Mech ANL 16.0 L02B Creep Plasticity
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Transcript of Mech ANL 16.0 L02B Creep Plasticity
April 16, 2015 1 Release 15.0
16.0 Release
ANSYS Mechanical Advanced Nonlinear Materials
Appendix 2B Creep with Plasticity
April 16, 2015 2 Release 15.0
• Chaboche nonlinear kinematic hardening can be combined with creep
Chaboche Metal Plasticity with Creep
April 16, 2015 3 Release 15.0
The Chaboche Kinematic Hardening is a nonlinear kinematic hardening model.
The evolution law which defines the shifting of the yield surface has a nonlinear term:
3 2
1
R
C/g
e
Limiting
Yield
Surface
Current
Yield
Surface
Limiting
value of
{a}
a
g iiplii C αεα 3
2
Where: epl is equivalent plastic strain
is accumulated plastic strain.
Review Nonlinear Kinematic Hardening
April 16, 2015 4 Release 15.0
Review Ratchetting & Shakedown
Time
0 2 4 6 8 10
Pla
stic s
train
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Plastic strain
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
Str
ess (
MP
a)
-1500
-1000
-500
0
500
1000
1500
2000
Time
0 2 4 6 8 10
Pla
stic s
train
0.000
0.002
0.004
0.006
0.008
Plastic strain
0.000 0.002 0.004 0.006 0.008
Str
ess (
MP
a)
-1500
-1000
-500
0
500
1000
1500
2000
Ratchetting
Shakedown Loading
Controlled Stress
Unsymmetry
t
Ratchetting modeled
with Chaboche using:
n=1
R=980,
C1=224000
g1 400
Shakedown modeled with
Chaboche using:
n=2
R=980
C1=224000 , C2=20000
g1=400, g2=0
April 16, 2015 5 Release 15.0
Review Creep
• Creep:
– Vacancies migrate from regions of high to low concentrations.
– Thermally activated (1/3 to 1/2 melt temp for common metals).
– Can occur at low stresses (no yield surface).
– Rate dependent
• The dependency of creep deformation on stress, strain, time, and temperature are generally modeled with a form similar to the following:
• The functions f1-4 are dependent on the creep law selected.
Tftfffcr 4321 ee
April 16, 2015 6 Release 15.0
… Review Creep
April 16, 2015 7 Release 15.0
• Be careful to maintain unit consistency!
• Mechanical does not convert creep units automatically, but will issue an error message when creep units defined in Engineering Data do not match solver unit system.
/TCCC
cr etσCε 432
1
… Review Creep
April 16, 2015 8 Release 15.0
The creep strain calculation can be turned on or off during an analysis.
• This is useful to establish initial conditions. In this situation, a very small ending TIME
value (e.g., 1e-8) should be set, and creep effects turned off. Solve initial stress state
as 1st load step. Then, to turn creep effects ON and specify the real end time for load
step 2.
Load Step 1 Load Step 2
… Review Creep
April 16, 2015 9 Release 15.0
Because creep is a path-dependent phenomenon, it is important to ensure that the response is adequately captured.
• One measure of this which the solver uses is the Creep Strain
Ratio defined as:
• Where ecr is the equivalent creep strain increment and eet is the
modified equivalent elastic strain (see Ch. 4.2/4.3 of the Theory
Manual for details).
• Creep Limit Ratio is the maximum allowable limit to the Creep
Strain Ratio
et
cr
sCe
e
… Review Creep
April 16, 2015 10 Release 15.0
If, during a timestep, the solver calculates a Creep Strain Ratio larger than the Creep Limit Ratio (default =1.0), then the solution is automatically bisected until the creep limit is satisfied or the minimum time step is reached.
… Review Creep
April 16, 2015 11 Release 15.0
• Chaboche nonlinear kinematic hardening can be combined with creep
Additive decomposition:
The elastic, creep, and plastic strains are all evaluated at the (current) stress state, but they are calculated independently.
Chaboche Metal Plasticity with Creep
crplel εεεε
/TCCC
cr etσCε 432
1
g iiplii C αεα 3
2
April 16, 2015 12 Release 15.0
• This Chaboche-creep combo can itself also be combined with other material models, such as nonlinear power law isotropic hardening, anisotropic plasticity, and multilinear isotropic hardening.
– Useful for large strain, nonsymmetric cyclic loading
– Only available via MAPDL
– Refer to Section 8.4.1 of MAPDL Structural Analysis guide
TB,CHAB,matid,1,,
TBDATA,1
TB,CREEP,matid,,,,
TBDATA,1,,,
TB,NLISO,matid,,,POWER
TBDATA,1,,,,,
• Combining creep with traditional multilinear kinematic models (TB,KINH, TB,MKIN, TB,PLAS,,,KIN) is still not supported.
─ Limitation related to theoretical sublayering technique used in MKIN making it very difficult to track stress state when combined with creep.
─ The material behavior is assumed to be composed of various portions (or subvolumes), all subjected to the same total strain, but each subvolume having a different yield strength
…Chaboche-Creep combinations
April 16, 2015 13 Release 15.0
• WB-Engineering Data now supports curve fitting of Chaboche plasticity model
─ Nonlinear regression process on yield surface evolution law:
Chaboche Curve fitting in Engineering Data
g iiplii C αεα 3
2
April 16, 2015 14 Release 15.0
• The MAPDL material curve-fitting tool now supports the power hardening law option for the nonlinear isotropic hardening specification (TB,NLISO,,,POWER) to curve fit plasticity data with Chaboche kinematic hardening..
Chaboche Curve fitting in MAPDL