VUMAT

1.2.17 VUMAT: User subroutine to define material behavior.

Product: Abaqus/Explicit

WARNING: The use of this user subroutine generally requires considerable

expertise. You are cautioned that the implementation of any realistic constitutive

model requires extensive development and testing. Initial testing on a single-element

model with prescribed traction loading is strongly recommended.

The component ordering of the symmetric and nonsymmetric tensors for

the three-dimensional case using C3D8R elements is different from the ordering

specified in “Three-dimensional solid element library,” Section 25.1.4 of the Abaqus

Analysis User’s Manual, and the ordering used in Abaqus/Standard.

References

• “User-defined mechanical material behavior,” Section 23.8.1 of the Abaqus Analysis User’s Manual

• *USER MATERIAL

Overview

User subroutine VUMAT:

• is used to define the mechanical constitutive behavior of a material;

• will be called for blocks of material calculation points for which the material is defined in a user

subroutine (“Material data definition,” Section 18.1.2 of the Abaqus Analysis User’s Manual);

• can use and update solution-dependent state variables;

• can use any field variables that are passed in; and

• can be used in an adiabatic analysis, provided you define both the inelastic heat fraction and the

specific heat for the appropriate material definitions and you store the temperatures and integrate

them as user-defined state variables.

Component ordering in tensors

The component ordering depends upon whether the tensor is symmetric or nonsymmetric.

Symmetric tensors

For symmetric tensors such as the stress and strain tensors, there are ndir+nshr components, and the

component order is given as a natural permutation of the indices of the tensor. The direct components are

first and then the indirect components, beginning with the 12-component. For example, a stress tensor

contains ndir direct stress components and nshr shear stress components, which are passed in as

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Component 2-D Case 3-D Case

1

2

3

4

5

6

The shear strain components in user subroutine VUMAT are stored as tensor components and not as

engineering components; this is different from user subroutine UMAT in Abaqus/Standard, which uses

engineering components.

Nonsymmetric tensors

For nonsymmetric tensors there are ndir+2*nshr components, and the component order is given as

a natural permutation of the indices of the tensor. The direct components are first and then the indirect

components, beginning with the 12-component. For example, the deformation gradient is passed as

Component 2-D Case 3-D Case

1

2

3

4

5

6

7

8

9

Initial calculations and checks

In the data check phase of the analysis Abaqus/Explicit calls user subroutineVUMATwith a set of fictitious

strains and a totalTime and stepTime both equal to 0.0. This is done as a check on your constitutive

relation and to calculate the equivalent initial material properties, based upon which the initial elastic

wave speeds are computed.

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VUMAT

Defining local orientations

All stresses, strains, stretches, and state variables are in the orientation of the local material axes. These

local material axes form a basis system in which stress and strain components are stored. This represents

a corotational coordinate system in which the basis system rotates with the material. If a user-specified

coordinate system (“Orientations,” Section 2.2.5 of the Abaqus Analysis User’s Manual) is used, it

defines the local material axes in the undeformed configuration.

Special considerations for various element types

The use of user subroutine VUMAT requires special consideration for various element types.

Shell and plane stress elements

You must define the stresses and internal state variables. In the case of shell or plane stress elements,

NDIR=3 and NSHR=1; you must define strainInc(*,3), the thickness strain increment. The

internal energies can be defined if desired. If they are not defined, the energy balance provided by

Abaqus/Explicit will not be meaningful.

Shell elements

When VUMAT is used to define the material response of shell elements, Abaqus/Explicit cannot calculate

a default value for the transverse shear stiffness of the element. Hence, you must define the element’s

transverse shear stiffness. See “Shell section behavior,” Section 26.6.4 of the Abaqus Analysis User’s

Manual, for guidelines on choosing this stiffness.

Beam elements

For beam elements the stretch tensor and the deformation gradient tensor are not available. For

beams in space you must define the thickness strains, strainInc(*,2) and strainInc(*,3).strainInc(*,4) is the shear strain associated with twist. Thickness stresses, stressNew(*,2)and stressNew(*,3), are assumed to be zero, and any values you assign are ignored.

Pipe elements

For pipe elements the stretch tensor and the deformation gradient tensor are not available. The axial

strain, strainInc(*,1), and the shear strain, strainInc(*,4), associated with twist are

provided along with the hoop stress, stressNew(*,2). The hoop stress is predefined based on your

pipe internal and external pressure load definitions (PE, PI, HPE, HPI, PENU, and PINU), and it should

not be modified here. The thickness stress, stressNew(*,3), is assumed to be zero and any value

you assign is ignored. You must define the axial stress, stressNew(*,1), and the shear stress,

stressNew(*,4). You must also define hoop strain, strainInc(*,2), and the pipe thickness

strain, strainInc(*,3).

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Deformation gradient

The polar decomposition of the deformation gradient is written as , where and

are the right and left symmetric stretch tensors, respectively. The constitutive model is defined in a

corotational coordinate system in which the basis system rotates with the material. All stress and strain

tensor quantities are defined with respect to the corotational basis system. The right stretch tensor, , is

used. The relative spin tensor represents the spin (the antisymmetric part of the velocity gradient)

defined with respect to the corotational basis system.

Special considerations for hyperelasticity

Hyperelastic constitutive models in VUMAT should be defined in a corotational coordinate system in

which the basis system rotates with the material. This is most effectively accomplished by formulating

the hyperelastic constitutive model in terms of the stretch tensor, , instead of in terms of the

deformation gradient, . Using the deformation gradient can present some difficulties because

the deformation gradient includes the rotation tensor and the resulting stresses would need to be rotated

back to the corotational basis.

Objective stress rates

The Green-Naghdi stress rate is used when the mechanical behavior of the material is defined using user

subroutine VUMAT. The stress rate obtained with user subroutine VUMAT may differ from that obtained

with a built-in Abaqus material model. For example, most material models used with solid (continuum)

elements in Abaqus/Explicit employ the Jaumann stress rate. This difference in the formulation will

cause significant differences in the results only if finite rotation of a material point is accompanied by

finite shear. For a discussion of the objective stress rates used in Abaqus, see “Stress rates,” Section 1.5.3

of the Abaqus Theory Manual.

Material point deletion

Material points that satisfy a user-defined failure criterion can be deleted from the model (see “User-

defined mechanical material behavior,” Section 23.8.1 of the Abaqus Analysis User’s Manual). You

must specify the state variable number controlling the element deletion flag when you allocate space

for the solution-dependent state variables, as explained in “User-defined mechanical material behavior,”

Section 23.8.1 of the Abaqus Analysis User’s Manual. The deletion state variable should be set to a value

of one or zero in VUMAT. A value of one indicates that the material point is active, while a value of zero

indicates that Abaqus/Explicit should delete the material point from the model by setting the stresses to

zero. The structure of the block of material points passed to user subroutine VUMAT remains unchanged

during the analysis; deleted material points are not removed from the block. Abaqus/Explicit will pass

zero stresses and strain increments for all deleted material points. Once a material point has been flagged

as deleted, it cannot be reactivated.

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User subroutine interface

subroutine vumat(C Read only (unmodifiable)variables -

1 nblock, ndir, nshr, nstatev, nfieldv, nprops, lanneal,2 stepTime, totalTime, dt, cmname, coordMp, charLength,3 props, density, strainInc, relSpinInc,4 tempOld, stretchOld, defgradOld, fieldOld,5 stressOld, stateOld, enerInternOld, enerInelasOld,6 tempNew, stretchNew, defgradNew, fieldNew,

C Write only (modifiable) variables -7 stressNew, stateNew, enerInternNew, enerInelasNew )

Cinclude 'vaba_param.inc'

Cdimension props(nprops), density(nblock), coordMp(nblock,*),

1 charLength(nblock), strainInc(nblock,ndir+nshr),2 relSpinInc(nblock,nshr), tempOld(nblock),3 stretchOld(nblock,ndir+nshr),4 defgradOld(nblock,ndir+nshr+nshr),5 fieldOld(nblock,nfieldv), stressOld(nblock,ndir+nshr),6 stateOld(nblock,nstatev), enerInternOld(nblock),7 enerInelasOld(nblock), tempNew(nblock),8 stretchNew(nblock,ndir+nshr),8 defgradNew(nblock,ndir+nshr+nshr),9 fieldNew(nblock,nfieldv),1 stressNew(nblock,ndir+nshr), stateNew(nblock,nstatev),2 enerInternNew(nblock), enerInelasNew(nblock),

Ccharacter*80 cmname

C

do 100 km = 1,nblockuser coding

100 continue

returnend

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Variables to be defined

stressNew (nblock, ndir+nshr)Stress tensor at each material point at the end of the increment.

stateNew (nblock, nstatev)State variables at each material point at the end of the increment. You define the size of this array by

allocating space for it (see “User subroutines: overview,” Section 15.1.1 of the Abaqus Analysis User’s

Manual, for more information).

Variables that can be updated

enerInternNew (nblock)Internal energy per unit mass at each material point at the end of the increment.

enerInelasNew (nblock)Dissipated inelastic energy per unit mass at each material point at the end of the increment.

Variables passed in for information

nblockNumber of material points to be processed in this call to VUMAT.

ndirNumber of direct components in a symmetric tensor.

nshrNumber of indirect components in a symmetric tensor.

nstatevNumber of user-defined state variables that are associated with this material type (you define this

as described in “Allocating space” in “User subroutines: overview,” Section 15.1.1 of the Abaqus

Analysis User’s Manual).

nfieldvNumber of user-defined external field variables.

npropsUser-specified number of user-defined material properties.

lannealFlag indicating whether the routine is being called during an annealing process. lanneal=0 indicatesthat the routine is being called during a normal mechanics increment. lanneal=1 indicates that this isan annealing process and you should re-initialize the internal state variables, stateNew, if necessary.Abaqus/Explicit will automatically set the stresses, stretches, and state to a value of zero during the

annealing process.

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stepTimeValue of time since the step began.

totalTimeValue of total time. The time at the beginning of the step is given by totalTime - stepTime.

dtTime increment size.

cmnameUser-specified material name, left justified. It is passed in as an uppercase character string. Some

internal material models are given names starting with the “ABQ_” character string. To avoid conflict,

you should not use “ABQ_” as the leading string for cmname.

coordMp(nblock,*)Material point coordinates. It is the midplane material point for shell elements and the centroid for

beam and pipe elements.

charLength(nblock)Characteristic element length. This is a typical length of a line across an element for a first-order

element; it is half of the same typical length for a second-order element. For beams, pipes, and trusses,

it is a characteristic length along the element axis. For membranes and shells it is a characteristic length

in the reference surface. For axisymmetric elements it is a characteristic length in the r–z plane only.

For cohesive elements it is equal to the constitutive thickness.

props(nprops)User-supplied material properties.

density(nblock)Current density at the material points in the midstep configuration. This value may be inaccurate in

problems where the volumetric strain increment is very small. If an accurate value of the density is

required in such cases, the analysis should be run in double precision. This value of the density is not

affected by mass scaling.

strainInc (nblock, ndir+nshr)Strain increment tensor at each material point.

relSpinInc (nblock, nshr)Incremental relative rotation vector at each material point defined in the corotational system. Defined

as , where is the antisymmetric part of the velocity gradient, , and .

Stored in 3-D as and in 2-D as .

tempOld(nblock)Temperatures at each material point at the beginning of the increment.

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stretchOld (nblock, ndir+nshr)Stretch tensor, , at each material point at the beginning of the increment defined from the polar

decomposition of the deformation gradient by .

defgradOld (nblock,ndir+2*nshr)Deformation gradient tensor at each material point at the beginning of the increment. Stored in 3-D as

( , , , , , , , , ) and in 2-D as ( , , , , ).

fieldOld (nblock, nfieldv)Values of the user-defined field variables at each material point at the beginning of the increment.

stressOld (nblock, ndir+nshr)Stress tensor at each material point at the beginning of the increment.

stateOld (nblock, nstatev)State variables at each material point at the beginning of the increment.

enerInternOld (nblock)Internal energy per unit mass at each material point at the beginning of the increment.

enerInelasOld (nblock)Dissipated inelastic energy per unit mass at each material point at the beginning of the increment.

tempNew(nblock)Temperatures at each material point at the end of the increment.

stretchNew (nblock, ndir+nshr)Stretch tensor, , at each material point at the end of the increment defined from the polar

decomposition of the deformation gradient by .

defgradNew (nblock,ndir+2*nshr)Deformation gradient tensor at each material point at the end of the increment. Stored in 3-D as ( ,

, , , , , , , ) and in 2-D as ( , , , , ).

fieldNew (nblock, nfieldv)Values of the user-defined field variables at each material point at the end of the increment.

Example: Using more than one user-defined material model

To use more than one user-defined material model, the variable cmname can be tested for different

material names inside user subroutine VUMAT, as illustrated below:if (cmname(1:4) .eq. 'MAT1') then

call VUMAT_MAT1(argument_list)else if (cmname(1:4) .eq. 'MAT2') then

call VUMAT_MAT2(argument_list)end if

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VUMAT_MAT1 and VUMAT_MAT2 are the actual user material subroutines containing the constitutive

material models for each material MAT1 and MAT2, respectively. Subroutine VUMAT merely acts as a

directory here. The argument list can be the same as that used in subroutine VUMAT. The material names

must be in uppercase characters since cmname is passed in as an uppercase character string.

Example: Elastic/plastic material with kinematic hardening

As a simple example of the coding of subroutine VUMAT, consider the generalized plane strain case for

an elastic/plastic material with kinematic hardening. The basic assumptions and definitions of the model

are as follows.

Let be the current value of the stress, and define to be the deviatoric part of the stress. The

center of the yield surface in deviatoric stress space is given by the tensor , which has initial values of

zero. The stress difference, , is the stress measured from the center of the yield surface and is given by

The von Mises yield surface is defined as

where is the uniaxial equivalent yield stress. The von Mises yield surface is a cylinder in deviatoric

stress space with a radius of

For the kinematic hardening model, R is a constant. The normal to the Mises yield surface can be written

as

We decompose the strain rate into an elastic and plastic part using an additive decomposition:

The plastic part of the strain rate is given by a normality condition

where the scalar multiplier must be determined. A scalar measure of equivalent plastic strain rate is

defined by

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The stress rate is assumed to be purely due to the elastic part of the strain rate and is expressed in terms

of Hooke’s law by

where and are the Lamés constants for the material.

The evolution law for is given as

where H is the slope of the uniaxial yield stress versus plastic strain curve.

During active plastic loading the stress must remain on the yield surface, so that

The equivalent plastic strain rate is related to by

The kinematic hardening constitutive model is integrated in a rate form as follows. A trial elastic

stress is computed as

where the subscripts and refer to the beginning and end of the increment, respectively. If the

trial stress does not exceed the yield stress, the new stress is set equal to the trial stress. If the yield

stress is exceeded, plasticity occurs in the increment. We then write the incremental analogs of the rate

equations as

where

From the definition of the normal to the yield surface at the end of the increment, ,

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This can be expanded using the incremental equations as

Taking the tensor product of this equation with , using the yield condition at the end of the increment,

and solving for :

The value for is used in the incremental equations to determine , , and .

This algorithm is often referred to as an elastic predictor, radial return algorithm because the

correction to the trial stress under the active plastic loading condition returns the stress state to the yield

surface along the direction defined by the vector from the center of the yield surface to the elastic trial

stress. The subroutine would be coded as follows:

subroutine vumat(C Read only -

1 nblock, ndir, nshr, nstatev, nfieldv, nprops, lanneal,2 stepTime, totalTime, dt, cmname, coordMp, charLength,3 props, density, strainInc, relSpinInc,4 tempOld, stretchOld, defgradOld, fieldOld,3 stressOld, stateOld, enerInternOld, enerInelasOld,6 tempNew, stretchNew, defgradNew, fieldNew,

C Write only -5 stressNew, stateNew, enerInternNew, enerInelasNew )

Cinclude 'vaba_param.inc'

CC J2 Mises Plasticity with kinematic hardening for planeC strain case.C Elastic predictor, radial corrector algorithm.CC The state variables are stored as:C STATE(*,1) = back stress component 11C STATE(*,2) = back stress component 22C STATE(*,3) = back stress component 33C STATE(*,4) = back stress component 12C STATE(*,5) = equivalent plastic strain

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CCC All arrays dimensioned by (*) are not used in this algorithm

dimension props(nprops), density(nblock),1 coordMp(nblock,*),2 charLength(*), strainInc(nblock,ndir+nshr),3 relSpinInc(*), tempOld(*),4 stretchOld(*), defgradOld(*),5 fieldOld(*), stressOld(nblock,ndir+nshr),6 stateOld(nblock,nstatev), enerInternOld(nblock),7 enerInelasOld(nblock), tempNew(*),8 stretchNew(*), defgradNew(*), fieldNew(*),9 stressNew(nblock,ndir+nshr), stateNew(nblock,nstatev),1 enerInternNew(nblock), enerInelasNew(nblock)

Ccharacter*80 cmname

Cparameter( zero = 0., one = 1., two = 2., three = 3.,

1 third = one/three, half = .5, twoThirds = two/three,2 threeHalfs = 1.5 )

Ce = props(1)xnu = props(2)yield = props(3)hard = props(4)

Ctwomu = e / ( one + xnu )thremu = threeHalfs * twomusixmu = three * twomualamda = twomu * ( e - twomu ) / ( sixmu - two * e )term = one / ( twomu * ( one + hard/thremu ) )con1 = sqrt( twoThirds )

Cdo 100 i = 1,nblock

CC Trial stress

trace = strainInc(i,1) + strainInc(i,2) + strainInc(i,3)sig1 = stressOld(i,1) + alamda*trace + twomu*strainInc(i,1)sig2 = stressOld(i,2) + alamda*trace + twomu*strainInc(i,2)sig3 = stressOld(i,3) + alamda*trace + twomu*strainInc(i,3)sig4 = stressOld(i,4) + twomu*strainInc(i,4)

C

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C Trial stress measured from the back stresss1 = sig1 - stateOld(i,1)s2 = sig2 - stateOld(i,2)s3 = sig3 - stateOld(i,3)s4 = sig4 - stateOld(i,4)

CC Deviatoric part of trial stress measured from the back stress

smean = third * ( s1 + s2 + s3 )ds1 = s1 - smeands2 = s2 - smeands3 = s3 - smean

CC Magnitude of the deviatoric trial stress difference

dsmag = sqrt( ds1**2 + ds2**2 + ds3**2 + 2.*s4**2 )CC Check for yield by determining the factor for plasticity,C zero for elastic, one for yield

radius = con1 * yieldfacyld = zeroif( dsmag - radius .ge. zero ) facyld = one

CC Add a protective addition factor to prevent a divide by zeroC when dsmag is zero. If dsmag is zero, we will not have exceededC the yield stress and facyld will be zero.

dsmag = dsmag + ( one - facyld )CC Calculated increment in gamma (this explicitly includes theC time step)

diff = dsmag - radiusdgamma = facyld * term * diff

CC Update equivalent plastic strain

deqps = con1 * dgammastateNew(i,5) = stateOld(i,5) + deqps

CC Divide dgamma by dsmag so that the deviatoric stresses areC explicitly converted to tensors of unit magnitude in theC following calculations

dgamma = dgamma / dsmagCC Update back stress

factor = hard * dgamma * twoThirds

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stateNew(i,1) = stateOld(i,1) + factor * ds1stateNew(i,2) = stateOld(i,2) + factor * ds2stateNew(i,3) = stateOld(i,3) + factor * ds3stateNew(i,4) = stateOld(i,4) + factor * s4

CC Update the stress

factor = twomu * dgammastressNew(i,1) = sig1 - factor * ds1stressNew(i,2) = sig2 - factor * ds2stressNew(i,3) = sig3 - factor * ds3stressNew(i,4) = sig4 - factor * s4

CC Update the specific internal energy -

stressPower = half * (1 ( stressOld(i,1)+stressNew(i,1) )*strainInc(i,1)1 + ( stressOld(i,2)+stressNew(i,2) )*strainInc(i,2)1 + ( stressOld(i,3)+stressNew(i,3) )*strainInc(i,3)1 + two*( stressOld(i,4)+stressNew(i,4) )*strainInc(i,4) )

CenerInternNew(i) = enerInternOld(i)

1 + stressPower / density(i)CC Update the dissipated inelastic specific energy -

plasticWorkInc = dgamma * half * (1 ( stressOld(i,1)+stressNew(i,1) )*ds11 + ( stressOld(i,2)+stressNew(i,2) )*ds21 + ( stressOld(i,3)+stressNew(i,3) )*ds31 + two*( stressOld(i,4)+stressNew(i,4) )*s4 )

enerInelasNew(i) = enerInelasOld(i)1 + plasticWorkInc / density(i)

100 continueC

returnend

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