Widespread fatigue damage


Widespread fatigue damage 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. 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. The in-flight loss of part of the fuselage from Aloha Airlines Flight 243 was attributed to multi-site fatigue damage.

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:
  • Cracks associated with MSD and MED are so small initially that they cannot be detected with existing inspection methods.
  • Fatigue cracks related to WFD grow rapidly. Therefore operators are not able to detect the cracks before they cause structural failure.

Rule to predict the occurrence of WFD

First, a parameter called Limits Of Validity is defined. LOV is defined as “the period of time up to which WFD will not occur in aeroplane structure.”
The steps followed are:
  • Evaluation of structural configurations and determination of LOV based on fatigue test evidence.
  • Provide warnings to preclude the development of WFD up to LOV.
  • Adopt LOV values as a criterion to determine the life of aeroplane.
  • Stop the operation of aeroplanes when LOV is reached.