发电机定子线棒空心水冷导线断裂原因分析

借助扫描电子显微镜、光学显微镜和力学性能测试等分析手段对发电机定子线棒空心水冷断裂导线的断口、金相组织和力学性能进行了分析.结果表明,水冷导线是由于制造时未将水冷导线捆扎紧,造成线捆松动,在微动力的摩擦作用下,产生疲劳断裂.     

316L纤维尺寸和含量对HA-ZrO2(CaO)/316L纤维复合生物材料性能的影响

研究了316L纤维的长度、直径与含量对HA-ZrO_2（CaO）/316L纤维生物复合材料的力学性能的影响规律.结果表明：纤维直径为40μm的复合材料力学性能优于纤维直径为50μm的复合材料;纤维长度为0.8～1.2mm的复合材料力学性能优于纤维长度为2～3mm的复合材料;随着纤维体积分数增大,纤维之间相互接触而导致在复合材料中形成的微孔增多,并成为微裂纹源,导致材料力学性能下降.含20vol%直径为40μm、长度为0.8～1.2mm的316L纤维的HA-ZrO_2（CaO）/316L纤维生物复合材料的综合力学性能最佳,其抗弯强度、杨氏模量、断裂韧性和相对密度分别为140.1MPa、117.8GPa、5.81MPa·m~（1/2）和87.1%.复合材料微观组织随HA粉末和316L纤维成分的变化呈规律性变化,没有出现明显的裂纹或孔隙,316L纤维与HA-ZrO_2（CaO）基体紧紧地咬合在一起,其结合主要靠基体对316L纤维的物理附着力所致.基体中发生微量Fe元素扩散,但在316L纤维中不发生基体Ca、P元素的扩散.含5%316L纤维复合材料表现为脆性断裂,而含10%、20%、40%316L纤维复合材料均表现为韧性断裂,且韧性程度随316L纤维含量的增加而增大.     

复合绝缘子芯棒脆断故障调查及原因分析

通过对2起复合绝缘子芯棒断裂掉串事故的分析及同期运行的同批次220kV输电线路复合绝缘子的抽样试验,初步分析了发生复合绝缘子芯棒脆断的原因,并提出防止类似事故发生的主要措施:改进金具接头界面的密封工艺并对档距较大的跨越采取双串。     

聚氨酯改性环氧树脂体系的力学性能研究

研究了高弹性聚氨酯环氧改性双酚F型环氧树脂固化体系在室温和液氮温度77 K下的力学性能,观察研究了断裂面的微观形貌与力学性能的关系。结果表明:聚氨酯环氧能够有效地改善环氧树脂在室温和低温下的力学性能,尤其在低温下具有较好的增强增韧效果。当聚氨酯环氧的质量含量为30%时,综合性能达到最佳,拉伸强度在室温和77 K下分别为87.35和118.73 MPa,冲击强度分别为15.76和21.88 MPa。聚氨酯环氧的加入使体系玻璃化温度下降为92.4℃,能满足低温应用要求。     

Microstructure and press formability of a cross-rolled magnesium alloy sheet

Cross-rolling, in which the roll axis is tilted by 7.5° towards the TD-direction, was carried out on a commercial magnesium alloy. The (0002) texture intensity of the cross-rolled specimen was lower than that of the unidirectionally rolled specimen, and the (0002) texture of the cross-rolled specimen was inclined about 10° towards the TD-direction. Also, the grain size of the cross-rolled specimen was smaller than that of the unidirectionally rolled specimen. As a result of the Erichsen tests at 433-493 K, the press formability of the cross-rolled specimen was higher than that of the unidirectionally rolled specimen. The high formability of the cross-rolled specimen is attributed to both the modification of (0002) texture and the enhancement of grain refinement by the cross-rolling.     

Failure Analysis of a Diesel Engine Connecting Rod

A failure investigation has been conducted on a diesel engine connecting rod. The fracture occurred at the small head of the connecting rod. Visual and scanning electron microscopy observations show that a lot of axial grooves appear on the internal surface close to the fracture and the fatigue cracks initiated from the axial grooves. Fractography indicates that the multiple-origin fatigue fracture is the dominant failure mechanism. The machining or assembling process was responsible for the formation of the axial grooves.     

Analysis of mixed-mode dynamic crack propagation by interface element based on virtual crack closure technique

An interface element tailored for the virtual crack closure technique (VCCT) was used to study an example of dynamic crack propagation under mixed mode loading. Through this interfacial element approach, VCCT can be implemented into a commercial finite element analysis (FEA) code having user subroutines without interrupting the main code. Further, with the implementation of relevant fracture criteria, this interface element can be used to simulate a wide range of fracture problems by utilizing the enhanced capabilities available by the commercial FEA codes. For illustration, this element has been implemented with the commercial FEA software ABAQUS^® through the user defined element (UEL). One example of fast crack propagation at constant speed and under mixed-mode loading was examined by comparison to the other’s numerical results using singular moving elements. No convergence difficulty was encountered for all the cases with different values of crack velocity. Neither singular element, nor the collapsed element was required. Therefore, due to its simplicity, the VCCT interface element as demonstrated could be a potential tool for engineers to practice dynamic fracture analysis in conjunction with commercial FEA codes.     

Virtual crack extension method for calculating the second order derivatives of energy release rates for multiply cracked systems

In this paper, we further generalize the work of Lin and Abel [Lin SC, Abel JF. Variational approach for a new direct-integration form of the virtual crack extension method. Int J Fract 1988;38:217-35.] to the case of higher order derivatives of energy release rates for two-dimensional, multiply cracked systems. The direct integral expressions are presented for the energy release rates and their first and second order derivatives. The salient feature of this numerical method is that the energy release rates and their first and second order derivatives can be computed in a single analysis. It is demonstrated through a set of examples that the proposed method gives expectedly decreasing, but acceptably accurate results for the energy release rates and their first and second order derivatives. The computed errors were approximately 0.5% for the energy release rates, 3-5% for their first order derivatives and 10-20% for their second order derivatives for the mesh densities used in the examples. Potential applications of the present method include a universal size effect model and a probabilistic fracture analysis of cracked structures.     

Formulas for risk of failure

The concept of stress strength inference plays an important role in modeling problems with respect to engineering fracture mechanics, see, e.g. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] and [21]. The risk of failure of a system can be expressed by the probability R = Pr(X₂ < X₁), where X₁ and X₂ are some random variables representing the stress and the strength experienced by the system. In this note, we derive a comprehensive collection of formulas for R by assuming the most commonly known models for X₁ and X₂. We feel that this work could serve as a useful reference for engineering fracture mechanics.     

纳米碳纤维复合材料断裂电发射试验研究

采用超声波分散的方法制备了纳米碳纤维／环氧树脂复合材料，研究了纳米碳纤维／环氧树脂复合材料在拉伸和弯曲破坏时的电发射现象，并对其电发射机理进行了探讨。研究结果表明：纳米碳纤维／环氧树脂复合材料在受荷开裂和裂纹扩展过程中均存在明显的电发射现象，这一特性将为复合材料结构的在线监测及非损伤诊断提供新的方法和途径。