After the specimen for tensile test was measured and marked, it needs to be fastened safely in gripper of mobile and motionless heads of machine for tensile testing.
After setting the specimen in gripper, the increasing with constant speed tensile stress is applied to the specimen. If increasing speed of tensile stress is not constant, the test results can be unstable. Before the stress is applied to the marked points of work length, the device is fastened to the specimen, which is called extensimeter. The extensimeter measures lengthening value during stress action, which appears as a result of stress action. Data of stress and lengthening are printed by self-recorder, which builds lengthening changes curve depending on applied stress. This curve is called stress-strain diagram. However, usually received results 
of tensile tests are showed as stresses and deformations. The stress is proportional to the force, because it is calculated as division of applied stress to sectional area. The deformation is determined as the lengthening value for given length.
The diagram for values, studied during tensile tests for typical carbon steel is showed on the fir. 1.
Several significant elements can be chosen on the graph “stress-deformation”, which demand more detailed examination. At the beginning of tests both stress and deformations are equal to zero. With increasing of stress the deformation value increases linearly to the stress value. This area shows, what was before called an elastic work, under which the proportional connection between stress and deformation remains.
