Widespread fatigue damage explained

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.

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 (LOV) is defined.^[1] 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:

• 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.

Notes and References

1. Web site: Understanding the new Widespread fatigue damage rule . Amos W. . Hoggard . Stephen R. . Johnson . Boeing . 30 July 2019.
2. "Federal Register, Aging aeroplane program"