Question

Fatigue failure occurs due to

• A. Sudden application of stress
• B. Gradual application of stress
• C. Repeated application of stress
• D. Constant application of stress

Hint:

Associate "Fatigue" with cyclic loading, vibration, or fluctuating stress.
Examples include rotating shafts, bridges under rolling traffic, and aircraft wings.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Materials subjected to fluctuating, cyclic forces can fail at stress levels significantly below their static yield strength. This phenomenon is known as fatigue.

Step 3: Detailed Explanation:
Let us analyze the impact of different stress application types:
1.

Sudden application of stress: Causes impact loading or shock, potentially leading to immediate brittle or ductile fracture.
2.

Gradual application of stress: Matches standard static tensile testing, leading to yield or ultimate structural failure.
3.

Repeated application of stress: Creates microscopic cracks at stress concentrators over many cycles. These cracks propagate progressively until the remaining cross-section can no longer bear the load, resulting in sudden fatigue failure.
4.

Constant application of stress: Leads to elastic/plastic deformation or long-term creep under high temperatures.
Thus, fatigue failure is specifically caused by repeated, cyclic application of stress.

Step 4: Final Answer:
The correct option is 3, which corresponds to "Repeated application of stress".