Tuesday, September 2, 2008

Hotspot Method For Bicycle Fatigue Life

Some of the most critical areas of any metal alloy bicycle are its joints, where tubing is welded using standard industry procedures. Everyone talks highly of new and improved materials used in building a bike, but if the joints are weak, sorry, you've got a poor bicycle!

So then the interesting question is, how do you define the fatigue life of a welded joint? Modern FEM analysis techniques are used for this purpose but adequate modeling of the welded joint has not yet been mastered.

One of the approaches to determining such stresses is on the basis of strictly local stresses determined in the potential point of crack initiation. This is called Hotspot stress method, and was first introduced for the fatigue analysis of steel offshore structures. It is recommended for situations where strains can be measured using instrumentation or estimated using FEM techniques near the welded joint.

You can delve into some basic reading on the Hot Spot Finite Element Analysis here [Fatigue Design of Aluminum Components And Structures]

Make sure you also check out the VelUS research team's work in this area. They are a group of Canadian/French researchers aiming to study and adapt Hotspot methods to bicycles to get better estimates of fatigue life in frames and components. In doing this, they have also partnered with industry names in bicycles such as the Canadian Cycles Devinci.

Stain gauges used to measure HotSpot strain measurement. These tubes have been tested and cracks can be noticed near the root of the weld [Source]

Chain of Strain Gauges to measure strain gradient near the root of a weld [Source]


Chris V. said...

It's always cool to see how much engineering work goes into different products, being an ME myself. It reminds me of some crack propagation calculations I saw for jet engine parts when was working for Pratt & Whitney as a contractor a couple of years back when I was just out of school. It's the same stuff that keeps oil rigs running and jet engines flying that keeps your bike from suddenly fracturing under you.

I only found your blog recently, but I really enjoy reading it - especially the technical stuff. Keep up the good work!

fish said...

Great post!! Clearly something to look into to avoid injury.