Fracture surface of 7075 T6

[will123123]

I would be interested to hear comments/ thoughts/ opinions on the attached images of a fracture surface of a 7075 T6 component. The part was machined from a piece of plate and failed in the short transverse direction. It appears to have cleaved at the grain boundaries; I am confident the material has been not been subject to anything that would cause SCC etc, but I cannot see typical features of an overload such as MVC. Other images will follow

 

[will123123]

SEM image

 

[will123123]

Higher mag

 

[will123123]

Crack before opening

 

[Tmoose]

What was stuffed into that threaded hole?
What were the circumstances of the failure? That is, I assume the failure occured when a fastener etc was in the threaded hole. Did the part crack during assembly, or "in service?"
How was the thread generated ?
It appears to be threaded darned close to the end. Even bottoming taps need a few threads to run-out. And fasteners should only engage 100% fully formed threads.

 

[Compositepro]

It appears that the screw was tightened into an oil-filled hole and hydraulic pressure caused the fracture. Some of the threads are damaged/deformed as the screw moved after the fracture.

 

[will123123]

Cracking was not noticed during or immediately after installation of the stud, but was discovered after a short in-service period. It is a cut thread. I am more interested in the appears of the fracture surface shown in the SEM images - it is not typical of ductile overload.

 

[WKTaylor]

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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