挤压AZ31B镁合金圆棒不同取向的疲劳裂纹扩展

使用紧凑拉伸(CT)试样,研究了挤压AZ31B镁合金圆棒三个方向的组织及疲劳裂纹扩展性能。结果表明,疲劳裂纹以穿晶为主的混合方式沿滑移带扩展。T-L方向裂纹呈直线扩展,扩展速率最高,T-R方向孪生和滑移协同塑性变形,裂纹局部偏转,呈波浪形沿径向扩展,扩展速率较低,强织构和不均匀组织引起L-T方向裂纹分叉和偏离,降低了裂纹尖端有效驱动应力强度因子幅,裂纹扩展速率最低,在疲劳裂纹扩展速率(da/dN)与应力强度因子幅(ΔK)的关系曲线上出现水平段。在ΔK较小时加载频率对裂纹扩展速率的影响不大,ΔK>3 MPa m时裂纹扩展速率随着加载频率的提高而减小。     

Ti75合金板材拉伸性能和冲击韧性各向异性的研究

通过分析组织和织构研究了Ti75合金板材拉伸性能和冲击韧性的各向异性。结果表明,Ti75板材横向(transverse direction,TD)的抗拉强度、屈服强度、伸长率和冲击韧性均优于轧向(rolling direction,RD)的对应指标。由于板材横向的屈服强度远大于轧向的屈服强度,使得板材横向屈强比(Rp0.2/Rm)远大于轧向的屈强比。Ti75板材为B/T(basal/transverse)织构类型,主要织构组分为{0002}1120(B织构)、{1013}1120(B31织构)和{1120}1010(T织构),织构造成横向和轧向拉伸时棱柱面滑移的Schmid因子不同。Ti75板材横轧向屈服强度的差异主要与织构引起晶粒滑移系启动的难易不同有关,抗拉强度主要取决于元素的强化作用,主要影响因素的不同造成了板材不同方向屈强比存在较大的差异。     

TC4ELI钛合金在3.5%NaCl溶液中的应力腐蚀行为研究

应力腐蚀是海洋装备用钛合金面临的主要威胁之一。为了给海洋装备的安全服役提供技术支撑，以海洋环境用TC4ELI钛合金为研究对象，根据恒位移WOL(楔形张开加载)试样测定K_ISCC的原理，系统开展了不同组织状态下TC4ELI钛合金在3.5%NaCl溶液中的应力腐蚀行为研究。用光学显微镜和扫描电镜观察了应力腐蚀试样断口形貌和裂纹扩展路径，用二次离子质谱仪分析了裂纹部位元素分布情况。结果表明：TC4ELI双态组织(α相与β相)板材的应力腐蚀门槛值K_ISCC为56.01 MPa·m^1/2,魏氏组织板材的应力腐蚀门槛值K_ISCC大于67.48 MPa·m^1/2,应力腐蚀试样断口主要由基体、疲劳预制区和撕裂区组成，双态组织试样疲劳预制区与撕裂区界面存在应力腐蚀区，裂纹尖端除主裂纹外还有部分次生裂纹出现；魏氏组织试样未发现明显应力腐蚀开裂特征，魏氏组织的抗应力腐蚀能力优于双态组织的。TC4ELI合金双态组织试样的应力腐蚀开裂以穿晶断裂为主，氢元素易于在裂纹尖端富集。     

方形和菱形路径下35CrMoA拉扭复合微动疲劳行为

研究了35CrMoA合金钢在接触应力为150 MPa,等效应力幅值为400 MPa时方形和菱形路径下的微动疲劳特性,包括循环应力响应特征、疲劳寿命、微动斑及微动疲劳断口的形貌特征。结果表明,方形路径下,35CrMoA钢经缓慢循环软化、快速软化到达最后的稳定阶段,而菱形加载下,材料快速软化之后直接到达稳定阶段;两种路径下的疲劳寿命差别不大;方形加载的滑移区较宽,粘着区较窄,而菱形加载则相反;方形路径下裂纹垂直于试样表面扩展,而菱形加载路径下的微裂纹是曲折的,沿与轴线成一定角度的方向上扩展。     

Ti-6Al-4V合金包覆叠轧薄板的轧制工艺对力学性能的影响

主要研究了包覆叠轧加工及热处理工艺对Ti-6Al-4V合金室温拉伸及疲劳性能的影响规律,用金相显微镜和扫描电镜观察分析了疲劳裂纹的扩展路径及断口形貌.结果表明,在交叉轧制状态下,Ti-6Al-4V合金板材的织构较弱,且该状态下的疲劳性能最好,疲劳裂纹尖端有塑性钝化;而在较低轧制温度和单向轧制状态下,合金板材具有强烈的织构,疲劳裂纹呈快速扩展.     

AZ31镁合金的变形织构和协调变形机理

进行变形速率可控的单向拉伸试验,研究了变形织构与滑移和孪生等协调变形机理对AZ31镁合金综合性能的影响。结果表明:在沿挤压方向拉伸过程中,变形织构使{0002}晶面Schmid因子较低,基面滑移难以开动,屈服强度高。在沿45°拉伸过程中,变形织构使柱面取向晶粒处于发生{0002}滑移的最佳位置,基面取向晶粒的棱柱面滑移也处于最佳位置,屈服强度低而延伸率高。沿横向拉伸的力学性能主要受孪晶影响,由于大量孪晶诱发裂纹,延伸率最低。试样在45°和横向拉伸时产生的大量拉伸孪晶,是出现{0002}双峰织构的诱因。     

爆炸应力波作用下空孔周围裂纹群的扩展规律

为探究爆炸载荷下空孔应力集中区裂纹群扩展规律,采用爆炸加载透射式焦散线系统,研究单炮孔作用下空孔周围预制裂纹扩展的动态行为。研究结果表明:在空孔与炮孔连线方向上,空孔迎爆侧、背爆侧预制裂纹起裂并扩展,其余方向预制裂纹没有扩展,背爆侧裂纹扩展长度大于迎爆侧裂纹扩展长度;迎爆侧、背爆侧裂纹不会同时起裂,两者起裂时间差为40μs左右,迎爆侧裂纹在扩展阶段的速度和应力强度因子峰值大于背爆侧裂纹相应值;空孔对于爆生裂纹的导向作用随炮孔与空孔距离L的增大越来越不明显,迎爆侧、背爆侧裂纹扩展速度、应力强度因子值随着L的增大而减小。     

18—8奥氏体不锈钢应力腐蚀破裂前端微区电化学行为的研究

本工作利用KCl-Ag/AgCl微参比电极和钨微电极研究了在试样预裂纹顶部应力集中的微小区域里,应力对1Cr18Ni9Ti不锈钢在3.5％NaCl溶液中电化学及化学状态(溶液酸度)的影响。研究表明,在应力腐蚀过程中,微区的电化学和化学状态与应力的存在有关。在发生塑性变形的条件下,应力他材料发生滑移,导致表面钝化膜的破坏,使微区电位下降,每次施加应力均使微区电位突降约40毫伏,当再钝化过程受阻,裂纹开始形成时,由于金属的大量溶解,使这里的pH值迅速下降到1～2之间,造成裂纹形成时它周围的低电化、低pH环境。     

单向围压作用下切缝药包爆破爆生裂纹扩展规律的研究

为了研究单向围压作用下切缝药包爆破爆生主裂纹的动态力学行为,采用新型数字激光动态焦散线实验系统开展多组爆破光测实验,同时使用LS-DYNA有限元软件分析初始应力对爆炸应力场的影响。结果表明:裂纹的扩展方向和延伸长度受控于切缝角度和压力的共同作用,当压力垂直和平行切缝方向时,分别能够抑制和促进爆生主裂纹的萌生及演化,降低和提升扩展速度及动态应力强度因子,当压力与切缝成一定角度时,主裂纹偏向压力方向扩展,动态力学参数减小。数值分析结果显示,当压力分别垂直和平行切缝方向时,初始围压静加载在环向形成压缩和拉伸预应力,能够降低和提高爆炸冲击在环向的拉伸作用,从而削弱和增强爆炸荷载的冲击致裂能力。     

外加电位对X80钢在玉门土壤模拟溶液中应力腐蚀的影响

采用慢应变速率拉伸实验研究X80钢在土壤模拟溶液中的应力腐蚀敏感性。以玉门地区的碱性土壤为基础,分析不同电位对应力腐蚀的影响。用扫描电镜对断口及二次裂纹形貌进行观察。结果表明:阳极电位下X80钢应力腐蚀敏感性不高。高阳极电位下,阳极溶解在一定程度上抑制了应力腐蚀。阳极电位较弱以及开路电位下,阳极溶解较弱,裂尖和其他表面存在溶解性差异,这些因素有利于裂纹扩展。但是较慢的溶解速率以及相对高的应变速率使得裂纹没有足够时间发生有效扩展,应力腐蚀敏感性仍然较低。当外加阴极电位时,裂尖发生阳极溶解而其他位置受到阴极反应抑制,应力腐蚀敏感性增加。随着阴极电位降低,不断增加的氢影响裂纹萌生和扩展,应力腐蚀敏感性随外加电位的降低而增大。     

The effects of texture in titanium alloys for engineering components under fatigue

The mechanical response of textured Ti 6/4 plate material is assessed through an evaluation of monotonic properties under tension and torsion loading and fatigue testing of plain section and notched specimen geometries. Significant variations in modulus, yield strength, ultimate tensile strength and ductility are demonstrated for testpieces taken from the plate materials parallel to either the transverse or longitudinal rolling direction. Cyclic performance is also shown to be sensitive to orientation with different cyclic stress–strain curves defined in the two orientations. The relationship between the principal stress axis and the dominant basal plane texture is shown to control fatigue crack initiation lives and the ultimate mode of fracture. Whilst loading parallel to the transverse direction offers the strongest monotonic and cyclic stress–strain response, fatigue tests performed on specimens orientated parallel to the longitudinal rolling direction provide the optimum cyclic life. These effects are discussed with reference to the inherent, anisotropic mechanical response of α+β titanium alloys, which results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.     

锈胀开裂钢筋混凝土粘结疲劳性能试验研究

重复加载和锈胀开裂均会导致钢筋混凝土粘结性能退化,进而对钢筋混凝土构件力学性能产生不利影响。该文开展了一系列偏心拔出试验,研究了重复加载以及锈胀开裂对粘结滑移性能的耦合影响规律。主要研究变量包括重复加载次数、应力水平以及钢筋锈蚀程度。结果表明:重复加载对非锈蚀试件和锈胀开裂试件粘结强度及峰值滑移没有显著影响,但会导致钢筋和混凝土之间不断累积残余滑移。重复加载后,粘结应力-滑移曲线形态特征与单调加载试件相似。该文还发现表面锈胀裂缝宽度对粘结强度、峰值滑移以及残余滑移的增长规律有明显影响。锈胀开裂会导致钢筋混凝土试件粘结疲劳寿命显著下降。基于试验数据及文献中研究结论,该文建立了重复及单调荷载作用下非锈蚀及锈胀开裂试件的局部粘结应力-滑移本构关系模型,推导得到了粘结疲劳寿命预测模型。     

7055铝合金T型型材的应力腐蚀行为研究

通过拉伸性能测试、C环应力腐蚀试验、金相分析、扫描电镜和透射电镜观察等研究了7055铝合金T型型材的应力腐蚀开裂(SCC)行为。结果表明:7055铝合金T型型材纵向试样的抗拉强度、屈服强度、伸长率及断面收缩率均大于横向试样的;在间浸腐蚀和恒温恒湿环境下,纵向C环试样的开裂时间均长于横向试样的。型材纵向截面晶粒变形特征明显,主要呈拉长状,横向试样的裂纹沿纵向扩展,裂纹一旦形成,沿着拉长晶界迅速扩展,裂纹平直,开裂时间短;型材横向截面晶粒呈等轴状,纵向试样的裂纹沿横向扩展,裂纹扩展过程中会沿着晶界发生偏折,开裂时间长。7055型材应力腐蚀机理为氢脆和电化学理论的共同作用。      

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Abstract

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.     

Orientation dependence of fracture in copper bicrystals with symmetric tilt boundaries

Experimental results (Wang and Anderson (1991), Acta Metall. 39, 779–792) show that the fracture behavior of Σ9 copper bicrystals depends on the cracking direction. Near-interface transgranular fracture surfaces were observed in the case of the crack growing in the [ 14] direction, with an essentially ductile failure mode, while the case of the crack growing in the [1 ] direction showed far less toughness and had an intergranular fracture surface with cleavage tongues. Asymptotic and finite element models for stationary cracks in ideally plastic and strain hardening materials have been used to examine this cracking direction dependency from a small strain continuum mechanics point of view. The tensile stress ahead of the crack tip was found to be essentially identical for the two growth directions, with the brittle orientation resulting in only slightly higher stress values at small distances from the crack tip. However, the strain field was found to be different for the two orientations, with the overall plastic zone size being much larger in the ductile case. Also, the orientations of the zones of concentrated shearing ahead of the crack, observed in the ideally plastic model, suggest two different dislocation shearing mechanisms. In the ductile case, this zone is parallel to the slip plane, resulting in a regular shearing mechanism in which dislocations can be nucleated at the crack tip and glide on the (111) slip plane. In contrast, this zone is perpendicular to the (111) slip plane in the brittle case, resulting in a kinking shear mode in which dislocations from other external sources expand in a dipole mode to produce macroscopically concentrated shearing. Thus, apart from the dislocation nucleation considerations, continuum mechanics does not seem to be able to fully explain this difference in directional dependency of fracture in the Σ9 copper bicrystals.     

Effect of Cyclic Loading on Cracking Behaviour of X-70 Pipeline Steel in Near-Neutral pH Solutions

The cracking behaviour of X-70 pipeline steel in near-neutral pH solutions was studied under different modes of cyclic loading. The crack propagation process of X-70 pipeline steel under low frequency cyclic loading condition was controlled mainly by stress corrosion cracking (SCC) mechanism. Under mixed-mode cyclic loading, both higher tensile stress and shear stress made cracks easier to propagate. Applied cathodic potentials and high content of carbon dioxide in solutions also promoted the propagation of cracks. The propagation directions of cracks were different under different cyclic loading conditions. Under mode I (pure tensile stress) cyclic loading condition, cracks were straight and perpendicular to the tensile stress axis, while under mixed-mode I/Ill (tensile/shear stress) cyclic loading, cracks were sinuous and did not propagate in the direction perpendicular to the main tensile stress axis. Under the mixed-mode cyclic loading, cracks were much easier to propagate, suggesting that shear stress intensified the role of tensile stress. In addition, shear stress promoted the interaction between cracks, resulting in easier coalescence of cracks.     

Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.     

SCC Behaviour of X-70 Pipeline Steel in Near-Neutral pH Solutions

The stress corrosion cracking (SCC) behaviour of X-70 pipeline steel in near-neutral pH solutions was studied via slow strain rate testing (SSRT). The results showed that the cracking mode of X-70 pipeline steel in near-neutral pH solutions was transgranular at different temperatures and applied potentials with the feature of quasi-cleavage. The pH value of the solution decreased with increasing the addition of CO2, which increased the susceptibility to SCC. SCC susceptibility increased as the applied potential moved towards the cathodic direction, suggesting that hydrogen induced cracking (HIC) dominated the cracking process at cathodic potentials. The slight decrease of pH values with decreasing temperature of the solution increased the susceptibility to SCC, which attributed to the change of solubility of CO2 in the solution at different temperatures. The propagating directions of SCC cracks were different at different potentials. At rather negative cathodic potentials, the cracks were almost perpendi     

敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀的影响

为了研究敏化处理对Z3CN20-09M不锈钢高温水应力腐蚀行为的影响,使用敏化处理的Z3CN20-09M不锈钢制成U弯试样,并置于250、290及320℃的高温水中进行应力腐蚀开裂实验,采用扫描电镜观察了高温水实验后试样的氧化膜厚度以及应力腐蚀裂纹的萌生及扩展行为.结果表明:敏化处理增加了氧化膜的厚度,降低了耐蚀能力,使SCC敏感性增大;温度较高时,敏化处理的影响较大;铁素体相容易被侵蚀,大多数点蚀坑产生于铁素体中;SCC裂纹优先在点蚀坑底部和奥氏体/铁素体相界位置处形成;相界面对SCC裂纹的影响取决于SCC裂纹相对于相界面的取向,SCC裂纹扩展方向平行于相界面时裂纹易沿着相界扩展,SCC裂纹扩展垂直于相界面方向时相界面对裂纹扩展起阻碍作用.     

Multiple transverse fracture in 90° cross-ply laminates of a glass fibre-reinforced polyester

Specimens of a 90° cross-ply glass-reinforced polyester were tested in tension in a direction parallel to one of the directions of reinforcement. Extensive cracking of the transverse ply occurred at strains much lower than the resin failure strain. These cracks formed in a direction parallel to the transverse reinforcement and showed a remarkably even crack spacing. Results of crack spacing measurements are presented against applied stress for specimens with differing transverse-ply thicknesses. The transverse-crack spacing was found to decrease with increasing applied stress and to increase with increasing transverseply thickness. There was no evidence of debonding between the plies during cracking and a multiple cracking theory in which the plies remain elastically bonded has been presented which can account for the results.