Fatigue Test - Destructive test

[Peyman 16150]

Introduction to Fatigue:-

A metal part subjected to repetitive fluctuating stresses will fail at a stress much lower than that required to cause fracture on a single application of load.
The resistance of metal to fatigue failure is called “Fatigue strength” or “Endurance Strength”.


Fatigue test:-

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Fatigue test determines the resistance of a metal to repeated loads. This test is made on cylindrical specimen (like round tensile test) by subjecting it to bending loads during their rotation. This develops reversing stresses (.i.e. tension and compression).
Fatigue testing machine consist of the following.
- Electric motor capable of running 10,000 r.p.m.
- Large bearing whose purpose is to release the motor of large bending movement which is applied to specimen.
- Collets (.i.e. Coupling) to hold the specimen.
- A rotating lever arm, subjected to down ward force in order to, place specimen in state of bending.
- The upper surface of specimen remains in tension while its lower surface will be in compression.
- The maximum value of the (tensile or compressive) stress is usually lower than the yield strength of material. He test is continued .i.e. the cycle of stress is applied until specimen fracture.


Importance of Fatigue Test:-

Fatigue is important factor in servicing of low temperature pressure vessels, bridges, ships, pipe lines, shafts and axels etc. For durable service life.