What is the function of an extensometer?

An extensometer is specifically designed to measure the amount that a specimen stretches, or deforms, during a tensile test. This device provides critical data on how materials respond to stress and strain, which is invaluable during material testing. When a tensile force is applied to a specimen, the extensometer captures the change in length relative to the original length of the sample, thereby allowing engineers to calculate tensile strain. This information is essential for determining the material's mechanical properties, such as its elasticity, yield strength, and ultimate tensile strength.

The other options, while related to material testing, do not accurately describe the primary function of an extensometer. For instance, measuring hardness is linked to other specialized equipment like durometers, and detecting surface cracks is done using non-destructive testing methods, rather than through direct measurement of deformation. Similarly, evaluating fatigue resistance involves observing how materials withstand repeated loading, which requires different equipment and methodologies.