23.10: Measurements of Strain
Strain quantifies the deformation of a material under force, typically measured as normal strain, which represents the change in length when compared with the original length. Electrical strain gauges are used for enhanced accuracy. These devices consist of a conductive wire mounted on a paper backing that adheres to the material's surface. These gauges operate on the piezoresistive effect, where the wire's electrical resistance changes in response to mechanical deformation. The strain gauge adjusts as the material elongates or compresses, altering the wire's resistance proportionally to the strain experienced.
To comprehensively understand strain at a specific material point, consider the normal and shearing strains, which account for shape changes at constant volume. This analysis requires multiple strain gauges arranged at various orientations. Strain measurements are typically conducted along lines set at defined angles to a horizontal axis, with these readings used to compute the overall strain components.
A strain gauge rosette, which positions several gauges in configurations such as 0°, 45°, and 90°, or 0°, 60°, and 120°, enables simultaneous multi-directional strain measurements. Such an arrangement is pivotal for detailed strain analysis at a point and is extensively employed in structural analysis, materials testing, and part validation. These methodologies are vital for engineers to evaluate material behavior, enhancing engineered structures' design and safety.
