Strain is the deformation caused by stress, and strain rate is the change in strain over time. Materials can experience reversible or permanent deformation. Elastic deformation is reversible, while plastic deformation is permanent. Strain rate varies between materials and can be affected by temperature and pressure. The way stress is applied can also affect strain rate, and it can be measured in the lab to ensure materials meet performance specifications.
Strain is a measure of the amount of deformation that occurs when an object is subjected to stress. Strain rate is defined as the change in strain with respect to the change in time. All materials will experience some change in their dimensions when exposed to stress. The deformation caused by stress can be completely reversible or permanent, depending on the amount of stress applied.
Elastic deformation occurs when a material under stress returns to its original size after the stress is removed. Plastic deformation occurs when an object has been exposed to very high levels of stress and will never return to its original shape after the stress has been removed. In many materials the reversal of elastic deformation is instantaneous, ie it occurs without a perceptible duration over time. Deformation that is fully recoverable, but occurs over time, is described in terms of strain rate.
Strain rate varies widely for different materials and often changes under different applied temperatures and pressures. Steel is an example of a material that returns to its original state soon after stress is removed. In contrast, in geology, stresses are applied over millions of years and the strain rate in rock is typically very low. A material whose strain rate changes greatly at different temperatures and pressures has a high strain rate sensitivity.
This speed also depends on how force or stress is applied. For many plastics, if a gradual stretching force is applied, the material will stretch significantly before breaking. This is because the molecules in the plastic have enough time to reorient and move past each other, causing them to stretch. If an impact or sudden force is applied to a plastic, it will immediately break and behave like a brittle material. The same plastic material can react very differently due to different strain rates caused by how stress is applied.
Strain rate can be measured in the laboratory using special test equipment that applies very precise loads to a specimen, measuring the strain and recovery that occurs after stress is removed. Because a material’s strain rate will affect its behavior, it is important to understand its sensitivity to the type of loading, amount of stress, and temperature. Understanding the strain rate of a material will ensure it meets the performance specifications required in the final application.
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