Segment to Segment Contact in Marc

Transcription

2 OVERVIEW Introduction Disadvantages of Node to Segment method Segment to Segment Method Description Pre processing Post processing Benefits Limitations Demo Copyright 2013 MSC.Software Corporation S1-2

3 INTRODUCTION Segment to Segment contact algorithm provides an easy, efficient and faster solution to Contact analysis problems No Master Slave concept Better stress continuity at Contact interfaces Supported in both Mentat & Patran GUI Copyright 2013 MSC.Software Corporation S1-3

4 DISADVANTAGES NODE TO SEGMENT (N2S) CONTACT Solution depends on selection of master and slave contact body Stress continuity is not maintained across the contact interface Primary output is Force not Stress Conflict with other MPC s and/or boundary conditions Double sided shell contact doesn t work Copyright 2013 MSC.Software Corporation S1-4

6 N2S CONTACT AND DOUBLE-SIDED SHELL CONTACT With node-segment contact a shell node can only touch another body at the top or at the bottom, not both. Body 2 cannot touch both Body 1 & Body 3 Copyright 2013 MSC.Software Corporation S1-6

7 SEGMENT TO SEGMENT CONTACT : PHASE 1 CONTACT DETECTION auxiliary points per contact segment are introduced to allow for a contact search Each contact segment has a number of auxiliary points, which are located at fixed positions on the contact segment and which are only used during the contact detection phase. Each segment (element edge/face) has a fixed number of auxiliary points; in 2D, we have 3 auxiliary points per segment, in 3D we have 9 auxiliary points (3x3 grid) per segment each auxiliary point can only touch one body at a time; at corners, multiple auxiliary points will be introduced Copyright 2013 MSC.Software Corporation S1-7

8 CONTACT DETECTION (CONTD.) Two-pass contact detection is performed for the auxiliary points Distance Check: check whether auxiliary point is inside the distance tolerance Direction Check: check whether the angle between the normal vector at the auxiliary point and the normal vector at the potentially contacted segment is larger than the threshold value α.the default value of α is 120º If both checks are passed, the segments are marked as potentially contacting Copyright 2013 MSC.Software Corporation S1-8

9 SEGMENT TO SEGMENT CONTACT : PHASE 2 CONTACT COSTRAINTS For each segment-segment combination within contact distance, polygons (polylines in 2D) will be generated element wise Each polygon has a number of polygon points These Polyline points does not coincide with the auxiliary points used of the contact detection before. Copyright 2013 MSC.Software Corporation S1-9

11 CONTACT CONSTRAINTS (CONTD.) In the same way as for the auxiliary points before, a second contact detection pass is performed for the polygon points (distance + direction check). At the end of this second pass, there is a set of polygon points being in contact and representing the contact area. The polygons can be treated as normal interface elements representing the contact stiffness. The polygon points build the basis for the numerical integration of the corresponding contact stiffness formulation. Copyright 2013 MSC.Software Corporation S1-11

12 SEGMENT TO SEGMENT CONTACT : PHASE 3 ADDING CONTACT CONSTRAINTS TO GLOBAL EQUATIONS For each polygon point in contact a weighted stiffness related to a penalty factor is added to a normal stiffness matrix. A tangential stiffness induced by sticking or glued contact is treated in a similar way. For each polygon point in contact a weighted contact force related to a Lagrange multiplier is added to a vector of normal forces. Tangential forces induced by friction or glued contact are treated in a similar way. Copyright 2013 MSC.Software Corporation S1-12

13 PENALTY FACTOR The penalty factor can be interpreted as a stiffness per area, applied to the normal interface elements. The default penalty stiffness is based upon the average initial material stiffness and a characteristic length L of the contacting bodies. For solid elements the characteristic length is L= 0.5 L_edge; L_edge = average edge length of all edges being part of the contact boundary. For shell elements L= 0.5 t; t = average thickness of all elements being part of a contact body. This choice gives good results in many application, particularly if the material stiffness of the contacting bodies is of the same order. In cases when a soft material touches a stiff material, or when two thin shell touch each other and are allowed to bend, the default penalty factor is possibly too high and may lead to convergence problems feature,10201 Can help. Copyright 2013 MSC.Software Corporation S1-13

14 AUGMENTATION In case that too large penetration occurs, the augmentation procedure may be used to iteratively adjust the contact normal stress so that the overlap of the contact bodies will be minimized. augmentation means that a gap function g defined by the contact gap (g=0 when gap is closed) will be minimized iteratively by adjusting the contact normal stresses p The augmentation procedure (adjustment of the spring base) forces additional recycles in the global Newton-Raphson algorithm. Copyright 2013 MSC.Software Corporation S1-14

15 AUGMENTATION (CONTD.) No augmentation. Recommended in most cases since it gives reasonable accurate results without additional recycles. Augmentation based on constant penetration field. Recommended for linear finite elements Augmentation based on (bi-)linear penetration field. Should be only used for quadratic elements. Automatic detection: choice is made based on the combination of elements corresponding to the contacting segments. Copyright 2013 MSC.Software Corporation S1-15

16 AUGMENTATION (CONTD.) An exact fulfillment of a zero gap is hardly achievable, therefore a threshold called penetration distance has been introduced to control the augmentation based iterations. If a gap exceeds the penetration distance, an additional recycle will be forced to reduce it. The default penetration distance beyond which an augmentation will be applied is defined as L; L = characteristic length of the contact pair. Users may redefine the penetration distance, either globally or individually per contact body pair. In the same way the augmentation method is applicable to adjust the tangential displacements under sticking conditions. Equivalent to the penetration distance, a slip distance is defined for the tangential augmentation procedure. Copyright 2013 MSC.Software Corporation S1-16

17 FINITE SLIDING Marc continuously monitors the relative displacements of the polygon points. Once a threshold value is exceeded, new polygon points will be created. The default value of the recreation threshold for the polygon points equals five times the contact tolerance. This value can be redefined by the user. As soon as new polygon points have been created, significant contact data like the contact stresses is mapped from the old to the new polygon points and used as a starting point to continue the analysis. Copyright 2013 MSC.Software Corporation S1-17

18 FRICTION & SEPARATION CONTROL Friction Only the bilinear coulomb and bilinear shear friction model based on stresses are available for S2S contact Separation control Separation is always based on absolute nodal stresses. If the contact normal stress is in tension, then the corresponding polygon point will separate, which implies that this point will no longer contribute to the global stiffness matrix and force vector. Copyright 2013 MSC.Software Corporation S1-18

19 SUMMARY S2S CONTACT ALGORITHM Contact Detection Auxiliary points located on each contact segment are used to detect contact between potential contact segments. A two-pass contact detection including a distance and a direction check is performed to check segments on contact. Contact Constraints Based on a projection from a contacting segment to a contacted segment, polygons representing the contact stiffness are built. Each polygon consists of a couple of polygon points building the basis for the numerical integration of the contact stiffness. The penalty factor represents the contact stiffness. It may be determined by Marc or defined by the user. Penetration may be limited by a correction procedure - augmentation. Copyright 2013 MSC.Software Corporation S1-19

25 POST PROCESSING Contact status: set to 1 as soon as contact has been detected in at least one polygon point on a contact segment to which the node belongs. So the contact status is shown on both contact bodies of a contact body pair. Contact normal force: computed by integration of the contact normal stresses in the polygon points. Contact friction force: computed by integration of the contact friction stresses in the polygon points. Contact normal stress: nodal vector representing the normal stresses in the polygon points based on a constant field approximation per contact segment of the polygon values. Contact friction stress: nodal vector representing the friction stresses in the polygon points based on a constant field approximation per contact segment of the polygon values. Copyright 2013 MSC.Software Corporation S1-25

26 BENEFITS Easy job setup Better stress continuity Better contact normal stress distribution Contact status shown in both bodies Faster solution time Run time comparison for Aerospace Buckling Application Contact Type Displacement # increments # iterations Wall Time (Sec) N 2 S S 2 S Copyright 2013 MSC.Software Corporation S1-26 S2S 62% Faster

27 LIMITATIONS AS OF MARC 2013 Anisotropic friction Deactivation of glued contact Breaking glue Brake squeal Pore pressure, fluid-solid and piezo-electric analyses Wear Domain Decomposition Method (DDM) Copyright 2013 MSC.Software Corporation S1-27