《材料科学基础英文版课件7.ppt

Chapter 9Failure of Materials Introduction Failure of engineering materials is a bad thing – causing life and economic losses, and also interfering with the availability of products and services Usual reasons for the failure: improper materials selection and processing, inadequate design of the component, misuse It is the responsibility of engineers to expect and plan for possible failure, and when failure occurs, to find out its reasons and take proper measures against future incidents Topics: simple fracture, fundamentals of fracture mechanics, impact fracture testing, ductile-to-brittle transition, fatigue, and creep Fundamentals of Fracture Simple fracture is the separation of a body into two or more pieces in response to an applied stress at low temperatures (relative to the melting temperature) The applied stress: tensile, compressive, shear, or torsional Two fracture modes: ductile and brittle, depending on the ability of a material to experience plastic deformation Any fracture process involves two steps – crack formation and propagation. Ductile fracture is corresponding to extensive plastic deformation near advancing cracks and relatively slow crack propagation rate (stable), showing considerable deformation on fracture surfaces (rough); brittle fracture to little plastic deformation and fast crack propagation rate (unstable), showing no apparent deformation on fracture surfaces (smooth) Brittle fracture often cause disasters because there is no warning signal before fracture and it is a very fast process. However, ductile fracture develops slowly and there is warning signal (considerable plastic deformation) before fracture Ductile fracture is always desirable. Under normal conditions, most metal alloys are ductile, but ceramics are considerably brittle and polymers may exhibit both types of fracture Ductile Fracture (1) Ductile Fracture (2) Ductile Fracture (3) Ductile Fracture (4) Ductile Fracture (5) Brittle Fracture (1) Brittle Fracture (2) B