Anatomy of a Failed Bicycle Spoke
The other day on the way to work I herd a "tink" from the front wheel sound as I came to a stop. I looked down and there was a spoke hanging off the hub. I got off the bike, removed the spoke then put it in my back pack. I loosened the brake adjuster a bit so the wheel would turn and continued to work.
When I got to work I had a look at the broken end and noticed that it had the classic marks of a brittle fatigue fracture. Not wanting to miss the chance to document a failure I took a couple of pictures. The wheel had over 7000 miles on it when the spoke. Using a circumference of 210cm, yields 5.7 million cycles on the spoke. This is plenty of cycles to cause a fatigue failure. This type of failure typically takes place in three phases.
- Initial crack initiation - This usually happens at some sort of stress riser. In this case it was the root of the thread.
- Progressive crack growth across the part - The crack will progress a tiny bit with each cycle perpendicular to the load direction. This leaves little marks on the fracture surface.
- Final sudden fracture of the remaining cross section - As the crack progresses the remaining bit of material has to bear the weight of the load. At some point the cross section is too small to take it and "tink" the part fails. This part of the fracture is usually ductile and is seen as a popcorn surface under high magnification.
Here is a side view of the broken spoke. The spoke broke right where the nipple ended. The manufacturer used rolled threads with a rounded root profile to reduce the amount of stress concentration. Still, there must have been some concentration as that is where the crack started.
The "beachmarks" are made when something unusual happens during the progress of the crack. Perhaps I rode in the rain, washed the bike or hit a big pot hole to cause these.The Scanning Electron Microscope (SEM) is the real deal for looking at the fracture surface. The three distinct phases of failure can be seen. Click on the picture to super size.Area "A" where the brittle fracture is progressing. The tiny striation marks are clearly visible.Area "B" where the final ductile failure occurred. The surface has a dimpled appearance in this region.