STRESS STRAIN CURVE EXPLAINED !!

Stress strain curve is the plot of stress and strain of a material or metal on the graph. In this the stress is plotted on y axis and its corresponding strain on the x axis. After plotting the stress and its corresponding strain on the graph, we get a curve, and this curve is called stress strain curve or stress strain diagram.

The stress strain curve for different material is different. It may vary due to the temperature and loading condition of the material.

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

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

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.

Generally people confuse a lot between proportional and elastic limit.Up to proportional limit material returns to its original shape (so in graph it is linear )but in elastic limit up on removal of load material returns to its original shape witha little deformation (so in graph it is non linear).

YIELD STRESS POINT

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

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 U material have very minute or zero strength to face further stress.

BREAKING STRESS (POINT OF RUPTURE)

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.

How to Draw Stress Strain Curve or Diagram

• A tensile test is done on the material for the drawing the stress strain curve. A specimen of specific dimension is taken generally a circular rod. A tensile test is than conducted on this rod by the use of tensile testing machine.
• In this test, the specimen is fixed at one ends and tensile load is applied on the other end. The value of load and the extension in the rod is noted down. As we have noted down the load and extension, the stress and the corresponding strain can be easily calculated.
• The formula that is used for the calculation of stress and strain are
• σ ( stress ) =load/area of cross section

1. strain=increase in length/original length

Where,

· σ = stress

· P = Load

· e = strain

· dL = extension produced in the rod

· L = original length

· A = cross section area

• We plot a graph between the stress and strain and a curve is obtained. This curve so obtained is called the stress strain curve or stress strain diagram.
• The stress strain curve for the same material is different for different temperature and loading condition of the material.
• In the graph the slope represents the young's modulus of the material.