《材料力學(xué)性能》大三教學(xué)PPT課件,材料力學(xué)性能,材料,力學(xué)性能,大三,教學(xué),PPT,課件 Strengthening and toughening of materialsLi ChenhuiGeneral Description of Strengthing A dislocation held up by a random array of slip-plane obstacles.The stress required to overcome the obstacles depends on the effective spacing(L)between the obstacles along the dislocation line and the angle(c)to which the dislocation bends before it breaks through them.The necessary shear stress for continued dislocation motion is(a)Dislocation topology appropriate to a strong obstacle.Here,c is small and the effective obstacle spacing(L)is approximately equal to the mean center-to-center spacing of the obstacles in the plane.(b)When the obstacles are weak,the dislocation line is nearly straight(i.e.,c=180),and the space between the obstacles is large in comparison to the mean obstacle center-to-center distance such that is approximately equal to the distance between obstacle intersections that a random line makes on the slip plane.Work Hardening=0.2(f.c.c);=0.4(b.c.c)L2=ConstantBoundary StrengtheningMicroyielding in a grain(Grain 1)favorably oriented for slip may precede macroscopic yielding.Macroscopic flow requires dislocation activity in all grains(e.g.,Grain 2),and this may be induced by the internal stress caused by the dislocation pileup at the boundary in Grain 1.This stress may cause dislocation emission from the boundary or may activate a dislocation source(at point r)in Grain 2.The magnitude of the stress concentration depends on the number of dislocations in the pileup,and increases with the grain diameter,d.Dislocation activation in the second grain occurs when:0 is the shear stress to initiate slip when grain boundaries offer no resistance and Ks is a coefficient which characterizes stress concentration at the tip of a slip band Solid-solution StrengtheningParticle HardeningA.Deforming particles 1.Coherency hardening 2.Modulus hardening 3.Chemical strengthening 4.Order strengthening B.Nondeforming particlesC.The tansition from cutting to bowing and the maximum particle hardeningDeforming particles:1.Coherency hardeningDeforming particles:2.Modulus hardeningDeforming particles:3.Chemical strengtheningB.Nondeforming particlesToughening in metals Toughening in ceramics1.Toughening due to crack deflection and geometry2.Microcrack toughening3.Transformation toughening4.Crack bridgingA.Crack bridging with brittle fibersB.Crack bridging a ductile phaseThermal stresses Toughening due to crack deflection and geometryMicrocrack tougheningProcess zone Transformation tougheningCrack bridgingCrack bridging with brittle fibersCrack bridging a ductile phase金屬陶瓷的常用增韌機(jī)理金屬陶瓷的常用增韌機(jī)理 （a a）加工區(qū)增韌；（）加工區(qū)增韌；（b b）裂紋橋連增韌；（）裂紋橋連增韌；（c c）裂紋偏轉(zhuǎn)增韌）裂紋偏轉(zhuǎn)增韌 粗粗顆顆粒粒（2.52.5m m）金金屬屬陶陶瓷瓷的的壓壓痕痕裂裂紋紋：（a a）裂裂紋紋穿穿過過粗粗顆顆粒粒硬硬質(zhì)質(zhì)相相；（b b）近近裂裂紋紋尖尖端端的的裂裂紋紋橋橋連連和和裂裂紋紋繞繞過過部部分分細(xì)細(xì)顆顆粒粒；（c c）裂裂紋紋連連續(xù)續(xù)穿穿過過多多個(gè)個(gè)硬硬質(zhì)質(zhì)相相顆顆粒?！癆 A”、“B B”、“C C”和和“D D”而而不不發(fā)發(fā)生生明明顯顯的的偏偏轉(zhuǎn)轉(zhuǎn) 圖圖3-9 3-9 粗（粗（2.52.5m m）金屬陶瓷的斷口形貌：（）金屬陶瓷的斷口形貌：（a a）大顆粒）大顆粒硬質(zhì)相的晶面劈裂；（硬質(zhì)相的晶面劈裂；（b b）大顆粒硬質(zhì)相的連續(xù)解理斷裂）大顆粒硬質(zhì)相的連續(xù)解理斷裂圖圖3-10 3-10 細(xì)顆粒（細(xì)顆粒（0.80.8m m）金屬陶瓷的壓痕裂紋形貌：（）金屬陶瓷的壓痕裂紋形貌：（a a）概貌；）概貌；（b b）裂紋橋連和裂紋穿過粗顆粒）裂紋橋連和裂紋穿過粗顆?！癆 A”、“D D”、“E E”和繞過細(xì)顆粒和繞過細(xì)顆?！癇 B”、“C C”圖圖3-11 3-11 細(xì)顆粒（細(xì)顆粒（0.80.8m m）金屬陶瓷的斷口形貌：（）金屬陶瓷的斷口形貌：（a a）概貌；）概貌；（b b）大顆粒的脆性斷裂現(xiàn)象）大顆粒的脆性斷裂現(xiàn)象 圖圖3-12 亞微米硬質(zhì)合金中的裂紋形貌；（亞微米硬質(zhì)合金中的裂紋形貌；（a）概貌；（）概貌；（b）遠(yuǎn)離裂紋尖端處；（遠(yuǎn)離裂紋尖端處；（c）裂紋尖端附近；（）裂紋尖端附近；（d）裂紋尖端）裂紋尖端 圖圖3-13 3-13 中晶粒硬質(zhì)合金中的裂紋形貌中晶粒硬質(zhì)合金中的裂紋形貌:（a a）近裂紋尖端；（）近裂紋尖端；（b b）裂紋尖端）裂紋尖端(a)界面局部脫粘樣品的顯微裂紋形貌（b)壓痕對角線方向的顯微裂紋(c)垂直壓痕側(cè)棱方向上的裂紋圖圖3 314 14 界面局部脫粘的金屬陶瓷裂紋形貌界面局部脫粘的金屬陶瓷裂紋形貌 （與與圖3 31313中金屬陶瓷源于同一中金屬陶瓷源于同一燒結(jié)樣品，但品，但圖 3 31414所示部分所示部分經(jīng)過特殊特殊熱處理工理工藝使之局部脫粘使之局部脫粘）硬質(zhì)合金的片晶增韌WC-CoCopper-Niobium Alloy(US2005092400)問題、為什么是最合適的合金化元素？（從原子半徑，模量、固溶度等方面考慮）、試從材料力學(xué)性能方面考慮，該專利中主要利用了哪些力學(xué)現(xiàn)象或效應(yīng)？壓力容器設(shè)計(jì)問題Consider the thin-walled spherical tank of radius r and thickness t that may be used as a pressure vessel.Schematic diagram showing the cross section of a spherical tank that is subjected to an internal pressure p,and that has a radial crack of length 2 a in its wall.設(shè)計(jì)準(zhǔn)則(a)One design of such a tank calls for yielding of the wall material prior to failure as a result of the formation of a crack of critical size and its subsequent rapid propagation.Thus,plastic distortion of the wall may be observed and the pressure within the tank released before the occurrence of catastrophic failure.Consequently,materials having large critical crack lengths are desired.(b)An alternative design that is also often utilized with pressure vessels is termed leak-before-break.Using principles of fracture mechanics,allowance is made for the growth of a crack through the thickness of the vessel wall prior to the occurrence of rapid crack propagation(Figure 9.15).Thus,the crack will completely penetrate the wall without catastrophic failure,allowing for its detection by the leaking of pressurized fluid.With this criterion the critical crack length ac(i.e.,one-half of the total internal crack length)is taken to be equal to the pressure vessel thickness t.Allowance for ac=t instead of ac=t/2 assures that fluid leakage will occur prior to the buildup of dangerously high pressures.For this spherical pressure vessel,the circumferential wall stress is a function of the pressure p in the vessel and the radius r and wall thickness t according to For both parts(a)and(b)assume a condition of plane strain.Solution(a)For the first design criterion,it is desired that the circumferential wall stress be less than the yield strength of the material.Substitution of y for in Equation 9.11,and incorporation of a factor of safety N leads towhere ac is the critical crack length.Solving for ac yields the following expression:Table 6.2 Ranking of Several Metal Alloys Relative to Critical Crack Length(Yielding Criterion)for a Thin-Walled Spherical Pressure VesselTherefore,the critical crack length is proportional to the square of the Kc/y ratio.The ranking is provided in Table 6.2,where it may be seen that the medium carbon(1040)steel with the largest ratio has the longest critical crack length,and,therefore,is the most desirable material on the basis of this criterion.(b)As stated previously,the leak-before-break criterion is just met when one-half of the internal crack length is equal to the thickness of the pressure vesseli.e.,when a=t.Substitution of a=t into Equation 9.11 givesAnd,from Equation:The stress is replaced by the yield strength,inasmuch as the tank should be designed to contain the pressure without yielding;furthermore,substitution of Equation 9.19 into Equation 9.18,after some rearrangement,yields the following expression:Hence,for some given spherical vessel of radius r,the maximum allowable pressure consistent with this leak-before-break criterion is proportional to KIc2/y.The same several materials are ranked according to this ratio in Table 9.3;as may be noted,the medium carbon steel will contain the greatest pressures.Of the eleven metal alloys that are listed in Table B.5,the medium carbon steel ranks first according to both yielding and leak-before-break criteria.For these reasons,many pressure vessels are constructed of medium carbon steels,when temperature extremes and corrosion need not be considered.疲勞裂紋擴(kuò)展裂紋尺寸對疲勞壽命的影響發(fā)動(dòng)機(jī)渦輪盤壽命計(jì)算A critical value of the elastic stress intensity c factor,IC,is used to describe brittle fracture,and the stress intensity factor range,is used to characterize fatigue crack propagation,a predominantly plastic process.Justify the use of K in both these cases,and hence explain the apparent contradiction.An aircraft wing spar,made of a 7475 Al-Mg-Zn alloy,is inspected for cracks every 20 flights.The non-destructive inspection(NDI)technique employed can detect surface defects greater than 1 mm deep.During a flight the stress in the spar fluctuates between 20 and 150 MPa,on average 1200 times.The fatigue crack growth behaviour of the alloy can be described by(for a in m,in MPa m1/2)Comment on the inspection interval.Would it be better to use a higher toughness alloy for the component or to purchase new NDI equipment with a resolution of 750(im?Give reasons for your answer.Data for the 7475 alloy:E=71 GPa,0.2%proof stress=540 MPa,XIC=43MPam1/2