However, strain is not mechanical stress. To find out what it is, two important points need to be considered first of all:
Temperature coefficient α of the material
When the ambient temperature changes, the material also changes. This change is designated by the temperature coefficient α. Example: When a steel cylinder is heated it expands, and with it the SG glued onto it. "This temperature-dependent material strain is precisely what we do not want to measure," says Boersch. To compensate for this effect, strain gauges are adapted to a specific material and developed so that they exhibit exactly the opposite temperature behavior. Ultimately the two effects balance out, thereby compensating for material strain so that the strain gauge measures only what it is intended to measure: the strain induced by external material loading. This is referred to as a self-compensated strain gauge or strain gauge with matched temperature response.
Modulus of elasticity (Young's modulus)
When a material is subjected to a load, it exhibits a mechanical stress. Mechanical stress is force divided by area. But how is it related to strain, which is recorded by a strain gauge? This correlation can be defined in the form of a characteristic curve for different materials by subjecting samples of materials to loads under controlled conditions. As a general rule, greater mechanical stress is matched by an increase in strain. Initially this correlation is linear. This is referred to as the elastic range and the correlation is described by the modulus of elasticity.
After a certain point, however, the material is so strongly deformed by the operant force that it is no longer able to return to its original condition. This plastic deformation continues until the material breaks. Only the linear range, where no plastic deformation occurs, is of interest for experimental stress analysis.
If the modulus of elasticity of a given material is known, the mechanical stress can be determined based on the strain: This is the objective of strain gauge measurements.