# RD-E: 5200 Creep and Stress Relaxation

The purpose of this example is to introduce how to use typical visco-elastic material to simulate creep and stress relaxation tests.

## Options and Keywords Used

- /MAT/LAW40 (KELVINMAX)
- Boundary condition (/BCS)
- Rigid body (/RBODY)
- Concentrated force load (/CLOAD)
- Imposed displacement (/IMPDISP)

## Input Files

## Model Description

- For stress relaxation test: The foam sample has been compressed until a given strain and kept in this state.
- For creep test: The foam sample has been tensile under constant force.

Units: mm, s, Mg, N, MPa

**Material Properties****Value**- Initial density
- 2e-9 [Mg/mm3]
- Bulk modulus
- 66.67 $\left[\mathrm{MPa}\right]$
- Long time shear modulus G
_{inf} - 10 $\left[\mathrm{MPa}\right]$
- Shear modulus G1
- 90 $\left[\mathrm{MPa}\right]$
- Decay constant
- $\beta $ 1 = 0.01 [1/ms]

### Model Method

For stress relaxation test: The foam sample has been compressed under constant displacement (/IMPDISP).

For creep test: The foam sample has been tensile under constant force (/CLOAD).

## Results

In LAW40 shear modulus is reduced with time and tends to G∞ after an infinite period of time. The softening speed is determined by relaxation parameter $\beta $ . Higher relaxation parameter means quick softening.

The general case of viscous materials represents time-dependent in elastic behavior. Creep is time-depended deformation and stress relaxation is a time-depended decrease in stress. Viscous material can describe these two phenomenons. In Radioss, the following material laws describe viscous:

### Visco-elastic Law

- /MAT/LAW34
- Visco-elastic generalized Maxwell model, Boltzmann (solids)
- /MAT/LAW35
- Visco-elastic generalized Maxwell-Kelvin-Voigt (shells + solids)
- /MAT/LAW38
- Visco-elastic tabulated (solids)
- /MAT/LAW40
- Visco-elastic generalized Maxwell-Kelvin (solids)
- /MAT/LAW42
- Ogden/Mooney-Rivlin with Prony viscosity (Hyperelastic materials)
- /MAT/LAW62
- Ogden (Hyperelastic materials)
- /MAT/LAW70
- Visco-elastic tabulated (solids)
- /MAT/LAW77
- Visco-elastic tabulated with porosity and air flow

### Visco-elastic Plastic Law

- /MAT/LAW33
- Visco-elastic plastic (solids) and user-defined yield function
- /MAT/LAW52
- Gurson, visco-elasto-plastic porous metals, and strain rate dependent
- /MAT/LAW66
- Semi-analytical plastic model. Yield surface built from curves in tension, compression and shear + /VISC/PRONY