StressGuy...
NOTE.
The accident report cited 'fatigue cracking' of the 4330 Steel Fuse Pin... NOT stress corrosion cracking [SCC].
The Dutch National Aerospace Laboratory was contracted to carry
out a metallurgic investigation of the fuse pin . The results of this
investigation are contained in report CR 93030 C: "Investigation of the
Outboard Midspar Fuse Pin from the Pylon of Engine #3 of El A l
1862" . The laboratory concluded the following:
1. A large fatigue crack was present at the outboard location of
minimum wall thickness of the fuse pin, which was of the "
bottle bore " configuration . This fatigue crack was up to 4
millimetre in depth and encompassed about 50% of the
inside circumference.
2. The fatigue crack had developed from multiple initiation
sites along poor quality machining grooves. There was no
evidence of corrosion pitting that could have contributed to
fatigue initiation.
3. The material of the fuse pin met the chemistry specification
for 4330 M steel. However, hardness measurements indicated
that the tensile strength was about 117 KSI, which is
lower than the specified range of 126- 139 KSI.
It should be noted that a low hardness does not mean the pin was
understrength, because sometimes the final machine cut is adjusted
based on the testing conducted in the sampling process.
Boeing also carried out a metallurgic investigation of the fuse pin.
The Boeing findings concur with the NLR findings.
Boeing was able to derive a crack growth curve of the fatigue fracture
surface as a function of total airplane cycles (flight cycles) versus
crack depth.
Based on this curve Boeing concludes that at the last inspection of
the fuse pin, 257 flights before the accident flight, the fatigue crack
would have had a depth of .14 inch. As the ultrasonic reference depth
is .085 inch a detectable crack existed at the last inspection.
NOTE.
15-5PH CRES HTxxx is used on the Boeing **-**** re-engine strut Fuse Pins. Similar design and manufacturing methodology as cited in the accident report. This aircraft Fuse Pins have deformed as the result of 'hard-landings' [a FEW very rare cases].... but has NEVER sheared due to overload or fatigue-cracking.
NOTE.
As I understand it, the 'giant aircraft manufacturer's' philosophy for pylon-engine attachment has changed... in-part due to this accident.
In-lieu-of allowing the pylon to fail benignly so the pylon-engine 'separates safely'.... pylons are now designed to resist failure and RETAIN the pylon-engine under foreseeable catastrophic circumstances. This is a real challenge with the monster fan engines... but is considered safer than allowing separation.
Regards, Wil Taylor
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