How to extract Stiffness matrix(s) for Non-linear FE Dynamic Implicit case in ABAQUS

[adas_11]

Hi,

I am studying the behavior of a soft robotic manipulator with elastomer material like rubber which has nonlinear material behavior.
For solving a nonlinear FE case with 'Nlgeom' ON in ABAQUS, the Stiffness matrix and external load vector are updated at each iteration by the relation. [K(x)]x = F_ext(x). how do I extract the Stiffness Matrix at each time step?

Regards,

Apurba

 

[Erik Panos Kostson]

As discussed in the previous posts, it does not seem to be possible, as it only outputs for linear perturbation analysis which can include initial stresses/stiffness effects, but that is not the same as obtaining the FE matrices from a full nonlinear analysis using a NR iterative solution

 

[FEA way]

I don’t think it’s possible (at least without some kind of subroutine). That’s because *Matrix generate keyword will only give you the stiffness matrix from the end of the step.

 

[Erik Panos Kostson]

Well it might be sort of possible if you have run your general nonlinear analysis up to the point to where you want to extract your FE matrices, and then input that last step/iteration as an initial condition to a linear perturbation analysis where you can output them (so one should get the correct stiffness matrix which was generated in the last increment of the NR iterations prior to the linear perturbation, because that will be used in the pert. analysis, not sure about other vectors).

Try it out on a simple model