RE: [CR]Never seen this happen before!


From: Mark Bulgier <mark@bulgier.net>
To: classicrendezvous@bikelist.org
Subject: RE: [CR]Never seen this happen before!
Date: Thu, 20 Mar 2003 09:53:14 -0800


Ray Homiski wrote:
> I have seen a lot of damaged bikes in my time. Some I could
> explain while others I could not. This one falls into the
> latter category. Guess it was just a bad brazing job. Look
> at the photo that says "Frame Crack"
> http://photos.yahoo.com/bc/planethouse/lst?.dir=/Benotto&.view=t

Looks like a classic fatigue crack to me. Though bad brazing could definitely be a contributing factor, it is possible for a well-brazed bike to crack that way too if it is ridden by a strong or heavy high-mileage rider, especially if the tubing is light. The crack in the photo is actually in one of the more common locations for bikes to break, in my experience.

Boring technical stuff follows: Many of you have heard of the "endurance limit", a property of steel and some other structural materials; a threshold stress level below which the part will theoretically never fatigue, even with infinite use. The problem is, stress levels vary though the part in use, some locations seeing higher stress levels. Those locations are often called "stress risers". An abrupt change in the thickness of the part, such as at the edge of the BB shell in the picture, is a classic stress riser.

Even if the average stress on the part is always below the endurance limit, often some stress riser exists that is seeing enough cycles above the endurance limit to fatigue. And once a crack starts, it will almost always propagate, because the tip of a crack is the perfect stress riser.

Mark Bulgier
Seattle, Wa
USA