Hydrogen embrittlement seems a likely candidate given the described failure mode.
Here is the Wiki on that:
Hydrogen embrittlement - Wikipedia, the free encyclopedia
I can remember some large spacer washers on the McDonnell F-4 that kept the stabilator centered on the fuselage. These things were way overdesigned to look at them, but they were failing in service because they didn't have the hydrogen driven out of the metal following plating operations. (Improper process specification) You could tell when one failed because the stabilator would then scar the side of the fuselage as it moved. Those massive washers behaved as if they were made of glass!