Question

The Young’s Modulus of Aluminium is around 70 GPa. Comment on any difference obtained from the experiment and the expected value of 70 GPa.

Answer 1

Youngs modulus is the ratio of stress to strain.

Upto elastic point, stress is directly proportional to strain, which gives a straight line upto certain length and then the slope changes. this point is taken as Elastic limit and the ratio of stress/strain at this point is considered as Youngs modulus.

stress is the ratio of Load applied / area of cross section.

This above curve is drawn considering the diameter of the specimen for tensile test is constant and does not reduce by the applied load. This curve is called Engineering stress-strain curve.

But in actual practice, when the load is applied, as the length of the specimen increases, the diameter of the specimen reduces, which affects the value of stress (P/A). Hence the actual/true curve deviates from the Engineering curve. Such curve drawn by considering the reduced diameter at every point is called True Stress-strain curve,

For all engineering applications, engineering stress strain curve is considered subjected to some factor of safety.

Because of this reason, the Experimental VAlue of Youngs modulus deviates from the True value of Youngs modulus.