Another text book example of a type of failure - fatigue. I took this pic last week with my cell phone. A snapped shaft due to fatigue.
Fatigue is the progressive damage that occurs when a material is subjected to cyclic loading. The maximum stress applied is less than the stress limit of the material yet the material is subjected to a constant or random elongation, bending or compression which will eventually take its toll and produce a failure. Notice the twist patterns as well as the color differences on the below pic:
Typically, a fractured piece will have a dark area which is an indication of slow crack growth, and a bright area which is an indication of sudden fracture. Fatigue begins with the crystalline structure experiencing dislocation. Eventually small micro-fissures develop which begin to compromise the physical integrity of the part, tool or material. Many materials will not normally recover if fatigue is relieved.
So, answer me this students-o-failure, why would the color be dark on the inside and not on the outside? How do you think the stress developed?