Extensometry
Extensometry is a technique used to measure deformations in materials and structures, making it possible to calculate the stresses associated with them, and is carried out using extensometers; sensors based on a grid of conductive material (usually thin metal alloys) embedded in an insulating adhesive material which operate on the basis of electrical resistance and which deform when a load or force is applied to a material, altering its electrical properties and can be: uniaxial, triaxial or high temperature.
Figure 1: Uniaxial (left) and triaxial (right) extensometers
The main change observed is the variation in the electrical resistance of the sensor, which is directly related to the deformation of the body in which the strain gauge is installed and can be accurately measured using the Wheatstone Bridge.
The physical principle that governs the operation of the extensometer is Hooke's Law, which states that the deformation of a material is directly proportional to the applied stress, as long as the material is within its elastic zone.
The mathematical relationship between stress and strain is expressed by the equations:
Strain: Stress:
- ΔL is the change in length of the material,
- L is the initial length,
- F is the force applied,
- A is the cross-sectional area of the material.
Figure 2: Stress x strain curve
These equations allow us to transform the strain measured by the extensometer into stress, using the modulus of elasticity of the material (Young's modulus), which is a constant that relates stress and strain in a linear fashion.
Its aim is to promote the Structural Integrity assets and equipment by detecting faults before they become critical. Possible applications include: stress analysis in metallic structures, monitoring of deformations in rail and road vehicles, and evaluation of the strength of materials. In addition, the technique can be combined with numerical methods, such as análise por Elementos Finitos (FEA), to create more accurate computer models and predict the behavior of structures under extreme conditions.
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