---
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]]"
source_refs:
  - source: "[[Abaqus-Analysis-User-s-Guide-Volume-III|Abaqus Analysis User's Guide Volume III]]"
    raw_path: ".raw/AbaqusAnalysisUserGuide3/"
    raw_files:
      - "AbaqusAnalysisUserGuide3_071.md"
      - "AbaqusAnalysisUserGuide3_070.md"
      - "AbaqusAnalysisUserGuide3_005.md"
      - "AbaqusAnalysisUserGuide3_004.md"
    md_indices:
      - 71
      - 70
      - 5
      - 4
    match: "heuristic-heading-keyword"
    confidence: high
---

# 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]]