In Understanding How Materials Fail: Stress v. Fatigue, forensic engineering expert witness Clyde C. Richard, Ph.D., P.E., writes on material failure:
Today, in most product liability cases in the court system or with insurance companies, the alleged cause of the accident is either misuse or material failure. Accidents including people falling off ladders, chairs or bicycles and the failure of machinery and motorized vehicles can be caused by or contributed by material failure. Materials such as metals, plastics, ceramics and glass can fail immediately or breakdown over time through two different mechanisms: stress and fatigue.
Stress, or what some experts call a fracture, is when someone or something applies a load that exceeds the ultimate strength of a material. An example would be a tow hitch on a car that is rated for 3,000 lbs. and the user pulls a load of 9,000 lbs.