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# Thermal Creep Viscoelastic Material Properties

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: = C1 xC2 x tC3

Rate-Independent Form: = A0 xA1 x tA2. When differentiated with respect to time and assuming that is constant during the time-step, this form yields = A0 xA1 x [A2 x tA2-1]. Equating the two equations yields C1=A0xA2, C2=A1, and C3=A2-1.

Double power law:

Rate-Dependent Form: = C1 xC2 x tC3 + C4 xC5x tC6

Rate-Independent Form: = A0 xA1 x tA2 + A3 xA4x tA5. When differentiated with respect to time and assuming that is constant during the time-step, this form yields = A0 xA1 x [A2 x tA2-1] + A3 xA4x [A5 x tA5-1]. Equating the two equations yields C1=A0xA2, C2=A1, C3=A2-1, C4=A3xA5, C5=A4, C6=A5-1.

For reference, the Garofalo creep law is not dependent on time, so the rate-dependent form and the software's form are identical:= A0 x [sinh(A1 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 C1/(C3+1) A0 A0 2nd Coefficient C2 A1 A1 3rd Coefficient C3+1 A2 A2 4th Coefficient C4/(C6+1) A3 5th Coefficient C5 A4 6th Coefficient C6+1 A5