Engineering:Widespread fatigue damage
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.
References
- ↑ 1.0 1.1 Hoggard, Amos W.; Johnson, Stephen R.. "Understanding the new Widespread fatigue damage rule". Boeing. http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/2/.
- ↑ "Federal Register, Aging aeroplane program"
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