Strain rate is the speed or velocity at which deformation of an object from its original shape occurs. Deformation can occur in any direction, depending on the way the force or stress is applied. Strain rate varies for different materials, and will often change at different temperatures and applied pressures. Strain rate can be measured using special test equipment such as an Instron, which applies very precise loads to a sample while measuring the deformation, timing and recovery occurring when stress is applied. Understanding strain rate of a material will ensure that it meets performance specifications required in the end-use application.
Write down the following equation for E strain rate, E = e ÷ t. Strain rate is defined as the change in strain e over the change in time t. Strain is the deformation of an object normalized to its original shape.
Record the formula for the change in strain e of the material where e = (L- L0) ÷ L0. The symbol L represents the length of the object after the deformation and L0 corresponds to the initial length of the object before the deformation occurred. The measurement of the change in strain is defined as the deformation of an object from its original shape due to an applied stress or force.
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Measure the initial length of the material using a ruler or caliper. Record this measurement as L. Use the testing equipment necessary to stretch the material, such as an Instron. Follow the manufacturer's instructions for the setup and running of the equipment. The Instron should be hooked up to a program which will record the time it took to stretch the material. Do not allow the material to break during the run, or you will have to repeat the test.
Measure and record the length of the stretched material while it is still hooked up to the test equipment and the time it took to stretch to that length. Many materials are very elastic, and if removed from the test equipment they will return to their initial length.
Substitute the change in strain equation into the strain rate formula and solve the formula using your measured values. The change in strain E would then be equal to E = (L- L0) ÷ (L0 x t). For example if the initial length of the material is 5.0 cm and the material stretched to 6.9 cm in 15 seconds than the strain rate E = (6.9 cm – 5.0 cm) ÷ (5.0 cm x 15 s) = .256 1/s.