The purpose of failure criteria is to predict or estimate the failure/yield of machine parts and structural members.
A considerable number of theories have been proposed. However, only the most common and well-tested theories applicable to isotropic materials are discussed here. These theories, dependent on the nature of the material in question (i.e. brittle or ductile), are listed in the following table:
|Ductile||Maximum shear stress criterion, von Mises criterion|
|Brittle||Maximum normal stress criterion, Mohr's theory|
All four criteria are presented in terms of principal stresses. Therefore, all stresses should be transformed to the principal stresses before applying these failure criteria.
|Non Stress-Based Criteria|
The success of all machine parts and structural members are not necessarily determined by their strength. Whether a part succeeds or fails may depend on other factors, such as stiffness, vibrational characteristics, fatigue resistance, and/or creep resistance.
For example, the automobile industry has endeavored many years to increase the rigidity of passenger cages and install additional safety equipment. The bicycle industry continues to decrease the weight and increase the stiffness of bicycles to enhance their performance.
In civil engineering, a patio deck only needs to be strong enough to carry the weight of several people. However, a design based on the "strong enough" precept will often result a bouncy deck that most people will find objectionable. Rather, the stiffness of the deck determines the success of the design.
Many factors, in addition to stress, may contribute to the design requirements of a part. Together, these requirements are intended to increase the sense of security, safety, and quality of service of the part.