文本講義教程comp-l10vcct

1、Lesson content:IntroductionVCCT CriterionLEFM Example using Abaqus/StandardLEFM Example using Abaqus/ExplicitOutputDuctile Fracture with VCCTVCCT Plug-inComparison with Cohesive BehaviorExamplesWorkshop 7: Analysis of a DCB using VCCT (Abaqus/Standard)Workshop 8: Analysis of a DCB using VCCT (Abaqus

2、/Explicit)Lesson 10: Virtual Crack Closure Technique (VCCT)3 hoursIntroduction (1/2)Motivation is aircraft composite structural analysis To reduce the cost of laminated composite structures, large integrated bonded structures are being considered.In primary structures, bondlines and interfaces betwe

3、en plies are required to carry interlaminar loads.Damage tolerance requirements dictate that bondlines and interfaces carry required loads with damage.Modeling debonding along skin-stringer interfaceIntroduction (2/2)VCCT uses LEFM concepts*Based on computing the energy release rates for normal and

4、shear crack-tip deformation modes.Compare energy release rates to interlaminar fracture toughness.See Rybicki, E. F., and Kanninen, M. F., A Finite Element Calculation of Stress Intensity Factors by a Modified Crack Closure Integral, Engineering Fracture Mechanics, Vol. 9, pp. 931-938, 1977.Mode II

5、treatedsimilarlyNode numbersare shownPure Mode IModified VCCT*An “enhanced” version of VCCT is available to model ductile fracture. This is discussed briefly later in this lecture.VCCT Criterion (1/4)The debond capability is used to perform the crack propagation analysis for initially bonded crack s

6、urfaces.The crack propagation analysis allows for five types of fracture criteria:Critical stress criterionCrack opening displacement criterionCrack length vs. time criterionVCCT criterionLow-cycle fatigue criterionDefining case 4, “VCCT criterion,” is the subject of this lecture. The details of cas

7、es 1, 2, and 3 are discussed in Appendix 1 “Additional Crack Propagation Analysis using the Debond Capability.” The details of case 5 will be discussed later in Lecture 11 “Low-cycle Fatigue.”54321VCCT Criterion (2/4)When using VCCT to model crack propagationYou must:define contact pairs for potenti

8、al crack surfaces;define initially bonded crack surfaces;activate the crack propagation capability; andspecify the VCCT criterion.You also may: define spatially varying critical energy release rates;use viscous regularization, contact stabilization, and/or automatic stabilization to e convergence di

9、fficulties for unstable propagating cracks;use a linear scaling technique to accelerate convergence for VCCT.4321VCCT Criterion (3/4)Abaqus/Standard vs. Abaqus/ExplicitVCCT is supported in both products. The user interface is different between the two, however. The key difference is that Abaqus/Stan

10、dard requires the use of contact pairs while Abaqus/Explicit the use of general contact.Therefore, VCCT can only be used with 3D models in Abaqus/ExplicitSummary of interface differences:Abaqus/StandardAbaqus/ExplicitCrack surfaces*Contact pair*ContactInitially bonded surfaces*Initial conditions, ty

11、pe=contact*Contact clearance*Contact clearance assignmentCrack propagation*Debond*Contact property assignmentVCCT criterion*Fracture criterion, type = VCCT*Surface interaction*Cohesive behavior*Fracture criterion, type = VCCTVCCT Criterion (4/4)Defining the VCCT criterion is supported in Abaqus/CAE

12、for Abaqus/Standard models only.A plug-in is available for Abaqus/Explicit models which allows you to interactively define the debond interface(s). The details of the VCCT plug-in will be discussed later in this lecture.Download from “VCCT plug-in utility,” Knowledge Base Article QA .LEFM Example us

13、ing Abaqus/Standard (1/16)Double cantilever beam (DCB)Analyze debonding of a DCB model using the VCCT criterion.Steps required for setting up the model include: Define slave (TopSurf) and master (BotSurf) surfaces along the debond interface.Define a set (bond) containing the initially bonded region

14、(part of TopSurf in this example).BotSurfTopSurfbondLEFM Example using Abaqus/Standard (2/16)Define contact pairs for potential crack surfaces Potential crack surfaces are modeled as slave and master contact surfaces. Any contact formulation except the finite-sliding, surface-to-surface formulation

15、can be used.Cannot be used with self-contact.*NSET, NSET=bond, GENERATE1, 121, 1*SURFACE, NAME=TopSurfTopBeam_S1, S1*SURFACE, NAME=BotSurfBotBeam_S1, S1*CONTACT PAIR, INTER=.TopSurf, BotSurfslave surfacemaster surfaceNote: The frictionless interaction property is assumed.1BotSurfTopSurfbondLEFM Exam

16、ple using Abaqus/Standard (3/16)Define initially bonded crack surfacesThe initially bonded contact pair is identified with initial conditions.BotSurfTopSurfbond*NSET, NSET=bond, GENERATE1, 121, 1*SURFACE, NAME=TopSurfTopBeam_S1, S1*SURFACE, NAME=BotSurfBotBeam_S1, S1*CONTACT PAIR, INTER=.TopSurf, Bo

17、tSurf*INITIAL CONDITIONS, TYPE=CONTACTTopSurf, BotSurf, bondslave surfacemaster surfacea list of slave nodes that are initially bonded2LEFM Example using Abaqus/Standard (4/16)The unbonded portion of the slave surface will behave as a regular contact surface.If the node set that includes the initial

18、ly bonded slave nodes is not specified, the initial contact condition will apply to the entire contact pair.In this case, no crack tips can be identified, and the bonded surfaces cannot separate.For the VCCT criterion, the initially bonded nodes are bonded in all directions.LEFM Example using Abaqus

19、/Standard (5/16)Activate the crack propagation capabilityThe debonding capability is used to model crack propagation in a given step and identify the surfaces to be debonded.BotSurfTopSurfbond*NSET, NSET=bond, GENERATE1, 121, 1*SURFACE, NAME=TopSurfTopBeam_S1, S1*SURFACE, NAME=BotSurfBotBeam_S1, S1*

20、CONTACT PAIR, INTER=.TopSurf, BotSurf*INITIAL CONDITIONS, TYPE=CONTACTTopSurf, BotSurf, bond*STEP, NLGEOM*STATIC.*DEBOND, SLAVE=TopSurf, MASTER=BotSurf3LEFM Example using Abaqus/Standard (6/16)Aside: StabilityRamp-down of debonding forceBy default, the traction between the two surfaces at the crack

21、tip is released immediately during the increment following debonding.Can optionally have the traction released gradually during succeeding increments after debonding to avoid a sudden loss of stability.Usage:*DEBOND, DEBONDING FORCE= STEP | RAMPLEFM Example using Abaqus/Standard (7/16)Specify the VC

22、CT criterionThe BK law model is used in this example. *NSET, NSET=bond, GENERATE1, 121, 1*SURFACE, NAME=TopSurfTopBeam_S1, S1*SURFACE, NAME=BotSurfBotBeam_S1, S1*CONTACT PAIR, INTER=.TopSurf, BotSurf*INITIAL CONDITIONS, TYPE=CONTACTTopSurf, BotSurf, bond*STEP, NLGEOM*STATIC.*DEBOND, SLAVE=TopSurf, M

23、ASTER=BotSurf*FRACTURE CRITERION, TYPE=VCCT, MIXED MODE BEHAVIOR=BK280.0, 280.0, 280.0, 2.284GICGIICGIIICBotSurfTopSurfbond4BK law:LEFM Example using Abaqus/Standard (8/16)Crack propagation analysis is carried out on a nodal basis. The crack-tip node debonds when the fracture criterion freaches the

24、value 1.0 within a given tolerance, ftol: 1 f 1 + ftolwhereGequiv is the equivalent strain energy release rate, andGequivC is the critical equivalent strain energy release rate calculated based on the user-specified mode-mix criterion and the bond strength of the interface.For the VCCT criterion, th

25、e default value of ftol is 0.2. Use following option to control ftol:*FRACTURE CRITERION, TYPE=VCCT, TOLERANCE=ftolLEFM Example using Abaqus/Standard (9/16)Aside: More on stabilityDebonding multiple nodesFor an unstable crack growth problem it is more efficient to allow multiple nodes at and ahead o

26、f a crack tip to debond in one increment without cutting back the increment size.Usage:*DEBOND*FRACTURE CRITERION, TYPE=VCCT, UNSTABLE GROWTH TOLERANCE = fUtolRecall debonding starts when f 1If 1 + ftol f 1 + fUtol a corresponding crack front node will be released. New attempts will be made with the

27、 same time increment instead of cutting back the time increment.This process is repeated until f 1 for all nodes ahead of the crack front.The number of attempts made within an increment is specified using time incrementation controls.LEFM Example using Abaqus/Standard (10/16)In the DCB model, the to

28、lerance is set to 0.1.*NSET, NSET=bond, GENERATE1, 121, 1*SURFACE, NAME=TopSurfTopBeam_S1, S1*SURFACE, NAME=BotSurfBotBeam_S1, S1*CONTACT PAIR, INTER=.TopSurf, BotSurf*INITIAL CONDITIONS, TYPE=CONTACTTopSurf, BotSurf, bond*STEP, NLGEOM*STATIC.*DEBOND, SLAVE=TopSurf, MASTER=BotSurf*FRACTURE CRITERION

29、, TYPE=VCCT, MIXED MODE BEHAVIOR=BK, TOLERANCE=0.1280.0, 280.0, 280.0, 2.284BotSurfTopSurfbondLEFM Example using Abaqus/Standard (11/16)In addition to the BK law model, Abaqus/Standard also provides two other commonly used mode-mix criteria for computing GequivC: the Power law and the Reeder law mod

30、els.An appropriate model is best selected empirically.Power lawReeder lawApplies only to three-dimensional problems*FRACTURE CRITERION, TYPE=VCCT, MIXED MODE BEHAVIOR=POWERGIC, GIIC, GIIIC, am, an, ao*FRACTURE CRITERION, TYPE=VCCT, MIXED MODE BEHAVIOR=REEDERGIC, GIIC, GIIIC, LEFM Example using Abaqu

31、s/Standard (12/16)Spatially varying critical energy release ratesThe VCCT criterion can be defined with varying energy release rates by specifying the critical energy release rates at all nodes on the slave surface.In this case, the critical energy release rates should be interpolated from the criti

32、cal energy release rates specified at the nodes with the *NODAL ENERGY RATE option. However, the exponents (e.g., ) are still read from the data lines under the *FRACTURE CRITERION option.*NODAL ENERGY RATEnode ID1, GIC, GIIC, GIIICnode ID2, GIC, GIIC, GIIIC.*STEP*STATIC.*FRACTURE CRITERION, TYPE=VC

33、CT, MIXED MODE BEHAVIOR=BK, NODAL ENERGY RATEGIC, GIIC, GIIIC, model dataLEFM Example using Abaqus/Standard (13/16)Viscous regularization for VCCTCan be used to e some convergence difficulties for unstable propagating cracks.In DCB example, set the value of the viscosity coefficient to 0.1.*NSET, NS

34、ET=bond, GENERATE1, 121, 1*SURFACE, NAME=TopSurfTopBeam_S1, S1*SURFACE, NAME=BotSurfBotBeam_S1, S1*CONTACT PAIR, INTER=.TopSurf, BotSurf*INITIAL CONDITIONS, TYPE=CONTACTTopSurf, BotSurf, bond*STEP, NLGEOM*STATIC.*DEBOND, SLAVE=TopSurf, MASTER=BotSurf*FRACTURE CRITERION, TYPE=VCCT, MIXED MODE BEHAVIO

35、R=BK, TOLERANCE=0.1, VISCOSITY=0.1280.0, 280.0, 280.0, 2.284BotSurfTopSurfbondLEFM Example using Abaqus/Standard (14/16)In addition, contact and automatic stabilization that are not specific to VCCT can be also used to aid convergence. They are built into Abaqus/Standard and are compatible with VCCT

36、.Note that the crack propagation behavior may be modified by the damping forces.Therefore, monitor the damping energy (ALLVD or ALLSD) and compare it with the total strain energy in the model (ALLSE) to ensure that the results are reasonable in the presence of damping.ALLVD stores the damping energy

37、 generated from viscous regularization.ALLSD stores the damping energy generated from contact stabilization and automatic stabilization.LEFM Example using Abaqus/Standard (15/16)Linear scaling to accelerate convergence for VCCTAbaqus provides a linear scaling technique to quickly converge to the cri

38、tical load state. This reduces the solution time required to reach the onset of crack growth.This technique works best for models in which the deformation is nearly linear before the onset of crack growth.Once the first crack-tip node releases, the linear scaling calculations will no longer be valid

39、 and the time increment will be set to the default value.Usage: *CONTROLS, TYPE = VCCT LINEAR SCALINGwhere is the coefficient of linear scaling. For details of linear scaling to accelerate convergence for VCCT, see “Crack propagation analysis,” Section 11.4.3 of the Abaqus Analysis Users Guide.LEFM

40、Example using Abaqus/Standard (16/16)Tips for using the VCCT criterionCrack propagation problems using the VCCT criterion are numerically challenging.To help you create a successful model, several tips for using the VCCT criterion are provided:The master debonding surfaces must be continuous. The ti

41、e MPCs should NOT be used for the slave debonding surface to avoid overconstraints.A small clearance between the debonding surfaces can be specified to eliminate unnecessary severe discontinuity iterations during incrementation as the crack begins to progress. Note: More tips are provided in “Crack

42、propagation analysis,” Section 11.4.3 of the Abaqus Analysis Users Guide.LEFM Example using Abaqus/Explicit (1/7)Double cantilever beam (DCB)For the sake of comparison, the DCB problem will be solved using Abaqus/Explicit.Illustrates the keyword interface for Abaqus/Explicit; the Abaqus/CAE interfac

43、e using a plug-in is illustrated in a workshop.Because the VCCT criterion must be defined in the context of a general contact interaction, a 3D model is requiredConventional shell elements are used in this exampleBotSurfTopSurfbondLEFM Example using Abaqus/Explicit (2/7)Define general contactUse con

44、tact inclusions to specify crack surfaces explicitly and assign pure slave and master rolesIn this case the top surface is assigned a slave role*Surface Interaction, name=coh*Cohesive Behavior, eligibility=ORIGINAL CONTACTS*Fracture Criterion, type=VCCT, mixed mode behavior=BK, tol=0.1280.0, 280.0,

45、280.0, 2.284*Contact Clearance, name=clear, search nset=bond*Step*Dynamic, Explicit:*Contact*Contact InclusionsTopSurf, BotSurf *Contact Formulation, type=PURE MASTER-SLAVETopSurf, BotSurf, SLAVE*Contact Property AssignmentTopSurf, BotSurf, coh*Contact Clearance AssignmentTopSurf, BotSurf, clearBotS

46、urfTopSurfbond1LEFM Example using Abaqus/Explicit (3/7)Define the initial crackRequires the identification of:Two surfaces that will be considered for VCCTA node set for the initially bonded nodes*Surface Interaction, name=coh*Cohesive Behavior, eligibility=ORIGINAL CONTACTS*Fracture Criterion, type

47、=VCCT, mixed mode behavior=BK, tol=0.1280.0, 280.0, 280.0, 2.284*Contact Clearance, name=clear, search nset=bond*Step*Dynamic, Explicit:*Contact*Contact InclusionsTopSurf, BotSurf *Contact Formulation, type=PURE MASTER-SLAVETopSurf, BotSurf, SLAVE*Contact Property AssignmentTopSurf, BotSurf, coh*Con

48、tact Clearance AssignmentTopSurf, BotSurf, clearBotSurfTopSurfbond2LEFM Example using Abaqus/Explicit (4/7)Specify the VCCT criterionDefine a VCCT-based cohesive behavior surface interaction Mixed mode behavior options:BK (default)PowerReeder*Surface Interaction, name=coh*Cohesive Behavior, eligibil

49、ity=ORIGINAL CONTACTS*Fracture Criterion, type=VCCT, mixed mode behavior=BK, tol=0.1280.0, 280.0, 280.0, 2.284*Contact Clearance, name=clear, search nset=bond*Step*Dynamic, Explicit:*Contact*Contact InclusionsTopSurf, BotSurf *Contact Formulation, type=PURE MASTER-SLAVETopSurf, BotSurf, SLAVE*Contac

50、t Property AssignmentTopSurf, BotSurf, coh*Contact Clearance AssignmentTopSurf, BotSurf, clearBotSurfTopSurfbond3LEFM Example using Abaqus/Explicit (5/7)Activate crack propagationSimply assign the surface interaction to the pair of surfaces*Surface Interaction, name=coh*Cohesive Behavior, eligibilit

51、y=ORIGINAL CONTACTS*Fracture Criterion, type=VCCT, mixed mode behavior=BK, tol=0.1280.0, 280.0, 280.0, 2.284*Contact Clearance, name=clear, search nset=bond*Step*Dynamic, Explicit:*Contact*Contact InclusionsTopSurf, BotSurf *Contact Formulation, type=PURE MASTER-SLAVETopSurf, BotSurf, SLAVE*Contact

52、Property AssignmentTopSurf, BotSurf, coh*Contact Clearance AssignmentTopSurf, BotSurf, clearBotSurfTopSurfbond4LEFM Example using Abaqus/Explicit (6/7)General observationsQuasi-static responseDynamic effects are of utmost relevance when assessing the results from a debonding analysis using the VCCT

53、criterion. You must ensure that the Abaqus/Explicit analysis generates low ratios of kinetic energy to internal energy if simulating a quasi-static processAvoid the use of mass scaling; reduced densities will help significantly in recovering a quasi-static responseRunning the analysis over a longer

54、period of time will not help in most cases because bond breakage is an inherently fast and localized process.Dynamic overshootThe bond breakage at a given location is associated with a dynamic overshoot beyond the static equilibrium position. If the vibrations are significant (kinetic energy is clea

55、rly observable), the dynamic overshoot at nodes behind the crack tip may lead to premature debonding of the crack tip.LEFM Example using Abaqus/Explicit (7/7)MeshingTo maximize the accuracy of the debonding simulation, use quad meshes between the slave and master surfaces of the debonding surfaces.

56、Avoid using elements with aspect ratios greater than 2. In most cases mesh refinement will help with obtaining a realistic result.Material dataHighly mismatched critical energy values between modes tend to induce crack propagation in continuously changing directions in a manner that may be unstable

57、and unrealistic, particularly for modes II and III. Do not use such values unless experimental data suggest so.ConstraintsAvoid the use of other constraints involving nodes on both surfaces of the debonding interface because the cohesive contact forces will compete with the constraint forces to achi

58、eve global equilibrium. Bond breakage might be hard to interpret in these cases.Output (1/4)The following output options are provided to support the VCCT criterion:The same output is available for Abaqus/Standard and Abaqus/Explicit.*OUTPUT, FIELD, FREQUENCY=freq*CONTACT OUTPUT, MASTER=master, SLAVE

59、=slave*OUTPUT, HISTORY, FREQUENCY=freq*CONTACT OUTPUT, (MASTER=master, SLAVE=slave)|(NSET=nset)Output (2/4)The following bond failure quantities can be requested as surface output:DBTThe time when bond failure occurredDBSFFraction of stress at bond failure that still remainsDBSStress in the failed b

60、ond that remainsOPENBCRelative displacement behind crack.CRSTSCritical stress at failure.ENRRTStrain energy release rate.EFENRRTREffective energy release rate ratio.BDSTATBond state (=1.0 if bonded, 0.0 if unbonded)CSDMGOverall value of the scalar damage variableAll of the above variables can be vis