Learning Forum

[milesvivian]

I just want to check my understanding of changing the substeps of the analysis. I'm assessing a bone in a 4 point bending test and I've applied a displacement to 2 cylinders above the bone (attached). For the sake of this explanation, let's say I applied a total displacement of 5mm. So as far as I'm aware:

• The initial substeps is the starting point of the analysis, so an initial substep of 5 means it will began the calculations from 5mm/5 = 1mm
• The minimum substep is the minimum number of steps for which the displacement will increase over, so a min substep of 4 will mean the programme will increase the displacement by a max of 5mm/4 = 1.25mm
• The maximum substep is the maximum number of steps for which the displacement will increase over, so a max substep of 20 will means the programme will increase the displacement by a max of 5mm/20 = 0.25mm

The programme will begin at the displacement of 1mm and initially increase the displacement in jumps of 1.25mm. If the results can't converge, it will increase the substep to 5 and do it again in jumps of 5mm/5 = 1mm. If it can't converge again it will increase the substep to 6 and do jumps of 5mm/6 = 0.83mm and so on. Am I on the right lines or am I way off? Appreciate anyone's help!

Screenshot 2020-12-23 121352.jpg

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[danielshaw]

Section 3.2 of the MAPDL Basic Analysis Guide has a good description of substeps. It appears that you are using automatic time stepping. You understand is basically correct, except the substep is not directly related to applied load. It is related to the solution time. The time step is the end time (time at end of load step) divided by the number of substeps. For example, if the load step end time is 1 sec and the number of substeps is 10, then the time step is 0.1 sec. Time step and subsets are reciprocals. Note: applied load can also be related to solution time, so substeps can be indirectly related to applied load.nAutomatic time stepping is only used in non-linear analysis. It uses internal logic to optimize the time step size for convergence and accuracy.nThe initial time step size (or 1/initial # substeps) is the initial time step size used for the first solve. At the end of the each solve, the solver decides if the time step size was too small, optimal, or too large. It then adjusts the time step size accordingly. The specified minimum time step size (or maximum # of substeps) sets the minimum allowable time step size. The specified maximum time step size (or minimum # of substeps) sets the minimum allowable time step size. The solver will not exceed those limitsn

[danielshaw]

Sorry for the typo. You understand is basically correct should be Your understanding is basically correct.n