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  • 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 favorgrain 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. Themovement 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 asa 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.

    A brief summary on creep

  • 2011 ANSYS, Inc. May 9, 20145

    A brief summary on creep

    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 =

    41 ff K

  • 2011 ANSYS, Inc. May 9, 20146

    The routine usercreep.F allows implementing uniaxialcreep 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. Implicitcreep 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 whichusers need to specify the following:

    Derivative of the incremental creep strain withrespect to the effective stress;

    Derivative of the incremental creep strain withrespect 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

    How is usercreep.F used?

    ( ) ( ) Tbmcrncr ek /0

    =

  • 2011 ANSYS, Inc. May 9, 201410

    How is usercreep.F used?

    TT

    tt

    bmnk ,,,0

    4 in this example

    cr

  • 2011 ANSYS, Inc. May 9, 201411

    How is usercreep.F used?

    state variables (defined by

    TB,STATE command)

    = crcr t

    cr

    cr

    cr

    cr t

    =

    =

    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 computethe 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 memk

    == /1

    0

    ( ) ( )

    ==

    crTbmcr

    ncr nenk /

    1

    0

    ( ) ( ) Tbmcrncr ek /0

    =

  • 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

    crm

    dcrda(1)=

    crn

    Ustatev(nstatv)= cr

  • 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 usermaterial 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 hasmaterial defined using the TB,CREEP,,,,100option. A 1% deformation is applied to both elements.

    The /POST26 processor results of stress components andcreep strain components are printed for both elements and

    are expected to be the same.

  • 2011 ANSYS, Inc. May 9, 201415

    Example of application

    Below is the description of the APDL syntax needed to use

    usercreep.F. In this specific example the user materialwas 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 implicitcreep, and TBOPT = 100 for user creep).

  • 2011 ANSYS, Inc. May 9, 201416

    Example of application

    /POST26 output: results are identical

  • 2011 ANSYS, Inc. May 9, 201417

    Example of application

    /POST26 output: printout of the evolution of statevariable #1 (equivalent creep strain).

  • 2011 ANSYS, Inc. May 9, 201418

    Example of application

    /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

    Example of application

    /POST1 output: contour plot of the equivalent creepstrain 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

    Example of application

    /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, 201421

    Example of application

    Output information:

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

  • 2011 ANSYS, Inc. May 9, 201422

    Example of application

    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