压缩载荷下孔隙结构变化的CT实验研究

为了研究孔隙对岩石力学性能的影响,利用自制的孔隙物理模型,通过 单轴压缩和CT扫描实验研究了受载条件下孔隙率对岩石孔隙结构的演化及其对外部物理力 学性能的影响,得到了不同加载阶段和不同CT观察尺度下孔隙模型的裂纹扩展规律以及孔隙 和固体介质的损伤变化情况. 实验结果表明：孔隙模型在受载条件下裂纹主要发生在峰值荷 载之后,主裂纹大都集中在孔隙密集的地方且伴随许多细小裂纹的产生;峰值载荷前出现了 少数微裂纹,微裂纹的产生与演化主要发生在孔隙周边.     

裂纹分析中的单节点二次边界元

提出一种新的边界单元；单节点二次元，利用这种单元，位移及其沿边界的切向导数在正规单元端点的连续条件自然得到满足，单节点二次元能很好地模拟角点处面力多值条件，特殊裂纹尖端单节点二次单元包括近裂纹尖端位移近似级数展开第二项。由于每个单元只有一个节点，计算程序大大简化。对直裂纹，圆弧裂纹和边裂纹进行了计算。数值计算结果表明，单节点二次边界单元计算精度高，收敛性好。     

疲劳短裂纹萌生及发展的细观过程和理论

本文综述了疲劳短裂纹研究的进展和现状,包括关于短裂纹的属性,短裂纹萌生和发展的细观过程以及描述短裂纹行为的若干模型。      

不同倾角预制裂纹缺陷与运动裂纹的相互作用

为了研究运动裂纹与不同倾角预制裂纹缺陷之间的作用机制,采用数字激光动态焦散线方法对含不同倾角预制裂纹缺陷的三点弯曲梁进行冲击实验。研究结果表明,在冲击载荷作用下,预制裂纹缺陷尖端均产生了次生裂纹;当运动主裂纹与预制裂纹缺陷贯通时,次生裂纹不会立即起裂,而是经过0~10 μs的能量积蓄后,次生裂纹才起裂;运动主裂纹应力强度因子峰值与次裂纹起裂时的应力强度因子值均随预制裂纹倾角的增大而增大。     

摩擦材料在无围压和加围压条件下的形变比较

研究铜基粉末冶金摩擦材料在无围压和加围压条件下的动态力学特性及其形变特征。试验在Hopkinson压杆上完成,通过试验发现,材料在无围压、应变率低于1000-1s时,有应变率强化效应;在更高应变率下,材料有损伤软化效应。微观分析可以看到试件上大量的平行滑移裂纹导致了材料破坏,形变是滑移伴随少量的孪晶;裂缝中有大量的碳纤维组织,这些纤维状组织对裂纹的扩展起抑制作用。而材料在加围压下的性能得到了很大的改善,同一应变率下最大应力、屈服极限和动态杨氏模量均有提高。在高应变率时,无论是加围压还是无围压,破碎的硬质颗粒都是长条状的。由此得出推论,球状、细化的硬质颗粒,可提高基体的强度。     

近场波动的数值模拟

本文评述了近年来波源（或散射体）及其邻域内波动数值模拟的主要研究结果，着重讨论了人工边界条件这一关键问题的研究进展     

低温环境下列车车轮材料磨损与损伤演变行为研究

利用低温环境装置和轮轨模拟试验机开展了室温(约20℃)与-40℃温度下列车车轮材料的滚动磨损试验,研究了-40℃下车轮材料磨损和表面与剖面损伤随循环次数的演变规律.结果表明：温度的降低对车轮材料磨损和损伤机制有明显影响.与室温相比,-40℃时车轮材料疲劳磨损明显减轻,磨损率下降.-40℃工况下车轮材料磨损与损伤的形成具有明显演变特征.在磨损初期,轮轨界面发生材料转移并在轮轨界面形成稳定的摩擦膜;摩擦膜的存在降低了车轮材料磨损率.随循环次数增加,由于低磨损率,车轮表层材料在滚动载荷作用下持续累积塑性变形.在磨损后期,累积了高塑性变形的车轮材料将促进裂纹萌生.因此在车轮试样亚表层萌生大量裂纹,亚表层裂纹相互汇合,从而加速疲劳裂纹扩展.     

Effects of lateral loads and constraints on buckling of cracked thin plates under compressive edge loads

Critical buckling load is important in investigation of behaviors of thin plates or shells. The presence of cracks in these structures can affects their safety factor with respect to the common modes of failure such as buckling. Some analytical solutions were obtained for un-cracked plates with different boundary conditions. Their numerical results have good agreement with these solutions. In this paper, we also studied the effects of lateral loads and constraints on the critical buckling load of cracked plates under axial compression, experimentally and numerically. Effects of length and orientation of cracks are investigated in presence of the lateral loads. Finally, tests data are compared with the results of numerical calculations.     

Application of ultrasonics to detection of fatigue cracks

An ultrasonic system was developed and used to observe the formation of fatigue cracks in centernotched sheet specimens of unalloyed aluminum, two aluminum alloys, a mild steel, and a nickel-base alloy tested in axial tensile fatigue. S-N curves of life-to-initial detectable cracks as well as life-to-fracture were obtained. With the reflection technique, fatigue cracks that ranged in length from 0.0005 to 0.005 in. were detected while the test was in progress. Cracks were detected within approximately 1 to 3 percent of total specimen life for all of the materials considered over the range of stresses considered. The through-transmission technique was utilized to measure relatively long cracks.     

油润滑下钢严重磨损表面的分形特征

本文在ＭＨＫ－５００型环－块试验机上于２０号内燃机油润滑下对４５＾＃钢（炉冷）、４５＾＃（水淬）、ＧＣｒ１５钢和Ｗ１８Ｃｒ４Ｖ钢进行了滑动磨损试验，得到了表征这４种钢试样由轻微磨损向严重磨损转变的Ｐ（临界载荷）－Ｖ（临界滑动速度）曲线。对严重磨损表面进行扫描电子显微镜观测的结果表明，在一定的标度范围内其表现裂纹具有分裂特征。在给定的试验条件下，利用Ｓａｎｄｂｏｘ法测量出上述４种钢严重磨损表面裂纹的     

钢筋混凝土构件试验的动态数值模拟方法

本文提供由计算机图形仿真系统构成的模拟试验环境．其主要特色是：１．采用裂纹扩展的分形弯折模型描述裂纹的动态扩展，裂纹位置采用随机处理技术，使单元裂纹更接近实际；２．采用动画显示技术，模拟钢筋混凝土构件从开始加载到变形破坏的试验全过程；３．用户可以根据需要选取任意截面，显示该截面的应力应变曲线．     

钢轨打磨磨痕粗糙度与交叉磨痕对滚动接触疲劳损伤的影响

利用砂纸在钢轨试样表面打磨出不同粗糙度等级(1、4和7 μm)的单向磨痕和交叉磨痕(?45°/70°、?20°/90°和45°/70°),利用MMS-2A轮轨摩擦磨损试验机对打磨后的钢轨试样进行滚动试验并分析滚动接触疲劳损伤规律. 结果表明:打磨钢轨的滚动磨损表面粗糙度和损伤均比未打磨钢轨严重,随打磨磨痕粗糙度增加,钢轨滚动磨损表面粗糙度和损伤均呈减小趋势. 当打磨粗糙度为7 μm时,滚动接触疲劳裂纹为枝裂纹,裂纹深度较小;随打磨粗糙度减小,枝裂纹发生贯穿连通形成网状裂纹损伤,裂纹深度增加. 相较于单向打磨钢轨,交叉打磨磨痕钢轨滚动磨损表面粗糙度较低,滚动接触疲劳损伤较轻微. 此外,?45°/70°和45°/70°交叉磨痕的滚动损伤比?20°/90°交叉磨痕钢轨更加轻微.      

Acoustic-Emission Monitoring of the Deformation and Fracture of Metal–Composite Pressure Vessels

This paper presents the results of experimental studies of damage accumulation in a metal–composite pressure vessel by pneumatic strength tests. The deformation and fracture of the composite structure accompanied by matrix cracking and fiber rupture are analyzed. It is shown that the cracks and fractures generate acoustic-emission signals of various types. The results of acoustic-emission monitoring were used to develop a criterion for ranking vessels according to the strength characteristics of the pressure composite shell.     

Surface acoustic wave measurements of small fatigue cracks initiated from a surface cavity

A model for the low frequency scattering of a surface acoustic wave by a surface cylindrical cavity with two corner cracks is presented. It is applied to determine the depth of the small fatigue cracks initiated from a pit-type surface flaw. The general scattering formalism based on the elastodynamic reciprocity principle is employed. The effect of the cylindrical cavity on the surface wave reflection from cracks is considered using an approximate stress intensity factor for the corner cracks. In situ surface acoustic wave measurements have been performed during fatigue tests for an Al 2024-T3 sample. The surface wave signal was acquired continuously at different cyclic load levels. The model is verified by comparing calculated reflection signals and spectra with those from experiments. The depths of fully and partially open cracks are determined from the predicted and the measured surface wave reflections. The surface wave reflection is observed to be sensitive to crack closure.     

STUDY ON THE SURFACE CRACK GROWTH BEHAVIOR IN 14MnNbq BRIDGE STEEL

Three-dimensional crack closure correction methods are investigated in this paper.The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted.The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail.It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks.In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed.Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R=0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5.Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches.Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.     

A new look at an old technique for the measurement of fatigue crack-growth rates

An automated technique is described by which a sequence of ‘beachmarks’ (clam-shell markings) can be produced in a specimen during laboratory tests on the propagation of fatigue cracks. The subsequent use of the beachmarks for the measurement of the lengths of the cracks is described, and the intrinsic accuracy of the method is estimated. It is found that, although fewer data points are obtained by this method, they are more accurate and better characterized than those obtained by the more commonly used ‘indirect’ methods. In addition, valuable supplementary information is obtained from which it is possible to assess both the confidence intervals for the results, and the degree of crack front curvature.     

??????????????μ???????

在空气和腐蚀介质中对低合金钢分别的进行疲劳试验,获得了S-N曲线,并对疲劳试样表面和断口形貌进行了观察。结果表明：与空气相比,腐蚀介质使低合金钢的疲劳强度显著降低;空气中疲劳试样只有一个萌生于试样表面基体的裂纹源,而腐蚀介质中的疲劳试样一般有多个裂纹源,多数裂纹源均萌生于点蚀坑;空气中疲劳裂纹扩展区以疲劳辉纹为主,而腐蚀疲劳则以沿晶开裂等脆性特征为主;此外还对空气中的疲劳极限进行了预测,预测值与试验值比较吻合     

基于双向聚能拉张爆破理论的巷道顶板定向预裂

为在切顶卸压沿空留巷中获得巷道顶板预裂炮孔的最优间距,基于双向聚能拉张爆破技术,采用以LS-DYNA动力分析软件为基础的数值模拟和现场试验对巷道顶板定向预裂进行研究,优化炮孔间距。数值模拟结果表明,当炮孔间距为400 mm时,应力波叠加后产生有效的拉应力;当炮孔间距为500 mm时,应力波叠加后也能够产生有效的拉应力,该拉应力大于孔壁围岩的抗拉强度,能够使孔壁围岩沿聚能方向形成裂缝,且利于炮孔间裂纹的扩展;随着炮孔间距进一步的增大,当炮孔间距为600 mm爆破时,由于间距过大,应力波无法有效叠加,不能产生连续裂缝。现场试验表明,间距为400和500 mm的炮孔间隔爆破时,未爆破孔自炮孔孔底至孔口产生连续有效裂缝,裂缝长度达2.4 m,相邻炮孔间沿炮孔中心线均能够形成连续有效的切缝面,能够有效控制沿空巷道顶底板位移及沿空巷道顶板压力。综合分析,在3种不同炮孔间距的试验方案中,确定间隔爆破、500 mm的炮孔间距为同一地质条件下3种试验设计中的最优方案。     

An experimental and analytical treatment of matrix cracking in cross-ply laminates

The development of damage in cross-ply Hercules AS4/3502 graphite/epoxy laminates has been investigated. Specific endeavors were to identify the mechanisms for initiation and growth of matrix cracks and to determine the effect of matrix cracking on the stiffness loss in cross-ply laminates. Two types of matrix cracks were identified. These include both straight and curved cracks. The experimental study of matrix crack damage revealed that the curved cracks formed after the straight cracks and followed a repeatable pattern of location and orientation relative to the straight cracks. Therefore, it was postulated that the growth mechanism for curved cracks is driven by the stress state resulting from the formation of the straight cracks. This phenomenon was analytically investigated by a finite-element model of straight cracks in a cross-ply laminate. The finite-element results provide supporting evidence for the postulated growth mechanism. The experimental study also revealed that the number of curved cracks increased with the number of consecutive 90-deg plies. Finally, experimental results show as much as 10-percent degradation in axial stiffness due to matrix cracking in cross-ply graphite/epoxy laminates.