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1、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 servicesUsual reasons for the failure:improper materials selection and processing,inadequate design of the component,
2、misuseIt 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 incidentsTopics:simple fracture,fundamentals of fracture mechanics,impact fracture testing,ductile-to-brittle transition,fatigue
3、,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 torsionalTwo fracture modes:ductile and brittle,depending o
4、n the ability of a material to experience plastic deformationAny 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 deform
5、ation 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 proce
6、ss.However,ductile fracture develops slowly and there is warning signal(considerable plastic deformation)before fractureDuctile 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,Duc
7、tile 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 appearance,Ductile Fracture(3),Cup-and-cone
8、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 apparent plastic deformation,Brittle fracture in a steel,Br
9、ittle 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(3),For most brittle materials,cra
10、ck 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,Crack p
11、ropagation 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 crack propagation around its girth,Principles of Fracture Mechanics(1
12、),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
13、mechanisms,Stress Concentration,Measured fracture strengths for most brittle materials much lower than their theoretically calculated values on the basis of atomic bonding energies,Principles of Fracture Mechanics(2),The maximum stress at the crack tip:,For a microcrack,(a/t)1/2 may be very large,le
14、ading to a very large m,i.e.,a very large stress concentration,The ratio m/o is denoted as the stress concentration factor Kt,Principles of Fracture Mechanics(3),It is seen from the above that stress concentration occursNot only at microscopic flaws or cracksbut also at macroscopic internal defects
15、like voids,at sharp corners or at notches,For a brittle material,there is nearly no plastic deformation occurring at the crack tip.Thus the theoretical stress concentration factor will result,For a ductile material,plastic deformation occurs when the maximum stress m exceeds the yield strength,Princ
16、iples of Fracture Mechanics(4),For a brittle material,when an applied tensile stress exceeds the critical stress,the crack will propagate,leading to failure,For the defect-free metallic or ceramic whisker,what about its measured fracture strength?,Question:,Principles of Fracture Mechanics(5),Fractu
17、re Toughness,In the light of principles of fracture mechanics,the relation of the critical stress for crack propagation(c)to the crack length(a)can be expressed as,where Kc is the fracture toughness in units of(MPa m1/2),and Y is a parameter associated with both crack and specimen sizes and geometri
18、es as well as the manner of load application,Kc is a material property,which is the measure of the materials resistance to brittle fracture when a crack exists,Principles of Fracture Mechanics(6),An internal crack in a plate of infinite width,A surface crack in a plate of semi-infinite width,Y=1.0,Y
19、=1.1,Principles of Fracture Mechanics(7),When the specimen is not thick enough compared with the crack length,the value of Kc changes with specimen thickness,When the specimen is sufficiently thick,the value of Kc will no longer change with specimen thickness(a constant).Under these conditions,a con
20、dition of plane strain exists around the crack tip,i.e.there is no strain component perpendicular to the front and back faces,The Kc value in this plane strain condition is called the plane strain fracture toughness KIc,which is defined as,Principles of Fracture Mechanics(8),Mode I,opening or tensil
21、e mode,Mode II,sliding mode,Mode III,tearing mode,Three modes of crack surface displacement,Principles of Fracture Mechanics(9),Main factors influencing the plane strain fracture toughness,Temperature KIc decreases with decreasing temperatureStrain rate KIc decreases with increasing strain rateMicro
22、structure KIc increases with reducing grain size(toughening).An increase in yield strength brought about by solid solution strengthening,precipitation hardening,or strain hardening causes a corresponding decrease in KIc(embrittling),Embrittlement Hardening embrittlement brittleness increases with st
23、rengtheningNon-hardening embrittlement brittleness increases without strengthening,Principles of Fracture Mechanics(10),Principles of Fracture Mechanics(11),Design using fracture toughness,In engineering design,if KIc and a are specified by application constraints,the design applied stress must be,If KIc and are specified by application requirements,the critical crack size may be determined by,Non-destructive evaluation(NDE)techniques can be used to detect the cracks to determine whether or not the crack size has reached the critical value,Impact Fracture Testing,BBB,
