The most basic test in the study of stress-strain relations is the simple tensile test wherein a cylindrical specimen of the type shown in Fig. 4.1 is subjected by a tensile test machine to a force F along the centerline of the specimen. The distance L between two points on the specimen is measured at all times by extensometer gauge (1) as seen in the diagram. Another extensometer gauge (2) meanwhile measures the diameter D of the cylinder. As the force F is varied, we measure Land D for each setting of F. Hence, at any setting, we have the following information.