Tuesday, November 15, 2011

Metal Fatigue, Cracks, and Turbo Mallards

Fatigue Failure with attempted repair

 Metal Fatigue occurs when the metal is subjected to repeated or alternating stresses below the  material's static yield strength. Fatigue failure occurs BELOW the material's ultimate tensile strength. Parts that are exposed to alternating stress cycles, such as engine crankshafts, may break even though they were never stressed near their ultimate strength. How does one know if a part is close to fatigue failure?

Fractured aircraft engine crankshaft. Beech Marks are a sign that a crack progressed across the part and failure was due to metal fatigue (red arrow). The white arrow shows the crack initiation point.

Fatigue life is determined by the number and magnitude of the stress cycles. Fatigue cannot be inspected -- unless you know the past history of a part, there is no method of determining how many stress cycles and therefore how close to fatigue failure the part is at.

Aircraft components that are described as being "zero timed", "like new", or "restored" are marketing terms that do not describe the remaining fatigue strength. That is the challenge of aging aircraft and purchasing critical stressed components where the past history is not known. 

Engineering Critical Assessment:
Without knowing the past loading history, the only method of evaluating the failure potential by fatigue is through an engineering critical assessment using Damage Tolerance methods such as fracture toughness, allowable flaw size;  and through this process inspecting for existing flaws and calculating the tolerable flaw size for the projected future loading spectrum. What this means to the mechanic is that maintenance cannot prevent fatigue failure, cannot inspect for fatigue failure, nor determine airworthiness from a metal fatigue basis without something to inspect; and without an Engineering Critical Assessment there is nothing to inspect. 

This is the issue that the mechanics of N2969, the Turbo Mallard who's wing broke off  killing all 20 people aboard. An old airplane with skin cracks -- where is the point of failure;  it could be the moment a crack is formed, or it could be a defined crack length based on an appropriate fracture mechanics analysis and following applicable codes.  It is safe to say that all aircraft have cracks and that not all cracks are a point of failure; in each case what is appropriate--replacement or repair?  Without unambiguous maintenance standards based on engineering analysis, fatigue failures will continue regardless of the intensity or quality of maintenance.

"Old designs are never proven for fatigue simply by virture of their longevity. Fatigue is wear-out. There is no guarantee that future failures will be confined to those seen in the past." Steve Swift, GNATS AND CAMELS - 30 Years of Regulating Structural Fatigue in Light Aircraft"

Comet 1 SN Diagram Animation

Additional Reading

Cracks in Aircraft Structures

Why Things Break - Getting to know Fracture Energy

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