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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 9,Failure of Materials,Chapter 9Failure of Materials,1,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,IntroductionFailure of enginee,2,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,Fundamentals of FractureSimple,3,Ductile Fracture(1),Features of ductile fracture,Macroscopically,Necking down to a point,Some necking,Highly ductile Considerable deformation,Moderately ductile Some deformation,Ductile Fracture(1)Features,4,Ductile Fracture(2),Microscopically,Initial necking,Microvoids formation,Coalescence of microvoids to a crack,Crack propagation,Final shear fracture at a 45,o,angle relative to the tensile direction,The fracture surface has a fibrous appearance,Cup,Cone,Ductile Fracture(2)Microscopi,5,Ductile Fracture(3),Cup-and-cone fracture in aluminum,Ductile Fracture(3)Cup-and-co,6,Ductile Fracture(4),Typical ductile fracture surface appearance(microscopically)-1,Sowing many dimples,SEM photograph,Ductile Fracture(4)Typical d,7,Ductile Fracture(5),Typical ductile fracture surface appearance(microscopically)-2,Ductile Fracture(5)Typical d,8,Brittle Fracture(1),No necking,no apparent plastic deformation,Crack propagation nearly perpendicular to the applied stress-flat fracture surface,Brittle fracture in a steel,Brittle Fracture(1)No necking,9,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 the crack,Brittle Fracture(2)Fracture s,10,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 fractograph for a cleavage fracture,Transgranular fracture,Brittle Fracture(3)For most b,11,Crack propagation is along grain boundaries-,Intergranular fracture,Brittle Fracture(5),(d),SEM fractographs for intergranular fracture(3-D nature of grains is present),Crack propagation is along gra,12,Brittle Fracture(6),An oil tank that fractured in a brittle manner by crack propagation around its girth,Brittle Fracture(6)An oil tan,13,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 material properties,stress level,crack-producing flaws,and crack propagation mechanisms,Stress Concentration,Measured fracture strengths for most brittle materials much lower than their theoretically calculated values on the basis of atomic bonding energies,Because of microscopic flaws or cracks,which always exist at the surfaces and in the interior of a material,An applied stress may be concentrated at the crack tip,the magnitude of which is dependent on crack orientation and geometry,Principles of Fracture Mechani,14,Principles of Fracture Mechanics(2),The maximum stress at the crack tip:,Showing the stress concentration around a crack,Curvature Radius,For a microcrack,(,a,/,t,),1/2,may be very large,leading to a very large,m,i
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