1. Surface-to-Surface Elements Chapter Six

2. Training Manual October 15, 2001 Inventory # 001567 6-2 Surface-to-surface elements The surface-to-surface contact elements are the most versatile contact elements in the ANSYS program. Because they are robust, feature-rich, and user-friendly, they have become the contact element of choice for most ANSYS users.

3. Training Manual October 15, 2001 Inventory # 001567 6-3 … Surface-to-surface elements In this chapter, we will present a comprehensive treatment of the ANSYS surface-to-surface contact elements via the following topics: A. Overview B. Advanced options for special problems C. Rigid surface considerations D. Creating surf-surf contact without the Wizard E. Troubleshooting

4. Training Manual October 15, 2001 Inventory # 001567 6-4 Surface-to-surface elements A. Overview Surface-to-surface elements, as their name implies, are ideal for modeling contact between any two surfaces. The surfaces can have any shape. –For point-to-surface, line-to-surface, or edge-to-surface contact, node-to-node and node-to-surface elements are better suited. Surface-to-surface elements transmit pressures between surface Gauss points. –The other elements transmit discrete forces, at the nodes. –This underlying technological advance is responsible for many of the advantages of the surface-to-surface elements.

5. Training Manual October 15, 2001 Inventory # 001567 6-5 Surface-to-surface elements... Overview The surface-to-surface elements are the most widely used contact elements in ANSYS, due to the many advantages that they have over the other contact elements: –Compatible with both lower order and higher order elements. –Support large deformations with significant amounts of sliding and friction efficiently. –Provide better contact results (easier to postprocess contact pressure and frictional stresses). –Can account for shell and beam thickness, as well as shell thickness changes. –Semi-automatic contact stiffness calculation. –“Pilot node” control of rigid surface. –Intelligent default settings, Contact Wizard (easy to use). –Thermal contact capability. –Numerous advanced options for overcoming difficult problems.

6. Training Manual October 15, 2001 Inventory # 001567 6-6 Surface-to-surface elements B. Advanced options With 20 possible real constants, 2 material properties, and 30 possible element option settings, these elements offer a rich library of features that can be used to model special effects and overcome difficult convergence situations. However, as was seen in the Basic Structural Nonlinearities training manual, the intelligent default settings for these many options allow these elements to solve many contact problems with relatively little user intervention. –Before you start exercising the advanced options, first try using the default settings. Simply specify the penalty stiffness, penetration tolerance, and the number of substeps, and then run the analysis. –Use the advanced options only if you encounter difficulties using the default settings.

7. Training Manual October 15, 2001 Inventory # 001567 6-7 Surface-to-surface elements... Advanced options All of the element options and parameters can be controlled through the Contact Wizard:

8. Training Manual October 15, 2001 Inventory # 001567 6-8 Surface-to-surface elements... Advanced options Surface to surface contact elements offer three DOF options: –Structural only: UX, UY, and UZ –Structural and Thermal : UX, UY, UZ, and TEMP –Thermal only: TEMP (for a pure thermal contact problems) When contact pairs are created via the wizard, the contact element DOF option is automatically set based on the DOF of the underlying elements. No adjustment is necessary. Outside the wizard, surface to surface contact element DOFs can be set via: –Main Menu > Preprocessor > Element Type > Add/Edit/Delete …options...

9. Training Manual October 15, 2001 Inventory # 001567 6-9 Contact Wizard Basic Tab –Contains all general parameters related to contact behavior and convergence. Specifies whether input constant is a scaling factor or an absolute value Surface-to-surface elements... Advanced options When specifying options with the Real Constant menu, a negative value denotes an absolute; positive denotes a factor.

10. Training Manual October 15, 2001 Inventory # 001567 6-10 Contact Wizard Friction Tab Contains all parameters related to static and dynamic friction at the contact interface; also allows for the definition of a maximum friction stress at the contact interface. MU = MUK*(1+(FACT-1)exp(-dc*VREL) Where: MU = coefficient of friction MUK = dynamic coefficient of friction VREL = relative slipping rate between the two contacting surfaces FACT = ratio of static to dynamic COEFF. dc = exponential decay ratio Surface-to-surface elements... Advanced options

11. Training Manual October 15, 2001 Inventory # 001567 6-11 Contact Wizard Initial Adjustment TAB Contains advanced penetration and surface offset parameters Surface-to-surface elements... Advanced options

12. Training Manual October 15, 2001 Inventory # 001567 6-12 Contact Wizard MISC TAB Features for Spurious contact prevention,Beam/Shell Thickness effect and location of contact detection point, gauss or node Surface-to-surface elements... Advanced options

13. Training Manual October 15, 2001 Inventory # 001567 6-13 Contact Wizard Enhancements Contact Wizard Rigid target TAB For specifying customize constraints and higher order target element option

14. Training Manual October 15, 2001 Inventory # 001567 6-14 Contact Wizard Enhancements Contact Wizard Thermal Tab All thermal contact parameters, including contact conductance, near field surface radiation, frictional heating factors

15. Training Manual October 15, 2001 Inventory # 001567 6-15 Surface-to-surface elements... Advanced options These same items can also be specified through the usual Real Constant and Element Option menus.

16. Training Manual October 15, 2001 Inventory # 001567 6-16 Surface-to-surface elements... Advanced options We will discuss these options in the order in which you might logically address them as you set up your model. This is not necessarily the same order in which they appear in the Contact Wizard. One of the first decisions you might make would concern how to enforce contact compatibility. You can include the augmented Lagrange method (default) or switch to a pure penalty method in the contact wizard’s BASIC Tab. –The augmented Lagrangian works well for most models. –The penalty method is recommended for models with very distorted elements, a high coefficient of friction, and/or poor convergence behavior with the augmented Lagrangian.

17. Training Manual October 15, 2001 Inventory # 001567 6-17 Surface-to-surface elements... Advanced options The most important option is the Normal penalty stiffness, or contact stiffness. –We have already discussed this option in the BASIC Structural Nonlinearities training manual, and in Chapter 3 of this manual. To reiterate: Try a starting value (factor) of 1.0 for bulky contact, 0.1 for flexible contact. Monitor convergence behavior, verify the validity of your results, and adjust this value as needed: –Larger for better accuracy; smaller for better convergence.

18. Training Manual October 15, 2001 Inventory # 001567 6-18 Surface-to-surface elements... Advanced options Sometimes it is advantageous to run the analysis with an initial soft contact stiffness and then gradually tighten the stiffness (by changing the specified normal penalty stiffness value) over a series of load steps. –This will, in effect, “ramp” the contact stiffness value, improving convergence behavior. –Incrementally increasing to a stiff value at the last load step will improve accuracy. In addition, the program can automatically change the contact stiffness from substep to substep, to account for large strain effects (i.e., necking down) that change the underlying element’s stiffness.

19. Training Manual October 15, 2001 Inventory # 001567 6-19 Surface-to-surface elements... Advanced options Element options to enable the program to update the contact stiffness can be set via the wizard BASIC Tab or the real constant menu. Allow user-specified changes to stiffness. Allow both automatic and user-specified changes.

20. Training Manual October 15, 2001 Inventory # 001567 6-20 Surface-to-surface elements... Advanced options The Penetration tolerance is usually the second most important option in terms of its effect on convergence and accuracy. By default, the penetration tolerance is a factor times the underlying element depth (h). –For highly varying mesh density, using a factor will enforce a tighter tolerance on some portions of the contact surface. An absolute value might be better in such cases. –Never use too tight a tolerance, because it will always have a detrimental effect on convergence.

21. Training Manual October 15, 2001 Inventory # 001567 6-21 Surface-to-surface elements... Advanced options In Chapter 5, we discussed auto time step control for impending contact status change. If you turn this control ON at the Solution Control level, it will be governed by the element option setting : –No Control: Time step size is unaffected. This is usually sufficient for static problems when automatic time stepping is on. –Auto Bisection: The time step is bisected if the contact status changes dramatically. For dynamic problems, auto cutback usually is sufficient. –Reasonable: A more expensive algorithm than auto cutback. –Minimum: This option predicts minimum time increment for the next substep (very computationally expensive, not recommended).

22. Training Manual October 15, 2001 Inventory # 001567 6-22 Surface-to-surface elements... Advanced options The Pinball region affects the contact status determination, as well as a number of other contact features. The pinball region is a circle (2D) or sphere (3D) that surrounds the contact element and that delineates the boundary between “near” and “far” regions around the contact element. By default, the pinball radius is 4 x depth (rigid-to-flex) or 2 x depth (flex-to-flex) of the underlying element. Contact Target Pinball radius

23. Training Manual October 15, 2001 Inventory # 001567 6-23 Surface-to-surface elements... Advanced options You can specify a different value for the pinball radius.

24. Training Manual October 15, 2001 Inventory # 001567 6-24 Surface-to-surface elements... Advanced options Several different Contact surface behaviors are available via the contact wizard BASIC Tab or the real constants menu. These options enable you to model special physical effects. The available options include: –Standard: normal contact closing and opening behavior, with normal sticking/sliding friction behavior. –Rough: normal contact closing and opening behavior, but no sliding can occur (similar to having an infinite coefficient of friction).

25. Training Manual October 15, 2001 Inventory # 001567 6-25 Surface-to-surface elements... Advanced options –No Separation: Target and contact surfaces are tied once contact is established (sliding is permitted). –Bonded: Target and contact surfaces are “glued” once contact is established. –No Separation (always): Any contact detection points initially inside the pinball region or that come into contact are tied in the normal direction (sliding is permitted). –Bonded Contact (always): Any contact detection points initially inside the pinball region or that come into contact are bonded for the remainder of the analysis. (Design Space Default) –Bonded Contact (initial contact): Bonds surfaces ONLY in initial contact, initially open surfaces will remain open.

26. Training Manual October 15, 2001 Inventory # 001567 6-26 Surface-to-surface elements... Advanced options The following parameters affect some of the surface behavior options: –The Contact opening stiffness (open-gap stiffness) enforces the no-separation and bonded behavior by tying the surfaces together with a spring that has nonzero stiffness even when the gap is open. This spring stiffness equals the normal penalty stiffness by default. Its effect is similar to that of the normal penalty stiffness. –Too soft can be inaccurate; too stiff can cause convergence problems.

27. Training Manual October 15, 2001 Inventory # 001567 6-27 Surface-to-surface elements... Advanced options –The Coefficient of Friction affects the basic friction behavior. MU = 0 specifies frictionless behavior. MU > 0 determines the sliding force. –The Maximum friction stress specifies the maximum value of shear stress that can be transmitted at the friction interface. (The shear yield of the underlying material.)

28. Training Manual October 15, 2001 Inventory # 001567 6-28 Surface-to-surface elements... Advanced options The Contact cohesion specifies the value of friction stress at which sliding begins when there is no normal pressure.

29. Training Manual October 15, 2001 Inventory # 001567 6-29 Surface-to-surface elements... Advanced options Friction leads to an unsymmetric stiffness matrix. Because an unsymmetric matrix is computationally more difficult to handle (and therefore results in a slower solution), the program automatically implements a symmetrization routine. Sometimes, better convergence can be obtained with the unsymmetric matrices: Remember that the sparse or frontal solvers must be used. This same option can also be globally activated for every element that supports unsymmetric matrices, using Solution>Unabridged Menu >Analysis Options. Set the Newton-Raphson option to “Full N-R unsymm”.

30. Training Manual October 15, 2001 Inventory # 001567 6-30 Surface-to-surface elements... Surface behavior – Workshop Please refer to your Workshop Supplement for instructions on: W5. No Separation Contact.

31. Training Manual October 15, 2001 Inventory # 001567 6-31 Surface-to-surface elements... Advanced options If you have created a model using beam or shell elements, the contact surfaces can be shifted to account for the beam or shell thickness. –For Shell181 elements, changes in thickness due to large-strain deformation are also taken into account. mid-plane of shell element shell element thickness Contact Target contact detected on shifted surface

32. Training Manual October 15, 2001 Inventory # 001567 6-32 Surface-to-surface elements... Advanced options Initial Interference There are several techniques for modeling initial interference contact problems (such as shrink fit assemblies). They use either the initial geometric penetration, a specified offset value, or both. To specify an offset value:

33. Training Manual October 15, 2001 Inventory # 001567 6-33 Surface-to-surface elements... Advanced options Specified offset (CNOF) The real constant CNOF is the contact surface offset. –Positive CNOF increases interference. –Negative CNOF decreases interference or results in gap. CNOF can be combined with a geometric penetration.

34. Training Manual October 15, 2001 Inventory # 001567 6-34 Surface-to-surface elements... Advanced options Automated CNOF Adjustment Allows ANSYS to automatically provide the value for CNOF based on the initial interference. –Results in “just touching” configuration –ICONT (discussed later) defaults to zero

35. Training Manual October 15, 2001 Inventory # 001567 6-35 Surface-to-surface elements... Advanced options The initial interference options include: –Include everything: Include an initial interference from the geometry and the specified offset (if any). –Exclude everything: Ignore all initial-interference effects. –Include with ramped effects: Ramp the interference, to enhance convergence. –Include offset only: Base initial interference on specified offset only. –Include offset only w/ ramp: Base initial interference on specified offset only, and ramp the interference effect to enhance convergence.

36. Training Manual October 15, 2001 Inventory # 001567 6-36 Surface-to-surface elements... Advanced options If a model contains an initial geometric penetration, the contact forces will immediately be “stepped” to a large value. –Remember that sudden large changes in load will often cause convergence difficulties. It is desirable to have a mechanism to ramp the initial interference effect. The Include with ramped effects and Include offset only w/ ramp options helps overcome convergence difficulties by ramping a large initial penetration over the first load step. –For best results, no other loads should be applied in the first load step.

37. Training Manual October 15, 2001 Inventory # 001567 6-37 Surface-to-surface elements... Advanced options Interference fit problems with widely varying contact normal directions can result in non-physical behavior. In this case try switching the target and contact surfaces. Or, try removing problem contact elements. Contact surface Target surface This can result in non-physical behavior

38. Training Manual October 15, 2001 Inventory # 001567 6-38 Surface-to-surface elements... Interference fit – Workshop Please refer to your Workshop Supplement for instructions on: W6. Initial Interference.

39. Training Manual October 15, 2001 Inventory # 001567 6-39 Surface-to-surface elements... Advanced options Rigid Body Modes In a static analysis of two (or more) bodies that are initially unconnected, a rigid body motion can result before the contact is established. F In this force controlled example the cylinder has no applied displacement constraints. The constraints on the cylinder are established by the contact between the cylinder and the plate.

40. Training Manual October 15, 2001 Inventory # 001567 6-40 Surface-to-surface elements... Advanced options If at any point in the solution the two bodies are disconnected, the stiffness matrix will become singular. ANSYS will issue a negative pivot warning message. There are several options for overcoming rigid body modes due to initially unconnected bodies: –Build the Geometry in the “Just-Touching” Position –Dynamics –Displacement Control –Weak Springs –Use No-Separation Contact (KEYOPT(12) discussed later) –Adjusting Initial Contact Conditions

41. Training Manual October 15, 2001 Inventory # 001567 6-41 Surface-to-surface elements... Advanced options Just Touching This requires you to know what the “just touching” position is. This can be difficult if the surfaces are curved or irregular. F Small gaps or penetrations can exist between the bodies due to numerical round-off in the finite element meshes. This can cause non-convergence or breaking away of the bodies in contact. As we will see later, two bodies located node-to-node are not necessarily “just touching”.

42. Training Manual October 15, 2001 Inventory # 001567 6-42 Surface-to-surface elements... Advanced options Dynamics In a dynamic analysis inertial effects prevent a rigid body motion. One option to overcome rigid body motion is to solve the problem dynamically. You will need to add mass and damping in order to convert the solution from a static to a dynamic solution. This is known as a slow dynamic solution. You must make sure that the system has truly come to rest at the conclusion of your analysis. –Otherwise, nonzero accelerations and velocities will cause artificial inertial and damping forces to affect the equilibrium!

43. Training Manual October 15, 2001 Inventory # 001567 6-43 Surface-to-surface elements... Advanced options Displacement Control This technique uses imposed displacements to move the two bodies into contact. The solution can then be switched from displacement control to force control through the use of a null load step. Small imposed displacement.

44. Training Manual October 15, 2001 Inventory # 001567 6-44 Surface-to-surface elements... Advanced options Displacement Control Load Step 1 –Use a small imposed displacement to initialize contact. Load Step 2 –Switch from displacement control to force control. Delete the imposed displacements, apply the reaction forces, and solve in one substep. (This load step should converge in one or two iterations, there has been no change to the system.) Load Step 3 –Continue with the load history.

45. Training Manual October 15, 2001 Inventory # 001567 6-45 Surface-to-surface elements... Advanced options Weak Springs This technique uses weak springs to ground to prevent the rigid body motion. F The spring stiffnesses should be negligible compared to the stiffness of the system. By connecting the springs to ground, the reactions at the grounded nodes can be compared to the total reaction forces to ensure that the springs have no effect on the solution. The open-spring contact option could also be used.

46. Training Manual October 15, 2001 Inventory # 001567 6-46 Surface-to-surface elements... Advanced options Although these are all valid analysis techniques, they can be difficult to implement. –“Just Touching” - Small gaps or penetrations can exist because of numerical round-off in the finite element mesh. –Dynamics - Not always easy to damp out unwanted dynamic effects in a “static” model. –Displacement Control - Not always obvious which displacements to impose for a complicated loading. –Weak Springs - The initial load has to be small enough to cause a small deflection on the springs such that the contact elements recognize the penetration. Can require some experimentation as to not “pass through” the contact surfaces.

47. Training Manual October 15, 2001 Inventory # 001567 6-47 Surface-to-surface elements... Advanced options Three advanced contact features allow you to adjust the initial contact conditions to prevent rigid body modes. –Automated CNOF adjustment: the program calculates CNOF to close the gap. –Initial contact closure (ICONT): moves the nodes on the contact surface, within the adjustment band, to the target surface. –Initial allowable penetration range: (PMIN & PMAX): physically moves the rigid surface into the contact surface.

48. Training Manual October 15, 2001 Inventory # 001567 6-48 Surface-to-surface elements... Advanced options Adjusting the Initial Contact Conditions (ICONT) The real constant ICONT can be used to specify an “adjustment band” around the target surface. Any contact points within the adjustment band are shifted to the target surface. Only a small correction is recommended, as discontinuities can occur as a result of too large a value of ICONT. If the real constant ICONT is not specified ANSYS provides a small default value of ICONT according to the dimensions of the model. –To turn ICONT off, you must set it to a very small value (1e-20). –Setting it to zero triggers the nonzero default value.

49. Training Manual October 15, 2001 Inventory # 001567 6-49 Surface-to-surface elements... Advanced options Contact surface adjustment, shown before and after adjustment:

50. Training Manual October 15, 2001 Inventory # 001567 6-50 Surface-to-surface elements... Initial contact closure – Workshop Please refer to your Workshop Supplement for instructions on: W7. Rigid Body – ICONT

51. Training Manual October 15, 2001 Inventory # 001567 6-51 Surface-to-surface elements... Advanced options Adjusting the Initial Contact Conditions (PMIN and PMAX) The real constants PMIN and PMAX specify an initial penetration range. ANSYS physically moves the entire target surface (and the attached deformable body) to be within the range of penetration specified by PMIN and PMAX. If the target surface has prescribed constraints of zero, the initial adjustment using PMIN and PMAX will not be performed. The initial adjustment is an iterative process. ANSYS uses a maximum of 20 iterations to bring the target surface within the range of PMIN and PMAX.

52. Training Manual October 15, 2001 Inventory # 001567 6-52 Surface-to-surface elements... Advanced options When the target surface is moved, the bodies are no longer separated by a gap, but are in an initial state of closed contact. Contact Surface Target Surface before and after adjustment

53. Training Manual October 15, 2001 Inventory # 001567 6-53 Surface-to-surface elements... PMIN & PMAX – Workshop Please refer to your Workshop Supplement for instructions on: W8. Rigid Body – PMIN & PMAX.

54. Training Manual October 15, 2001 Inventory # 001567 6-54 Surface-to-surface elements... Advanced options In an ANSYS static analysis, if a force is applied to a DOF that has mathematically perfect zero stiffness, the program ignores the load. An initially unconstrained free body that is composed of deformable elements will usually have enough mathematical “noise” in the stiffness matrix to prevent the load from being ignored. However, an initially unconstrained rigid target surface will generally have mathematically perfect zero stiffness. –Forces and moments applied to the pilot node of such a surface will be ignored. –Use weak springs to add a small amount of stiffness in this case, to prevent such “loss” of load.

55. Training Manual October 15, 2001 Inventory # 001567 6-55 Surface-to-surface elements... Advanced options A specified offset (real constant CNOF, discussed previously) or automatic offset can also be very effective for closing an initially open gap and preventing rigid-body motion.

56. Training Manual October 15, 2001 Inventory # 001567 6-56 Surface-to-surface elements... Advanced options Self Contact Using asymmetric contact is more efficient for self contact, but it may be difficult to predict the contact and target surfaces. To use symmetric contact for self contact, simply place the target elements and contact elements on the same surface. Center Hyperelastic Ring Contacts Itself

57. Training Manual October 15, 2001 Inventory # 001567 6-57 Contact is erroneously detected Surface-to-surface elements... Advanced options Prevention of Spurious Contact If an external corner on a surface with symmetric contact (both contact and target elements on the same surface; frequently used for self-contact analyses) folds along a crease of less than 90°, spurious contact may result. No contact is detected

58. Training Manual October 15, 2001 Inventory # 001567 6-58 Surface-to-surface elements... Advanced options You can activate an advanced option to suppress spurious contact detection for this situation:

59. Training Manual October 15, 2001 Inventory # 001567 6-59 When building a contact pair between two bodies with the Contact Wizard, the DOF of the contact elements will be consistent with that of the underlying elements. No Keyopt adjustment is required by user. –For underlying structural element PLANE82, the Wizard automatically assigns the contact element DOF for structural, UX and UY, (KEYOPT(1)=0). –For underlying coupled-field element SOLID5, the Wizard, automatically assigns the contact element DOF for structural and temperature, (KEYOPT(1)=1). –For underlying pure thermal element PLANE77, the Wizard, automatically assigns the contact element DOF for temperature only, (KEYOPT(1)=2). Surface-to-surface elements... Advanced options

60. Training Manual October 15, 2001 Inventory # 001567 6-60 Coupled-field solids (PLANE13, SOLID5,SOLID98) and surface-to- surface contact elements with KEYOPT(1) = 1 (DOF = UX,UY,UZ,and temp) –Key features: Heat conduction between contacting surfaces Heat generation due to frictional dissipated energy Heat convection and/or radiation –Between surfaces with small gap –From free surface to environment Heat flux input at open gap Differential Thermal Expansion effects Thermally induced stresses Contact pressure at interface –NOTE: Coupled field elements listed above do not support plasticity Surface-to-surface elements... Advanced options

61. Training Manual October 15, 2001 Inventory # 001567 6-61 ANSYS can simulate this behavior and other complex thermal problems using … –Coupled-field structural-thermal Contact Supports general thermal contact analysis capability. Typical applications: –Metal forming (sequential Multi-Physics) –Assembly contact –Gas turbine Thermal Assembly Frictional Heating Surface-to-surface elements... Advanced options

62. Training Manual October 15, 2001 Inventory # 001567 6-62 Pure thermal solids (PLANE77, SOLID90) and surface-to- surface contact elements with KEYOPT(1) = 2 (DOF = temp) –Key features: Heat conduction between contacting surfaces Heat convection and/or radiation –Between surfaces with small gap (within pinball region) –From free surface to environment Heat flux input at open gap Surface-to-surface elements... Advanced options

63. Training Manual October 15, 2001 Inventory # 001567 6-63 Contact Resistance Two surfaces (at different temperatures) in contact experience a temperature drop across the interface. The drop is due to imperfect contact between the two surfaces. The imperfect contact, and hence the contact resistance, can be influenced by many factors such as: –surface flatness –surface finish –oxides –entrapped fluids –contact pressure –surface temperature –use of conductive grease TT T x Surface-to-surface elements... Advanced options

64. Training Manual October 15, 2001 Inventory # 001567 6-64 With the temperature DOF added (Keyopt(1)=1 or 2), the REAL constants menu will have additional thermal options… This topic and all associated REAL constants are also discussed in the Heat Transfer Training Manual Surface-to-surface elements... Advanced options

65. Training Manual October 15, 2001 Inventory # 001567 6-65 … and the Contact Wizard Options will have the Thermal TAB activated with the same data entry capability. Surface-to-surface elements... Advanced options

66. Training Manual October 15, 2001 Inventory # 001567 6-66 Heat conduction:q = TCC * (T T - T C ) –TCC is thermal contact conductance coefficient (real constant) Can be a table parameter by using the %TABLE% option (function of pressure and/or temperature) –T T and T C are target and contact surface temperatures –Heat flows when contact status is closed –Model temperature discontinuity across contact interface Surface-to-surface elements... Advanced options

67. Training Manual October 15, 2001 Inventory # 001567 6-67 Frictional heat generation: q = FHTG *  * v –FHTG is fraction of energy converted to heat (real constant) –  is the equivalent frictional stress –v is the sliding rate –Heat can be distributed unequally between contact and target: q c = FWGT * FHTG *  * v q t = (1-FWGT) * FHTG *  * v FWGT is weight factor for distribution of heat between the contact and target (real constant) Surface-to-surface elements... Advanced options

68. Training Manual October 15, 2001 Inventory # 001567 6-68 Convection:q = CONV * (T E - T C ) –CONV is convection coefficient (SFE table parameter load) –T E is target temperature or bulk temp for free surface (SFE) –T C is contact temperature –Heat flows between contact and target when 0 < gap < pinball –Heat flows from contact to environment for free surface Free surface is recognized for any of these conditions: –Open far-field contact (gap > pinball) –Contact elements only (omit target elements) –If target elements are present, Keyopt(3)=1 for target elements Surface-to-surface elements... Advanced options

69. Training Manual October 15, 2001 Inventory # 001567 6-69 Radiation: q = RDVF * EMIS * SBCT * [(T E + TOFFST) 4 - (T C + TOFFST) 4 ] –RDVF is radiation view factor (real constant) RDVF can be table parameter (function of time, temp, gap distance) –EMIS is surface emissivity (material property) –SBCT is Stefan-Boltzmann constant (real constant) –TOFFST is temperature offset from absolute zero (TOFFST command) –Heat flows between contact and target when 0 < gap < pinball –Heat flows from contact to environment for free surface Free surface recognized as for convection Surface-to-surface elements... Advanced options

70. Training Manual October 15, 2001 Inventory # 001567 6-70 External heat flux input –SFE applied to contact surface only (not target surface) Heat flux acts only if contact status is open For near-field contact, the flux acts on both contact and target For free surface flux acts only on contact element –Free surface recognized as for convection –Cannot be applied simultaneously with convection on a given element Surface-to-surface elements... Advanced options

71. Training Manual October 15, 2001 Inventory # 001567 6-71 Thermal contact tips –Conductance TCC Has units of heat / (time*degree*area) Typically less than the equivalent conductance of contacting solids –For frictional heating, TIME must have true chronological units However, if structural inertia and damping are unimportant, turn them off with TIMINT,STRUC,OFF for faster solution –Unsymmetric solver option may benefit frictional sliding NROPT,UNSYM Surface-to-surface elements... Advanced options

72. Training Manual October 15, 2001 Inventory # 001567 6-72 Surface-to-surface elements... PURE THERMAL – Workshop Please refer to your Workshop Supplement for instructions on: W9. Pure Thermal Contact.

73. Training Manual October 15, 2001 Inventory # 001567 6-73 Surface-to-surface elements C. Rigid surface considerations There are some additional modeling considerations for using a rigid surface in a contact problem. These include: –Pilot Nodes –Meshing Rigid Surfaces –Changing the Shape of the Target Surface –Connecting other Elements to Rigid Surfaces

74. Training Manual October 15, 2001 Inventory # 001567 6-74 Surface-to-surface elements... Rigid surface considerations Pilot Node The rigid surface can be associated with a “pilot node” whose motion governs the motion of the target surface. Forces, displacements and/or rotations for the entire surface can be specified on the pilot node. You can think of the pilot node as a handle for the entire rigid surface. If you define a pilot node, ANSYS checks for boundary conditions only on the pilot node and ignores the constraints on the other nodes of the target surface. The pilot node can be generated either by meshing a keypoint, or by direct generation using the same element attributes as the target elements.

75. Training Manual October 15, 2001 Inventory # 001567 6-75 Surface-to-surface elements... Rigid surface considerations The pilot node can be specified at any location. This allows for general rotations of the rigid target surface. Only the pilot node can connect to other elements. To account for the mass of a rigid body, define a mass element (MASS21) at the pilot node. Each target surface can have only one pilot node. Rigid surface rotated

76. Training Manual October 15, 2001 Inventory # 001567 6-76 Surface-to-surface elements... Rigid surface considerations Constraining a Rigid Target By default a rigid target is auto constrained, if the following conditions are met: –No explicit boundary conditions are defined. –The target surface is not connected to other elements. –No couples or constraint equations are defined.

77. Training Manual October 15, 2001 Inventory # 001567 6-77 Surface-to-surface elements... Rigid surface considerations If the Auto constrained option is selected, the internal settings for the constraints on the rigid surface are reset at the end of each load step. Check the model carefully when restarting the solution. If User specified is selected, ANSYS expects that all appropriate boundary conditions for the rigid target surface are defined by the user. –For this option, if you neglect to define DOF constraints, the surface will act as an unconstrained free body, with all the associated problems that pertain.

78. Training Manual October 15, 2001 Inventory # 001567 6-78 Surface-to-surface elements... Rigid surface considerations Meshing Complex Target Surfaces A rigid target can be a combination of a primitive target segment (cylinder, cone, sphere, etc.) and general segments. For an arbitrary surface, the quality of the mesh shape is not important. However, it is important that the target elements represent the geometry well. Excessively coarse discretization of the target surface can result in convergence problems.

79. Training Manual October 15, 2001 Inventory # 001567 6-79 Surface-to-surface elements... Rigid surface considerations Recommendations for meshing complex target surfaces: If the rigid target can be represented by primitives, use primitives. (Fewer elements are required, more efficient.) –For complex surfaces, use mapped meshing wherever possible. –If there is no curvature on one edge of the surface, assign one element division on that edge. –Use lower order targets for flat or nearly flat surfaces. Use higher order elements if the target surface is curved.

80. Training Manual October 15, 2001 Inventory # 001567 6-80 Surface-to-surface elements... Rigid surface considerations Below are examples of meshing patterns for a rigid surface using both lower order and higher order elements. Lower order elements are less expensive computationally but more elements are required. Higher order elements are expensive, but fewer elements are necessary. Lower Order Higher Order

81. Training Manual October 15, 2001 Inventory # 001567 6-81 Surface-to-surface elements... Rigid surface considerations Changing the Shape of the Target Surface For a target surface meshed with either higher order or lower order target segments (not primitives), the target nodes can be rotated into a local coordinate system. This allows a target surface to change shape within an analysis. For example, if an expanding rigid cylinder was desired. The cylinder could be meshed with higher order quadrilaterals, the target nodes rotated, and radial displacements imposed on the rigid target to expand the cylinder.

82. Training Manual October 15, 2001 Inventory # 001567 6-82 Surface-to-surface elements D. Creating elements w/o the Wizard The Contact Wizard is an extremely popular tool for creating a contact pair with minimal user intervention. However, from time to time an unanticipated modeling situation arises for which the Wizard was not designed. –Although such cases are rare, if you encounter one, you will need to be able to create contact elements without using the Wizard. There is an element attribute that is unique to target elements, the target element shape (TSHAP). You need to specify TSHAP whenever you create target elements by direct generation. (The program automatically uses the correct TSHAP when you create target elements by meshing.) In order to apply TSHAP, you need to become familiar with the available element shapes.

83. Training Manual October 15, 2001 Inventory # 001567 6-83 Surface-to-surface elements... Creating elements w/o the Wizard 2D Target Elements TARGE169 can have the following target segment types: line, parabola, arc (clockwise or counterclockwise), circle, or pilot node. J I I J K I J K I I Arc Pilot Node Circle Line Parabola

84. Training Manual October 15, 2001 Inventory # 001567 6-84 Surface-to-surface elements... Creating elements w/o the Wizard 3D Target Elements TARGE170 can have the following target segment types: three node triangle, four node quad, six node triangle, eight node quad, cylinder, cone, sphere, and pilot node.

85. Training Manual October 15, 2001 Inventory # 001567 6-85 Surface-to-surface elements... Creating elements w/o the Wizard 2D Surface-to-Surface Contact Elements CONTA171 is a 2-D, 2-node lower order line element that can be located on the surface of 2-D solid, shell, or beam elements. CONTA172 is a 2-D, 3-node, higher order parabolic element that can be located on the surfaces of 2-D solid elements with midside nodes. CONTA171CONTA172

86. Training Manual October 15, 2001 Inventory # 001567 6-86 Surface-to-surface elements... Creating elements w/o the Wizard 3-D Surface-to-Surface Contact Elements CONTA174 is a 3-D, 8-node, higher order quadrilateral element that can be located on the surfaces of 3-D solid elements with midside nodes. It can also degenerate to a 6-node triangle. CONTA173 is a 3-D, 4-node lower order quadrilateral element that can be located on the surface of 3-D solid or shell elements. It also can degenerate to a 3-node triangle.

87. Training Manual October 15, 2001 Inventory # 001567 6-87 Surface-to-surface elements... Creating elements w/o the Wizard Surface-to-Surface Procedure For the surface-to-surface contact elements one surface is designated as the “target” and the other as the “contact” surface. For rigid-to-flexible contact the rigid surface is always designated as the target surface. For flexible-to-flexible contact both the contact and target surface are associated with the deformable bodies. Contact elements are constrained against penetrating the target surface. However, target elements can penetrate through the contact surface.

88. Training Manual October 15, 2001 Inventory # 001567 6-88 Surface-to-surface elements... Creating elements w/o the Wizard Procedure for creating a contact pair without the Wizard: 1.Create the underlying finite element model. 2.Designate contact and target surfaces. 3.Specify element options and real constants. 4.Set element attributes and create the target surface elements. 5.Set element attributes and create the contact surface elements.

89. Training Manual October 15, 2001 Inventory # 001567 6-89 Surface-to-surface elements... Creating elements w/o the Wizard Designating Contact and Target Surfaces The goal is to maximize the number of contact detection points. For rigid-to-flexible contact the target surface is always the rigid surface. For flexible-to-flexible contact the choice of the target and contact surface can cause a different amount of penetration and thus affect the solution accuracy. Target Surface Contact Surface The target can penetrate the contact surface.

90. Training Manual October 15, 2001 Inventory # 001567 6-90 Surface-to-surface elements... Creating elements w/o the Wizard Guidelines for Target/Contact Surface –If a convex surface comes into contact with a flat or concave surface, the flat or concave surface should be the target surface. –If one surface has a coarse mesh and the other a fine mesh, the surface with the coarse mesh should be the target surface. –If one surface is stiffer than the other, the stiffer surface should be the target surface. –If one surface is higher order and the other is lower order, the lower order surface should be the target surface. –If one surface is larger than the other, the larger surface should be the target surface.

91. Training Manual October 15, 2001 Inventory # 001567 6-91 Surface-to-surface elements... Creating elements w/o the Wizard Surface-to-Surface Procedure Step 1. Create the underlying solid model and its mesh. Step 2. Designate the contact and the target surfaces. In this example one surface is considered rigid, thus the target and contact surface designation is clear. Contact Surface Hyperelastic Seal Insertion Rigid Target Surface

92. Training Manual October 15, 2001 Inventory # 001567 6-92 Surface-to-surface elements... Creating elements w/o the Wizard Step 3. Set the element options, and real constants. IMPORTANT! Contact pairs are identified by the real constant set number. The target and contact elements must share the same real constant set.

93. Training Manual October 15, 2001 Inventory # 001567 6-93 Surface-to-surface elements... Creating elements w/o the Wizard Step 4. Create the target surface elements. In this step the method used will depend on whether the target surface is rigid or deformable. –For a Rigid Target use: Direct generation (E) or, Automatic meshing (LMESH, AMESH) –For a Deformable Target use: Create > Elements > Surface-to-Surface (ESURF)

94. Training Manual October 15, 2001 Inventory # 001567 6-94 Surface-to-surface elements... Creating elements w/o the Wizard Step 4. Create the target surface elements. For direct generation of a rigid target surface, an additional element attribute, TSHAP, needs to be set before creating the target elements. TSHAP defines the shape of the target elements.

95. Training Manual October 15, 2001 Inventory # 001567 6-95 Surface-to-surface elements... Creating elements w/o the Wizard Step 4. Create the target surface elements. TSHAP is not required for automatic meshing of a rigid target surface. ANSYS will determine the appropriate shape of the target element from the solid model. –Mesh Keypoints (KMESH) - pilot node –Mesh Lines (LMESH) - 2-D rigid target surface –Mesh Areas (AMESH) - 3-D rigid target surface

96. Training Manual October 15, 2001 Inventory # 001567 6-96 Surface-to-surface elements... Creating elements w/o the Wizard Step 4. Create the target surface elements. To automatically create target elements over a deformable mesh, select the deformable surface nodes, and create target elements on the deformable body: Preprocessor > Create > Elements > Surf to Surf ANSYS will determine the shape and outward normal direction of the target elements from the underlying mesh. (ESURF)

97. Training Manual October 15, 2001 Inventory # 001567 6-97 Surface-to-surface elements... Creating elements w/o the Wizard Step 4. Create the target surface elements. Check outward normal direction (very important for rigid targets if automatic meshing was used): –Plot elements with element coordinate system symbol turned on. –Fix with ESURF (Node Num reverse). Line Counter-clockwise arc Pilot node

98. Training Manual October 15, 2001 Inventory # 001567 6-98 Surface-to-surface elements... Creating elements w/o the Wizard Step 5. Create the contact surface elements. Set contact element attributes, select deformable surface nodes, and create contact elements on the deformable body (same procedure as for the target elements on a deformable mesh): Preprocessor > Create > Elements > Surf to Surf These contact elements have the same order (lower or higher order) as the underlying elements. (ESURF)

99. Training Manual October 15, 2001 Inventory # 001567 6-99 Surface-to-surface elements... Creating elements w/o the Wizard Step 5. Create the contact surface elements. Note that for target or contact element creation over shell or beam elements, there is an option to create the elements on the top or the bottom surfaces of the beam or shell elements. Can control element location

100. Training Manual October 15, 2001 Inventory # 001567 6-100 Surface-to-surface elements... Creating elements w/o the Wizard From this point on, the procedure for applying loads and boundary conditions, solving, and postprocessing, is exactly the same as if the elements had been created using the Wizard.

101. Training Manual October 15, 2001 Inventory # 001567 6-101 Surface-to-surface elements... Creating elements – Workshop Please refer to your Workshop Supplement for instructions on: W10. Creating a Contact Pair without the Wizard

102. Training Manual October 15, 2001 Inventory # 001567 6-102 Output File Information The output file will print a contact summary, identifying all contact pairs in the model together with the settings used. *** NOTE *** Rigid-deformable contact pair identified by real constant set 1 has been set up. Please verify constraints on target nodes which may be automatically fixed by ANSYS. Contact stiffness factor FKN 0.10000 Default penetration tolerance factor FTOLN 0.10000 Default initial closure ICONT will be used. Default pinball region factor PINB 3.0000 Default Max. friction stress TAUMAX 0.10000E+21 Default tangent stiffness factor FKT 1.0000 Warning: Initial penetration is included. **************************************** *** NOTE *** Rigid-deformable contact pair identified by real constant set 2 has been set up. The DOF of rigid surface is driven by pilot node 255. Please verify constraints on the pilot node which may be automatically fixed by ANSYS. Contact stiffness factor FKN 0.10000 Default penetration tolerance factor FTOLN 0.10000 Default initial closure ICONT will be used. Default pinball region factor PINB 3.0000 Default Max. friction stress TAUMAX 0.10000E+21 Default tangent stiffness factor FKT 1.0000 Warning: Initial penetration is included. **************************************** Surface-to-surface elements E. Troubleshooting

103. Training Manual October 15, 2001 Inventory # 001567 6-103 Surface-to-surface elements... Troubleshooting Output File Information The output file will also echo the initial contact closure selected for each contact pair. *** NOTE *** Min. Initial gap 3.914707351E-02 was detected between contact element 603 and target element 1 specified by real constant set 1. *** NOTE *** You may move entire target surface specified by real constant set 1: x= 3.902639615E-02, y= -3.071443974E-03, z= 0,to bring it in contact. *** WARNING *** An initial contact closure factor (ICONT) 3.E-02 has been selected for real constant set 1.

104. Training Manual October 15, 2001 Inventory # 001567 6-104 Surface-to-surface elements... Troubleshooting Rigid-to-Flexible and Flexible-to-Flexible contact can be defined in the same model. Be sure to use separate real constants to define the contact pairs. The output file information can be helpful to check your contact pairs. Be sure to check the outward normal direction of the contact and target elements (via Main Menu > Preprocessor > Create > Contact Pair > View and Edit…) Contact occurs on the positive outward normal side of the contact and target elements. Improper Proper

105. Training Manual October 15, 2001 Inventory # 001567 6-105 Surface-to-surface elements... Troubleshooting Ensure that the target and contact surface definitions extend far enough to cover the full expected range of motion for the analysis. Be sure to adequately discretize a rigid target surface. Excessively coarse discretization can cause convergence difficulties. Ensure that the appropriate surface pairs are initially in contact with either ICONT, PMIN/PMAX or CNOF to prevent rigid body motions.

106. Training Manual October 15, 2001 Inventory # 001567 6-106 Surface-to-surface elements... Troubleshooting Sharp convex corners can cause convergence difficulties in large sliding problems. Use a line or an area fillet to smooth a sharp convex corner. Outward Normal Smoothing Radius Rigid Target Surface

107. Training Manual October 15, 2001 Inventory # 001567 6-107 Surface-to-surface elements... Troubleshooting Reset solution options, number of substeps, number of equilibrium iterations, unsymmetric solver, etc. And finally, FKN and FTOLN need to be set appropriately: –FKN usually will be between 0.01 and 10. Use a value of 1.0 (default) for bulk deformation problems, and 0.1 for bending dominated problems. Do NOT set FTOLN too small, too tight a penetration tolerance can result in divergence. –A model that ran successfully using ANSYS 5.5.x might not converge using ANSYS 5.6 or higher, because of changes to the semi-automatic contact-stiffness algorithm. If you have this problem, try decreasing FKN to 10%-20% of the value used at 5.5, and rerun.

108. Training Manual October 15, 2001 Inventory # 001567 6-108 Surface-to-surface elements... Troubleshooting