What does the extensometer measure in a specimen during testing?

An extensometer is a precise instrument used to measure the elongation or change in length of a specimen as it undergoes deformation during testing. This elongation is directly related to the material's strain, which is defined as the change in length per unit length of the original specimen. When a specimen is stretched or compressed, the extensometer captures these movements, allowing for an accurate assessment of how much the material deforms under load.

Strain is a dimensionless quantity, typically expressed as a ratio or percentage, and it provides critical information about the material's behavior under various stress conditions. By measuring strain, the extensometer helps engineers and material scientists understand the material's performance characteristics, such as ductility and elasticity, essential for applications in welding and structural integrity assessments.

Other options, such as mass, stress, and temperature, refer to different physical properties that are not directly measured by an extensometer during the testing process.