will...
1. RE the cross-section photo...
There is one probable fatigue crack initiation site in the upper-most thread on the 'right-side' [photo] of the tapped-hole [thread just below the surface].
The mangled upper ~3 threads 'suggest' some initial cross-threading by the much harder steel-stud thread.
NOTE
a. A fine-thread helical-coil provides benefits relative to preventing cross threading tapped-aluminum holes in this situation.
2. What is Your plate material spec and EXACT temper [-T6, -T651, etc] designation?
NOTES.
b. Thick 7075-T6 plate is highly susceptible to SCC especially in the grain-wise direction thru thickness. Plate -T651 is slightly better in this regard.
c. From Your photos, plate grain' orientation is not obvious nor marked... but I suspect that the crack [initiation, at least] is oriented along the grain.
d. In general 7075-T7351 plate would have been a better choice for this application.
e. A die-forging or hand-forged-blank, 7075-T73 or -T7352, would have been a 'best choice' for a higher durability assembly... although there are even better alloys available, now. Most high durability aluminum alloy aerospace fluid fittings are [now] made from die-forged 7073-T73 [DF 2014-T6 is obsolete]
3. What is/are the stud and tapped-hole thread sizes/specifications. Where they carefully inspected to validate 'spec' mating. Were the tapped-threads fluorescent dye-penetrant inspected to ensure freedom from pre-existing tears or fractures due to the tapping process?
4. Compositepro also made a very good observation: tightening any male thread part into blind female tapped-threads... especially with any incompressible fluids such as lubricating oil or 'wet' sealant or retaining compound... and with a very small end-cavity... creates a trapped pocket which can generate very high static 'pressure' forces. This is one reason tapered pipe threads are useful.
NOTE.
f. A narrow/radiused groove cut along the stud threads [ axially or in a spiral]... has been traditionally used to allow pressure/fluid venting. Another alternative is to have a stud with an axial hole for pressure venting... or to have a pilot-hole 'thru the end of the cavity'... that is subsequently plugged for environmental sealing.
5. One thing to consider for a stud install like this... is 'loading'. From a 'photo thumb-check', thread engagement appears to be ~1.7-Thread Dia... Which might not be adequate for substantial stud bending in plate... especially if loading is cyclically coincident with grain orientation.
Regards, Wil Taylor
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