2材料科学基础英文版课件_(15).ppt
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1、Chapter 9 Failure 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 compo
2、nent, 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 tran
3、sition, 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
4、 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)
5、, 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 f
6、racture 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 poly
7、mers may exhibit both types of fracture,Ductile Fracture (1),Features of ductile fracture,Macroscopically,Necking down to a point,Some necking,Ductile Fracture (2),Microscopically,Initial necking,Final shear fracture at a 45o angle relative to the tensile direction,The fracture surface has a fibrous
8、 appearance,Ductile Fracture (3),Cup-and-cone fracture in aluminum,Ductile Fracture (4),Typical ductile fracture surface appearance (microscopically) - 1,Sowing many dimples,Ductile Fracture (5),Typical ductile fracture surface appearance (microscopically) - 2,Brittle Fracture (1),No necking, no app
9、arent plastic deformation,Brittle fracture in a steel,Brittle Fracture (2),Fracture surface markings for brittle fracture,With naked eyes: V-shaped markings near the center of fracture surface which point back towards the origin of the crack,With naked eyes: ridges which radiate from the origin of t
10、he crack,Brittle Fracture (3),For most brittle materials, crack propagation is along specific crystal planes. The planes are called cleavage planes and the fracture is call cleavage fracture. This type of fracture is Transgranular fracture,Grainy or faceted texture seen in the SEM micrograph,SEM fra
11、ctograph for a cleavage fracture,Transgranular fracture,Crack propagation is along grain boundaries - Intergranular fracture,Brittle Fracture (5),(d),SEM fractographs for intergranular fracture (3-D nature of grains is present),Brittle Fracture (6),An oil tank that fractured in a brittle manner by c
12、rack propagation around its girth,Principles of Fracture Mechanics (1),Brittle fracture of normally ductile materials requires us to have a good understanding of the mechanisms of fracture. To do this, we need to know the knowledge of fracture mechanics,Concerned with the relationship between materi
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