i am trying to implement a simple VUMAT traction separation code in my cohesive zone model by using the relation, separation=straininc*charlength. but while implementing i am getting the error as "zero or negative initial dilatational modulus caused by bad material data". i have changed my charlength=1 as it tends for constitutive thickness in traction separation approach. although the dilation error being removed, still i am not able to validate with with my GUI solution of abaqus.
waiting for a reply..
Dear Pran,
Are you in general familiar with writing a VUMAT subroutine?
A VUMAT for a cohesive element works the same way as a progressive damage law for a conventional element. You can derive everything you need from a progressive damage law for continuum material. The implementation is even easier for a cohesive element, as the stiffness matrix is usually diagonal.
The difference is that Abaqus provides you only three components in the strain increment. From these you can calculate the separation as you described. However, what you need to provide is the traction (three stress components).
The characteristic length that you receive in the VUMAT will be your specified constitutive thickness.
In case you're still struggling with the code, just provide a minimal example, and we might be able to assist you.
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