STRESS CORROSION CRACKING OF 321 STAINLESS STEEL SINGLE CRYSTAL UNDER MODE Ⅱ LOADING

研究了不锈钢Ⅱ型单晶试样在４２％沸腾ＭｇＣｌ＿２溶液中的应力腐蚀。结果表明，对任何晶面取向的试样，其应力腐蚀裂纹均在最大正应力处形核，而在最大剪应力位置并不发生应力腐蚀．在最大正应力位置的周围虽有滑移线，但在裂纹形核的区域并无滑移线，而在有滑移线的地方却没有宏观裂纹。观察表明，有些滑移线上存在很多蚀坑，在蚀坑较轻微的滑移线上，有许多微裂纹，这些裂纹与滑移线成一定角度（约２０°）．在应力腐蚀裂纹的形核与扩展中起作用的是位错局部塞积所形成的应力集中．位错塞积应力与电化学过程的联合作用下导致了应力腐蚀开裂，金属表面的滑移台阶虽然也会发生腐蚀，但并不起主要作用。     

DOUBLE PEAK OF HYDROGEN ACCUMULATION NEAR NOTCH TIP UNDER Ⅰ＋Ⅱ LOADING MODE

利用ＫＹＫＹ离子探针微分析仪和有限元定量计算发现，２１Ｃｒ７Ｎｉ９ＭｎＮ奥氏体钢在复合型（Ⅰ＋Ⅱ）载荷下，缺口顶端无论在开裂方向还是塑性区尺寸最大方向上，皆有两个氢浓度的富集峰，且随着Ⅱ型分量的增加，远离缺口的氢富集峰有下降且移向缺口顶端的趋势．钢中原始氢浓度的改变、并不严重影响缺口顶端双氢峰的形成。缺口前端塑性应变和静水应力的梯度分布是导致其氢浓度再分布的主要因素．据此提出了复合型载荷缺口顶端双氢峰富集模型．     

DISTRIBUTION OF HYDROGEN NEAR NOTCH TIP UNDER MODE Ⅰ LOADING

Distribution of hydrogen near crack tiphas long been recognized as a major factor inhydrogen induced cracking.It governs manyaspects,such as the location where the prima-ry crack occurs,the way that crack propa-gates,etc.Therefore understanding of distri-bution of hydrogen near crack tip is of im-portant significance.The first relationship be-     

STRESS CORROSION CRACKING IN HIGH STRENGTH STEEL UNDER MODE Ⅲ LOADING

研究了超高强度钢Ⅲ型缺口试样在水介质中的应力腐蚀,并与动态充氢条件作了对比。结果表明,超高强度钢Ⅲ型试样在水介质中能产生应力腐蚀,而且裂纹沿着和原缺口面成45°的平面形核和扩展,获得沿晶断口。这些都和Ⅲ型试样动态充氢时氢致开裂的情况相同。水介质中加硫脲,能使Ⅲ型和Ⅰ型的应力腐蚀门槛值明显下降,如动态充氢,则使裂纹的孕育期和门槛值进一步下降,实验也表明,无论动态充氢还是应力腐蚀,Ⅲ型试样的门槛值均比Ⅰ型要高,这可用Ⅲ型和Ⅰ型的平衡氢浓度不同来解释。     

DEFORMATION AND FRACTURE OF TiAl PST CRYSTALS UNDER MONOTONIC AND CYCLIC LOADING

ＤＥＦＯＲＭＡＴＩＯＮＡＮＤＦＲＡＣＴＵＲＥＯＦＴｉＡｌＰＳＴＣＲＹＳＴＡＬＳＵＮＤＥＲＭＯＮＯＴＯＮＩＣＡＮＤＣＹＣＬＩＣＬＯＡＤＩＮＧＹ．Ｕｍａｋｏｓｈｉ；Ｔ．ＮａｋａｎｏａｎｄＨ．Ｙ．Ｙａｓｕｄａ（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃ...     

INFLUENCES OF HEAT-TREATING PROCESS ON DUCTILE FRACTURE MECHANISM UNDER MIXED MODE LOADINGS

１ＩＮＴＲＯＤＵＣＴＩＯＮＩｎｅｎｇｉｎｅｅｒｉｎｇｐｒａｃｔｉｃｅ，ａｌｏｔｏｆｃｒａｃｋｐｒｏｂｌｅｍｓｃａｎｂｅｉｎｃｌｕｄｅｄｉｎｄｉｆｅｒｅｎｔｃｏｍｂｉｎａｔｉｏｎｓｏｆｔｈｒｅｅｂａｓｉｃｆｒａｃｔｕｒｅｍｏｄｅｓ，ｉ．ｅ．ｍｉｘｅｄ...     

AN INVESTIGATION ON THE BRITTLE FRACTURE OF K_Ⅰ-K_Ⅱ COMPOSITE MODE CRACKS

本文用三点弯曲和四点弯曲试样,在K_Ⅱ/K_Ⅰ=0—14的范围内,对GC-4超高强度钢、30Cr2MoV中强度转子钢和稀土球墨铸铁进行了线弹性平面应变复合型脆断试验。结果表明:复合型裂纹的脆断开裂方向与现有的三种复合型理论符合较好;但是,复合型裂纹扩展阻力随K_Ⅱ/K_Ⅰ比值增加而增大,与现有复合型理论差别较大。 本文从塑性区的形状、大小和裂纹顶端在开裂方向的应力状态等方面对此问题进行了分析并指出,现有理论关于复合型裂纹扩展阻力与变形特征无关的假设,对于具有一定塑性变形能力的金属材料,是不合适的。文章还对简单且偏于安全的处理复合型问题的工程分析方法作了讨论。     

EVOLUTION OF STATISTICAL DISTRIBUTION OF PIT GEOMETRIC PARAMETERS UNDER FATIGUE LOADING

The pits and pit-associated fatigue crack initiation of the offshore structure steel A537cLlhave been investigated under salt water spray condition.The tests were interrupted on sched-ule to make plastic replica of corroded specimen surface on which many randomly distributedpits can be found.The development of the geometric parameters of pits was obtained and thestatistical distributions of these parameters were determined.Instead of depth,the ratio ofdepth to width is the most dominant parameter in relation to crack initiation.Also,the pittingwas enhanced when low frequency or high stress range was applied and although stress rangehas no effect on the nucleation of pits,it can accelerate the initiation of cracks from pits.     

MECHANISM ANALYSIS OF THICKNESS EFFECT ON MIXED MODE Ⅰ/Ⅱ FRACTURE OF LC4-CS ALUMINUM ALLOY

Mixed mode Ⅰ/Ⅱ fracture experiments of LC4-CS aluminum alloy were conducted by using tension-shear specimens with thicknesses of 2, 4, 8 and 14mm. Fracture mechanisms of thickness effect on mixed mode Ⅰ/Ⅱ fracture were first examined from fracture surface morphology to correlate with the macroscopic fracture behavior and stress state. It is found that specimen thickness has a strong influence on mixed mode fracture. As thickness varies from thin to thick the macroscopic fracture surfaces appear the characteristics of plane stress state (2mm, 4mm-thick specimen), threedimensional stress state (8mm-thick specimens), and plane strain state (14mm-thick specimens), respectively. The specimens of all kinds of thicknesses are typical of tensile type failure under mode I loading condition and shear type failure under mode Ⅱloading condition. Two distinct features coexist on the fracture surfaces under mixed mode loading conditions, and the corresponding proportion varies with loading mixity. Void-growth processes are the failure mechanism in both predominately tensileand shear-type fractures. The size and depth of dimples on the fracture surface vary greatly with thickness. Therefore, it is extraordinary necessary to take into account the thickness effect when a mixed mode fracture criterion is being established.     

MECHANISM ANALYSIS OF THICKNESS EFFECT ON MIXED MODE Ⅰ/Ⅱ FRACTURE OF LC4-CS ALUMINUM ALLOY

Mixed mode Ⅰ/Ⅱ fracture erperiments of LC4-CS aluminum alloy were conductedby using tension--shear specimens with thicknesses of 2, 4, 8 and 14mm. Fracturemechanisms of thickness effect on mixed mode Ⅰ/Ⅱ fracture were first examined fromfracture surface morphology to correlate with the macroscopic fracture behavior andstress state. It is found that specimen thickness has a strong influence on mixed modefracture. As thickness varies from thin to thick the macroscopic fracture surfacesappear the characteristics of plane stress state (2mm, 4mm--thick specimen), three--dimensional stress state (8mm--thick specimens), and plane strain state (14mm--thickspecimens), respectively. The specimens of all kinds of thicknesses are typical of ten-sile type failure under mode Ⅰ loading condition and shear type failure under mode Ⅱloading condition. Two distinct features coexist on the fracture surfaces under mixedmode loading conditions, and the corresponding proportion varies with loading mix-ity. Void--growth     

AN EXPERIMENTAL-NUMERICAL METHOD FOR MEASURING CRACK PROPAGATING VELOCITIES UNDER STRESS WAVE LOADING

An experimental-numerical method for measuring dynamic crack propagating velocities under stress wave loading is established in this paper. The experiments of the three-point bend specimen are done on the improved Hopkinson bar. Deflection of loading point, dynamic load and instantaneous crack length are measured, then crack propagating velocities are calculated. Experiments on 40Cr steel show that the results given by this method have a good agreement with that obtained by the resistance fracture gage method. Therefore this method is feasible for measuring crack propagating velocities under high loading rate and will have wide application.     

FRACTAL ANALYSIS OF FRACTURE SURFACE OF WELDED JOINT UNDER LOW CYCLE FATIGUE

Hausdorff dimension of fracture surface roughness of welded joint,both welding metal andheat-affected zone.of pressure vessel steel 16MnR,tested under strain-controlled low-cyclefatigue,was examined with computer vision srstem and by two-dimensional variation method.Results show that it decreases with the increase of cyclic hysteresis energy.The Hausdorff di-mension variation at heat-affected zone is greater than that of weld metal.It is believed thatthe greater the fractal dimension is,the longer the fatigue life will be.     

FRACTURE MODE AND PROCESS OF SiC_w B_4C_p/MB15 MAGNESIUM MATRIX COMPOSITE

ＦＲＡＣＴＵＲＥＭＯＤＥＡＮＤＰＲＯＣＥＳＳＯＦＳｉＣｗ＋Ｂ４Ｃｐ／ＭＢ１５ＭＡＧＮＥＳＩＵＭＭＡＴＲＩＸＣＯＭＰＯＳＩＴＥ①ＳｕＹｉｎｇ，ＺｈａｎｇＧｕｏｄｉｎｇ，ＷａｎｇＨｕｉｍｉｎ，ＺｈａｎｇＤｉ，ＱｉｎＪｉｎｉｎｇＳｔａｔｅＫｅｙＬａｂｏｆＭ...     

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

It is obtained in this paper that the fatigue threshold value of mode H was 1.9 times of that ofmode Ⅰ in dual-phase steel(DPS),and the normal stress intensity factor range oJ mode Ⅱbranch crack tip was 2.2 times of that of mode Ⅰ.Above results illustrate that the resistance ofmode Ⅱ crack growth was higher than that of mode Ⅰ,the former resulting fromroughness-induced shear resistance,the latter,crack closure.The mode Ⅱ component can play two important roles in near-threshold fatigue crackgrowth:(1)increasing crack tip plasticity which accelerates the crack growth and(2)intro-ducing crack surface contact and rubbing to reduce the crack propagation rate.By means ofcrack closure,the quantity of shear resistance was easily solved in this paper.The frictionshear stress strength factor range of mode Ⅱ,K_,is still much higher than the closure stressstrength factor range of mode Ⅰ,K_(Ⅰ,cl).This illustrated that the roughness enlarged the secondrole and played a role of shielding crack tip from mode Ⅱ crack.     

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

通过混合(Ⅰ/Ⅱ)型疲劳裂纹扩展门槛值和裂纹闭合效应的测试.获得双相钢纯Ⅱ型疲劳门槛值是其纯Ⅰ型的1.9倍:其纯Ⅱ型分枝裂尖正应力强度因子范围为纯Ⅰ型的2.2倍说明由同一粗糙度引入裂纹面的剪切阻力,对Ⅱ型裂纹扩展的阻碍程度将大于此粗糙度诱导裂纹闭合效应,降低裂尖Ⅰ型有效驱动力的贡献,即粗糙度对Ⅱ型裂纹扩展的阻碍比对Ⅰ型的大Ⅱ型分量在近门槛值的疲劳裂纹生长过程中能够起两个重要的作用:(1)增大裂尖塑性以至加速裂纹生长;(2)引入裂纹表面接触以至减小裂纹扩展速率本文通过裂纹闭合效应的测试,定量估算了剪切阻力,Ⅱ型摩擦剪切应力强度因子范围(△K_(Ⅱ.f))要远远大于纯Ⅰ型的闭合应力强度因子范围(△K_(ICl)),说明粗糙度的引入加大了第二种作用,从Ⅱ型裂纹角度出发起到了屏蔽裂尖的功能.     

IN SITU OBSERVATION OF DEFORMATION AND FRACTURE FOR SUPERALLOY GH169 UNDER COMBINED FATIGUE-CREEP ACTION

In Situ observation of deformation and fracture for superalloy GH169 under combinedfatigue-creep action is made by using high temperature metalloscope,it is shown that underthe test conditions the deformution takes place by merely of slipping,twinning and grainhoundary sliding,and the mode of failure depends on the microstructure of specimen.lntergranular cracks arise.from W-type voids produced by the stress concentration at triplepoint which could not be relaxed by the interior deformation of grains and the localdeformathm region along grain bounaries.And the crack propagation mechanism is thenucleation,growth and linkage of carities at the grain boundaries.Transgranular cracks formfrom deformatiom damages within the grain,and its propagation mechanism is shear rupturealong the slip phme.     

CORROSION FATIGUE CRACK INITIATION IN A MODE Ⅱ NOTCH SPECIMEN

用有限元法通过J积分计算了Ⅱ型试样的应力场强度因子K_Ⅱ·在此基础上,用高强度钢40CrNiMoA Ⅱ型缺口试样研究了疲劳和腐蚀疲劳裂纹形核的规律,并探讨了氢的影响.结果表明,疲劳裂纹不在最大剪应力位置(θ=80°)形核,而是在最大正应力位置(θ=-110°)形核,裂纹取向和最大正应力垂直(α=-80°)。如充氢或在水中腐蚀疲劳,则裂纹形核位置和取向发生了改变,即θ=-85°,α=-55°。动态充氢使缺口形成疲劳裂纹的门槛值从28.8降为10.2MPa·m~(1/2).当△K_Ⅱ较低时,腐蚀疲劳裂纹沿晶形核,这和空气中疲劳时完全不同.     

(Ⅰ+Ⅱ)MIXED MODE HYDROGEN INDUCED CRACKING

Hydrogen induced cracking(HIC)of 0.3% C,1% Cr,1% Mn,1% Si high strength steel hasbeen studied under simple mode Ⅰ,mode Ⅱ and(Ⅰ+Ⅱ)mixed mode loading conditions.Af-ter being hydrogen-charged in IN H_2SO_4 solution,the material behaved hydrogenembrittlement in all the cases studied.The threshold K_(ⅡH)/K_(ⅡX) of HIC under mode Ⅱ load-ing was 0.27,which was nearly the same as that K(ⅠH)/K_(ⅠX)=0.29 under mode Ⅰ loading.While the thresholds of-HIC under(Ⅰ+Ⅱ)mixed mode loading were 0.36,0.41 and 0.37 cor-responding to the K_Ⅱ/K_Ⅰ ratio of 0.27,0.4 and O.81.The results show that simple mode Ⅰor mode Ⅱ loading is more susceptible to hydrogen embrittlement than(Ⅰ+Ⅱ)mixed mode.For explaining the experimental results,the effects of triaxial stress as well as plasticdeformation ahead of crack tip has been discussed.     

MECHANISM ANALYSIS OF THICKNESS EFFECT ON MIXED MODE I／II FRACTURE OF LC4-CS ALUMINUM ALLOY

Mixed mode I//II fracture experiments of LC4-CS aluminum alloy were conducted by using tension-shear specimens with thicknesses of 2, 4, 8 and 14mm. Fracture mechanisms of thickness effect on mixed mode I//II fracture were first examined from fracture surface morphology to correlate with the macroscopic fracture behavior and stress state. It is found that specimen thickness has a strong influence on mixed mode fracture. As thickness varies from thin to thick the macroscopic fracture surfaces appear the characteristics of plane stress state (2mm, 4mm-thick specimen), three-dimensional stress state (8ram-thick specimens), and plane strain state (14mm-thick specimens), respectively. The specimens of all kinds of thicknesses are typical of tensile type failure under mode I loading condition and shear type failure under mode II loading condition. Two distinct features coexist on the fracture surfaces under mixed mode loading conditions, and the corresponding proportion varies with loading mixity. Void-growth processes are the failure mechanism in both predominately tensile and shear-type fractures. The size and depth of dimples on the fracture surface vary greatly with thickness. Therefore, it is extraordinary necessary to take into account the thickness effect when a mixed mode fracture criterion is being established.