Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics
Writing a VUMAT
Appendix 3
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics A3.2
Overview
• Introduction
• Motivation
• Steps Required in Writing a VUMAT
• VUMAT Interface
• Example: VUMAT for Kinematic Hardening Plasticity
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics
Introduction
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics A3.4
Introduction
– ABAQUS/Explicit has an interface that allows you to implement general
constitutive equations.
• In ABAQUS/Explicit the user-defined material model is implemented in
user subroutine VUMAT.
– Use VUMAT when none of the existing material models included in the
ABAQUS/Explicit material library accurately represents the behavior of the
material to be modeled.
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics A3.5
Introduction
– These interfaces make it possible to define any (proprietary) constitutive
model of arbitrary complexity.
– User-defined material models can be used with any ABAQUS/Explicit
structural element type.
– Multiple user materials can be implemented in a single VUMAT routine and
can be used together.
– In this lecture the implementation of material models in VUMAT will be
discussed and illustrated with an example.
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics
Motivation
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics A3.7
Motivation
– Proper testing of advanced constitutive models to simulate experimental
results often requires complex finite element models.
• Complex material modeling
– Special analysis problems occur if the constitutive model simulates
material instabilities and localization phenomena.
– The material model developer should be concerned only with the
development of the material model and not with the development and
maintenance of the FE software.
• Developments unrelated to material modeling
• Porting problems with new systems
• Long-term program maintenance of user-developed code
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics
Steps Required in Writing a VUMAT
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics A3.9
Steps Required in Writing a VUMAT
– Proper definition of the constitutive equation, which requires one of the
following:
• Explicit definition of stress (Cauchy stress for large-strain applications)
• Definition of the stress rate only (in corotational framework)
– Furthermore, it is likely to require:
• Definition of dependence on time, temperature, or field variables
• Definition of internal state variables, either explicitly or in rate form
Copyright 2005 ABAQUS, Inc.
ABAQUS/Explicit: Advanced Topics A3.10
Steps Required in Writing a VUMAT
– Transformation of the constitutive rate equation into an incremental
equation using a suitable integration procedure:
• Forward Euler (explicit integration)
• Backward Euler (implicit integration)
• Midpoint method