Implementing User-Programmable Features (UPFs) in ANSYS · Implementing User-Programmable Features...

© 2011 ANSYS, Inc. May 9, 20141

Lecture 6Implementing implicit creep: using usercreep.F

Implementing User-Programmable Features (UPFs) in ANSYS

© 2011 ANSYS, Inc. May 9, 20142

Lecture overview

• A brief summary on creep

• What is usercreep.F used for?

• How is usercreep.F used?

• Implementing a primary creep model

• Example of application

• Where to find additional information

© 2011 ANSYS, Inc. May 9, 20143

Creep is defined as material deforming under load over time

in such a way as to tend to relieve the stress. Creep may also

be a function of strain and stress rates. The term relaxation

has also been used interchangeably with creep.

In crystalline materials, such as metals, creep mechanism is

linked to diffusional flow of vacancies and dislocation

movement. In particular:

• Vacancies are point defects, and they tend to favor

grain boundaries that are normal, rather than

parallel, to the applied stress.

A brief summary on creep

© 2011 ANSYS, Inc. May 9, 20144

• Dislocations in grains are line defects. The

movement of dislocations (climb, glide, deviation)

tend to be activated by high stresses, although it

may also occur at intermediate temperatures.

• Grain boundary sliding is sometimes considered as

a separate mechanism which also contributes to

creep deformation.

Although a detailed discussion of material science is beyond

the scope of this class, it may suffice to say that the

aforementioned physical mechanics contribute to creep.


© 2011 ANSYS, Inc. May 9, 20145


The dependency of creep deformation on stress, strain, time,

and temperature are generally modeled with a form similar

to the following:

being functions dependent on the creep law selected

and whose associated creep constants are usually obtained

through tensile tests conducted at different rates and

temperatures.

However, the type of material being analyzed determines

the choice of a specific creep equation.

( ) ( ) ( ) ( )Tftfffcr 4321 εσε =•

41 ff K

© 2011 ANSYS, Inc. May 9, 20146

The routine usercreep.F allows implementing uniaxial

creep laws that will be generalized to the multi-axial state

by the general time-dependent viscoplastic material

formulation implemented in the program.

usercreep.f is for modelling implicit creep. Implicit

creep is efficient, robust, accurate and recommended for

general use.

The ANSYS installation includes an example which

corresponds to the primary creep function TBOPT=1(strain hardening) and that can be used as reference.

What is usercreep.F used for?

© 2011 ANSYS, Inc. May 9, 20147

Such a routine can be found in the following folder:

… \ANSYS Inc\v150\ansys\customize\user

What is usercreep.F used for?

© 2011 ANSYS, Inc. May 9, 20148

The routine usercreep.F provides a template in which

users need to specify the following:

• Derivative of the incremental creep strain with

respect to the effective stress;

• Derivative of the incremental creep strain with

respect to the creep strain.

These derivatives are required by ANSYS in order to

calculate the material tangent stiffness matrix correctly. In

this respect their evaluation is crucial because they impact

both convergence behaviour and accuracy. If these

quantities cannot be computed directly, then numerical

differentiation can be employed.

How is usercreep.F used?

© 2011 ANSYS, Inc. May 9, 20149


( ) ( ) Tbm

cr

n

cr ek /0

−•

= εσε

© 2011 ANSYS, Inc. May 9, 201410


TT∆

tt∆

bmnk ,,,0

4 in this example

crεσ

© 2011 ANSYS, Inc. May 9, 201411


state variables (defined by

TB,STATE command)

•

∆=∆ crcr t εε

cr

cr

cr

cr tεε

εε

∂∂∆=

∂∆∂

•

σε

σε

∂∂∆=

∂∆∂

•

crcr t

© 2011 ANSYS, Inc. May 9, 201412

Implementing a primary creep model

In view of implementing the primary creep function which is

already available in ANSYS (TBOPT=1), we need to compute

the first derivatives of the incremental creep strain with

respect to the effective stress and creep strain. In particular,

if we consider the following expression for the incremental

creep strain:

then we get:

( ) ( )cr

crTbm

cr

n

cr

cr memkεεεσ

εε

•

−−

•

==∂∂ /1

0

( ) ( )σεεσ

σε

•

−−

•

==∂

∂ crTbm

cr

ncr nenk /1

0

( ) ( ) Tbm

cr

n

cr ek /0

−•

= εσε

© 2011 ANSYS, Inc. May 9, 201413

Implementing a primary creep model

prop(1)= 0k prop(2)= n

prop(3)= m prop(4)= b

dcrda(2)=

cr

crmεε

•

dcrda(1)= σε

•

crn

Ustatev(nstatv)= crε

© 2011 ANSYS, Inc. May 9, 201414

Example of application

The available creep model, accessible via the TB,CREEP,

…,…,…,1 command, is used to demonstrate the user

material subroutine usercreep.F .

The example is a two-element test case under simple

tension. Element 1 has material defined using the

TB,CREEP,…,…,…,1 option, while Element 2 has

material defined using the TB,CREEP,…,…,…,100option. A 1% deformation is applied to both elements.

The /POST26 processor results of stress components and

creep strain components are printed for both elements and

are expected to be the same.

© 2011 ANSYS, Inc. May 9, 201415


Below is the description of the APDL syntax needed to use

usercreep.F . In this specific example the user material

was assigned to material #2 (we also have to specify the

creep constants):

TB,CREEP,2,1,4,100TBDATA,1,0.0001,1,0.5,0

We notice that the syntax is very similar to that used to

specify any creep model. In particular, the only difference is

the option 100 to be specified at the TBOPT location

(TBOPT= 0 for explicit creep, TBOPT = 1-13 for implicit

creep, and TBOPT = 100for user creep).

© 2011 ANSYS, Inc. May 9, 201417


/POST26 output: printout of the evolution of state

variable #1 (equivalent creep strain).

© 2011 ANSYS, Inc. May 9, 201418


/POST26 output: graph of the evolution of state variable(*) #1

(equivalent creep strain).

(*) More information on the use of state variables can be found in Lecture 4.

© 2011 ANSYS, Inc. May 9, 201419


/POST1 output: contour plot of the equivalent creep

strain at last substep. We notice that the results are

identical for both models.

(A) structure with available

creep material model

(B) structure with user-defined

creep model

APDL commands:

/POST1

PLESOL,EPCR,EQV,0,1.0

(A)

(B)

© 2011 ANSYS, Inc. May 9, 201420


/POST1 output: contour plot of state variable #1

(equivalent creep strain) at last substep. We notice that it is

non-zero only when considering the structure (cube on the

right) with the user-defined routine.

(A) structure with available

creep material model

(B) structure with user-defined

creep model

APDL commands:

/POST1

PLESOL,SVAR,1

(A)

(B)

© 2011 ANSYS, Inc. May 9, 201422


Information regarding the number of the state variables,

inputted via the TB,STATE syntax, is provided as well.

Output information:

TB,CREEP,…,…,…,1 vs. TB,CREEP,…,…,…,100

© 2011 ANSYS, Inc. May 9, 201423

Where to find additional information

Additional information on creep can be found in the online

manual:

Also, the ANSYS training course “ANSYS Mechanical –

Advanced Nonlinear Materials” provides detailed

information regarding creep modelling and, in this respect, it

is recommended.

ANSYS Documentation > Mechanical APDL > Material Reference > Structural Analysis Guide > 8. Nonlinear Structural Analysis >8.4. Modeling Material Nonlinearities

ANSYS Documentation > Mechanical APDL > Material Reference >

3. Material Models > 3.5. Rate-Dependent Plasticity (Viscoplasticity)

ANSYS Documentation > Mechanical APDL > Theory Reference >4. Structures with Material Nonlinearities > 4.3. Rate-Dependent Plasticity

(Including Creep and Viscoplasticity)

ANSYS Documentation > Mechanical APDL > Material Reference > 5. Material Curve Fitting // 5.3. Creep Material Curve Fitting

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