锅炉水冷壁管开裂失效分析

针对锅炉水冷壁管的开裂原因进行了失效分析。采用宏微观形貌分析、化学成分分析、金相显微组织分析等手段,分析了锅炉水冷壁管开裂的原因,结果表明,锅炉水冷壁管的开裂类型为高温蠕变开裂,使用中超温是导致该锅炉水冷壁管开裂的主要原因。     

304不锈钢酸性气体压力管道开裂失效分析

针对304不锈钢酸性气体压力管道开裂的开裂原因进行了失效分析。采用宏微观形貌分析、化学成分分析、金相显微组织分析等手段,分析了压力管道失效的原因,结果表明,管道的开裂类型为晶间腐蚀开裂,不锈钢管道焊接热影响区敏化是导致该不锈钢管道开裂的主要原因。     

不锈钢换热管U形部位开裂分析

针对某石化厂换热器运行较短时间后多根换热管U形部位出现环向开裂的现象,在开裂换热管开裂部位与远离开裂部位分别取样,通过宏观检查、光谱分析、显微硬度分析、金相分析、断口扫描电镜形貌和能谱分析,得出该换热管的开裂原因为:换热管在含氢环境下,产生大量氢致马氏体,导致产生氢致裂纹;裂纹在扩展的过程中,S、Cl~-及非金属夹杂物的存在对开裂起到了一定的促进作用,最终发生开裂。     

催化装置外取热器管程出口三通焊缝开裂失效分析

针对某催化装置外取热器管程出口三通焊缝开裂失效的情况,通过化学成分分析、有限元应力分析、扫描电镜分析等手段进行了失效分析。结果表明,三通的开裂性质为应力腐蚀开裂。     

压力表管接头焊缝开裂原因分析

分析了不锈钢压力表管接头焊缝开裂的原因,认为此种开裂具有应力腐蚀开裂、沿晶断裂特征。     

裂解汽油加氢二段反应器急冷剂管道开裂原因分析及对策

对裂解汽油加氢二段反应器急冷剂管道因化学腐蚀和应力腐蚀引起的开裂进行了溯源分析。通过对1号外管宏观及低倍金相检验、样管化学成分分析、样管显微组织分析、电镜观察及能谱分析等方式进行综合分析急冷剂管道开裂原因。确定氯离子应力腐蚀开裂起主导作用,焊缝及热影响区敏化加速了开裂进程,采取相应对策有效地防止管道开裂发生,为乙烯裂解汽油加氢装置长周期安全稳定运行提供了保障。     

法兰不锈钢垫片开裂的失效分析

通过化学成分分析、金相检查和断口分析等手段,对一法兰不锈钢垫片的开裂进行了失效分析。实验测试和数据分析表明,该垫片开裂的主要原因为氯离子引起的应力腐蚀开裂。     

加热炉烟道的腐蚀开裂分析

针对加热炉烟道发现的腐蚀开裂进行了化学成份、金相、能谱和扫描电镜分析,确认烟道开裂是由于烟道气中硫及硫化物引起的沿晶腐蚀开裂。     

氧气气瓶开裂原因分析

通过宏观检测、化学分析、金相分析、扫描电镜分析等方法对开裂氧气瓶进行了检测,并根据检验检测结果对发生开裂的原因进行了综合分析。结果表明,氧气瓶开裂主要由瓶肩内腔表层折叠裂纹引起,裂纹在设备运行时多次扩展,直至部分穿透导致氧气瓶泄漏。     

304不锈钢盘管开裂失效分析

采用宏观检查、微观分析、力学性能、断口微观形貌等方法对某换热器304不锈钢盘管开裂进行了分析。结果表明:该盘管的开裂属于高温高压水蒸汽应力腐蚀开裂,盘管工作应力和壳程水蒸汽中的溶解氧共同作用下发生应力腐蚀,最终导致盘管开裂。     

PC／PE共混合金尖锐缺口试样的冲击断裂分析

本文研究了ＰＣ／ＰＥ共混合金尖锐缺口试样的冲击断裂行为，并用扫描电镜观察了尖锐缺口试样的微观断裂特征。结果表明：ＰＣ／ＰＥ共混合金尖锐缺口试样在室温下的冲击断裂是一个单一过程的韧性断裂，裂纹扩展时产生毫米级的过程区，裂纹尖端存在着强烈钝化，过程区形变的实质为界面空洞化及基体ＰＣ的剪切形变。本文还给出了ＰＣ／ＰＥ共混合金尖锐缺口试样的宏观及微观断裂模型。     

Fatigue Crack Growth in Ferroelectric Ceramics Driven by Alternating Electric Fields

Electric-field-induced fatigue crack growth in ferroelectric ceramic PZT-5 with precracks was investigated. The experimental results showed that there were two distinct characteristics in the crack growth under electric loading. Under low electric loads, microcracks located ahead of the main crack emerged and grew and, as a result, impeded the growth of the main crack. On the other hand, under high electric loads, microcracks were absent, and the main crack was the only mode of fatigue cracking. The main crack grew macroscopically along the original path perpendicular to the electric field. Microscopically, the crack grew along the grain boundaries and grain breakaway was observed. The crack growth rate was nonlinearly related to the cyclic electric load. Similar to mechanical fatigue, there existed a crack growth threshold in the applied electric-field amplitude below which the crack ceased to grow. A steady crack growth occurred when the applied electric field exceeded this threshold. An empirical model for crack growth was obtained. Domain-switching effect and fracture-mechanics concepts were used to explain the observed crack closure and crack growth under electric loads.     

Unique crack behaviors of glass BK7 occurred in successive double scratch under critical load of median crack initiation

Successive double scratch tests under the critical load of median crack initiation were conducted to investigate the crack behaviors under the influence of interaction between the arranged grits during grit-arranged grinding BK7 glass. After the inspection of the subsurface crack morphology by the advanced focused ion beam, it is found that with the separation distance increasing, the crack behaviors under the 2nd scratch experience three stages: a new type of crack—premature lateral crack initiation, no crack initiation and median crack reappearing. The crack behaviors that occurred in the former two stages can alleviate the subsurface damage. The initiation mechanisms of the unique crack behaviors were analyzed by a stress field analytical model. Also, the benefit of these unique crack behaviors on the grinding process was discussed.     

Crack-Propagation Rates in β"-Alurnina

Specimens of sodium and silver β"-alumina were electrolyLed for 3ariom times and subsequently broken in bending. Using the Griffith equation, the lengths of the cracks after electrolysis were estimated as a function of electroljsis time. It way obsened that the crack length vs time plots were bimodal. During the early Ytages of crack propagation, the crack velocity war found to he independent of crack length. The rate of crack propagation in Na-β"-alumina was about three times batter than that in Ag-β"-alumina. Furthermore, in Na-β"-alumina, the rate of crack propagation was found to increase with increasing temperature. The dependence of crack velocity on temperature, the difference in crack-growth rates in Na-β-alumina and Ag-β"-alumina, and the fact that crack velocity is independent of crack length were rationalized in terms of a model that takes into account the transient nature of ionic flux to the growing crack.     

Geometry of crack network and its impact on transport properties of concrete

This paper investigates the crack network geometry in concretes subjected to cyclic axial loading. A total of 24 cylinder specimens of two concretes (OPC and HVFC) were cast and eight levels of cracking extents were created for each concrete. Disks were extracted from the cylinders and the crack geometry was evaluated for sections both perpendicular and parallel to loading. The crack geometry is quantified by crack density, length, orientation and connectivity. The crack length is found to obey log-normal distribution, and the crack orientation and connectivity are correlated strongly with crack density. The volumetric density is identified as a consistent parameter to describe the impact of crack network on altered transport properties. The effective porosity, capillary sorptivity, gas permeability and electrical conductivity all have strong dependence on crack density. In particular, the gas permeability is proved to be sensitive to both small range and large range of crack density.     

Indentation Crack-Shape Evolution during Subcritical Crack Growth

Indentation crack-shape evolution during subcritical crack growth was examined in soda-lime glass. Crack-arrest markings were generated on the fracture surfaces by temporary unloading during subcritical crack growth. Crack shapes were determined from these crack-arrest markings by optical microscopy. The shapes were found to be semielliptical, but the ellipticity changed significantly with crack extension under an applied bending stress. Crack-shape evolution was predicted by using the stress intensity factor equation for a semielliptical crack for the applied stress distribution and the subcritical crack growth parameters for the material/environment system. Experimental results were found to be in excellent agreement with the predictions. The influence of the initial crack size and shape, the loading configuration, and the subcritical crack growth exponent n on crack-shape evolution was also investigated in the simulations. It was found that, for the particular loading configuration and material, there is an equilibrium shape associated with the subcritically growing crack. The variation in crack shape with its extension during subcritical crack growth was found to be an important factor in lifetime prediction.     

Crack Initiation and Arrest in a SiC Whisker/A12O3 Matrix Composite

The fracture initiation and arrest stress intensity factors were determined for a SiC-whisker-reinforced AI₂O₃ matrix composite. A chevron-notched, three-point-bend specimen was used to genera e the load/displacement curve, which exhibited repeated crack initiation, followed by crack arrest behavior. Corresponding stress intensity factors were determined for both situations using the compliance technique. Calculated crack arrest positions were in agreement with fractographic observations. Both the crack arrest and the crack initiation stress intensity factors exhibited a rising R -curve with increasing crack length, suggesting the presence of wake toughening effects on the crack growth resistance.     

Measurement of Crack Tip Toughness in Alumina as a Function of Grain Size

Crack profile measurements near the crack tip in the SEM were used to measure crack tip toughness of alumina as a function of grain size (average grain size 0.9–16 μm). For comparative tests, two crack configurations were included in the present study: straight cracks (CT specimen) loaded with an in situ device; and radial indentation cracks. The measured crack tip toughness values were independent of crack geometry, and no grain size dependence could be discerned. A mean crack tip toughness of 2.3 MPam^1/2 was evaluated. The crack tip toughness determined from crack profile measurements is significantly lower than the toughness evaluated with conventional indentation techniques (e.g., indentation strength bending).     

Determination of Subcritical Crack Growth Parameters by In Situ Observation of Indentation Cracks

A technique to determine stress intensity factor—crack velocity (K— v ) relationships for subcritical crack growth from in situ observation of indentation cracks is described. To minimize the effect of residual contact stresses and lateral crack interaction, measurements were made only on cracks that had undergone significant subcritical crack growth. Crack shapes were determined fractographically from crack-arrest markings, produced by temporary unloading during the crack extension process. The subcritical crack growth parameters obtained by this technique were in excellent agreement with those determined from dynamic fatigue and previous studies.     

Characterisation of bending cracks in R/FRC using image analysis

In this study, experiments were conducted to induce bending cracks of specific surface crack width to reinforced concrete beams made of plain concrete (RC) and reinforced concrete beams made of fibre reinforced concrete (R/FRC). After injecting and impregnating the cracks with dyed epoxy resin, image processing and analysis were employed to investigate the internal crack morphology. Several crack features including crack width (accumulated, effective and maximum), branching and tortuosity were defined and quantified. The results showed that in addition to arrested crack development, the presence of fibres yielded a distinctive change in the internal crack pattern, including increased branching and tortuosity, both of which have positive implications regarding concrete permeation. Likewise, specimens with fibres exhibited reduced maximum individual crack widths near the rebar, potentially increasing the ability of autogenous crack healing and reducing the risk of corrosion initiation.