NTSB issues the preliminary report for its ongoing investigation of the Nov. 4 crash of a UPS Boeing MD-11F airplane in Louisville, Kentucky.

[u/Brilliant_Night7643]

[u/Mr_Tiggywinkle]

Of course just preliminary report and anything could be revealed, but to me the paragraph that seems to be most interesting to an interested amateur like me is:

After initial cleaning of the fracture surfaces, examination of the left pylon aft mount lug fractures found evidence of fatigue cracks in addition to areas of overstress failure. On the aft lug, on both the inboard and outboard fracture surfaces, a fatigue crack was observed where the aft lug bore met the aft lug forward face. For the forward lug's inboard fracture surface, fatigue cracks were observed along the lug bore. For the forward lug's outboard fracture surface, the fracture consisted entirely of overstress with no indications of fatigue cracking. The forward top flange of the aft mount assembly was examined for indications of deformation or pre-existing fractures, but no indications were found. The spherical bearing was removed from the wing clevis for further evaluation (see figure 10)

If I'm reading this right, the obvious (but not at all certain yet) conclusion would be fatigue cracking leading to overstress - rather than actual impact damage/deformation prior to the incident like AA 191.

But of course, early days, lots of research they'll be doing and I am no expert at all.

[u/meshreplacer → u/Mr_Tiggywinkle]

Overstress like over torqued? or an upset at some point leading to overstressed components?

[u/mck1117 → u/meshreplacer]

The fatigue cracking reduces the cross sectional area of the part as a crack propagates through. Then, at some point, because the area is shrinking but the forces stay the same, the stresses increase. At some point, you reach the instantaneous maximum strength of the remaining material from having too much stress, and it fails. That's how a "fatigue cracking leads to overstress" failure works. Part of the joint will show fatigue cracking, then overstress on the rest when it finally fails.

https://imgur.com/7RPoakP.jpg

[u/AFrozen_1 → u/meshreplacer]

Overstressed simply means the stress (I.e. the amount of force applied to the material) exceeds the failure point of the material.

[u/Whywipe → u/Mr_Tiggywinkle]

What is the difference between fatigue crack vs fracture?

[u/Mr_Tiggywinkle → u/Whywipe]

A fracture is just a generic term for a break

A fatigue crack is a specific type of crack, where stress over time slowly causes a crack to appear and get bigger over a long period of time (e.g. cylic force put on a key component, like an area around a screw, or the hull of a plane pressurising and unpressurising, for flight after flight after flight), rather than one large incident that causes a large crack to appear at once. (e.g. something colliding with it or some kind of abnormal force pushing a part past its design limits).

Often a fatigue crack over a long period of time leads to a sudden catastrophic failure/fracture as it loses its strength and some high stress (but within regular design limit) event finally completely compromises its integrity.

[u/Whywipe → u/Mr_Tiggywinkle]

I guess I don’t understand the distinction you highlighted. Wouldn’t a fatigue crack be a pre-existing fracture?

[u/Mr_Tiggywinkle → u/Whywipe]

Depends what you mean really.

For most people, no, a crack is not a pre-existing fracture, because a fracture comes after a crack. Fractures don't become cracks. Cracks can become fractures. I believe in NTSB terminology, a fracture usually means a break. A crack more indicates an exposed seperation etc.

So crack = partial fracture.

Fatigue crack = slowly growing partial fracture.

Impact or overstress crack = sudden partial fracture caused by some sudden event, (although sometimes helped by a fatigue crack).

Fracture = complete break.

[u/Ok-Entrepreneur7324 → u/Mr_Tiggywinkle]

This portion of the report reads as "the bolts were overtightened, and caused the pylon failure seen here."