Strain Rate Sensitivity for Linear Mobility Model

Linear Mobility

The question has been rased: "What are the consequences in assuming a linear mobility law?" Wei suggested that, under the assumption of linear mobility, the stress strain curve produced would be subjected to unrealistic constraints. It was implied that this was linked to the strain rate dependence, but he could not remember exactly what the claim was.

Elastoplastic Response in One Dimension

In order to answer the above questions, we will start with a simple analytical model in one dimension. This avoids the unnecessary complications that occur with tensors and ultimately should be easily generalized. Thus, we will deal with one dimensional strain, stress, and Young's modulus.

From an experimental point of view, we generally impose a constant strain rate and measure the materials stress response. The strain rate can generally be decomposed into elastic and plastic parts

${\dot {e}}={\dot {e}}^{el}+{\dot {e}}^{pl}$

The elastic strain rate must be related to the stress rate through Hooke's law

${\dot {e}}^{el}={\frac {\dot {\sigma }}{E}}$

and the plastic strain rate can be approximated as

${\dot {e}}^{pl}=\rho vb$

where $\rho$ is the dislocation density, v is the average dislocation velocity, and b is the burger's vector.