Strain is a measure of the amount of deformation that occurs when an object is placed under stress. Strain rate is defined as the change in strain over the change in time. All materials will undergo some change in their dimensions when exposed to stress. The deformation caused by stress can be fully reversible or permanent, depending on the amount of stress applied.
Elastic strain occurs when a material under stress returns to its original dimensions once the stress is removed. Plastic strain occurs when an object has been exposed to very high levels of stress and will no longer return to its original shape after the stress is removed. In many materials, the reversal of elastic strain is instantaneous, meaning it occurs without a perceptible duration of time. Deformation that is fully recoverable, but occurs over time, is described in terms of the strain rate.
Strain rate varies widely for different materials, and will often change at different temperatures and applied pressures. Steel is an example of a material that returns to its original state immediately after stress is removed. Conversely, in geology, stresses are applied over millions of years and the rate of strain in rock is typically very low. A material whose rate of strain changes a large amount at different temperatures and pressures has high strain rate sensitivity.
This rate is also dependent on the way the force or stress is applied. For many plastics, if a gradual stretching force is applied the material will elongate a large amount before it breaks. This is because the molecules in the plastic have enough time to reorient themselves and move past each other, causing the stretching to occur. If an impact or sudden force is applied to a plastic, it will break immediately and behave like a brittle material. The same plastic material can react very differently because of the different rates of strain caused by the way the stress is applied.
Strain rate can be measured in the laboratory using special test equipment that applies very precise loads to a sample, while measuring the deformation and recovery that occurs after the stress is removed. Since the strain rate of a material will influence how it behaves, it is important to understand its sensitivity to the type of load, amount of stress, and temperature. Understanding the strain rate of a material will ensure that it meets the performance specifications required in the end-use application.