A steel is defined in the standards by a description of the mechanical properties and chemical composition of the steel grade. Yield strength is a mechanical property. As described in the previous chapter, it is used as the delimiter between mild steels and the various levels of high strength steels.


Definition of Yield strength, Tensile strength and Elongation

When a steel is subjected to external forces, its form will change to a greater or lesser extent. The magnitude of the deformation is dependent on the material properties and on the magnitude of the force to which the steel is subjected.

Deformation can be classified into two types, i.e. elastic or plastic. The characteristic of elastic deformation is that the steel resumes its original form when the force is relieved.

Plastic deformation is characterized by the deformation remaining even after the force has been relieved.

A tensile testing machine is used for determining the relationship between force and deformation in tension, and the machine measures the tensile force and the elongation.

In tensile testing, the test piece is subjected to a stress. This is specified as force per unit of area and is designated N/mm2 or MPa. Elongation is measured in % of the original length by which the length of the steel has increased during the tensile test.

The properties evaluated during tensile testing are the yield strength (Re), tensile strength (Rm) and elongation (A5/A80). In daily speech, we often use the concept of "strength" for describing these factors.

Tensile test curve

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Stress, N/mm2
Failure
Elongation, %
Elongation to rupture