散斑干涉微区应变测量术在腐蚀疲劳裂尖形变研究中的应用

建立了计算机控制自动化散斑干涉微区应变测量系统,介绍了工作原理及其在腐蚀疲劳裂尖形变行为研究中的应用,该方法可测定材料裂尖局部应变场。通过比较A537钢充氢前后裂尖应变场的变化与阳极溶解对纯Cu裂尖应变分布的改变、研究了腐蚀疲劳过程中裂尖材料的化学-力学效应,结果表明:充氢使裂尖应变减小、阳极溶解促进变形     

阳极电流对纯铜裂尖表面形变和总体形变影响的对比

采用微机控制激光散斑干涉技术，对多晶纯铜平板预裂纹试样在空气及在３．５％ＮａＣｌ溶液中外加阳极电流条件下的裂纹张开位移进行了原位测量，并对比了这两种条件下，裂尖表面应变场的变化，研究了阳极溶解对裂尖材料总体形变和表面形变的影响。结果表明：加阳极电流后，裂尖表面应变量和应变范围都明显增大，而反映裂尖总体形变的张开位移变化很小。阳极溶解对金属内部材料的形变没有直接的影响，只能缓解金属表面的变形硬化，促进金属的表面变形．     

阴极充氢对结构钢裂尖变形行为的影响

本文采用单边缺口试样,原位研究了A537结构钢在3.5%NaCl水溶液中,-1400mV(SCE)电位下阴极充氢6小时前后,疲劳裂纹自由面张开位移和裂尖表面应变场的变化.同时还对充氢与未充氢条件下光滑圆柱试样的应力应变行为进行了对比研究,用扫描电镜观察了疲劳断口形貌.结果表明:由于氢的进入,疲劳裂尖材料变形困难,从而导致了裂尖塑性区尺寸与裂纹相对张开位移的减少.     

A537钢在3.5%NaCl溶液中低周腐蚀疲劳的裂纹萌生

利用三电极技术研究了Ａ５３７低合金钢在３．５％ＮａＣｌ活化体系中的低周腐蚀疲劳的裂纹萌生行为．结果表明，应变速率越小，或者应变幅越大，裂纹萌生寿命越短；阳极极化和强阴极极化可以大大缩短裂纹萌生寿命，阴极保护条件下的裂纹萌生寿命与空气中的相似．裂纹主要在软的铁素体内萌生，然后穿过珠光体连接．剧烈的阳极溶解破坏了表面形变形貌，阴极析氢导致表面产生许多小裂纹．     

Fracture in strain gradient elasticity

Recent experiments have shown that the microscale material behavior is very different from that of bulk materials, and displays strong size effects when the characteristic length associated with the deformation is on the order of microns. Conventional continuum theories, however, can not predict this size dependence because they do not have an intrinsic length in their constitutive models. A new continuum theory, namely the strain gradient theory, has been proposed to investigate the deformation of solids at the microscale. For materials undergoing plastic deformation, the basis of strain gradient theory is the dislocation theory in materials science, and strain gradient plasticity has agreed remarkably well with experiments. For elastic materials with microstructures, it has also been established that the material behavior can be represented by an elastic strain gradient theory. A general approach to investigate fracture of materials with strain gradient effects is established. Both the near-tip asymptotic fields and the elastic full-field solutions are obtained in closed form. Due to stain gradient effects, stresses ahead of a crack tip are significantly higher than those in the classical K field. The plastic zone size surronunding a crack tip is estimated by elastic near-tip fields, as well as by the Dugdale model. It is established that the plastic zone is, in general, much more round and larger than that estimated from the classical K field.     

INFLUENCE OF APPLIED POTENTIAL AND LOADING WAVEFORM ON FATIGUE CRACK GROWTH FOR STEEL A537 IN 3.5% NaCl SOLUTION

Influence of applied potentials and loading waveform on the fatigue crack growth forsteel A537 in 3.5% NaCl solution,and corresponding straining electrode behaviour havebeen studied.Under the applied potentials over or below -800mV(SCE),the anodicdissolution or the hydrogen embrittlement is predominatant,respectively.For appliedanodic potential,the acceleration effect of continuous loading pattern on the CF crackgrowth mainly appeared at the range of low ΔK values,while for cathodic potential,itappeared at the high ΔK values.The continuous straining causes a decrease of naturalpotential and an increase of anodic dissolution current.     

Effect of stress on electrochemical characteristics of pre-cracked ultrahigh strength stainless steel in acid sodium sulphate solution

The electrochemical behaviors of ultrahigh strength stainless steel Cr12Ni5MoCo14 at crack tip under applied stress were studied by micro-electrochemical measurements as well as finite element analysis. The non-uniform distribution of the stress and strain induces a higher electrochemical activity at crack tip and promotes the anodic dissolution rate. The corrosion rate increases with increasing applied stress. In the elastic stress range, the effect of the applied stress on the electrochemical behaviors of Cr12Ni4Mo2Co14 steel is small. In the plastic stress range, the plastic deformation has a dramatic effect on the mechanical–electrochemical interaction and enhances the anodic activity.     

腐蚀疲劳过程中裂尖阳极溶解对裂纹扩展的作用

在模拟腐蚀疲劳裂尖介质中,分别用铂丝和40CrNiMo钢为辅助电极测定了在不同拉伸速率下中温回火的40CrNiMo钢的腐蚀电流密度i_(a)与应变速率、塑性应变量的关系,其结果为:利用疲劳裂纹尖端应变的弹塑性有限元计算,得到腐蚀疲劳裂尖阳极溶解引起的裂纹扩展与△K的关系,结果表明:中温回火的40CrNiMo钢在3.5％NaCl水容液中裂尖阳极溶解对腐蚀疲劳裂纹扩展的直接贡很小,阳极溶解的主要作用是影响为裂尖断裂区氢致开裂提供氢原子的阴极过程。     

阳极电位下小幅载荷在腐蚀疲劳裂纹扩展中的作用

研究了A537钢在3.5%NaCl水溶液中,阳极溶解条件下以不同方式叠加在主载荷ΔP上的小幅载荷对疲劳裂纹扩展da/dN～ΔK曲线的影响,并与空气中的结果进行比较,讨论了阳极溶解影响小幅载荷行为的机理.结果表明:空气中幅度小于30%ΔP的小载荷叠加在主载荷底部时对裂纹扩展没有影响,而阳极溶解能使这些主载荷底部的小幅载荷对裂纹扩展过程产生重要作用,在阳极溶解条件下,即使是幅度为零的保持载荷(B0)对裂纹扩展也具有明显的加速作用;虽然阳极溶解对主载荷顶部的幅度为30%ΔP(T30)和70%ΔP(T70)的小载     

The effects of film formation and mechanical factors on the initiation of stress corrosion cracking of unalloyed steels in carbonate solutions

Investigations into the intergranular stress corrosion cracking (SCC) of unalloyed steel St 36 in carbonate-bicarbonate solutions have shown that SCC susceptibility is limited to the potential range of magnetite formation for both constant load and constant strain rate tests. Experiments to determine the creep behaviour of the steel in the corrosive solution show that creep rates corresponding to the critical strain rates in the constant strain rate test occur during the crack initiation state after loading of the tensile specimen with a constant crack initiating load. These experimental findings establish, therefore, that critical crack initiating strain rates of the unalloyed steel are always present during the initial stage for SCC regardless of the test method used. The cause of crack initiation at grain boundaries can be attributed to the fact that the magnetite layers feature faults and grooves in the oxide along the grain boundaries in the entire critical potential range. These oxide grooves, which are also observed on unstressed steel, cause a notch-like localized corrosion attack. The depth and crack tip radius of the resultant grain boundary notches appear to be sufficient to initiate propagating cracks under the single action of predominately pure stresses without macroscopic deformation of the steel. This does not necessarily mean, however, that a purely stress induced corrosion process is involved, for it is not out of question that the concentration of stress in the base of grain boundary notches is sufficient to cause localized deformation processes required for crack initiation.     

310奥氏体不锈钢应力腐蚀的透射电镜原位观察

用自制的恒位移加载台，在透射电镜中原位观察310奥氏体不锈钢在纯水中局部溶解前后裂尖位错组态的变化以及微裂纹的形核和扩展。结果表明，310奥氏体不锈钢在室温纯水中局部阳极溶解能促进位错发射，增殖和运动，在低应力下，纳米级微裂纹在无位错区中连续或不连续形核，由于介质的作用，纳米级微裂纹并不钝化成空洞或缺口，而是解理扩展。     

高强度结构钢的韧性启裂和稳态扩展

研究了具有一定韧度的高强度结构钢裂纹早期扩展阻力与材料的微观组织结构、应力状态、变形历史和应变特性之间的关系。结果表明:裂纹扩展阻力(指J_R阻力曲线中的J_i和dJ/da)主要决定于裂纹尖端的塑性约束程度和应力应变场中消耗的弹性能和塑性功。这与三轴应力状态函数σ_mσ~(1/2)有关。材料的流变曲线描述了形变过程中的变形历史和应变特性,有明显的阶段性,其中第Ⅱ阶段的变形历史和应变特性制约着裂纹的启裂过程,第Ⅲ阶段的变形历史和应变特性制约着裂纹的扩展过程。应变硬化率dσ/dε可以敏感地反映出材料内部的应力分布和微观断裂机制。     

SCC MECHANISM OF STAINLESS STEEL AISI 321 IN ACIDIC CHLORIDE SOLUTION

It is verified that stainless steel AISI 321 is in the active anodic dissolution state in 0.5mol/L HCll+0.5 mol/L NaCl solution at 55℃.The SCC of the steel in the solution cannot be reasonably explained by passive film rupture-nepasivation theory and by hydrogenembrittlement theory.There are evidences that the fracture stress at the tip of the cracksis reduced by anodic dissolution due to its role in relieving strain hardening layer at cracktip.     

阳极溶解促进位错发射和运动的TEM原位观察

用自制的恒位移加载台在ＴＥＭ中原位观察了３１０不锈钢在水中阳极溶解前后加载裂尖前方位错形貌的变化．在排除室温蠕变影响后发现，阳极溶解能明显促进位错发射、增殖和运动．３１０不锈钢薄膜试样在室温水中能产生应力腐蚀．     

Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging

We investigated the hydrogen embrittlement of a Fe–18Mn–1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10^?6 s^?1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe–Mn–C twining-induced plasticity steel.     

DUCTILE CRACK INITIATION AND STEADY-STATE PROPAGATION OF HIGH STRENGTH STRUCTURAL STEEL

The resistance to crack propagation at earlier stage for a high strength structural steel withcertain ductility and its correlation to microstructures,stress states,deformation history andstrain characteristics have been investigated.The resistance to crack propagation is mainly de-termined by the plastic constrain ahead of the crack tip,the elastic energy and plastic workabsorbed in the stress-strain field.These are connected with the state function of triaxialstress.The deformation history and strain characteristic during deformation of material aredescribed by the flow line in which the deformation history and strain characteristic restrainthe crack initiation at stage Ⅱ and the crack propagation at stage Ⅲ.The strain hardeningrate may sensitively reflect the stress distribution and micro-fracture mechanism in the interi-or of material.     

强度错配双界面约束条件下纯铝层TEM原位拉伸观察

利用透射电镜（TEM）中的原位拉伸台对纯金属铝及受强度错配双界面约束的纯铝层拉伸过程进行观察。结果表明：纯金属铝拉伸过程中裂端形成微孔洞后即与主裂纹连接，在界面约束条件下，裂端形成微孔后并非立即与主裂纹连通，而是在更前沿位置产生更多的微孔洞，即发生连续孔洞化。拉伸过程中，裂尖钝化与锐化的现象共存，随着中间纯铝层厚度的降低，裂端发射的位错在其周围晶界形成越来越严重的塞积，大大提高了晶界及裂端前沿应力场强度，引起中间纯铝层的脆化。     

黄铜在氨水中的溶解过程对其局部塑性变形的促进作用

用激光云纹干涉法研究了黄铜在氨水溶液中应力腐蚀过程中缺口前端位移场和应变场的变化。结果表明，在应力腐蚀裂纹萌生前，溶解（腐蚀）过程本身能使缺口前端性区增大，与此同时也使塑性区中各点的塑性变形量增大，即溶解过程本身能促进塑性变形。     

焊接接头裂纹尖端应变场的云纹干涉法

以海洋平台用钢Ａ５３７采用ＬＢ５２ＮＳ焊条施焊的高匹配中心裂纹试件为例，本文引入一种新的光力学方法－反射式贴片云纹干涉法及其剪切技术，对该方法的基本原理与试验技术进行了系统论述，并结合小型宽板拉伸试验成功地应用于接头裂纹尖端应变场的测试，由于该方法有高灵敏度（虚栅频率≥６００Ｌ／ｍｍ）和极宽的可调量程，对横向应变场的测试更为有利。     

腐蚀促进位错发射、运动及SCC形核的TEM原位观察（Ti_3Al＋Nb──甲醇体系）

在ＴＥＭ中研究了Ｔｉ_３Ａｌ＋Ｎｂ在甲醇中的应力腐蚀开裂（ＳＣＣ）过程，结果表明，腐蚀过程能促进位错发射、增殖和运动；当其发展到临界条件时，纳米级应力腐蚀裂纹就在无位错区或裂尖形核。