Question

Assertion: Brittle materials do not exhibit an identifiable yield point; rather, they fail by brittle fracture.
Reason: The Value of the largest stress in tension and compression defines the ultimate strength.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are correct.

Answer 1

In order to solve this question we need to understand stress and strain. Stress is defined as force acting per unit area whereas strain is defined as change in length per unit length whenever a force is applied. So those forces which deform the material are known as deforming force so when this deforming force applied continuously then at some point material’s atoms cannot counterbalance tension and it breaks, also when the atoms could not be compressed more as by compressing we are increasing potential energy of configuration by which atoms can be as at distance of closest approach but when they compressed beyond this then to conserve energy it breaks.

Complete answer:
Brittle materials are those materials having their atoms very closely packed and tightly bound with each other. These kinds of materials do not deform easily as they counterbalance external force but we apply external force for a long time or very strong external force above its elastic strength then it ruptures suddenly, so brittle materials do not have specific yield point or breaking point.

Ultimate stress is the maximum amount of external force that is applied per unit area beyond which material ruptures.So both statements are correctBut non identification of yield point is not due to application of max external force rather it is due to external force just above its elastic strength.

So, Correct option is B.

Note: It should be remembered that a while deforming a material its atoms are actually compressed or elongated so from rule of newton body opposes this deformation in its state so it exerts force in opposite direction of external force known as elastic strength but when deforming force is greater than elastic strength then brittle material ruptures.