分布系统中多米诺效应的分析与消除

分布系统中发生故障后,错误信息在进程间的传播,以及由此引起的系统恢复过程中的多米诺效应是分布系统容错恢复技术所面临的最重要与最困难的问题.目前各种研究工作偏重于具体的系统动态恢复技术.本文的研究重点在于首次揭示多米诺效应本身固有的数量特征及其与分布系统技术参数间的联系,给出多米诺效应动态消除技术,引入系统额外开销的下限,并针对动态技术的缺点,探讨多米诺效应的静态消除技术,为解决困难开辟新途径.     

T-stress in orthotropic functionally graded materials: Lekhnitskii and Stroh formalisms

A new interaction integral formulation is developed for evaluating the elastic T-stress for mixed-mode crack problems with arbitrarily oriented straight or curved cracks in orthotropic nonhomogeneous materials. The development includes both the Lekhnitskii and Stroh formalisms. The former is physical and relatively simple, and the latter is mathematically elegant. The gradation of orthotropic material properties is integrated into the element stiffness matrix using a “generalized isoparametric formulation” and (special) graded elements. The specific types of material gradation considered include exponential and hyperbolic-tangent functions, but micromechanics models can also be considered within the scope of the present formulation. This paper investigates several fracture problems to validate the proposed method and also provides numerical solutions, which can be used as benchmark results (e.g. investigation of fracture specimens). The accuracy of results is verified by comparison with analytical solutions.     

Study on ASTM E399 and ASTM E1921 standards

Costs of ASTM E399 and ASTM E1921 tests, which were developed to determine the fracture toughness (K_Ic) and the ductile–brittle transition temperature of ferritic steels, respectively, are considered high and the procedures are also very complicated. In this study, a method, which is more cost‐effective and easier to carry out, is proposed.     

Failure analysis of three reconditioned rail car couplers

Three reconditioned rail car couplers, arbitrarily identified as couplers, failed during service due to cracking of the front face. The three couplers were reportedly upgraded by quenching and tempering to enhance strength and toughness. The objective of this investigation was to determine the mode of crack propagation and the root cause of the observed failures. Verl E. (Bud) Kahle, P.E., was a professional failure analyst for more than forty years. After receiving his Bachelor of Science degree in physical metallurgy from Washington State University 1957, Bud was employed by the General Electric Company at a facility in Hanford, Washington. He later was employed by the Aerojet General Corporation and eventually was head of the Southern Pacific Materials Science Laboratory in Sacramento. After retirement from Southern Pacific he owned and operated his own firm, Kahle Metallurgical Consulting Services, for fifteen years. He came to be known as “the man with the answers” to clients in a variety of industries. Bud was a Life Member of ASM International. Many of Bud’s clients remarked on the high quality of his failure analysis case histories and encouraged him to have them published. After Bud passed away in 1999, his wife, Suzanne, approached ASM International to see if the society would be interested in publishing any of his work. Mrs. Kahle was able to supply a large number of reports, which Bud had carefully archived over the years. Journal of Failure Analysis and Prevention Associate Editor Michael Stevenson read through the collected case histories, selected the ones to be included in this issue, and edited them for journal publication. ASM International wishes to thank Mrs. Kahle for providing these case histories for publication.     

An investigation of crack closure and the propagation of semi-elliptical fatigue cracks in Q1N (HY80) pressure vessel steel

The results of an experimental investigation of the effect of crack closure on the propagation of semi-elliptical fatigue cracks are presented. Load-shedding fatigue threshold tests were carried out at stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure was measured at the surface and depth positions using backface strain gauges, near-tip gauges, and a clip gauge. Differences between the surface and depth growth behaviour are explained by considerations of the effects of the transition from plane stress conditions at the surface to plane strain conditions at the depth. The effects of stress ratios are attributed largely to differences in the crack opening displacement, which result in asperities coming into contact to induce roughness-induced crack closure.     

Synergism of Toughening Mechanisms in Whisker-Reinforced Ceramic-Matrix Composites

The improved fracture resistance of whisker-reinforced ceramic-matrix composites involves more than one energy-absorbing mechanism. The possible mechanisms are reviewed and a micromechanical model evaluating the relative contributions to the overall toughness is presented. The mechanisms involve microcracking, load transfer, bridging, and crack deflection. The synergism of these mechanisms is examined using an energy release rate balance equation. The basic assumption of the proposed model is that the load transfer between the matrix and the whiskers is due to Coulomb friction. The model has been applied to an Al₂O₃/SiC whisker composite and shows reasonable agreement with reported experimental results. The role of the thermal residual stresses is also examined in light of the frictional load transfer assumption.     

Some experimental findings in compression-after-impact (CAI) tests of CF/PEEK (APC-2) and conventional CF/epoxy flat plates

Compression-after-impact (CAI) tests have been conducted for quasi-isotropic thick plates with 48 plies by using the NASA method and on plates with 32 plies by using the SACMA method. Specimens are made of CF/PEEK (APC-2) and conventional CF/epoxy for the NASA plates and CF/epoxy for the SACMA plates. In the NASA CAI tests, the sequence of delamination buckling and its propagation is clearly revealed through various experimental techniques. One major technique is moiré topography, and the other is thermo-mechanical stress analysis with a high-accuracy infrared sensor. The arrest of delamination propagation just before catastrophic failure due to high fracture toughness is clearly captured by the moiré camera. This behavior provides good CAI values of CF/PEEK. The initial buckling properties of the delaminated region by the impact are then extensively discussed. Numerical predictions of initial buckling stress have been obtained by modelled geometry of the delaminated region simplified from its precise structure clarified by ultrasonic C-scanning. They agree fairly well with the experimental results. The in-plane stress distribution in the delaminated region before initial buckling is measured by an infrared stress graphic system. This compared favorably with finite element predictions. Two types of symmetric buckling modes with respect to the central plate surface, twin and single peak ones, are experimentally captured.     

SA299钢冷脆行为的研究

某锅炉厂生产的锅炉汽包,在冬季制造过程中发生破裂,汽包材料为 SA299钢,对断口的初步分析发现为脆性解理断口,据此认为是冷脆破坏。本文依据冷脆特征温度 Tc 的概念对 SA299钢的冷脆行为进行了细致的分析。     

Fatigue crack growth analysis of welded aluminium RHS T-joints with manipulated residual stress level

The fatigue life of a welded aluminium T‐joint made from beams with rectangular hollow section (RHS) has been predicted using a crack propagation analysis and compared with experimental results from joints with different residual stress levels. To include the effect of the residual stresses, the stress ratio was calculated at the weld toe and, via Walker's equation, introduced into the analysis. How to obtain the Walker exponent has been discussed in detail. The introduction of a stress ratio at the weld toe provides good agreement between the experimentally and analytically found S–N curves. The effect of the residual stress was successfully included in the analysis.     

精对苯二甲酸反应器桨叶断裂分析与修复

为了保证进口精对苯二甲酸装置氧化反应器中钛制搅拌器的正常工作，对钛制搅拌器桨叶在运行中断裂脱落的原因进行了分析，并叙述了桨叶修理的方法和要求。经修理后的桨叶投入生产使用，达到了预期的目标。