What Are UMAT and VUMAT in Abaqus? Abaqus UMAT (User Material) and Abaqus VUMAT are Fortran-based subroutines used to define custom constitutive material models in Abaqus/Standard and Abaqus/Explicit, respectively. They allow engineers and researchers to program advanced mechanical behaviors—such as complex elasticity, plasticity, and composite damage—when built-in Abaqus material libraries are insufficient.
This CAE Assistant guide provides a definitive roadmap for developing, implementing, and executing these subroutines. By exploring input/output architectures, step-by-step coding for elastic and orthotropic models, and a direct UMAT vs. VUMAT comparison, readers will gain the practical expertise needed to write accurate custom algorithms. Access the provided free PDF guides and Fortran code templates to accelerate advanced material modeling projects.
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What Are UMAT and VUMAT in Abaqus?
Abaqus comes with many built-in materials, but in real projects these are often not enough. That is why engineers use user subroutines. With Abaqus UMAT and Abaqus VUMAT, you can write your own material model in Fortran and link it directly to the solver. In simple terms, you are teaching Abaqus how your material reacts when it deforms.
Think of it like this: every time Abaqus takes a step, it calls your subroutine and asks, “What is the updated stress, and how should the material respond?” In a UMAT, this happens inside the implicit solver (Abaqus/Standard). In a VUMAT, it happens inside the explicit solver (Abaqus/Explicit).
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UMAT: Runs with Abaqus/Standard, best for static or slow-loading problems. It also provides the material Jacobian (tangent stiffness), which makes convergence faster.
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VUMAT: Runs with Abaqus/Explicit, ideal for dynamic events such as crash, impact, or large deformation. It updates stresses in blocks for many points at once.
Figure 1: UMAT subroutine workflow
Typical learning path
If you are starting from scratch, the usual path is simple:
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Begin with a basic elastic model (so you can learn the structure and test your setup).
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Add complexity step by step—plasticity, hardening, or viscoelastic behavior.
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Once you’re comfortable, extend your code to include damage, failure, or rate-dependent effects.
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Finally, try porting your UMAT logic into a VUMAT when your problem involves dynamics or large deformations.
This blog will guide you through that first step: writing and testing a minimal elastic UMAT, and then showing how the same idea extends to a VUMAT. By the end, you’ll know not just the theory but also the workflow needed to grow into more advanced models.
