Question

The resistance to fatigue of a material is measured by

• A. elastic limit
• B. endurance limit
• C. Young’s modulus
• D. ultimate tensile strength

Hint:

For fatigue design, endurance limit is more important than ultimate strength.

Answer 1

The Correct Option is B

Step 1: Understanding fatigue.
Fatigue is the failure of a material due to repeated or fluctuating stresses, often at stress levels much lower than the ultimate tensile strength.
Step 2: Definition of endurance limit.
The endurance limit is the maximum stress level below which a material can endure an infinite number of load cycles without fatigue failure.
Step 3: Eliminating incorrect options.
Elastic limit indicates the limit of elastic behavior, not fatigue resistance.
Young’s modulus measures stiffness, not fatigue strength.
Ultimate tensile strength represents maximum static strength, not cyclic performance.
Step 4: Conclusion.
Hence, the resistance of a material to fatigue is measured by its endurance limit.

Answer 2

Fatigue failure occurs after a material experiences many cycles of fluctuating stress, often at stress levels well below its static strength. The S-N (stress-life) curve, which plots applied stress amplitude against the number of cycles to failure, is the standard way engineers characterize a material's fatigue behaviour. Let's check each option against this curve.

1. Elastic limit: This is a static-loading property, the stress up to which a single, one-time application of load produces no permanent deformation; it says nothing about how a material behaves under thousands or millions of repeated stress cycles, so it does not capture fatigue behaviour.
2. Endurance limit: On the S-N curve, many materials (particularly ferrous ones) show a stress level below which the curve becomes essentially flat, meaning the material can withstand an effectively infinite number of cycles without failing. This flat asymptote is precisely the endurance limit, and it is the standard quantity used to characterize resistance to fatigue.
3. Young's modulus: This measures the stiffness of a material, i.e. how much it stretches elastically under a given stress, under a single static load; it is unrelated to how many repeated cycles the material can withstand before fatigue cracking initiates.
4. Ultimate tensile strength: This is the maximum stress a material can withstand in a single, monotonically increasing static tensile test; components subjected to cyclic loading typically fail by fatigue at stresses far below this value, so ultimate tensile strength does not by itself indicate fatigue resistance.

Since the S-N curve's flat asymptote (the stress level for indefinite cyclic life) is what defines resistance to repeated loading, the correct measure is the endurance limit.

Therefore, the correct answer is endurance limit.