3 ms·

If a part's made of aluminum and undergoes cyclic stress, even staying within its instantaneous modulus of elasticity, it will eventually crack. Note the alumi

If a part's made of aluminum and undergoes cyclic stress, even staying within its instantaneous modulus of elasticity, it will eventually crack. Note the aluminum curve in the graph here [0], it never levels off horizontally. My understanding is this effectively means the modulus of elasticity for aluminum parts is forever diminishing. Aluminum is a metal that's "work hardened", it's basically becoming increasingly brittle the more cyclic stress it experiences. I'd been told in the past that commercial planes are regularly X-rayed for cracks as part of their maintenance, because of their extensive use of aluminum. None of this should be a surprise except that it's supposedly "unexpected" in the pickle fork. These vehicles have always been high maintenance machines, in order for them to be safe all the structural aluminum must be regularly inspected for cracks, the cracks should be expected and parts replaced as needed. [0] https://en.wikipedia.org/wiki/Fatigue_limit https://en.wikipedia.org/wiki/Fatigue_limit Edit: Out of curiosity I did some digging for photos of this part and more information, this thread delivers some insightful links and citations: https://www.pprune.org/rumours-news/625886-737ngs-have-cracked-pickle-forks-after-finding-several-jets.html https://www.pprune.org/rumours-news/625886-737ngs-have-crack...

dredmorbius 7y ago
AFAIU, ultrasound is far more typically used than x-rays, as the acoustic properties of fractured castings are more distinctive than their xradiographs. Apparently induction testing is also used (likely on a similar basis to ultrasound: properties deviate sharply from norm/spec).
corbani 7y ago
The modulus of elasticity of aluminum does not change with cycling (or basically with anything else but big temperature differences). Fatigue is a damage accumulation process, that does not show in the most obvious properties, like modulus or strength. The aviation regulations changed a long time ago from requiring to avoid cracks to requiring to have structures that can operate with cracks long enough to for them to be detected, as is exactly the case here. Of course nobody design planes to crack, but the regulations reflect the realization that completely avoiding cracks is impossible ...

dredmorbius 7y ago

AFAIU, ultrasound is far more typically used than x-rays, as the acoustic properties of fractured castings are more distinctive than their xradiographs. Apparently induction testing is also used (likely on a similar basis to ultrasound: properties deviate sharply from norm/spec).

corbani 7y ago

The modulus of elasticity of aluminum does not change with cycling (or basically with anything else but big temperature differences). Fatigue is a damage accumulation process, that does not show in the most obvious properties, like modulus or strength. The aviation regulations changed a long time ago from requiring to avoid cracks to requiring to have structures that can operate with cracks long enough to for them to be detected, as is exactly the case here. Of course nobody design planes to crack, but the regulations reflect the realization that completely avoiding cracks is impossible ...