Strain

Engineering definition

For engineers, strain is defined as the elongation per unit length (normal) or by the amount of elongation per unit length perpendicular to the elongation direction (shear). Engineering strain values may be calculated or measured. Strain can be calculated by measuring the elongation of an entire member and dividing this by the initial length of the member prior to elongation, ε=ΔL/L. This technique yields an average strain and may be used to determine the strain in members with a constant cross-section in the case of normal strain. In addition, strain gauges are often used to measure strain. These devices consist of a flexible membrane with embedded conductors. It is attached to the surface of the structure being measured and it elongates at the same rate as the material to which it is attached.

Electronics are used to measure the strain in the gauge, which is essentially equal to the strain in the part in the small area over which the gauge is attached. One advantage of strain gauges is that they can be used to measure parts with complex geometry. In engineering, the material under strain retains its homogeneous material properties. If cracking occurs in the material, the strain calculation become invalid, and different methods must be used to predict how the cracked material will behave. Moreover, because the amount of strain depends on the load applied (causing elongation), when the load is removed the strain is reduced to zero.

Confusion

To engineers, “strain” refers to a temporary condition that results from the application of force. The material under strain retains its homogeneous material properties, and once the force is removed, the strain is reduced to zero. But in medicine, this term “strain” describes a more lasting condition of damage that exists even after the force (in this case, tissue elongation) is removed. Clinicians equate “strain” with tissue damage of a muscle, ligament, or tendon caused by overextension, high forces, or overuse. Generally this damage is a tear or multiple tears in the tissue.