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


• Chaboche nonlinear kinematic hardening can be combined with creep

Chaboche Metal Plasticity with Creep


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


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


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


… Review Creep


• 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


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


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


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


• 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


• 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


• 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


• 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

Mech ANL 16.0 L02B Creep Plasticity

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