封面图片说明

正本期封面图片出自文章"合金成分对Ag-Cu二元合金制备纳米多孔银的影响",是由辽宁石油化工大学吴明教授和王旭博士,油气管道腐蚀与防护技术的研究、纳米材料的防护技术、轻质高强油气管线的金属材料开发等的研究总结。海洋油气储运管道受微生物硫酸盐还原菌腐蚀严重,SRB新陈代谢可以产生H_2S,代谢产物水解氢原子向金属内部扩散,氢原子不断地向裂纹尖端高应力区域聚集,裂纹尖端区域的氢浓度、氢压、应力强度因子随时间呈指数型增长,加速了金属材料的氢脆腐蚀敏感性。针对微生物对海洋复     

脆性材料复合型裂纹的断裂准则

实际工程中,脆性材料中的裂纹多处于复合型受力状态,因此,确定脆性材料中的复合型裂纹起裂角和临界荷载有着重要的理论意义和实用价值。以复合型裂纹为研究对象,将裂纹尖端的最小无量纲塑性区尺度ρmin和广义合成偏应力强度理论相结合,建立脆性材料复合型裂纹的断裂准则,预测裂纹起裂角及临界荷载,将其结果与最大周向应力准则和应变能密度因子准则相比较发现,基于该文方法得到的临界荷载曲线大于最大周向应力准则得到的临界荷载曲线,与应变能密度因子准则得到的临界荷载曲线比较接近。因而,表明了用该文的方法来预测脆性材料复合型裂纹起裂角和临界荷载是行之有效的。     

基于裂纹尖端应力比值的含裂纹功能梯度材料圆筒应力强度因子计算方法

由于功能梯度材料(FGM)性质的特殊性,现有含裂纹FGM结构应力强度因子计算方法难以避免复杂的矩阵运算以及数值积分。该文针对含外表面环向裂纹FGM圆筒,利用FGM圆筒与均匀材料圆筒裂纹尖端应力之间的比例关系,将复杂的FGM圆筒应力强度因子求解问题转化为简单的应力值提取问题以及经验公式计算问题,仅由均匀材料圆筒应力强度因子经验公式、均匀材料圆筒和FGM圆筒裂纹尖端应力比值即可得到任意含裂纹FGM圆筒应力强度因子。该方法仅需建立2D轴对称模型即可满足计算要求,在保证精度的基础上成功回避了传统方法中的复杂矩阵运算以及数值积分,且适用于不同FGM、筒体尺寸、裂纹深度等情况下的应力强度因子计算。通过多组算例对比分析,证明该方法计算精度高、计算过程简便,便于工程应用。     

超细晶粒对纳晶材料断裂韧性的影响

为了研究纳米晶体材料的断裂韧性,该文建立了一个包含两种晶粒的材料模型:超细晶粒(2nm~4nm)和普通纳晶晶粒(20nm~100nm)。超细晶粒可以看作普通纳晶晶粒三晶交的组成部分,并称包含超细晶粒的三晶交为超级三晶交,且均匀地分布在普通纳晶的基体中。裂纹尖端的应力集中会引起晶间滑移,晶间滑移又会导致超级三晶交处刃型位错的产生。该文研究了超级三晶交处的位错对临界应力强度因子的影响,结果表明超细晶粒的存在有效地提高了纳米晶体材料的断裂韧性。     

裂纹面荷载作用下多裂纹应力强度因子计算

该文基于比例边界有限元法计算了裂纹面荷载作用下平面多裂纹应力强度因子.比例边界有限元法可以给出裂纹尖端位移场和应力场的解析表达式,该特点可以使应力强度因子根据定义直接计算,同时不需要对裂纹尖端进行特殊处理.联合子结构技术可以计算多裂纹问题的应力强度因子.数值算例表明该文方法是有效且高精确的,进而推广了比例边界有限元法的...     

薄壁结构剩余强度预测方法研究进展

综述了目前比较重要的薄壁结构剩余强度预测方法,介绍了采用应力强度因子、裂纹尖端张开位移δCTOD、裂纹尖端张开角度ψCTOA、内聚力模型参数、能量耗散、临界拉伸应力以及GTN损伤模型等方法预测剩余强度的详细过程,并比较了各种方法在预测薄壁结构剩余强度时的优缺点。此外对整体结构和非整体结构在预测时的一些细节问题也进行了分析,包括多位置损伤、裂纹偏移和分叉、焊接残余应力和强度适配以及粘接结构的问题等。     

同轴单搭接接头破坏过程研究

在实验所得结果之基础上研究了预偏角对单搭接胶接接头破坏机制的影响,用弹塑性有限元法模型得到了不同裂纹长度时接头整体和胶层中心等效应力沿裂纹尖端的分布,并研究了外载大小、裂纹长度对应力强度因子的影响.结果表明,同轴接头和标准接头的破坏均从胶层界面开始,但同轴接头以形成较多小裂纹为主,而标准接头中裂纹沿着胶层向中部扩展,最终导致破坏;随裂纹长度的增加,接头上等效应力渐增,胶层中心峰值应力也增大,裂纹尖端的应力远高于其他部位;当裂纹扩展到临界尺寸时,接头会迅速破坏.     

纳米晶双峰材料的本构行为

由微米级粗晶颗粒和纳米级纳晶颗粒组成的纳米晶双峰材料不仅具有高强度,还具有较高的延性。根据Taylor强度理论和Johnson-Cook模型提出纳米晶双峰材料的一个新的本构模型,研究了晶粒尺寸和纳米裂纹对纳晶双峰材料本构及失效行为的影响,并进行了数值计算。结果表明,模型预测的结果与实验结果有很好的一致性。由计算结果可知:在纳晶双峰材料中,纳晶基体能提供高强度,粗晶能有效提高材料延性;纳米裂纹的存在不会导致破坏,反而对应变硬化起积极作用。     

不同热输入下F460钢焊接粗晶热影响区韧脆转变的内在机理

使用Gleeble 3800热模拟试验机模拟F460钢单道次焊接条件下焊接粗晶热影响区的热循环过程,通过光镜(OM)、扫描电镜(SEM)分析热影响区的显微组织、确定临界事件,通过ABAQUS软件计算临界解理断裂应力σf,进而系统分析不同焊接热输入E下韧脆转变温度变化的内在机理。结果表明:随着E的提高,焊接粗晶热影响区显微组织依次为少量板条马氏体和大量细密的板条贝氏体,板条贝氏体较多的板条/粒状贝氏体,粒状贝氏体较多的板条/粒状贝氏体,粗大的粒状贝氏体。原始奥氏体晶粒、贝氏体团的最大尺寸随着E的提高而变大。在完全解理断裂的冲击断口上,寻找停留在缺口尖端附近的残留裂纹,通过对比残留裂纹长度、原始奥氏体晶粒大小、贝氏体团尺寸,发现不同E下解理断裂的临界事件尺寸都是贝氏体团大小,而临界事件尺寸越小,韧脆转变温度越低。此外,通过有限元模拟缺口尖端的应力分布得到σf,σf越大冲击韧度越好,随着E的提高σf降低,故进一步说明随着E的提高韧脆转变温度Tk上升的内在机理。     

压力容器表面裂纹疲劳扩展的数值计算

本文通过Franc3D软件,计算了压力容器表面裂纹失稳扩展的临界尺寸及不同尺寸的初始裂纹的扩展,发现当容器表面的初始椭圆形裂纹尺寸小于3 mm×20 mm及5 mm×10 mm时,在裂纹穿透壁厚之前,裂纹尖端的应力强度因子都小于压力容器用钢的断裂韧性KIC,故该裂纹不会发生失稳扩展,且上述两个初始裂纹穿透容器壁厚时的循环载荷次数分别为40 688、45 560次。     

On the Effect of Hydrogen on the Fracture Toughness of Steel

A model is developed to quantify the effect of hydrogen on the critical stress intensity factor or fracture toughness of steels. The stress-assisted hydrogen diffusion model proposed by Liu (1970) is assumed and combined with the elastic stress field around the crack tip for quantifying the hydrogen concentration at the crack tip. Introducing a fracture criterion as the critical hydrogen concentration at a critical distance ahead of the crack tip, this model is successfully applied to the interpretation of hydrogen embrittlement behavior in a piping material. Experimental data at constant temperature were used to validate the model. With further development, the model has the potential to predict fracture toughness values at temperatures other than the test temperature.     

Effect of grain size on crack propagation of high strength steel in gaseous hydrogen atmosphere

Abstract

The effect of prior austenite grain size on the crack propagation behaviour of tempered martensitic steels having tensile strength of about 2 GN m^?2 was studied in hydrogen gas at pressures in the range from 98 to 784 kPa using modified compact tension specimens. The crack propagation rate da/dt in hydrogen decreased as the prior austenite grain size increased from 45 to 450 μm. The dependence of da/dt on hydrogen pressure at a given applied stress intensity was examined. The permeation of hydrogen from the crack tip surface was estimated to decrease with increasing grain size. However, the fractographic study suggested that the degree of embrittlement of grain boundaries increases with grain size. Consequently, the inverse effect of grain size on da/dt may be caused by a decrease of the average concentration of hydrogen along grain boundaries at the crack tip with increasing grain size.

MST/1060     

Multi-scale simulation of hydrogen influenced critical stress intensity in high Co–Ni secondary hardening steel

Hydrogen embrittlement was an important and long-standing problem in the fields of steels, especially ultra-high strength steels. In order to simulate the ability of hydrogen embrittlement resistance for high Co–Ni secondary hardening steels, a multi-scale simulation method with four steps was used to calculate the critical stress intensity (K_IC) and hydrogen influenced critical stress intensity (K_ISCC). For the four steps: the atomic scale and nm scale simulation were mainly used to simulate the effect of stress-assisted hydrogen diffusion at the crack tip; the μm scale simulation was used to handle the effect of microstructure; the cm simulation was used to analyze the size effect. As the effect of hydrogen concentration at the crack tip, the simulation results of critical cohesive strength of the Fe(110) at the crack tip decreased by 82.3%. The μm scale simulation showed the improvement of fracture toughness with the help of austenite layer between martensite laths. Compared with the mechanical properties of 300 M and AerMet100 steels, the accuracy of this simulation method was proved.     

NUCLEATION,BLUNTING OR PROPAGATION OF A NANOCRACK IN DISLOCATION-FREE ZONES OF THIN CRYSTALS

Nucleation, blunting and propagation of nanocracks in dislocation-free zones (DFZs) ahead of crack tips in ductile and brittle metals have been investigated by tensioning in situ with a TEM, and analysed using microfracture mechanics. The results show that in either ductile or brittle metals, many dislocations could be emitted from a loaded crack tip and a DFZ formed after equilibrium. The stress in the DFZ may be up to the cohesive strength of the material, and then a nanocrack is initiated in the DFZ or directly from the crack tip. In ductile metals, the nanocrack is blunted into a void or notch during constant displacement. In brittle metals, the nanocrack propagated as a cleavage microcrack rather than being blunted.     

Effect of cooperative grain boundary sliding and migration on dislocation emitting from a semi-elliptical blunt crack tip in nanocrystalline solids

A theoretical model is established to investigate the interaction between the cooperative grain boundary (GB) sliding and migration and a semi-elliptical blunt crack in deformed nanocrystalline materials. By using the complex variable method, the effect of two disclination dipoles produced by the cooperative GB sliding and migration process on the emission of lattice dislocations from a semi-elliptical blunt crack tip is explored. Closed-form solutions for the stress field and the force acting on the dislocation are obtained in complex form, and the critical stress intensity factors for the first dislocation emission from a blunt crack under mode I and mode II loadings are calculated. Then, the influence of disclination strength, curvature radius of blunt crack tip, crack length, locations and geometry of disclination dipoles, and grain size on the critical stress intensity factors is presented detailedly. It is shown that the cooperative GB sliding and migration and the grain size have significant influence on the dislocation emission from a blunt crack tip.     

Is stress concentration relevant for nanocrystalline metals?

Classical fracture mechanics as well as modern strain gradient plasticity theories assert the existence of stress concentration (or strain gradient) ahead of a notch tip, albeit somewhat relaxed in ductile materials. In this study, we present experimental evidence of extreme stress homogenization in nanocrystalline metals that result in immeasurable amount of stress concentration at a notch tip. We performed in situ uniaxial tension tests of 80 nm thick (50 nm average grain size) freestanding, single edge notched aluminum specimens inside a transmission electron microscope. The theoretical stress concentration for the given notch geometry was as high as 8, yet electron diffraction patterns unambiguously showed absence of any measurable stress concentration at the notch tip. To identify possible mechanisms behind such an anomaly, we performed molecular dynamics simulations on scaled down samples. Extensive grain rotation driven by grain boundary diffusion, exemplified by an Ashby-Verrall type of grain switching process, was observed at the notch tip to relieve stress concentration. We conclude that in the absence of dislocations, grain realignment or rotation may have played a critical role in accommodating externally applied strain and neutralizes any stress concentration during the process.     

Modelling the strongest grain size in nanocrystalline FCC metals

A physical model is proposed to predict the critical grain size at which nanocrystalline FCC metals reach a maximum steady state flow stress. The model considers that nanocrystalline metals are composed of two phases. One is the grain boundary phase and the other is the grain interior phase. The grain boundary phase has specific deformation mechanism different to the grain interior phase. The critical grain size with the maximum steady state flow stress is predicted to decrease with deformation temperature and to increase with strain rate. Both normal and abnormal Hall–Petch relations can be described simultaneously by the model.     

Evaluation and modeling of hydrogen-induced fracture in structural steels

From the macroscopic point of view, the results of hydrogen embrittlement of pearlitic steel strongly depend on compressive residual stresses generated in the vicinity of the crack tip by fatigue precracking of the specimens (the relevant variable is the maximum stress intensity factor during the last stage of fatigue precracking. As far as the kinetic crack growth curve is concerned, the threshold stress intensity value for hydrogen-assisted cracking does not have an intrinsic character but depends on the distribution of compressive residual stresses in the vicinity of the crack tip and, therefore, on the maximum stress intensity factor during the last stage of fatigue precracking. The fractographic analysis of hydrogen embrittlement tests on precracked and notched specimens of high-strength pearlitic steel reveals the existence of a special microscopic mode of fracture associated with hydrogen-induced microdamage: the so-called tearing topography surface or TTS, which can be regarded as the process zone. Hydrostatic stress is the macroscopic variable governing the extension of the TTS zone. It is demonstrated that the distribution of hydrostatic stresses in notched specimens is practically independent of the loading process and the point of maximum hydrostatic stress is a characteristic of the geometry of the problem and never changes its position. The depth of the TTS zone is a function of the electrochemical potential, the maximum fatigue precracking load, and the duration of the test, which reveals its correlation with the process of hydrogen embrittlement. These phenomenological relations can be explained by using a model of stress-assisted diffusion of hydrogen. The role of diffusion as the main mechanism of hydrogen transport in pearlitic steel is demonstrated by comparing the hydrogen-affected region and the plastic zone, which reveals no relationship between them. Hydrostatic stresses play an important role in accelerating the diffusion of hydrogen. The hydrogen-induced fracture of notched specimens of pearlitic steel is a time-dependent phenomenon admitting the kinematic modeling of the process as a function of the strain rate. In this conceptual framework, the local strain rate in the vicinity of the notch tip is the relevant variable controlling the process. For cold-drawn prestressed steel wires, the chemical modeling of the diffusion of hydrogen emphasizes the role of residual stresses generated in smooth wires in the process of manufacturing as well as the role of stresses induced by fatigue loads in precracked wires. In this case, their magnitude depends on the load (maximum stress intensity factor). In notched specimens of austenitic stainless steel, hydrogen damage can be described as multicracking in the area surrounding the notch and failure is induced by plastic instability. In this case, the action of hydrogen can be mechanically simulated as a geometric enlargement of the notch in the form of extended microdamage.     

Fracture toughness and hydrogen embrittlement of pipes with notches

A technique for experimental determination of fracture toughness and hydrogen embrittlement of pipes made of API 5L X52 steel is described. The tests were performed using arc-shaped specimens with a notch cut out from pipes under the conditions of a three-point bend. The fracture toughness was determined in terms of the J-integral and the stress intensity factor at the notch tip. The value of K _ρ,c was established using the volumetric method based on the experimentally measured critical load and the results of the FEM calculation of the distribution of elastic-plastic stresses ahead of the notch tip, and J _ρ,c was determined using the method of separable functions. The effect of hydrogen embrittlement was studied using electrolytically prehydrogenated specimens.