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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 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.
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 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
How is usercreep.F used?
( ) ( ) Tbm
cr
n
cr ek /0
−•
= εσε
© 2011 ANSYS, Inc. May 9, 201410
How is usercreep.F used?
TT∆
tt∆
bmnk ,,,0
4 in this example
crεσ
© 2011 ANSYS, Inc. May 9, 201411
How is usercreep.F used?
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
Example of application
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, 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 state
variable #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 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
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