The tension can exceed the elasticity

Stress-strain diagram

A standardized metal rod is slowly pulled apart until the rod gives way under the load and breaks. The material to be tested behaves differently from the beginning of the tensile test until it breaks, which is shown in the stress-strain diagram.

In addition, the elastic deformation limit, the plastic deformation limit, the maximum tensile strength and the breaking limit can be read off. These values ​​are particularly important for design engineers, as they can be used to find out how much force a material can absorb in relation to the cross-section used without permanently deforming.

The graph in the diagram shows how much the material stretches under the increasing stress. If you take a closer look at the course of the curve, in most cases the behavior of the material can be divided into four phases.

These are called:

  • elastic deformation
  • plastic deformation / flow zone
  • plastic deformation / work hardening
  • plastic deformation / necking

designated. The tensile test ends when the metal rod breaks.

This means that the material only returns to its initial state in the area of ​​the first phase, the elastic deformation, without deformation or damage when the tensile force is reduced again. In the following phases, the deformation is permanent and therefore irreversible.

The data of a stress-strain diagram are reliable. However, the specified values ​​depend on some variables that have a direct influence on the measurement results. This includes the type of material production, the material composition, microscopic imperfections and the temperature. For this reason, the diagram of each tensile test is slightly different and several tensile tests are always necessary in order to create a reliable stress-strain diagram with the determined average values.