Question

The Stress-Strain graph of a metal wire is shown in figure up to pint E. The wire returns to its original state 0 along EPO when it is gradually unloaded point B corresponds to the fracture of wire.

1. Up to what point of the curve Hooke’s Law is obeyed? 

2. Which point on the curve corresponds to yield point or elastic limit. 

3. Indicate the Elastic region and plastic region. 

4. Describe what happens when the wire Is loaded up to the stress corresponding to point A and then unloaded gradually. In particular to the dotted line. 

5. What is peculiar about portion of curve from C to B? Upto what stress can be applied without causing fracture?

Answer 1

1. Since the graph is linear till point E. Hooke’s Law is obeyed till point E. 

2. The point E corresponds to the elastic limit because till there the body returns to its original state along EPO. 

3. Elastic - O to E; Plastic - E to B 

4. Up to point E an almost linear increase in strain with stress. After point E, if it is loaded till A, the strain increases faster for some increase in stress. But on unloading the body returns along line AO to a permanent strain (change in shape) of O'. 

5. In the curve C to B, the body undergoes a strain even if it is being unloaded and then fractures. To present fracture the value of stress has to be just less than that of C.