Learning Forum

[David Weed]

Hi Afrah First, I'll address the role of the number of substeps or time step size for SMART static crack growth. For static crack growth, the number of substeps should be set by the user such that in two or three substeps, the SIF/Jint threshold value is reached. Once static crack growth begins, the time step values set under "Step Controls for Crack Growth" in the details of the SMART crack object should take over. Furthermore, in order to utilize the SMART step controls, under Analysis Settings you need to set Auto Time Stepping to 'On'. Otherwise, if you explicitly control the number of substeps, the SMART step controls will never be triggered. This in turn will lead to widely different results for let's say 10, 50, or 100 substeps for the same analysis. In general, setting the substep controls for static crack growth differs from fatigue crack growth in that for fatigue crack growth you explicitly set the number of substeps to calculate a given number of delta_a values (and you turn off Auto Time Stepping). One technique that users employ is to first carry out a fracture analysis without SMART (just a regular quasi-static analysis) in order to determine at what pseudo time the critical SIF/Jint value is reached. This will then give them an idea of what size to make the initial time step so that this threshold is reached in just a couple of time steps. Then, you simply use that initial time step value in your actual SMART crack growth analysis while setting Auto Time Stepping to 'On'.
In general, make sure that the mesh is refined enough around the crack area to yield reasonable results. It may help to first perform a mesh convergence study without without SMART. There are some relevant sections on fracture meshing in the Mechanical User's Guide: section Performing a Fracture Analysis -> Fracture Meshing and in section Troubleshooting -> Problem Situations -> Fracture Meshing Problems.
I hope that this helps. If anything can be clarified, please let me know.