Question - Describe How A Fatigue Crack May Be Initiated ?
Answer -
The initiation site will be where the local stress is high enough to increase the minute cracks which occur on the metal surface. The stress can be increased locally by a surface defect, or even an extreme stress concentration caused by high applied stress.
The main causes of fatigue cracks are:
Stress raisers:
These can be reduced by ensuring a smooth surface finish to all area where high stress is applied especially in the web/pin radii area
Oil holes:
These should be minimised whenever possible, and the oil hole opening have wide and smooth radii
Tensile stresses:
Fatigue strength is reduced when tensile stress are present, so the radii areas are often cold rolled to ensure that the fatigue strength in these areas are increased.
Stress applied on hardened materials:
Fatigue cracks can grow faster when the material is harder, as the dislocations in the metal concentrate the stress on a smaller area of the material structure, hence any hardening of the crank pins must not be applied to the highly stressed radii areas.