Bicycle axle material change, Uni project!

[matt_matthew]

Hi mates, I need some help in a project. In short, I have a part of a mechanical assembly, and this part broke due to material failure. I need to propose a solution only by changing the material, not the desing (at least, the main focus have to be only change the metal...).
This is an axle from a BMX jump bike, it failed on me suddenly. Actually, I didn't realize at the moment that it broke into 2 pieces. A friend of mine asked for the bike and I gave it to him, he jumped in and asked what's wrong with the front hub since it was like "loose"... The nuts were in place, all seemed fine but it really had a little gap. When we were loosening the nut, we realized that we had two separate pieces of the axle, lol.
In the link below are some amplified images of the axles and the fractures from the two perspectives. I noticed that the failure started right in the last thread of the axle, meaning that it had stress concentration caused by the thread cavity. What information can you deduce when looking at the fracture? The axle is made of Chromoly (I think it's 4130 or 4140 Cr-Mo, correct me if I'm wrong). The diameter (almost uniform along the axle) is 3/8" of an inch (around 0.95 mm). The total lenght of the axle is around 16.25 mm = a little bit more than 5/8 inch.
I will attach a diagram to show up what's the situation. The words are "gancho"=dropout, where the fork of the bike sits the axle. "Apoyo rodamiento" = where the bearing seats on the axle. The distance (c to c) of the two sites is 18.5 mm = almost 3/4 inch, so I assume that there's some bending involved due to the separation, and it's not only shear stress. There's also a lot of fatigue because of the abusive nature of BMX tricks (mostly street BMX tricks, when you ussually land tricks with your bike from more than 5 ft).
Looking at the image where the 2 pieces are, we can see the threaded piece and at left there's the another one, the mirror-like polished site right there is where the bearing sits (it's a sealed catridge bearing). The fracture occured closer to the bearing seat, on the first thread like I said before (stress concentration?)...


parte_roscada_3.jpeg

Thank you in advance for any reply!
Matt.-

[Kelly_Bramble]

Looks like a fatigue crack turned failure over repeated applied stress. The root of the thread is a classic failure point -assuming the design and installation did not have shear force on the thread area you should review torque requirements and installation practices on installation.

[matt_matthew]

Thank you for your answer, Kelly!

I don't think that the installation was the problem, since the axle simply sits on the fork dropout and only a nut is adjusted (with low torque) to hold things in place.

And yeah, the root of the thread was obviously the starting point.

But I can't find info on this particular type of fracture, I only found that it's very similar to a rotating-bending fracture under low nominal stress but with severe stress concentration. Also, the fractures of "tension-tension or tension-compression" under the same stress conditions seems very similar. I'll attach the diagram where I found it.



Thanks guys!