Definition: A ductile fracture experiences significant permanent deformation before rupture. Large amounts of energy are required to initiate the fracture.
Identification: A ductile fracture can be identified by a surface that is dull or gray (dark). Adjectives used to describe a ductile fracture surface include silky, matte, smooth, fine, and fiborous or stringy. The fracture surface is often slanted with respect to the tooth, and occurs on a plane of maximum shear stress. Shear lips are often observed on the edges of the fracture surface, and there is typically appreciable plastic deformation.
Discussion: Although fractures of any type are not good, ductile fractures are the preferred overload failure mode. Ductile failures often provide some advance warning, in the form of increased noise or vibration due to the plastic deformation of components.
Prevention: Ductile fractures can be prevented by avoiding high loading rate events.
More Information: For more information on ductile fracture failures, please submit a question to our “Ask an Expert” link. For assistance with a gearbox failure analysis, please visit our “Consulting” page.