Widespread fatigue damage (WFD) in a structure is characterised by the simultaneous presence of cracks at multiple points, that are of sufficient size and density such that, the structure will no longer meet its damage tolerance requirement and could fail. For example, small fatigue cracks developed along a row of fastener holes coalesce, this moves to adjacent sites and propagates. The objective of a designer is to determine when large numbers of small cracks could degrade the joint strength to an unacceptable level.
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Categories of WFD
Several factors can influence the occurrence of WFD, like Design issues and Probabilistic parameters like manufacturing, environment etc. Two categories of WFD are:
Multi-Site Damage (MSD)
MSD is the simultaneous presence of fatigue cracks in the same structural element.
Multi-Element Damage (MED)
MED is the simultaneous presence of fatigue cracks in similar adjacent structural elements.
Difficulty in determining WFD occurrence
Main difficulties involved are:
Rule to predict the occurrence of WFD
First a parameter called Limits Of Validity (LOV) is defined. LOV is defined as: “the period of time (in flight cycles, hours or both) up to which WFD will not occur in aeroplane structure.”
The steps followed are: