Simple Stress and Strain MCQ with answer and solution

Concept

Fatigue cracks start at the surface, so surface finish and residual stresses control fatigue strength most.

Step-by-step solution

1. Fatigue failure starts as a small crack at a stress concentration. 2. Such points are mostly on the surface (scratches, roughness). 3. Residual stress also matters: compressive residual stress improves fatigue life, tensile residual stress reduces it. 4. Young's modulus and thermal conductivity do not control crack start. 5. So the answer is surface roughness and residual stresses.

Explanation

Fatigue cracks begin at the surface where roughness gives stress concentration. Residual stresses add to or reduce the applied stress. Processes like shot peening (compressive stress) and polishing increase fatigue strength.

Why the other options are wrong

A: Composition has some effect, but surface condition is more significant in practice. C: Young's modulus affects stiffness, not fatigue strength. D: Thermal conductivity has almost no effect.

Shortcut

Fatigue means surface crack, so choose surface finish and residual stress.

Common mistake

Choosing chemical composition because it decides the static strength.