Introduction

An increase of strain rate raises the flow stress of most materials. The amount of the effect depends on the material and the temperature. In most metallic materials, the effect near room temperature is small and is often neglected. A factor of ten increase in strain rate may raise the level of the stress-strain curve by only 1% or 2%. On the other hand, at elevated temperatures the effect of strain rate on flow stress is much greater. Increasing the strain rate by a factor of ten may raise the stress-strain curve by 50% or more.

Strain localization occurs very slowly in materials that have a high strain-rate dependence because less-strained regions continue to deform. Under certain conditions, the rate dependence is large enough for materials to behave superplastically. Tensile elongations of 1000% are possible.

There is a close coupling of the effects of temperature and strain rate on the flow stress. Increased temperatures have the same effects as deceased temperatures. This coupling can be understood in terms of the Arrhenius rate equation.

Strain-Rate Dependence of Flow Stress

The average strain rate during most tensile tests is in the range of 10−3 to 10−2/s. If it takes 5 min during the tensile test to reach a strain of 0.3, the average strain rate is. At a strain rate of a strain of 0.3 will occur in 30 seconds.