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MattMatthew

Bicycle axle material

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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.

Here 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?)...

 

Images at https://filebin.net/6j8yeglbvyylqgwg

 

Thank you in advance for any reply!

Matt.-

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That's a near perfect example of fatigue stress crack propagation.

Basically, the repeated impact loading from the jumping has caused a crack to propagate from the thread extend to the point that the material has suffered catastrophic failure. the "tree-ring" appearance  on the RHS of the image shows this...but that said, the vertical cracking is a little "odd" and out of place...it's not a simple as it first appears.

Whilst it is difficult to be sure with out a full analysis, I would suggest that this is a minor manufacturing defect that has been the propagation source, and that this is unlikely to repeat, but again...

Basically, if properly preloaded, the direct load on the axle is minimal, and you should get a cup/cone fracture if any....ie a failure in tension. This is a stress propagation failure, suggesting that one of two possibilities...
1: there was a minor flaw which was the source of a fatigue stress crack and ultimately the failure.
2: the bolt was not suitably preloaded, allowing the stress to act in a non-design way.
Thinking through....I think its a combination of both 1 and 2.

If preloaded, the "cones" (or the bearings - subject to the design) take the loading by friction on the load-faces and the axle, only tension. The image sort of suggests (and again there is insufficient information to be 100% sure) that the loading has been a flexing one for a significant period of time. The other failure mode would be shear...which this sort of indicates...but again this would be down to a bolt not tight enough as the friction of the load-faces should take all of the loading thus eliminating the shear.

I'd suggest just getting hold of a new standard axle and tighten the bolts properly...and if jumping regularly...checking it is tight regularly!

All the best

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