I agree that it is a shame that good in-focus close-ups of the bolt faces are missing. But there are some interesting clues in one photo

Notice the "flat" part of the crack. This suggests a very flat fatigue crack (no 1 arrow) that then resulted in a final overlaod failure (no 4 arrow). There are also suggestions in arrow no 2 and arrow no 3 that there were other fatigue cracks that started but did not progess as far as the "primary" fatigue crack (arrow no. 1).

This is all speculation since the evidence is obscured.

Ask me how I know - oh yeah, fatigue failures of my 2006 Porsche Caymen connecting rod bolts which destroyed my engine (and many others had similar failures which are frequently but usually wrongly attributed to oil starvation), which is why I am running a turbo Miata now. BTW, in 2009 Porsche increased the size of the connecting rod bolts. I wonder why?

 

It looks like the hex holes in the studs got stripped out on some, those shouldn’t be seeing enough torque to start stripping them either.

 

According to OP, the engine never ran, which makes fatigue very unlikely since even an under-tensioned stud would see only a hundred or so cycles.

 

I missed that part. You are correct. Still wish there were better close-up photos of the fracture faces.

 

Seems to be a lot of brown. Looks like corrosion, mixed with copper spray gasket and maybe some never seize? My guess is the issue had something to do with the engine sitting so long. Or like others have said returned knockoffs.

 

question to OP :
What did ARP say after you sent them to them for examination ?

 

Still waiting on reply.

 

I still have not heard back from ARP but here are some better pics.

 

Thanks for posting the additional photos.

The dull matte gray region in the annotated photo is the final overload failure. Note that this is only about ½ of the original cross-section area.

The flat portion (“original crack”) is slightly discolored due to some oxidation which indicates that this portion of the crack has been there for some time (but cannot estimate how long). Although there photos are reasonable quality, there is not enough close-up detail to provide a good evaluation (I realize that you had to reduce to photo file size for the post). I cannot see any evidence of “beachmarks”, which are definitive evidence of fatigue cracking. However, such beachmarks may sometimes only be visible under high magnification. The fact that these bolts are deemed to be new would completely rule out fatigue.

Although it could be just a surface scratch, there appears to be evidence of another existing “original” crack in the bolt just below the failure, presumably much smaller than the “original” crack at the failure location. Since you do not have the bolts anymore, this cannot be checked by you.

There is a small chance that the original cracks are quench cracks. A more complete metallurgical analysis would be needed to identify this.

My preliminary guess is the bolts were cracked before you installed them. Since they were new to you, how and when they were originally cracked is a mystery.

 

ARP called me today and asked more question and said they think it was "Hydrogen embrittlement" but were not sure yet as they have not begun metallurgy testing on the bolts. They will contact me as soon as testing is done.

I'll save you the Google:

Hydrogen embrittlement is a metal's loss of ductility and reduction of load bearing capability due to the absorption of hydrogen atoms or molecules by the metal. The result of hydrogen embrittlement is that components crack and fracture at stresses less than the yield strength of the metal.

https://en.wikipedia.org/wiki/Hydrogen_embrittlement

 

Today I learned that you should not use ARP head studs to hold together the pressure vessel of your nuclear reactor, as gaseous hydrogen tends to be present in the primary-loop cooling water. https://www.researchgate.net/publica..._Power_Systems

 

As far as I'm aware that's typically a much bigger problem for welding than just heat treatments. I doubt these are quenched in water (or if they are, I have bigger questions).

That said, as soon as you start touching/handling the fracture surface of the part it's rather difficult to get an exact determination of what happened. I imagine ARP is just going to test hardness and maybe do a quick stress/strain test with the non-fractured portion of the bolt to compare to specs. Ideally you'd have a unmolested fracture surface and toss it in the SEM to get a better look at things before etching, measuring grain size/distribution with some micrographs, and alloy composition with a mass spec.

 

Certain platings must be followed quickly (hours) by a low temp bake to remove the hydrogen and prevent embrittlement.

 

Yeah absolutely, I imagine any type of electroplating would need that step. Maybe electroless nickel plating too depending on the specific process.

 

Did ARP ever get back with results of their testing??