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

Fatigue is progressive damage caused by many cycles of fluctuating stress, so the property that measures resistance to it must come from cyclic (not static) test data.
Elastic limit and Young's modulus are both determined from a single static loading test and don't reflect how a material behaves after repeated cycling.
Ultimate tensile strength is also a static, one-time-loading property; many parts fail by fatigue at stresses well below this value.
The endurance limit, in contrast, is obtained directly from cyclic (S-N) testing and represents the stress level a material can withstand for an effectively unlimited number of cycles, making it the correct measure of fatigue resistance.
So the matching term is endurance limit.

Answer 2

Consider a steel component with an ultimate tensile strength of, say, 600 MPa. Under a single static pull test it would survive stresses up to that value without breaking. But if the same component is subjected to a fluctuating stress of only 250 MPa repeated millions of times, it can still crack and fail, well below its static strength, purely because of the cyclic nature of the loading. This shows static strength values cannot, by themselves, predict fatigue life. Testing each option against this example:

1. Elastic limit: Like the ultimate strength in the example, this is measured from a single loading event and gives no information about how many cycles at a lower stress the part can survive.
2. Endurance limit: This is precisely the stress level (found from repeated-cycle testing) below which the same component in the example could be cycled indefinitely without fatigue failure, directly answering how much repeated stress the material can take.
3. Young's modulus: This only describes how much the material stretches per unit stress; it says nothing about how many cycles of loading the part can endure before cracking.
4. Ultimate tensile strength: As illustrated, a part can fail by fatigue at stresses much lower than this value, showing this static property overstates the safe cyclic stress level.

The example confirms that resistance to fatigue must be judged from cyclic-test data, which is exactly what the endurance limit provides.

Therefore, the correct answer is endurance limit.