脆性断裂-自学材料

1、Fundamentals of Materials Science and Engineering Chapter 8 FailureWhat is the real nature for the different appearance of fracture section?Fundamentals of Materials Science and Engineering Why study failure?Air plane or car crash, bridge breaking, machine breaking down are all, to some sense, relat

2、ed to the failure of engineering materials. The best way to prevent such in-service failures is to study the mechanics of the various failure modes and design the structures or components appropriately.Fundamentals of Materials Science and Engineering What should you be able to do after studying thi

3、s chapter?Have the basic knowledge of the major types of engineering materials failure.Understand the mechanics of various fracture modes.Fundamentals of Materials Science and Engineering Important terms and concepts:lFailure 失效失效lFracture 断口断口lBrittle fracture 脆性断裂脆性断裂lDuctile fracture 塑性断裂塑性断裂lDuc

4、tile-to-brittle transition 塑脆转变塑脆转变lCase hardening 表面强化表面强化lFatigue 疲劳疲劳lLow-cycle fatigue低周疲劳低周疲劳lHigh-cycle fatigue高周疲劳高周疲劳lCorrosion fatigue 腐蚀疲劳腐蚀疲劳lOrigin 裂纹源裂纹源lBeachmarks 沙滩印沙滩印lFatigue striations 疲劳辉纹疲劳辉纹lFibrous texture纤维组织纤维组织lMean stress平均应力平均应力lThermal fatigue 热疲劳热疲劳lFatigue crack 疲劳裂纹疲劳

5、裂纹lFatigue life 疲劳寿命疲劳寿命lFatigue limit 疲劳极限疲劳极限lEndurance limit 容许极限容许极限lFatigue strength 疲劳强度疲劳强度 lCreep 蠕变蠕变lNeck 颈缩颈缩lcup-and-cone 杯突形杯突形lCharpy test 夏氏试验夏氏试验lCrack initiation site 裂纹源裂纹源lResidual compressive stress 残余压缩应力残余压缩应力lConstant load 恒载荷恒载荷lPrimary creep/ transient creep 第一蠕变第一蠕变lSeconda

6、ry creep/Steady-state creep第二蠕第二蠕变变lTertiary creep 第三蠕变第三蠕变Fundamentals of Materials Science and Engineering lPlane strain 平面应变平面应变lPlane strain fracture toughness 平面应变断裂韧性平面应变断裂韧性lStress intensity factor应力强度因素应力强度因素lStress raiser 应力集中源应力集中源lFracture mechanics/mode 断裂机制断裂机制/模式模式lTransgranular fractu

7、re穿晶断裂穿晶断裂lIntergranular fracture沿晶断裂沿晶断裂lFracture toughness断裂韧性断裂韧性lImpact energy 冲击能冲击能 lStress ratio 应力比应力比lStress amplitude 应力幅应力幅lSN curve S-N曲线曲线lCrack initiation 裂纹萌生lCrack propagation/growth 裂纹扩展lCoalescence n. 合并lsmall cavities- microvoids 微孔ljeopardy peril or dangerlelliptical crack 椭圆形裂纹l

8、Dimples 韧窝lChevron 人字形lRidgelike 山脊状lPatterns 花样Fundamentals of Materials Science and Engineering lINTRODUCTIONlThe failure of engineering materials is almost always an undesirable event for several reasons; these human lives that are put in jeopardy (Risk of loss or injury; peril or danger), econom

9、ic losses, and the interference with the availability of products and services. lEven though the causes of failure and the behavior of materials may be known, prevention of failure is difficult to guarantee. Fundamentals of Materials Science and Engineering lThe usual causes are improper materials s

10、election and processing and inadequate design of the component or its misuse. lIt is the responsibility of the engineer to anticipate and plan for possible failure and, in the event that failure does occur, to assess its cause and then take appropriate preventive measures against future incidents.Fu

11、ndamentals of Materials Science and Engineering Fracture-the separation of body into two or more pieces in response to a imposed stress that is static and at low temperature. There are two fracture mode: ductile and brittle modes, both of which involve the formation and propagation of cracks. lFatig

12、ue - a form of failure that occurs in structures subjected to dynamic and fluctuating stresses, a common type of catastrophic failure.lCreep - deformation caused by which materials are placed in service at elevated temperatures and exposed to static mechanical stresses.Fundamentals of Materials Scie

13、nce and Engineering lDuctile fracture surfaces will have their own distinctive features on both macroscopic and microscopic levels. Figure 9.1 shows schematic representations for two characteristic macroscopic fracture profiles. lThe configuration shown in Figure 9.1a is found for extremely soft met

14、als, such as pure gold and lead at room temperature, and other metals, polymers, and inorganic glasses at elevated temperatures. These highly ductile materials neck down to a point fracture, showing virtually 100% reduction in area.lThe most common type of tensile fracture profile for ductile metals

15、 is that represented in Figure 9.1b, which fracture is preceded by only a moderate amount of necking. Fundamentals of Materials Science and Engineering Fundamentals of Materials Science and Engineering lThe fracture process normally occurs in several stages (Figure 9.2).lFirst, after necking begins,

16、 small cavities, or microvoids, form in the interior of the cross section, as indicated in Figure 9.2b. lNext, as deformation continues, these microvoids enlarge, come together, and coalesce to form an elliptical crack, which has its long axis perpendicular to the stress direction. The crack continu

17、es to grow in a direction parallel to its major axis by this microvoid coalescence process (Figure 9.2c). Fundamentals of Materials Science and Engineering lFinally, fracture ensues by the rapid propagation of a crack around the outer perimeter of the neck (Figure 9.2d), by shear deformation at an a

18、ngle of about 45 with the tensile axisthis is the angle at which the shear stress is a maximum.lSometimes a fracture having this characteristic surface contour is termed a cup and- cone fracture because one of the mating surfaces is in the form of a cup, the other like a cone. In this type of fractu

19、red specimen (Figure 9.3a), the central interior region of the surface has an irregular and fibrous appearance, which is indicative of plastic deformation.Fundamentals of Materials Science and Engineering Fig.9.2 stages in the cup-and-cone fracture. (a) Initial necking. (b) Small cavity formation. (

20、c) Coalescence of cavities to form a crack.(d) Crack propagation. (e) Final shear fracture at a 45angle relative to the tensile direction.Fundamentals of Materials Science and Engineering l For ductile fracture, evidence will exist of gross plastic deformation at the fracture surface. lIn tension, h

21、ighly ductile metals will neck down to essentially a point fracture; cup-and-cone mating fracture surfaces result for moderate ductility, where microscopically, dimples(韧窝韧窝) are produced. lCracks in ductile materials are said to be stable (i.e., resist extension without an increase in applied stres

22、s); usually non-catastrophic fracture will happen, this fracture mode is almost always preferred.Fundamentals of Materials Science and Engineering Fig.8.4 (a) Scanning electron fractograph showing spherical dimples characteristic of ductile fracture resulting from unixial tensile loads.(b) Scanning electron fractograph showing parabolic-shaped