Widespread fatigue damage
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Widespread fatigue damage (WFD) in a structure is characterised by the simultaneous presence of fatigue cracks at multiple points that are of sufficient size and density that while individually they may be acceptable, link-up of the cracks could suddenly occur and the structure could fail.[1] For example, small fatigue cracks developing along a row of fastener holes can coalesce increasing the stress on adjacent cracked sites increasing the rate of growth of those cracks. The objective of a designer is to determine when large numbers of small cracks could degrade the joint strength to an unacceptable level.[2] The in-flight loss of part of the fuselage from Aloha Airlines Flight 243 was attributed to multi-site fatigue damage.
Multi-Site Damage (MSD)
Several factors can influence the occurrence of WFD, like Design issues and Probabilistic parameters like manufacturing, environment etc. Two categories of WFD are:
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.