properties of materials
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. 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.Stress-strain diagram of a structural
steel, which is a ductile material, is given.
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
s = Ee
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
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 tretching, the
diameter of the specimen starts decreasing fast due to local instability and
this p henomenon is called necking.
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.