Stress
strain curve is a behavior of
material when it is subjected to load. In this diagram stresses are plotted
along the vertical axis and as a result of these stresses, corresponding
strains are plotted along the horizontal axis. As shown below in the stress
strain curve.
From
the diagram one can see the different mark points on the curve. It is because,
when a ductile material like mild steel is subjected to tensile test, then it
passes various stages before fracture.
These
stages are;
1. Proportional Limit
2. Elastic Limit
3. Yield Point
4. Ultimate Stress Point
5. Breaking Point
Proportional
limit is point on the curve up to which the value of stress and strain remains
proportional. From the diagram point P is
the called the proportional limit point or it can also be known as
limit of proportionality. The stress up to this
point can be also be known as proportional limit stress.
Hook’s
law of proportionality from diagram can be defined between
point OP. It
is so, because OP is a straight line which shows that
Hook’s law of stress strain is followed up to point P.
Elastic
limit is the limiting value of stress up to which the material is perfectly
elastic. From the curve, point E is the elastic limit point. Material
will return back to its original position, If it
is unloaded before the crossing of point E.
This is so, because material is perfectly elastic up to point E.
Yield
stress is defined as the stress after which material extension takes place more
quickly with no or little increase in load. Point Y is the yield point on the graph and
stress associated with this point is known as yield stress.
Ultimate
stress point is the maximum strength that material have to bear stress before
breaking. It can also be defined as the ultimate stress corresponding to the
peak point on the stress strain graph. On the graph point U is the ultimate stress point. After
point Umaterial have very minute or zero strength to
face further stress.
Breaking
point or breaking stress is point where strength of material breaks. The
stress associates with this point known as
breaking strength or rupture strength. On the stress strain curve, point B is the breaking stress point.