Definition: A brittle fracture is one with very little permanent deformation. Very little energy is absorbed in a brittle fracture, or put another way, very little work is required for a brittle fracture.
Identification: A brittle fracture can be identified by a surface that is bright or shiny. Adjectives used to describe a brittle fracture surface include grainy, coarse, and granular. The fracture surface is often flat and occurs in the plane of maximum principal stress, which in the case of a gear tooth is perpindicular to the applied load. Plastic deformation of the failed components is negligible.
Discussion: Brittle fractures are to be avoided if at all possible. While failures of any type are not good, when equipment fails due to brittle fracture, there can be very bad and even dangerous consequences. Brittle fractures, by their very nature, do not require much energy. This means that high loads that are present even for a very short period of time can result in failures. Once a crack has begun propagating in a brittle failure, it will continue to grow until rupture, even if the load is removed. Brittle fractures provide no warning in advance of their failure. Brittle fractures can result from several factors, working individually or in combination. These factors include the use of inherently brittle materials, high sulfur content, poor heat treatment practices, the presence of an inclusion, porosity, the geometry of the component, the presence of tensile residual stresses, and low operating temperatures.
Prevention: Brittle fractures can be avoided by using ductile materials (high in Ni and Mo, low in Cr, Mn, C, P, and S), avoiding low temperature operating conditions, avoiding high loading rate events, using clean materials, using good heat treatment practices that result in small grain size and minimizes tensile residual stress, and not designing stress concentrating features in components.
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