There are many production methods used to change the properties of metals. Annealing and work hardening are two ways in which the crystalline structures of the metals are manipulated in order to change the properties of a metal. These practices have a consequence though, failure of the material. Actually, any stress on the metal promotes failure. The two most common types of failure are metal fatigue and fracture. Here, we will discuss fatigue, and the two types of fracture, brittle and ductile fracture.

Fatigue

First, let's take a look at the primary causes of metal fatigue.

Over worked metal will most often result in metal fatigue. Fatigue is a major concern for engineers who wish to insure the longevity of a product under stress, as well as metallurgists trying not to over work the metal during production. Overworking may be the result of over-manipulation during production time or the result of excessive use by the consumer. Fatigue begins when the surface of the metal develops a deformation. Cracks work their way into the metal from these surface deformations. As these cracks develop, the metal actually tears apart. If undetected, a crack may eventually cause catastrophic failure by a fracture mechanism.

Fracture occurs when the metal experiences stress that exceeds its yield strength. Fractures occur as two different types, ductile fracture and brittle fracture. Brittle fracture occurs when the metal doesn’t yield before it breaks. Instead of the sheets of atoms in the metal sliding over each other as occurs in deformation, when stressed, the sheets of atoms pull completely apart. This type of fracture most often occurs in metals that are extremely hard. Brittle fracture almost always occurs at low temperatures.

Ductile fracture is the most common type of fracture in metal. Unlike what occurs in a brittle fracture, the metal yields before it breaks in a ductile fracture. The peak stress a metal can withstand before it breaks is called tensile strength. Ductile fracture is caused by the stress exerted on the metal actually work hardening the metal as it yields, cracks from fatigue develop, and then these cracks propagate very rapidly through the metal until complete failure occurs