Strain Rate

Strain rate is a measure of how quickly a component or material is being deformed. Its units are strain per unit time, but since strain is a unitless values, strain rate typically has units of 1/s. Although it can be difficult to know exactly which strain rate or strain rates occur during a certain event, an approximation can be given based on the event duration. Ballistic and explosive events occur in a few microseconds and have high strain rates (2000 /s and above). Conversely, dental braces, inflatables, and geological events all occur over minutes, days, and years and have low strain rates (0.1 /s and below). The majority of other events such as sporting collisions, metal work, and automotive accidents all occur between a few milliseconds to a few seconds. These events occur at standard strain rates (1 /s to 1000 /s).

Quasi-Static vs Dynamic Strain Rates

The general separation between these two occurs around or just below 1 /s. The physical difference between these two strain rate regimes is based on the effect of stress waves. At quasi-static strain rates, the waves are allowed enough time to come to equilibrium as the load is increased; whereas at dynamic strain rates, the stress waves are detectable and not constant through the material. At high strain rates, these waves have a low wavelength. As the strain rate is decreased, the waves become longer. Because of this, testing materials at standard strain rates often requires large machines that can generate and properly measure these long waves.

Why does strain rate matter?

Many factors affect a material’s performance and mechanical properties: chemistry, processing, temperature, etc. When a material is going to be used in a specific application, its chemistry and processing are usually determined first. After this, the material is tested mechanically at room temperature as well as at the high end and low end of temperatures that the material is likely to see in service. This temperature study is often required because it is well known that mechanical properties vary with respect to temperature. Mechanical properties vary with respect to strain rate as well.