脆性斷裂-自學(xué)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 斷口斷口lBrittle fracture 脆性斷裂脆性斷裂lDuctile fracture 塑性斷裂塑性斷裂lDuc

4、tile-to-brittle transition 塑脆轉(zhuǎn)變塑脆轉(zhuǎn)變lCase hardening 表面強化表面強化lFatigue 疲勞疲勞lLow-cycle fatigue低周疲勞低周疲勞lHigh-cycle fatigue高周疲勞高周疲勞lCorrosion fatigue 腐蝕疲勞腐蝕疲勞lOrigin 裂紋源裂紋源lBeachmarks 沙灘印沙灘印lFatigue striations 疲勞輝紋疲勞輝紋lFibrous texture纖維組織纖維組織lMean stress平均應(yīng)力平均應(yīng)力lThermal fatigue 熱疲勞熱疲勞lFatigue crack 疲勞裂紋疲勞

5、裂紋lFatigue life 疲勞壽命疲勞壽命lFatigue limit 疲勞極限疲勞極限lEndurance limit 容許極限容許極限lFatigue strength 疲勞強度疲勞強度 lCreep 蠕變?nèi)渥僱Neck 頸縮頸縮lcup-and-cone 杯突形杯突形lCharpy test 夏氏試驗夏氏試驗lCrack initiation site 裂紋源裂紋源lResidual compressive stress 殘余壓縮應(yīng)力殘余壓縮應(yīng)力lConstant load 恒載荷恒載荷lPrimary creep/ transient creep 第一蠕變第一蠕變lSeconda

6、ry creep/Steady-state creep第二蠕第二蠕變變lTertiary creep 第三蠕變第三蠕變Fundamentals of Materials Science and Engineering lPlane strain 平面應(yīng)變平面應(yīng)變lPlane strain fracture toughness 平面應(yīng)變斷裂韌性平面應(yīng)變斷裂韌性lStress intensity factor應(yīng)力強度因素應(yīng)力強度因素lStress raiser 應(yīng)力集中源應(yīng)力集中源lFracture mechanics/mode 斷裂機制斷裂機制/模式模式lTransgranular fractu

7、re穿晶斷裂穿晶斷裂lIntergranular fracture沿晶斷裂沿晶斷裂lFracture toughness斷裂韌性斷裂韌性lImpact energy 沖擊能沖擊能 lStress ratio 應(yīng)力比應(yīng)力比lStress amplitude 應(yīng)力幅應(yīng)力幅lSN curve S-N曲線曲線lCrack initiation 裂紋萌生lCrack propagation/growth 裂紋擴展lCoalescence n. 合并lsmall cavities- microvoids 微孔ljeopardy peril or dangerlelliptical crack 橢圓形裂紋l

8、Dimples 韌窩lChevron 人字形lRidgelike 山脊?fàn)頻Patterns 花樣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(韌窩韌窩) 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