講義教程案例conv-l07modeling

1、Lesson content:General RemarksOverconstraints Detected during Model ProcessingOverconstraints Detected during Analysis Execution Controlling the Overconstraint Checks Example: Four-bar LinkageNonconservative LoadsLesson 7: Constraints and Loading90 minutesGeneral Remarks (1/8)An overconstraint means

2、 applying multiple consistent or conflicting kinematic constraints.*TIE and symmetry boundary conditions (node 141 or 151)*FRICTION, ROUGH and symmetry boundary conditions (node 101)Intersecting *TIE definitions*TIE and contact nodes (node 801 or 901)*TIEsymmetry boundary conditionsrigid punchfixed

3、rigid surfacecontact with *FRICTION, ROUGH201151141101801901General Remarks (2/8)Consistent overconstraintsTwo or more compatible constraints are applied at the same node. Also referred to as redundant constraints.All overconstraints in the previous slide are consistent. Conflicting overconstraintsT

4、wo or more patible (inconsistent) constraints applied at the same node.Example: the boundary conditions and the contact constraint at the marked slave node are conflicting.*BOUNDARYrigid indenterGeneral Remarks (3/8)For an Abaqus/Standard analysisIf the model has consistent overconstraints:The solve

5、r cannot guarantee the correct solution of such a system.In some (lucky) cases the expected solution is computed.Results can be computer platform-dependent.If the model has conflicting overconstraints:A correct solution does not exist. The model is ill-posed.If convergence is achieved, the solution

6、has no meaning.Symptoms (“solution” or convergence) can be computer platform-dependent.General Remarks (4/8)For an Abaqus/Explicit analysisIf the model has consistent overconstraints not removed in the analysis preprocessor:A solution will be found, and the displacement solution is correct.Forces th

7、rough the constraints are not uniquedistribution based approximately on stiffnesses and masses of connected components.Results are not computer platform-dependent due to overconstraint.If the model has conflicting overconstraints not detected in the analysis preprocessor:A correct solution does not

8、exist. The model is ill-posed.Results will not satisfy conflicting constraints but are not computer platform-dependent.General Remarks (5/8)Abaqus automatically resolves a limited (but frequently encountered) set of consistent overconstraints:Check for overconstraints caused by combinations of: Sing

9、le-point constraints:*BASE MOTION *BOUNDARYMulti-point constraints:*EQUATION *MPC*KINEMATIC COUPLING*RIGID BODY*TIEContact: *CONTACT PAIR Connector elementsGeneral Remarks (6/8)Two categories of detected overconstraintsOverconstraints detected during model processing:If consistent, the unnecessary c

10、onstraints are eliminated (warning message).If conflicting, an error message is issued.Overconstraints detected and resolved during analysis execution:Involve contact and either the *TIE option or the *BOUNDARY option.The analysis may e overconstrained because of contact status changes.General Remar

11、ks (7/8)Zero pivot warningAn overconstraint that Abaqus has not been able to resolve will usually cause a zero pivot message to be issued in the message file:*WARNING: SOLVER PROBLEM. ZERO PIVOT WHEN PROCESSING ELEMENT 20025 INTERNAL NODE 1 D.O.F. 4 OVERCONSTRAINT CHECKS: An overconstraint was detec

12、ted at one of the Lagrange multipliers associated with element 20025. There are multiple constraints applied directly or chained constraints that are applied indirectly to this element. The following is a list of nodes and chained constraints between these nodes that most likely lead to the detected

13、 overconstraint. LAGRANGE MULTIPLIER: 2321 863: connector element 20025 type SLOT ALIGN constraining 2 translations and 3 rotations .2321 - 10007: *RIGID BODY (or *COUPLING - KINEMATIC) .10007 - 3159: *RIGID BODY (or *COUPLING - KINEMATIC)Constraint chain; discussed in detail laterGeneral Remarks (8

14、/8)The effect of a zero pivot is hard to predict.If the overconstraint involves a contact interaction:Abaqus may not be able to establish the proper contact conditions, which will lead to repeated cutbacks as a result of an excessive number of SDIs.If MPCs are involved:The solution may diverge.Occas

15、ionally the zero pivot will cause the time average force in the model to increase by 1020 orders of magnitude without otherwise causing convergence problems. In this case the convergence tests are effectively disabled, which is a problem in itself because incorrect solutions will likely be created.

16、For example, this situation can occur if a boundary condition conflicts with a *TIE constraint.Overconstraints Detected during Model Processing (1/7)Intersecting tie constraints (*TIE)*TIEthree nodes in the same location At the marked location: There are three constraints tying together the three no

17、des.Only two constraints are needed.One constraint will be eliminated automatically.Overconstraints Detected during Model Processing (2/7)Boundary conditions applied to tied nodes (*TIE + *BOUNDARY)In general, *BOUNDARY at *TIE dependent nodes will be ignored.If different boundary conditions are spe

18、cified at the paired nodes, inconsistent overconstraint: error message is issued.symmetry boundary conditions *TIE between gasket (red) and cover (blue) Overconstraints Detected during Model Processing (3/7)Overconstraints related to rigid bodiesIn many cases regions of the model that were originall

19、y deformable are declared rigid in subsequent runs.This can lead to overconstrained models if:Surfaces tied together (*TIE) belong to rigid bodies.Connector elements included in rigid bodies or used to connect rigid bodies.Element sets that are used to define the rigid bodies overlap.Boundary condit

20、ions were specified at nodes of the rigid bodies.Overconstraints Detected during Model Processing (4/7)Surfaces tied (*TIE) together belong to rigid bodies.Original modelBoth left and right regions are deformable.*TIE used between regions.Modified model 1User makes everything rigid using *RIGID BODY

21、.*TIE will be ignored. Modified model 2User creates two adjacent rigid bodies.BEAM connector placed between reference nodes.Modified model 3User keeps brick mesh region deformable and makes tet mesh region rigid.Surfaces on the *TIE data line are switched.*TIE1Overconstraints Detected during Model P

22、rocessing (5/7)Element sets that are used to define the rigid bodies overlapOriginal model All elements are deformable.The defined element sets overlap.Modified model 1Two rigid bodies created from the two element sets.The two element sets behave as one rigid body.Internally a BEAM connector is plac

23、ed between the two reference nodes.Modified model 2User creates a rigid cradle using an additional RIGID BODY definition including all elements.Internally one additional BEAM connector is placed between two reference nodes.2ELSET 1ELSET 2Overconstraints Detected during Model Processing (6/7)Connecto

24、r elements are included in rigid bodies or are used to connect rigid bodiesOriginal modelAll deformable components.Joined with WELD/BEAM connectors.Modified model 1:One rigid body definition for the entire cradle.Overconstrained because WELDs/BEAMs are inside the RIGID BODY.Abaqus deactivates the WE

25、LDs/BEAMs.Modified model 2:One rigid body definition for each colored part.Overconstrained because too many WELDs/BEAMs between RIGID BODYs.Abaqus deactivates some WELDs/BEAMs.3BEAM connectorsBEAM connectors WELD connectorsOverconstraints Detected during Model Processing (7/7)Boundary conditions are

26、 specified at nodes of the rigid bodies.Original model Deformable fan.Boundary conditions in vertical direction allow only for rotation about the vertical axis. Modified modelShaft made rigid with *RIGID BODY.Boundary conditions transferred to the reference node.Two additional rotational boundary co

27、nditions are generated to allow only for rotations about the vertical axis.Reaction forces appear only at the reference node.4Overconstraints Detected during Analysis Execution (1/4) Overconstraints due to contact interactionsX-symmetry conditionsY-symmetry conditionselastomersteel backing compressi

28、on stoppertransmission panExample:Transmission pan gasket analysisThe pan is bolted to a flat rigid surface.Overconstraints Detected during Analysis Execution (2/4) Contact interactions and tie constraints*TIEpangasketMismatched meshes are tied together (using *TIE) near the compression stopper.Cont

29、act slave nodes are constrained both by the contact and the tie constraints.The contact constraint will not be enforced for the slave nodes of the tie constraint.Overconstraints Detected during Analysis Execution (3/4) Contact interactions and boundary conditions*FRICTION, ROUGHX-symmetry boundary c

30、onditionsThe friction formulation is switched from ROUGH to PENALTY only at overconstrained nodes.Overconstraints Detected during Analysis Execution (4/4) Contact interactions and boundary conditionsdieblankcontact slave nodes with fixed boundary conditionspunchThe marked contact slave nodes have bo

31、undary conditions in the same direction as the contact constraint.The contact constraint will not be applied at the overconstrained slave node.Controlling the Overconstraint Checks (1/3) Default overconstraint checks:Abaqus will attempt to remove as many overconstraints as possible.If a conflicting

32、overconstraint is detected, an error message is issued and the analysis is stopped.If a consistent overconstraint is identified but cannot be removed, a detailed message is issued.Controlling the Overconstraint Checks (2/3)Controlling constraint checks*CONSTRAINT CONTROLS, PRINT=YESPrints the constr

33、aint chains to the message file (illustrated shortly).*CONSTRAINT CONTROLS, NO CHANGESNo automatic elimination of consistent overconstraints is performed.Error messages are issued if overconstraints are encountered.*CONSTRAINT CONTROLS, NO CHECKSNo overconstraints checks are performed.Not mended!Con

34、trolling the Overconstraint Checks (3/3)Best practice:The model must be changed to remove all identified overconstraints that could not be removed automatically.Scan the message and data files (or use Abaqus/Viewer) for messages starting with the string OVERCONSTRAINT CHECKS. These messages contain

35、useful information about the overconstraints in the model.Search the message file for zero-pivot warnings. Do not ignore them. They are almost always an indication of overconstraints.Example: Four-bar Linkage (1/13)A four-bar linkage is modeled using connector elements.The mechanism is actuated by p

36、rescribing a connector motion to one of the hinges HINGE = JOIN + REVOLUTE RIGID BODYReference node completely fixedConnector motionNumber of constraints: JOIN: 4x3= 12 constraintsREVOLUTE: 4x2= 8 constraintsBoundary: 6 constraints Connector motion: 1 constraintTotal constraints: 27 constraintsTotal

37、 number of DOFs: 4x6 = 24 DOFsWe have: 27 24 = 3 constraints too manyExample: Four-bar Linkage (2/13)Model details*Part, name=bar*Node 1, 0.0, 0.0 2, 1.0, 0.0*Element, type=B31, elset=bar 1, 1, 2*Node, nset=ref 3, 0.5, 0.0*Rigid Body, ref node=ref, elset=bar*End Part*Assembly*Instance, name=bottom,

38、part=bar*End Instance : :*End Assembly*Element, type=CONN3D2, elset=hinges 1, bottom.2, right.1 2, right.2, top.1 3, top.2, left.1 4, left.2, bottom.1*Connector Section, elset=hinges Hinge, hingeOri,*Orientation, name=hingeOri 0., 0., 1., 0., 1., 0. : : HINGE = JOIN + REVOLUTE RIGID BODYbottomtoprig

39、htleft111122221432Part instance nameElement node numberConnector element number3333Rigid body reference nodeExample: Four-bar Linkage (3/13)When Abaqus/Standard attempts to find a solution for this model, three zero pivots are identified in the first increment of the analysis suggesting that there a

40、re 3 too many constraints in the model (as expected). *WARNING: SOLVER PROBLEM. ZERO PIVOT WHEN PROCESSING ELEMENT 2 (ASSEMBLY) INTERNAL NODE 20 D.O.F. 1 *WARNING: SOLVER PROBLEM. ZERO PIVOT WHEN PROCESSING ELEMENT 2 (ASSEMBLY) INTERNAL NODE 20 D.O.F. 4 *WARNING: SOLVER PROBLEM. ZERO PIVOT WHEN PROC

41、ESSING ELEMENT 2 (ASSEMBLY) INTERNAL NODE 20 D.O.F. 5Example: Four-bar Linkage (4/13)The constraint chainsRequires *CONSTRAINT CONTROLS, PRINT=YESConsider the first zero pivot warning:*WARNING: SOLVER PROBLEM. ZERO PIVOT WHEN PROCESSING ELEMENT 2 (ASSEMBLY) INTERNAL NODE 20 D.O.F. 1 LAGRANGE MULTIPL

42、IER: 2 INSTANCE RIGHT 1 INSTANCE TOP: connector element 2 type HINGE .2 INSTANCE RIGHT - 3 INSTANCE RIGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE RIGHT - 1 INSTANCE RIGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE RIGHT - 2 INSTANCE BOTTOM: connector element 1 type HINGE .2 INST

43、ANCE BOTTOM - 3 INSTANCE BOTTOM: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE BOTTOM - pre-defined type *BOUNDARY .3 INSTANCE BOTTOM - 1 INSTANCE BOTTOM: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE BOTTOM - 2 INSTANCE LEFT: connector element 4 type HINGE .2 INSTANCE LEFT - 3 INSTANCE LE

44、FT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE LEFT - 1 INSTANCE LEFT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE LEFT - 2 INSTANCE TOP: connector element 3 type HINGE .2 INSTANCE TOP - 3 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE TOP - 1 INSTANCE TOP: *RIGID BO

45、DY (or *COUPLING - KINEMATIC) .2 INSTANCE LEFT - 1 INSTANCE BOTTOM: connector element 4 with *CONNECTOR MOTION in components 4 Constraint Chainstopright122Example: Four-bar Linkage (5/13)LAGRANGE MULTIPLIER: 2 INSTANCE RIGHT 1 INSTANCE TOP: connector element 2 ASSEMBLY_NAME ASSEMBLY type HINGE const

46、raining 3 translations and 2 rotations Indicates the zero pivot is associated with the connection between the RIGHT and TOP bars. Subsequent lines of the message trace how the nodes identified in the zero pivot warning are connected to the rest of the model.Use this as a starting point when trying t

47、o identify the overconstraint.topright122Example: Four-bar Linkage (6/13)Detailed analysis of constraint chain for node right.2First chain of constraints terminates in a boundary condition: .2 INSTANCE RIGHT - 3 INSTANCE RIGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE RIGHT - 1 INSTANCE RI

48、GHT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE RIGHT - 2 INSTANCE BOTTOM: connector element 1 type HINGE .2 INSTANCE BOTTOM - 3 INSTANCE BOTTOM: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE BOTTOM - pre-defined type *BOUNDARY .3 INSTANCE BOTTOM - 1 INSTANCE BOTTOM: *RIGID BODY (or *CO

49、UPLING - KINEMATIC) .1 INSTANCE BOTTOM - 2 INSTANCE LEFT: connector element 4 type HINGE .2 INSTANCE LEFT - 3 INSTANCE LEFT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE LEFT - 1 INSTANCE LEFT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE LEFT - 2 INSTANCE TOP: connector element 3 type H

50、INGE .2 INSTANCE TOP - 3 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE TOP - 1 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .2 INSTANCE LEFT - 1 INSTANCE BOTTOM: connector element 4 with *CONNECTOR MOTIONSince indentation level is the same, this indicates the end of the fi

51、rst chainright.2 right.3 right.1 bottom.2 bottom.3 *BOUNDARYFirst chain of constraints terminates in a boundary conditionExample: Four-bar Linkage (7/13)The second chain forms a closed loop: .2 INSTANCE RIGHT - 3 INSTANCE RIGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE RIGHT - 1 INSTANCE R

52、IGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE RIGHT - 2 INSTANCE BOTTOM: connector element 1 type HINGE .2 INSTANCE BOTTOM - 3 INSTANCE BOTTOM: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE BOTTOM - pre-defined type *BOUNDARY .3 INSTANCE BOTTOM - 1 INSTANCE BOTTOM: *RIGID BODY (or *C

53、OUPLING - KINEMATIC) .1 INSTANCE BOTTOM - 2 INSTANCE LEFT: connector element 4 type HINGE .2 INSTANCE LEFT - 3 INSTANCE LEFT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE LEFT - 1 INSTANCE LEFT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE LEFT - 2 INSTANCE TOP: connector element 3 type

54、HINGE .2 INSTANCE TOP - 3 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE TOP - 1 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .2 INSTANCE LEFT - 1 INSTANCE BOTTOM: connector element 4 with *CONNECTOR MOTIONright.2 right.3 right.1 bottom.2 bottom.3 bottom.1 left.2 left.3 lef

55、t.1 top.2 top.3 top.1 right.2The links that form the first chain until the indentation level is repeated also form part of the second chainRepeated indentation: end of the second chainExample: Four-bar Linkage (8/13)The third chain ends in a prescribed connector motion: .2 INSTANCE RIGHT - 3 INSTANC

56、E RIGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE RIGHT - 1 INSTANCE RIGHT: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE RIGHT - 2 INSTANCE BOTTOM: connector element 1 type HINGE .2 INSTANCE BOTTOM - 3 INSTANCE BOTTOM: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE BOTTOM - pre-d

57、efined type *BOUNDARY .3 INSTANCE BOTTOM - 1 INSTANCE BOTTOM: *RIGID BODY (or *COUPLING - KINEMATIC) .1 INSTANCE BOTTOM - 2 INSTANCE LEFT: connector element 4 type HINGE .2 INSTANCE LEFT - 3 INSTANCE LEFT: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE LEFT - 1 INSTANCE LEFT: *RIGID BODY (or *CO

58、UPLING - KINEMATIC) .1 INSTANCE LEFT - 2 INSTANCE TOP: connector element 3 type HINGE .2 INSTANCE TOP - 3 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .3 INSTANCE TOP - 1 INSTANCE TOP: *RIGID BODY (or *COUPLING - KINEMATIC) .2 INSTANCE LEFT - 1 INSTANCE BOTTOM: connector element 4 with *CONN

59、ECTOR MOTIONright.2 right.3 right.1 bottom.2 bottom.3 bottom.1 left.2 bottom.1 *Connector MotionExample: Four-bar Linkage (9/13)Removing the overconstraintNo unique way to remove the overconstraint in this model. bottomleft4The constraint chains indicate there are 3 overconstraints (1 translational

60、and 2 rotational). Analyze each constraint in isolation and track the overall impact on the model.Starting point is arbitrary; here we start with the hinge connection between the bottom and left bars:Recall that a hinge connection defines a join and a revolute constraintJoin: endpoints of the bars r