Potential crack surfaces are modeled as slave and master contact surfaces(see).Any contact formulation except the finite-sliding, surface-to-surfaceformulation can be used. The predetermined crack surfaces are assumed to beinitially partially bonded so that the crack tips can be identified explicitlybyAbaqus/Standard.Initially bonded crack surfaces cannot be used with self-contact.Define an initial condition to identify which part of the crack is initiallybonded. You specify the slave surface, the master surface, and a node set thatidentifies the initially bonded part of the slave surface. The unbonded portionof the slave surface will behave as a regular contact surface. For fracture criteria based on the critical stress, critical crack openingdisplacement, or crack length versus time, it is possible to bond the nodes inthe node set (or the contact pair if a node set is not defined) only in thenormal direction. In this case the nodes are allowed to move freely tangentialto the contact surfaces. Friction cannot be specified if the nodes are bonded only in the normal direction.Bonding only in the normal direction is typically used to model bondedcontact conditions in Mode I crack problems where the shear stress ahead of thecrack along the crack plane is zero.
The crack propagation capability must be activated within the stepdefinition to specify that crack propagation may occur between the two surfacesthat are initially partially bonded. You specify the surfaces along which thecrack propagates.If the crack propagation capability is not activated for partially bondedsurfaces, the surfaces will not separate; in this case the specified initialcontact conditions would have the same effect as that provided by the tiedcontact capability, which generates a permanent bond between two surfacesduring the entire analysis (see). InAbaqus/Explicitpotential crack surfaces are modeled as bonded general contact surfaces (see)in the context of surface-based cohesive behavior (see).Hence, the capability is available in three-dimensional analyses only and isimplemented using a pure master-slave formulation. As is the case inAbaqus/Standard,the predetermined crack surfaces are assumed to be initially partially bondedso that the crack tips can be identified explicitly.To identify which pair of surfaces determine the crack and which part of thecrack is initially bonded, you must define and assign a contact clearance (see).You first define a contact clearance to specify the node set that is initiallybonded, and then you assign this contact clearance to a pair of twosingle-sided surfaces that define the crack. The unbonded portion behaves as aregular contact surface.
The nodes in the node set are considered to beinitially bonded in all directions.The crack tip is identified only from the specified two surfaces and thenode set. No attempt is made to determine a crack tip from all surfacesincluded in the general contact domain. Consequently, to be able to identifythe crack tip, the surface including the specified node set must extend pastthe node set. Otherwise, the surfaces will not debond, and the crack cannotpropagate.You complete the definition of the crack propagation capability by defininga fracture-based cohesive behavior surface interaction. You activate the crackpropagation by assigning it to the pair of surfaces that are initiallypartially bonded. If the fracture criterion is met, crack propagation occursbetween these two surfaces.
Cohesive behavior is also used to specify theelastic behavior of the bonds (see).If a fracture-based surface interaction is not assigned to a pair ofsurfaces, the crack definition is incomplete. UnlikeAbaqus/Standardwhere the identified nodes will stay bonded if the crack is not activated, inAbaqus/Explicitthe nodes identified by the contact clearance definition will separate withoutgenerating any interface stress.Similar toAbaqus/Standard,cracks can propagate from single or multiple crack tips for the same pair ofsurfaces. F L L ≤ f ≤ f U L,where f U L = 1 + f t o land f L L = 1for VCCT, enhancedVCCT, and low-cycle fatigue criteria orf L L = 1 - f t o lfor other fracture criteria. You can specify the tolerancef t o l.InAbaqus/Standard,if f 1 + f t o l,the time increment is cut back such that the crack propagation criterion issatisfied except in the case of an unstable crack growth problem where multiplenodes at and ahead of a crack tip are allowed to debond without the cut back ofincrement size in one increment. The default value of f t o lis 0.1 for the critical stress, critical crack opening displacement, and cracklength versus time criteria and is 0.2 for theVCCT, enhancedVCCT, and low-cycle fatigue criteria. F = ( σ ^ n σ f ) 2 + ( τ 1 τ 1 f ) 2 + ( τ 2 τ 2 f ) 2, σ ^ n = max ( σ n, 0 ),where σ nis the normal component of stress carried across the interface at the distancespecified; τ 1and τ 2are the shear stress components in the interface; and σ fand τ 1 fare the normal and shear failure stresses, which you must specify.
The secondcomponent of the shear failure stress, τ 2,is not relevant in a two-dimensional analysis; therefore, the value ofτ 2 fneed not be specified. The crack-tip node debonds when the fracture criterion,f, reaches the value 1.0.If the value of τ 1 fis not given or is specified as zero, it will be taken to be a very largenumber so that the shear stress has no effect on the fracture criterion.The distance ahead of the crack tip is measured along the slave surface, asshown in.The stresses at the specified distance ahead of the crack tip are obtained byinterpolating the values at the adjacent nodes. The interpolation depends onwhether first-order or second-order elements are used to define the slavesurface. F = δ δ c,where δis the measured value of crack opening displacement andδ cis the critical value of the crack opening displacement (user-specified). Thecrack-tip node debonds when the fracture criterion reaches the value 1.0.You must supply the crack opening displacement versus cumulative cracklength data. InAbaqus/Standardthe cumulative crack length is defined as the distance between the initialcrack tip and the current crack tip measured along the slave surface in thecurrent configuration. The crack opening displacement is defined as the normaldistance separating the two faces of the crack at the given distance.You specify the position, n, behind the crack tipwhere the critical crack opening displacement is calculated.
The value of thisposition must be specified as the length of the straight line joining thecurrent crack tip and points on the slave and master surfaces. This criterion is available only inAbaqus/Standard.To specify the crack propagation explicitly as a function of total time, youmust provide a crack length versus time relationship and a reference point fromwhich the crack length is measured.
This reference point is defined byspecifying a node set.Abaqus/Standardfinds the average of the current positions of the nodes in the set to definethe reference point. During crack propagation the crack length is measured fromthis user-specified reference point along the slave surface in the deformedconfiguration. The time specified must be total time, not step time.The fracture criterion, f, is stated in terms of theuser-specified crack length and the length of the current crack tip. The lengthof the current crack tip from the reference point is measured as the sum of thestraight line distance of the initial crack tip from the reference point andthe distance between the initial crack tip and the current crack tip measuredalong the slave surface.Referring to,let node 1 be the initial location of the crack tip and node 3 be the currentlocation of the crack tip. The distance of the current crack tip located atnode 3 is given. F = l - ( l 3 - Δ l 23 ) Δ l 23,where l is the length at the current time obtained fromthe user-specified crack length versus time curve.
Abaqus Student
Crack-tip node 3 will debondwhen the failure function f reaches the value of 1.0(within the user-defined tolerance).If geometric nonlinearity is considered in the step , thereference point may move as the body deforms; you must ensure that thismovement does not invalidate the crack length versus time criterion.Abaqus/Standarddoes not extrapolate beyond the end points of your crack data. Therefore, ifthe first crack length specified is greater than the distance from the crackreference point to the first bonded node, the first bonded node will neverdebond and the crack will not propagate.
In this caseAbaqus/Standardwill print warning messages in the message (.msg) file. This criterion is available in bothAbaqus/StandardandAbaqus/Explicit.The Virtual Crack Closure Technique ( VCCT)criterion uses the principles of linear elastic fracture mechanics( LEFM) and, therefore, is appropriate forproblems in which brittle crack propagation occurs along predefined surfaces.VCCT is based on the assumption that thestrain energy released when a crack is extended by a certain amount is the sameas the energy required to close the crack by the same amount. For example,illustrates the similarity between crack extension from ito j and crack closure at j. F = G I G I C = 1 2 ( v 1, 6 F v, 2, 5 b d ) 1 G I C ≥ 1.0,where G Iis the Mode I energy release rate, G I Cis the critical Mode I energy release rate, b is thewidth, d is the length of the elements at the crack front,F v, 2, 5is the vertical force between nodes 2 and 5, and v 1, 6is the vertical displacement between nodes 1 and 6. Assuming that the crackclosure is governed by linear elastic behavior, the energy to close the crack(and, thus, the energy to open the crack) is calculated from the previousequation. Similar arguments and equations can be written in two dimensions forMode II and for three-dimensional crack surfaces including Mode III. F = G e q u i v G e q u i v C ≥ 1.0,where G e q u i vis the equivalent strain energy release rate calculated at a node, andG e q u i v Cis the critical equivalent strain energy release rate calculated based on theuser-specified mode-mix criterion and the bond strength of the interface.
Thecrack-tip node will debond when the fracture criterion reaches the value of1.0.Abaqusprovides three common mode-mix formulae for computing G e q u i v C:the BK law, the power law, and the Reeder law models. The choice ofmodel is not always clear in any given analysis; an appropriate model is bestselected empirically. 1 + f t o l ≤ f ≤ 1 + f t o l u,where f t o lis the tolerance described earlier in this section, rather than cut back theincrement size, more nodes at and ahead of the crack tip are allowed to debondin one increment until f. F = G e q u i v G e q u i v C ≥ 1.0.The crack-tip node debonds when the fracture criterion reaches the value of1.0. However, unlike the original VCCTcriterion, you can specify two different critical fracture energy releaserates: G Cfor the onset of a crack and G C Pfor the growth of a crack.
When the enhancedVCCT criterion is used in the general caseinvolving Mode I, II, and III fracture, the amount of energy dissipatedassociated with the release of the debonding force is controlled by thecritical equivalent strain energy release rate required to propagate the crack,G e q u i v C P,rather than by the critical equivalent strain energy release rate required toinitiate the crack, G e q u i v C.The formulae for calculating G e q u i v C Pare identical to those used for G e q u i v Cfor different mixed-mode fracture criteria. This criterion is available only inAbaqus/Standard.If you specify the low-cycle fatigue criterion, progressive delaminationgrowth at the interfaces in laminated composites subjected to sub-criticalcyclic loadings can be simulated. D a d N = c 3 Δ G c 4,where c 3and c 4are material constants.At the end of cycle N,Abaqus/Standardextends the crack length, a N,from the current cycle forward over an incremental number of cycles,Δ Nto a N + Δ Nby releasing at least one element at the interface. Given the materialconstants c 3and c 4,combined with the known node spacing Δ a N j = a N + Δ N - a Nat the interface elements at the crack tips, the number of cycles necessary tofail each interface element at the crack tip can be calculated asΔ N j,where j represents the node at thejthe crack tip.
The analysis is set up to release at leastone interface element after the loading cycle is stabilized. The element withthe fewest cycles is identified to be released, and itsΔ N m i n = m i n ( Δ N j )is represented as the number of cycles to grow the crack equal to its elementlength, Δ a N m i n = m i n ( Δ a N j ).The most critical element is completely released with a zero constraint and azero stiffness at the end of the stabilized cycle. Cara install windows 8 memakai flashdisk. As the interface element isreleased, the load is redistributed and a new relative fracture energy releaserate must be calculated for the interface elements at the crack tips for thenext cycle.
This capability allows at least one interface element at the cracktips to be released after each stabilized cycle and precisely accounts for thenumber of cycles needed to cause fatigue crack growth over that length.If G m a x G p l,the interface elements at the crack tips will be released by increasing thecycle number count, d N,by one only. When you use the critical stress, critical crack opening displacement, orcrack length versus time fracture criteria, you can define how this force is tobe reduced to zero with time after debonding starts at a particular node on thebonded surface. You specify a relative amplitude, a, as afunction of time after debonding starts at a node. Thus, suppose the forcetransmitted between the surfaces at slave node N isT N 0when that node starts to debond, which occurs at time t N 0.Then, for any time t t N 0the force transmitted between the surfaces at node N isa ( t - t N 0 ) T N 0.The relative amplitude must be 1.0 at the relative time 0.0 and must reduce to0.0 at the last relative time point given.The best choice of the amplitude curve depends on the material properties,specified loading, and the crack propagation criterion.
If the stresses areremoved too rapidly, the resulting large changes in the strains near the cracktip can cause convergence difficulties. For large-strain problems severe meshdistortion can also occur.
For problems with rate-independent materials alinear amplitude curve is normally adequate. For problems with rate-dependentmaterials the stresses should be ramped off more slowly at the beginning ofdebonding to avoid convergence and mesh distortion difficulties. To reduce thelikelihood of convergence and mesh distortion difficulties, you can reduce thevalue of the debond stress by 25% in 50% of the time to debond. The solutionshould not be strongly influenced by the details of the unloading procedure; ifit is, this usually indicates that the mesh should be refined in the debondregion. For the VCCT and the enhancedVCCT criteria, when the energy release rateexceeds the critical value at a crack tip, you can either release the tractionbetween the two surfaces at the crack tip immediately during the followingincrement or release the traction gradually during succeeding increments withthe reduction of the magnitude of the debonding force being governed by thecritical fracture energy release rate. The latter approach is sometimesrecommended to avoid sudden loss of stability when the crack tip is advanced.The enhanced VCCT criterion is meaningful onlywhen used in conjunction with the latter approach. When the former approach isused, the results obtained by using the enhancedVCCT criterion are identical to those obtainedby using the original VCCT criterion.
When automatic incrementation is used for any criteria other thanVCCT, enhancedVCCT, or low-cycle fatigue, you can specifythe size of the time increment used just after debonding starts. By default,the time increment is equal to the last relative time specified.
However, if afracture criterion is met at the beginning of an increment, the size of thetime increment used just after debonding starts will be set equal to theminimum time increment allowed in this step.For fixed time incrementation the specified time increment value will beused as the time increment size after debonding starts ifAbaqus/Standardfinds it needs a smaller time increment than the fixed time increment size. Thetime increment size will be modified as required until debonding is complete. The simulation of structures with unstable propagating cracks is challengingand difficult. Nonconvergent behavior may occur from time to time. While theusual stabilization techniques (such as contact pair stabilization and staticstabilization) can be used to overcome some convergence difficulties, localizeddamping is included for VCCT or enhancedVCCT by using the viscous regularizationtechnique.
Viscous regularization damping causes the tangent stiffness matrixof the softening material to be positive for sufficiently small timeincrements. For most crack propagation simulations usingVCCT or the enhancedVCCT criterion, the deformation can be nearlylinear up to the point of the onset of crack growth; past this point theanalysis becomes very nonlinear.
In this case a linear scaling method can beused to effectively reduce the solution time to reach the onset of crackgrowth.Suppose that an applied “trial” load at increment t = t iis just a fraction of the critical load at the onset time of crack growth,t = t c r i t.The following algorithm is used inAbaqus/Standardto quickly converge to the critical load state. Δ t i + 1 = ( β i G e q u i v C G e q u i v - 1 ) t i,where initially β iwould be set between 0.7 and 0.9 depending on the degree of nonlinearity (thedefault value is 0.9). When Δ t i + 1becomes smaller than 0.5% (indicating that the load is within 0.5% of itscritical value), the next β iis automatically set to 1.0 to cause the most critical crack-tip node toprecisely reach the critical value at the next increment.
After the firstcrack-tip node releases, the linear scaling calculations are no longer validand the time increment is set to the default value. Cutback is then allowed. Crack propagation problems using the VCCTor enhanced VCCT criterion are numericallychallenging. The following tips will help you create a successfulAbaqus/Standardmodel:.An analysis with the VCCT or enhancedVCCT criterion requires small time increments.Abaqus/Standardtracks the location of the active crack front node by node when theVCCT or enhancedVCCT criterion is used. Therefore, the crackfront is allowed to advance only a single node forward in any single increment(although such an advance may take place across the entire crack front inthree-dimensional problems). Because an analysis using theVCCT or enhancedVCCT criterion provides detailed results ofthe growth of the crack, you will need small time increments, especially if themesh is highly refined.Three different types of damping can be used to aid convergence for amodel using the VCCT or enhancedVCCT criterion: contact stabilization,automatic or static stabilization, and viscous regularization.
Contact andautomatic stabilization are not specific toVCCT; they are built intoAbaqus/Standardand are compatible with VCCT. Setting thevalue of the damping parameters is often an iterative procedure. If yourVCCT model fails to converge due to unstablecrack propagation, set the damping parameters to relatively high values andrerun the analysis. If the parameters are high enough, stable incrementationshould return.
However, the crack propagation behavior may have been modifiedby the damping forces and may not be physically correct. To monitor the energyabsorbed by viscous damping, plot the damping energy and compare the results tothe total strain energy in the model ( ALLSE).When set properly, the value of the damping energy should be a small fractionof the total energy. Monitor the damping energy to ensure that the results ofthe VCCT simulation are reasonable in thepresence of damping. When you use contact or automatic stabilization,Abaquswrites the damping energy to the variable ALLSD in the output database (. Whenyou use viscous regularization,Abaquswrites the damping energy to the variable ALLVD.To maximize the accuracy of the debonding simulation, try to use matchedmeshes between the slave and master surfaces of the debonding contact pair.If you do use a mismatched mesh, you can maximize the accuracy of thesimulation by using the small-sliding, surface-to-surface formulation for thecontact pair (see).Printing contact constraint information to the data(.dat) file allows you to review the status of thedebonding contact pair at the beginning of the analysis. By printing detailedcontact conditions to the message (.msg) file, you cantrack the incremental behavior of the advancing crack front during theanalysis. Crack propagation problems using the VCCTcriterion analyzed inAbaqus/Explicitbenefit from the robustness of the general contact algorithm in the context ofan explicit time integrator.
Nevertheless, as is the case inAbaqus/Standard,these analyses remain challenging given the discontinuous nature of thefracture phenomenon. The following tips will help you create a successfulAbaqus/Explicitmodel:.Dynamic effects are of utmost relevance when assessing the results froma debonding analysis using the VCCT criterion.In most cases experimental and/or theoretical data are available inquasi-static settings. You must ensure that theAbaqus/Explicitanalysis generates low ratios of kinetic energy to internal energy (1% orless).
In practical terms this requirement often translates into avoiding theuse of mass scaling in the model. Use smooth amplitudes to drive the loading tohelp reduce the kinetic energy in the model. Running the analysis over a longerperiod of time will not help in most cases because bond breakage is aninherently fast and localized process.If appropriate, use damping-like behavior in the materials associatedwith the debonding plates to reduce dynamic vibrations. UnlikeAbaqus/Standard,where a pure static equilibrium is achieved at the end of a convergedincrement, inAbaqus/Explicitthe bond breakage at a given location is associated with a dynamic overshootbeyond the static equilibrium position.
If the vibrations are significant(kinetic energy is clearly observable), the dynamic overshoot at nodes behindthe crack tip may lead to premature debonding of the crack tip.To maximize the accuracy of the debonding simulation, use quad meshesbetween the slave and master surfaces of the debonding surfaces. Avoid usingelements with aspect ratios greater than 2.
In most cases mesh refinement willhelp with obtaining a realistic result.Highly mismatched critical energy values between modes tend to inducecrack propagation in continuously changing directions in a manner that may beunstable and unrealistic, particularly for modes II and III. Do not use suchvalues unless experimental data suggest so.Use frequent field output requests to evaluate the debonding evolutionas the analysis progresses. In some cases this can point to nontrivial modelingdeficiencies that are difficult to identify from a simple data check analysis.Avoid the use of other constraints involving nodes on both surfaces ofthe debonding interface because the cohesive contact forces will compete withthe constraint forces to achieve global equilibrium. Bond breakage might behard to interpret in these cases.
ComparingVCCTand cohesive elements. VCCTCohesive ElementsSimulation (mechanics)-driven crack propagation along a knowncrack surface.Simulation (mechanics)-driven crack propagation along a knowncrack surface. However, cohesive elements can also be placed between elementfaces as a mechanism for allowing individual elements to separate.Models brittle fracture usingLEFM only.Model brittle or ductile fracture forLEFM or EPFM.Very general interaction modeling capability is possible.Uses a surface-based framework. Does not require additionalelements.Require definition of the connectivity and interconnectivity ofcohesive elements with the rest of the structure. For accuracy, the mesh ofcohesive elements may need to be smaller than the surrounding structural meshand the associated cohesive zone. As a result, cohesive elementsmay be more expensive.Requires a pre-existing flaw at the beginning of the cracksurface. Cannot model crack initiation from a surface that is not alreadycracked.Can model crack initiation from initially uncracked surfaces.
The following predefined fields can be specified in a crack propagationanalysis, as described in:.Although temperature is not a degree of freedom in stress/displacementelements, nodal temperatures can be specified as predefined fields. Thespecified temperature affects temperature-dependent critical stress and crackopening displacement failure criteria, if specified.The values of user-defined field variables can be specified. Regular, rectangular meshes give the best results in crack propagationanalyses.
Results with nonlinear materials are more sensitive to meshing thanresults with small-strain linear elasticity.First-order elements generally work best for crack propagation analysis.Line spring elements cannot be used in crack propagation analysis.The VCCT, enhancedVCCT, and low-cycle fatigue criteria not onlysupport two-dimensional models (planar and axisymmetric) but alsothree-dimensional models with contact pairs involving first-order underlyingelements (solids, shells, and continuum shells). InAbaqus/Standarduse of the VCCT or enhancedVCCT criterion in two-dimensional models withcontact pairs involving higher-order underlying elements is limited to crackfronts that are aligned with the corner nodes of the higher-order elementfaces. Use of the low-cycle fatigue criterion with contact pairs involvinghigher-order underlying elements is not supported. Unless otherwise stated, the following discussions in this section areapplied only to the critical stress, critical crack opening displacement, andcrack length versus time criteria.At the start of an analysisAbaqus/Standardwill scan the partially bonded surfaces and identify all of the crack tips thatare present in the model.
Abaqus Crack Propagation
The initial contact status of all of the slavesurface nodes is printed in the data (.dat) file. At thisstageAbaqus/Standardwill explicitly identify all the crack tips and mark them as crack 1, crack 2,etc.
The slave and master surfaces that are associated with these cracks arealso identified.The initial contact status of all of the slave surface nodes is also printedin the data (.dat) file for theVCCT, enhancedVCCT, and low-cycle fatigue criteria. The following bond failure quantities can be requested as surface output(see,and)for all fracture criteria:DBTThe time when bond failure occurred. For theVCCT, enhancedVCCT, and low-cycle fatigue criteria, this isthe time when debonding initiates.DBSFFraction of stress at bond failure that still remains.DBSAll components of remaining stress in the failed bond.DBS1 i1 i component of stress in the failed bond thatremains ( i = 1, 2).For the VCCT, enhancedVCCT, and low-cycle fatigue criteria, thefollowing additional variables can be also requested as surface output (see):CSDMGOverall value of the scalar damage variable.BDSTATBond state. The bond state varies between 1.0 (fully bonded) and 0.0 (fullyunbonded).OPENBCRelative displacement behind crack when the fracture criterion is met.CRSTSAll components of critical stress at failureCRSTS1 i1 i component of critical stress at failure( i = 1, 3).ENRRTAll components of strain energy release rate.ENRRT1 i1 i component of strain energy release rate( i = 1, 3).EFENRRTREffective energy release rate ratio, G e q u i v G e q u i v C.Surface output requests provide the usual output of contact variables inaddition to the above quantities. The bond failure quantities must be requestedexplicitly; otherwise, only the default output for contact will be given.Abaqus/CAEprovides support for the visualization of time-history plots andX– Y plots of the variables that arewritten to the output database. Contour integrals can be requested for two-dimensional crack propagationanalyses performed using the critical stress, critical crack openingdisplacement, or crack length versus time fracture criteria.
If the contoursare chosen so that the crack tip passes through the contour, the contour valuewill go to zero (as it should). Therefore, in crack propagation analysiscontour integrals should be requested far enough from the crack tip that thecrack tip does not pass through the contour, which is easily done by includingall nodes along the bond surface in the crack-tip node set specified. Seefor details on contour integral output. Benzeggagh, M., and M. Kenane, “Measurement of Mixed-Mode DelaminationFracture Toughness of Unidirectional Glass/Epoxy Composites with Mixed-ModeBending Apparatus,” Composite Science andTechnology, vol. 56 439, 1996. Reeder, J., S.
Kyongchan, P.B. Ambur, “Postbuckling and Growth of Delaminations inComposite Plates Subjected to AxialCompression ” 43rd AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and Materials Conference, Denver, Colorado,vol. 10, 2002. Wu, E.M., and R.C. ReuterJr., “Crack Extension in Fiberglass ReinforcedPlastics, ” T and M Report, University ofIllinois, vol.
Contact interactions for contact pairs and general contact are defined byspecifying surface pairings and self-contact surfaces. General contactinteractions typically are defined by specifying self-contact for the defaultsurface, which allows an easy, yet powerful, definition of contact.(Self-contact for a surface that spans multiple bodies implies self-contact foreach body as well as contact between the bodies.)At least one surface in an interaction must be a non-node-based surface, andat least one surface in an interaction must be a non-analytical rigid surface.Additional restrictions and guidelines for contact surfaces are discussed foreach contact definition approach.
The definition of contact pairs is discussedin detail inand.The definition of general contact interactions is discussed in detail inand. Contact interactions in a model can refer to a contact property definition,in much the same way that elements refer to an element property definition. Bydefault, the surfaces interact (have constraints) only in the normal directionto resist penetration.
The default algorithmic controls for contact analyses are usuallysufficient, but you can adjust numerical controls for some special cases. Forexample, depending on the contact algorithm used, the numerical controls forthe contact formulation, the master and slave roles for the contact surfaces,and the sliding formulation are provided. Information on contact formulationsand numerical methods used by the contact algorithms is provided inand.The available numerical controls for the various contact algorithms arediscussed in,and. Contact pairs and general contact combine to provide the followingcapabilities inAbaqus/Standard:.Contact between two deformable bodies. The structures can be either two-or three-dimensional, and they can undergo either small or finite sliding.Examples of such problems include the assembly of a cylinder head gasket andthe slipping between the two components of a threaded connector.Contact between a rigid surface and a deformable body.
The structurescan be either two- or three-dimensional, and they can undergo either small orfinite sliding. Examples of such problems include metal forming simulations andanalyses of rubber seals being compressed between two components.Finite-sliding self-contact of a single deformable body. An example ofsuch a problem is a complex rubber seal that folds over on itself.Small-sliding or finite-sliding interaction between a set of points anda rigid surface. These models can be either two- or three-dimensional. Anexample of this type of problem is the pull-in of an underwater cable that isresting on the seabed, with the seabed modeled as a rigid surface.Contact between a set of points and a deformable surface.
These modelscan be either two- or three-dimensional. An example of this class of contactproblem is the design of a bearing where one of the bearing surfaces is modeledwith substructures.Problems where two separate surfaces need to be “tied” together so thatthere is no relative motion between them. This modeling technique allows forjoining dissimilar meshes.Coupled thermomechanical interaction between deformable bodies withfinite relative motion. The analysis of a disc brake is an example of such aproblem.Coupled thermal-electrical-structural interaction between deformablebodies with finite relative motion. An example of this type of problem is theanalysis of resistance spot welding.Coupled pore fluid-mechanical interaction between bodies.
An example ofthis type of problem is the analysis of the interfaces between layered soilmaterial at a waste disposal site.Coupled thermomechanical and coupled thermal-electrical-structuralinteractions can be included in any of the above examples as long as both ofthe surfaces are deformable. For most contact problems you have a choice of whether to define contactinteractions using general contact or contact pairs.
InAbaqus/Standardthe distinction between general contact and contact pairs lies primarily in theuser interface, the default numerical settings, and the available options. Thegeneral contact and contact pair implementations share many underlyingalgorithms.The contact interaction domain, contact properties, and surface attributesare specified independently for general contact, offering a more flexible wayto add detail incrementally to a model. The simple interface for specifyinggeneral contact allows for a highly automated contact definition; however, itis also possible to define contact with the general contact interface to mimictraditional contact pairs. Conversely, specifying self-contact of a surfacespanning multiple bodies with the contact pair user interface (if thesurface-to-surface formulation is used) mimics the highly automated approachoften used for general contact.InAbaqus/Standardtraditional pairwise specifications of contact interactions may result in moreefficient analyses as compared to an all-inclusive self-contact approach todefining contact.
Therefore, there is often a trade-off between ease ofdefining contact and analysis performance.Abaqus/CAEprovides a contact detection tool that greatly simplifies the process ofcreating traditional contact pairs forAbaqus/Standard(see). Default settings for general contact and contact pairsDifferences in default settings for general contact and contact pairs inAbaqus/Standardinclude the following:.Contact formulation: General contact uses the finite-sliding,surface-to-surface formulation supplemented by the finite-sliding,edge-to-surface; finite-sliding, edge-to-edge; and finite-sliding,vertex-to-surface formulations. For crack surfaces on enriched elements, theedge-to-edge and vertex-to-surface formulations are not available. Contactpairs use the finite-sliding, node-to-surface formulation by defaultexcept when the contact detection tool inAbaqus/CAEis used to create the contact pairs, in which case the finite-sliding,surface-to-surface formulation is used by default. Seefor a discussion of contact formulations.Treatment of shell thickness and offset: General contact automaticallyaccounts for thicknesses and offsets associated with shell-like surfaces.Contact pairs that use the finite-sliding, node-to-surface formulation do notaccount for shell thicknesses and offsets.
Seeandfor discussions of surface properties for contact inAbaqus/Standard.Contact constraint enforcement: General contact uses the penaltymethod to enforce the contact constraints by default. Contact pairs that usethe finite-sliding, node-to-surface formulation use a Lagrange multipliermethod to enforce contact constraints by default in most cases.
Seefor a discussion of contact constraint enforcement methods.Treatment of initial overclosures: General contact eliminates initialoverclosures with strain-free adjustments by default. Contact pairs treatinitial overclosures as interference fits to be resolved in the first incrementof the analysis by default. See,andfor more information on contact initialization inAbaqus/Standard.Master-slave assignments: General contact automatically assigns puremaster and slave roles for most contact interactions and automatically assignsbalanced master-slave roles to other contact interactions.
Contact betweenopposing surfaces of a crack on enriched elements supports only the balancedmaster-slave approach. The user must assign master and slave roles for mostcontact pairs. Seeandfor discussions of master and slave roles for contacting surfaces.
Abaqus/Standardprovides a library of contact elements that can be used to model certainclasses of problems. Examples of such problems are:.Contact interaction between two pipelines or tubes modeled with pipe,beam, or truss elements where one pipe lies inside the other (such as a J-tubepull in offshore piping installation) or the pipes lie next to each other(available in both two and three dimensions; see).Alternatively, this category of problems can be simulated using general contactwith the beam-to-beam formulation.Contact between two nodes along a fixed direction in space. An exampleof such a problem is the interaction of a piping system with its supports (see).Simulations using axisymmetric elements with asymmetric deformations, CAXA n and SAXA n elements.
Seefor details.Heat transfer analyses where the heat flow is one-dimensional. Anexample of such a problem is the heat flow in a piping system that isdiscontinuous. The thermal interaction in this problem is one-dimensional, sono surfaces can be defined (see).
Defining a contact simulation using contact elementsThe steps required for defining a contact simulation using contactelements are similar to those needed when defining a surface-based contactsimulation:.create the contact elements or slide lines;.assign element section properties to the contact elements;.associate sets of contact elements with the slide lines if applicable;and.define the contact property models for the contact elements.The first three steps are discussed inin the sections for each type of contact element. The contact property modelsfor contact elements are identical to those used for surface-based contact. Abaqus/Explicitprovides two algorithms for modeling contact interactions. Contact definitions are not entirely automatic with the general contactalgorithm but are greatly simplified. The generality of this algorithm isprimarily in the relaxed restrictions on the surfaces that can be used incontact.
There are fundamental differences in the mechanical contact algorithms inAbaqus/StandardandAbaqus/Expliciteven though the input syntax is similar.