A tensile test is generally conducted on a standard specimen to obtain the relationship between the stress and the strain which is an important characteristic of the material.
In the test, the uniaxial load is applied to the specimen and increased gradually. The corresponding deformations are recorded throughout the loading.
Based on load application and strain induced, stress-strain diagram is constructed and then material properties are analysed.
Stress-strain diagrams of materials vary widely depending upon whether the material is ductile or brittle in nature.
If the material undergoes a large deformation before failure, it is referred to as ductile material or else brittle material.
In above figure, the stress-strain diagram of a structural steel, which is a ductile material, isgiven. Initial part of the loading indicates a linear relationship between stress and strain, and the deformation is completely recoverable in this region for both ductile and brittle materials. This linear relationship, i.e., stress is directly proportional to strain, is popularly known as Hooke’s law.
σ = Eε
The co-efficient E is called the modulus of elasticity or Young’s modulus. Most of the engineering structures are designed to function within their linear elastic region only.
After the stress reaches a critical value, the deformation becomes irrecoverable. The corresponding stress is called the yield stress or yield strength of the material beyond which the material is said to start yielding.
In some of the ductile materials like low carbon steels, as the material reaches the yield strength it starts yielding continuously even though there is no increment in external load/stress.
This flat curve in stress strain diagram is referred as perfectly plastic region.
The load required to yield the material beyond its yield strength increases appreciably and this is referred to strain hardening of the material.
In other ductile materials like aluminum alloys, the strain hardening occurs immediately after the linear elastic region without perfectly elastic region. After the stress in the specimen reaches a maximum value, called ultimate strength, upon further stretching, the diameter of the specimen starts decreasing fast due to local instability and this phenomenon is called necking.
The load required for further elongation of the material in the necking region decreases with decrease in diameter and the stress value at which the material fails is called the breaking strength. In case of brittle materials like cast iron and concrete, the material experiences smaller deformation before rupture and there is no necking.