 Implicit analyses in LS-DYNA Other sources of useful information regarding implicit analyses in LS-DYNA: search conference papers DYNAmore / LSTC also gives courses in implicit analyses: Non-linear implicit analysis in LS-DYNA (T. Borrvall) NVH & Frequency domain analysis in LS-DYNA (A. Jonsson) See also: Please report bugs and errors! To support@dynamore.se 55

45 Thank you! Your LS-DYNA distributor and more 56

46 Alternative procedures for applying bolt pre-tensioning Apply thermal loading to strain the bolt. Model the bolt too short. Stretch the bolt to nominal length using for example *BOUNDARY_PRESCRIBED_MOTION. Let the contact be born. Model the pre-tensioning as a press-fit. Use IGNORE = 3 (or 4) on the MORTAR contact, or alternatively INTERFERENCE contact. Either model the bolt too short as above, or use contact surface augmentation in MORTAR. By these methods, some iterations will be needed to find the right pretensioning. 57

47 Bolt pre-tension issues (IZSHEAR) From R10.0, a new feature is added on *INITIAL_STRESS_SECTION: IZSHEAR Add shear resistance to the cross section In the dev version, the average stress is prescribed instead of constant per integration point. MAT_24 bolt R10.1 dev Pre-tensioning techniques in LS-DYNA-implicit

48 Bolt pre-tension issues (IZSHEAR) From R10.0, a new feature is added on *INITIAL_STRESS_SECTION: IZSHEAR Add shear resistance to the cross section In the dev version, the average stress is prescribed instead of constant per integration point. The temporary remedy for R10 may be to switch to *MAT_ELASTIC in the bolt Original geo R10.1, MAT_24 R10.1, MAT_ELASTIC Pre-tensioning techniques in LS-DYNA-implicit

49 Troubleshooting convergence problems Start out with the suggested control card settings. Check the model integrity Element quality, element types, un-intended cracks in the mesh, element connectivity (beams attached directly to solids by common node(s)) etc. Also beware of element connectivity when coupling *ELEMENT_BEAM elform 6 to other elements. Many pre-processors have built-in tools for model checking Check model connectivity Perform eigenvalue analysis - *CONTROL_IMPLICIT_EIGENVALUE In ANSA: Check > Connectivity > Detect unconnected assemblies Avoid the use of release conditions on Constrained nodal rigid bodies (use joints instead) Also, if possible, on beam elements (use joints or trusses instead) Check that consistent units are used (for materials, loadings, accelerations etc.) Use a moderate (not too small) initial time step. The automatic time step control can only decrease time step size upon RETRY. Check material models 60