2: Fatigue Material Properties
Fatigue tests can provide us with a number of key properties of the material under investigation.
Fatigue Life – The test demonstrates the number of cycles that the material will survive in service with a given cyclic stress.
Fatigue Strength – If we only require a material to last for a certain number of cycles then the maximum stress can be determined under which the material will not prematurely fail.
Fatigue Limit – The stress can be determined for any given material for which failure never occurs. Determination of this maximum stress is the preferred option for many applications as the material will therefore never require replacement.
Fatigue Ratio – Fatigue resistance is largely influenced by the tensile strength of the material at its surface. The ratio of fatigue limit to tensile strength is known as the fatigue ratio and is given by:
An increase in the tensile strength at the surface results in an increase in the fatigue resistance.
Notch Sensitivity – Failure by fatigue occurs through the growth of cracks and defects. A fatigue test can determine the susceptibility of a material to fatigue from defects already present on the material surface.