The thermal creep viscoelastic material model is available for 2D, brick and tetrahedral elements. This material model will account for creep. Creep occurs when a part deforms over time after a thermal load has been applied. To properly use this material model, the creep parameters must be defined in the Thermal tab of the Element Definition dialog. The thermal creep viscoelastic material properties are listed below. The material properties are input in a manner similar to the Temperature Dependent Material Properties. In addition to these properties, it may be necessary to define some Isotropic Material Properties
The implementation of creep is not based the rate-form of the equation. Instead of 
= f(
), our implementation is based on 
= f(). When this implementation is differentiated with respect to time to get the strain-rate equation, this affects the constants (and the material properties input) as follows:
Power-law:
Rate-Dependent Form: 
= C
1
x
C2
x t
C3
Rate-Independent Form: = A
0
x
A1
x t
A2
. When differentiated with respect to time and assuming that is constant during the time-step, this form yields = A
0
x
A1
x [A
2
x t
A2-1
]. Equating the two equations yields C
1
=A
0
xA
2
, C
2
=A
1
, and C
3
=A
2
-1.
Double power law:
Rate-Dependent Form: = C
1
x
C2
x t
C3
+ C4 x
C
5
x t
C
6
Rate-Independent Form: = A
0
x
A1
x t
A2
+ A
3
x
A
4
x t
A
5
. When differentiated with respect to time and assuming that is constant during the time-step, this form yields = A
0
x
A1
x [A
2
x t
A2-1
] + A
3
x
A
4
x [A
5
x t
A5-1
]. Equating the two equations yields C
1
=A
0
xA
2
, C
2
=A
1
, C
3
=A
2
-1, C
4
=A
3
xA
5
, C
5
=A
4
, C
6
=A
5
-1.
For reference, the Garofalo creep law is not dependent on time, so the rate-dependent form and the software's form are identical:= A
0
x [sinh(A
1
x )]
A2
.
Coefficients:
Enter the coefficients corresponding to the selected creep law in the Viscoelastic Properties spreadsheet. Note the interpretation of the coefficients depend on whether the material properties are derived from the effective creep strain-rate or the total effective creep strain . The 4th through the 6th coefficients only need to be entered for the double-power creep law.
| Input in the Viscoelastic Properties spreadsheet |
Value when material properties are derived from the effective creep strain-rate |
Value when material properties are derived from the total effective creep strain |
|
|
Power or Double-Power Law |
Garofala |
||
|
1st Coefficient |
C 1 /(C 3 +1) |
A 0 |
A 0 |
|
2nd Coefficient |
C 2 |
A 1 |
A 1 |
|
3rd Coefficient |
C 3 +1 |
A 2 |
A 2 |
|
4th Coefficient |
C 4 /(C 6 +1) |
A 3 |
|
|
5th Coefficient |
C 5 |
A 4 |
|
|
6th Coefficient |
C 6 +1 |
A 5 |
|