Figure 35 comprises two photographs of a bicycle crank that has snapped into two pieces. The first picture shows the two pieces placed together. The crank in this picture is designed to also support the chain wheel. It has a further five short arms that radiate from one end and the chain wheel is bolted to these arms. This means that the bicycle crank suddenly increases in thickness and width towards the chain wheel end and it is at this sudden change in section that the crank has snapped. The second picture is a close-up view of the broken surface of the crank arm. The distinctive feature of this surface is that it has two areas of coloration – one dark and one light. Closer inspection of the darker surface reveals a series of radial rings emanating from a corner of the cross section. The darker surface is evidence of a fatigue crack and the rings show how it is grown in size over time. It started at one corner and over time grew to cover about 40% of the cross section. Over this time the crack has been exposed to atmosphere which has oxidised its surface. Hence the darker coloration! As the crack grew, the cross-sectional area of the crank actually capable of transmitting the forces was progressively getting smaller until it is not large enough to support the resulting stress. At this point the crank has snapped and the resulting surface is virgin material and no rings are evident. As well as being much lighter in colour it would feel rough to the touch with a few irregular jagged edges.

 2.6.3 Industrial design