Mechanical Engineering Ch.4: Fatigue Failure and Design Against Fatigue

In dynamic loads, the stress intensity changes constantly but the direction of the stress remains constant.

Components subjected to dynamic loads experience cyclic or random stress.

The strength parameters determined from tensile, compression, and bending tests are useful for material selection and component dimensioning for dynamically loaded components.

Failure in application occurs when the applied load on the material causes permanent deformation.

Fatigue failure occurs when the intensity of the stress increases slowly but steadily until the load finally fractures the specimen.

The S–N Curve is a property obtained from the fatigue test.

Fatigue failure occurs only when the material is subjected to loads higher than the tensile or yield strength for a static load.

Fatigue failure is more likely to occur after a lengthy period of repeated stress or strain cycling.

The strength limits to be met are higher under dynamic load than under static load.

For fatigue to occur, the stress in the material must be compressive in nature.

Fatigue failure is ductile and allows for significant plastic deformation before failure occurs.

Fatigue failure commonly occurs in stationary structures such as buildings.

Cyclic stresses account for only one fluctuating stress-time mode that leads to fatigue failures.

A reversed (pure alternating) stress cycle involves stress that does not alternate from a maximum tensile stress to a minimum compressive stress.

Repeated (alternating) stress cycle always involves asymmetric maximum and minimum stresses.

Fatigue failure can occur in biomedical implants and consumer products that experience repetitive stresses below the yield strength of the material.