Which of the following is not a characteristic observed in material failure by fatigue fracture ?

ANS:B - Initiation of crack from below the surface does not occur.

"Initiation of crack from below the surface does not occur" is incorrect in the context of fatigue failure. In fatigue failure, cracks often initiate from below the surface of the material due to cyclic loading. This is one of the key characteristics of fatigue failure. The repetitive application of stress, even if it's below the yield strength of the material, can lead to the accumulation of microscopic defects, which eventually coalesce into cracks below the surface. These cracks then propagate gradually until catastrophic failure occurs. Therefore, the correct statement regarding the initiation of cracks in fatigue failure is: "Initiation of crack occurs below the surface of the material."