R087ABAQUS资料-COMP-W07-DCB-VCCT-Std

Workshop 7
Analysis of a DCB using VCCT (Abaqus/Standard)
Introduction
In this workshop we will consider crack growth in the double cantilever beam (DCB) specimen studied earlier using the virtual crack closure technique (VCCT). We will employ the same geometry, material properties as well as the mesh, boundary conditions and sets and surfaces defined in the cohesive surfaces model from the earlier workshop (see Figure W7–1 for geometry and load details).
Figure W7–1 Schematic of the double cantilever beam specimen. Preliminaries
In this workshop, you will bond the two halves of the specimen and study crack growth resulting from the applied displacement. Previously, this problem was analyzed using both element-based and surface-based cohesive behavior. As part of this workshop, we will compare the results obtained from the VCCT model with those obtained using cohesive behavior.
Open the model database file edited earlier (dcb-cohesive.cae). Begin by copying the model named coh-surfs to a model named vcct. If you did not complete the exercises with cohesive surfaces (Part 2 of the earlier workshop), follow the instructions given there to define the surfaces and sets, step, mesh, and contact properties and interaction before proceeding.
The instructions that follow apply to the vcct model.
W7.2
Deleting obsolete features and defining the bond properties
You will begin by deleting features defined in the previous model that are not required in the VCCT model. Edit the contact interaction properties to delete the cohesive and damage properties inherited from the coh-surfs model. You will also define the bond properties at this stage.
1.In the Model Tree, expand the Interactions Properties container and double-
click the interaction property named coh.
2.In the Edit Contact Property dialog box that appears, select Cohesive Behavior
from the list and click to delete it.
3.In a similar fashion, delete Damage.
4.In the editor, select Mechanical→Fracture Criterion and enter the data as shown
in Figure W7–2:
a.Select VCCT as the type.
b.Select BK as the mixed mode behavior.
c.Enter 0.1 as the tolerance.
d.Enter 280 as the critical release rate for each mode and 2.284 as
the exponent.
Figure W7–2 Bond properties.
Editing contact
Any contact formulation other than the finite sliding, surface-to-surface discretization can be used in conjunction with VCCT. As we used finite sliding for the cohesive surfaces model, we must change this to use small sliding.
1.Expand the Interactions container in the Model Tree and double-click coh.
2.In the Edit Interaction dialog box (see Figure W7–3) that appears, change the
sliding formulation to Small sliding.
3.In the Clearance tabbed page, select Uniform value across slave surface from
the drop down menu next to Initial clearance. Enter 1e-7 in the data field, and
click OK.
Note: The part instances are so positioned that there is no initial clearance between the bonded surfaces. After meshing, the nodes may have small initial overclosures that can cause several unnecessary severe discontinuity iterations. This is avoided by specifying a small initial clearance.
Figure W7–3 Editing the predefined interaction for VCCT.
Defining the bond
You will now define the bond.
1.From the main menu bar, select Special→Crack→Create.
2.In the Create Crack dialog box, select Debond using VCCT.
3.In the Edit Crack dialog box:
a.Select Step-1 as the initiation step.
b.Select coh as the contact pair interaction.
4.The editor appears as shown in Figure W7–4. Click OK to complete the operation.
Figure W7–4 Bond definition
Output
Edit the default field output request to also include bond state BDSTAT and strain energy release rates ENRRT, found under the Failure/Fracture subsection (see Figure W7–4). These variables allow one to track the progression of damage at the interface through the course of a simulation, and are available for the slave surface.
Figure W7–4 Editing the default field output request
Job
1.In the Model Tree, double-click Jobs to create a job for this model. Name the job
dcb-vcct.
2.Save your model database.
3.Click mouse button 3 on the job name and select Submit from the menu that
appears. From the same menu, you may also select Monitor to monitor the
progress of the job and Results to automatically open the output database file for this job (dcb-vcct.odb) in the Visualization module.
Results
When the job is complete, open dcb-vcct.odb in the Visualization module.
1.Plot the deformed shape and contour the stress distribution in the specimen.
Animate the response (increasing the scale factor so that the deformation in the
early stages can be seen more clearly). Figure W7–8 shows the final specimen
configuration and the Mises stress distribution at the end of the job.
Figure W7–8 Mises stress distribution
2.To locate the crack tip more precisely, contour the output variable BDSTAT (bond
state), which is a numerical estimate of the extent of damage at the interface. It
ranges from 0 for slave nodes that are fully detached from the master surface to 1 for fully bonded slave nodes. Animating the response allows you to track crack-
propagation. Figure W7–9 shows a contour plot of BDSTAT for a magnified view of the crack roughly half way through the simulation (t=0.5). One can also use the strain energy release rates (ENRRT) for the same purpose.
Figure W7–9 Contour plot of bond state BDSTAT at time = 0.5
3.Create a force-displacement plot as described earlier.
4.Save the curve as VCCT and plot it.
5. A comparison of the force-displacement response obtained using VCCT with that
obtained using element-based and surface-based cohesive behavior is shown in
Figure W7–10. The results are in excellent agreement.
Figure W7–10 Comparison of force-displacement response between VCCT and element-based and surface-based cohesive methods.
Note: A script that creates the complete model described in these instructions is available for your convenience. Run this script if you encounter difficulties following the instructions outlined here or if you wish to check your work. The script is named
ws_composites_dcb_answer.py
and is available through the Abaqus fetch utility.。