Failure Analysis

MH20

Well-Known Member
Posts
358
Reaction
4
This is my brother's moutain bike crank. He broke it today after doing a 3ft drop off a ledge. What do you guys think of the way it broke and what the metal looks like inside of it. It was made out of T6 6061Cold Forged Aluminum. Is this a material flaw or just pure abuse?

1993 Ford Ranger Supercab 4.0L 2WD
 

Attachments

Kritter

Krittro Campbell
Posts
10,914
Reaction
296
If you look at the bottom quarter or so of the crack it looks different then the rest of the crack (or at least on my crappy monitor) so it was probablky cracked befor ehe jumped and then when he jumped it could no longer hold togetther and broke. The crack propogated enough to exceed the fracture toughness of the material. Fracture toughness is proportional to the root square of the crack length. For aluminum to hold on as well as it appeared to...I would say he was lucky for it only to break on a 3 footer. It also looks as if there is a sharp radius AKA stress riser where the crack started at. I am sure Dave G can tell you if the process is a good one or not.

Kris
<A target="_blank" HREF=http://www.dmsrace.com>www.dmsrace.com</A>
"Jesus loves you, everybody else thinks you're an A-hole"
 

Dave_G

Well-Known Member
Posts
2,859
Reaction
307
Well, without seeing the parts in person it's kind of hard to tell by the photos. But in one pic I can see some discoloration in the upper part of the photo that may indicate that the part was cracked for a period of time before it failed. It may also be just a simple case of exceeding the ultimate tensil strength of the material.

Dave

"I know it all, but I can't remember most of it..."
 

MH20

Well-Known Member
Posts
358
Reaction
4
Thanks guys, I'll take some better pics tomorrow when its brighter and I don't have to use the flash.



1993 Ford Ranger Supercab 4.0L 2WD
 

ntsqd

Well-Known Member
Posts
2,429
Reaction
27
I agree with the analysis posted so far. I always assume anything made from aluminum has a very set lifespan. Which is why I ride steel frames. That, or a I'm a Retrogrouch.
If he's Freeriding, DSing, or DHing those cranks, it was a forgone conclusion that they'd fail fairly quickly. 3 ft drops make me think he is. For that kind of riding he really needs a crank designed for it. Something with more cross section to the arm like a RaceFace Turbine LP or NorthShore/DH, or the DH/Freeride TruVativ crank offerings.

TS

"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages
 
Top