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UMAT: Tangent stiffness for linear perturbation steps / steady state dynamics / vibration analysis

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Dear community,

having been an avid reader of this forum for quite some time, I'm posing my first question today, hoping you can help me with my problem:

I'm currently working on the modelling of rubbers and the implementation of some material models as UMATs in Abaqus. However, I can observe the following:

I'm able to replicate some of the built-in hyperelastic models in Abaqus (NeoHooke, Mooney-Rivlin, ...). Stress results from built-in models are identical to my UMAT and also the DDSDDE when comparing it with the NeoHookean example subroutine from the Abaqus verification guide. So far so good. For linear perturbation steps (Direct solution steady-state analysis) on the other hand, my results are off significantly for prestrained as well as non-prestrained vibrations.

Does Abaqus require a modified DDSDDE for linear-perturbation steps? Am I missing something here? Has anyone been working on something similar and might have some pointers for me?

Really looking forward to your suggestions!

Thanks in advance and kind regards