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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 compone
2、nt, 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 transiti
3、on, 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 torsionalTwo fracture modes: ductile and b
4、rittle, depending on 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), showi
5、ng 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
6、 and it is a very fast process. 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
7、 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 appeara
8、nce,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 apparent pl
9、astic 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 the crack
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
11、 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 crack pro
12、pagation 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 material prope
13、rties, 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,Principles of Fracture Mechanics (2),The maximum stre
14、ss at the crack tip:,For a microcrack, (a/t)1/2 may be very large, leading 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 onl
15、y at microscopic flaws or cracksbut also at macroscopic internal defects 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
16、deformation occurs when the maximum stress m exceeds the yield strength,Principles 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 it
17、s measured fracture strength?,Question:,Principles of Fracture Mechanics (5),Fracture 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
18、(MPa m1/2), and Y is a parameter associated with both crack and specimen sizes and geometries 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
19、 crack in a plate of infinite width,A surface crack in a plate of semi-infinite width,Y = 1.0,Y = 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, th
20、e value of Kc will no longer change with specimen thickness (a constant). Under these conditions, a condition 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 strai
21、n fracture toughness KIc, which is defined as,Principles of Fracture Mechanics (8),Mode I, opening or tensile 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,
22、Temperature KIc decreases with decreasing temperatureStrain rate KIc decreases with increasing strain rateMicrostructure KIc increases with reducing grain size (toughening). An increase in yield strength brought about by solid solution strengthening, precipitation hardening, or strain hardening caus
23、es a corresponding decrease in KIc (embrittling),Embrittlement Hardening embrittlement brittleness increases with strengtheningNon-hardening embrittlement brittleness increases without strengthening,Principles of Fracture Mechanics (10),Principles of Fracture Mechanics (11),Design using fracture tou
24、ghness,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,
