Abaqus user-defined material behavior lets analysts implement mechanical or thermal constitutive laws through material subroutines when built-in material models are insufficient.
How It Works
For mechanical behavior, Abaqus/Standard calls UMAT at material points during each iteration and requires updated stresses, solution-dependent state variables, and the material Jacobian matrix. The Jacobian quality strongly influences Newton convergence and computational efficiency. Abaqus/Explicit calls VUMAT on blocks of material points and passes information suited to explicit vectorized updates.
For thermal behavior, UMATHT defines constitutive thermal response. User materials can allocate state variables, output them through SDV identifiers, and use state variables to control element deletion. In Abaqus/Explicit, deleted material points remain in the subroutine block but receive zero stresses and strain increments after deletion.
The guide also describes practical combinations and limitations. User-defined mechanical materials can often be combined with density, thermal expansion, permeability, and heat-transfer properties, while stiffness-proportional damping must be handled through the user material in some cases.
Why It Matters
User materials are the most direct bridge from finite element theory to production implementation. They offer maximum constitutive flexibility but move correctness burdens onto the analyst: stress update, state evolution, tangent consistency, deletion logic, heat generation, and compatibility with elements and procedures.
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