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+---
+type: concept
+title: "Abaqus User-Defined Material Behavior"
+complexity: advanced
+domain: computational-mechanics
+created: 2026-06-01
+updated: 2026-06-01
+address: c-000103
+aliases:
+  - Abaqus UMAT
+  - Abaqus VUMAT
+  - Abaqus UMATHT
+  - Abaqus user material
+  - Abaqus user-defined material
+tags:
+  - concept
+  - finite-element-method
+  - abaqus
+  - user-subroutines
+  - materials
+  - implementation
+status: current
+related:
+  - "[[Abaqus-Analysis-User-s-Guide-Volume-III|Abaqus Analysis User's Guide Volume III]]"
+  - "[[Abaqus User Subroutines and Utility Routines]]"
+  - "[[Abaqus Constitutive Integration]]"
+  - "[[Abaqus Material Library and Data Definition]]"
+  - "[[Finite Element Program Implementation]]"
+sources:
+  - "[[Abaqus-Analysis-User-s-Guide-Volume-III|Abaqus Analysis User's Guide Volume III]]"
+---
+
+# Abaqus User-Defined Material Behavior
+
+## Definition
+
+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.
+
+## Connections
+
+- [[Abaqus User Subroutines and Utility Routines]] is the broader compiled-extension workflow.
+- [[Abaqus Constitutive Integration]] explains why stress updates and material tangents matter.
+- [[Finite Element Program Implementation]] provides the general FE code architecture context.
+
+## Sources
+
+- [[Abaqus-Analysis-User-s-Guide-Volume-III|Abaqus Analysis User's Guide Volume III]]
+