基于Lamb波群速度的铝合金疲劳损伤检测

从理论上分析Lamb波群速度识别铝合金疲劳损伤的可行性。搭建主动Lamb波结构健康监测系统,采用基于Lamb波群速度的结构损伤检测方法,研究金属疲劳引起的损伤形变的识别过程;通过波包分解技术将接收信号中混叠的激励波包分解出来,从而准确地提取波包的到达时刻,实现Lamb波传播群速度的计算。在实际7050铝合金板材结构上进行了实验验证,并与短时傅里叶变换法进行对比。实验结果表明,采用波包分解技术能准确获取波包的传播时间,通过对比群速度识别金属疲劳的存在,其有效性高于短时傅里叶变换法。     

Degradation modeling of 2 fatigue‐crack growth characteristics based on inverse Gaussian processes: A case study

Most modern products that are highly reliable are complex in their inner and outer structures. This situation indicates quality characterization by the interaction of multiple performance characteristics, which motivates the utilization of robust reliability models to obtain robust estimates. It is paramount to obtaining substantial information about a product's life cycle; therefore, when multiple performance characteristics are dependent, it is important to find models that address the joint distribution of performance degradation of such. In this paper, a reliability model for products with 2 fatigue‐crack growth characteristics related to 2 degradation processes is developed. The proposed model considers the dependence among degradation processes by using copula functions considering the marginal degradation processes as inverse Gaussian processes. The statistical inference is performed by using a Bayesian approach to estimate the parameters of the joint bivariate model. A time‐scale transformation is considered to assure monotone paths of the degradation trajectories. The comparison results of the reliability analysis, under both dependent and independent assumptions, are reported with the implementation of the proposed modeling in a case study, which consists of the crack propagation data of 2 terminals of an electronic device.     

Effects of load and thermal conditions on Pb-free solder joint reliability

Reliability of lead-free solder joints has been a hot topic widely debated in the electronic industry. Most published data indicate that a change to lead-free soldering has the potential benefit of more reliable solder joints than the current Sn-Pb eutectic solder joints. However, in reality many mechanical, metallurgical, thermal, and environmental factors affect the service reliability of solder joints. This paper tries to shed some light on the effects of mechanical loading and thermal conditions on solder joint reliability. These conditions are determined not only by external environments but also by the solder alloy itself and the joint geometry. Analyses with first principles are carried out on solder joints of both areal array and peripheral packages. Effects on fatigue life of solder joint geometry, thermal and mechanical characteristics of components and substrate materials, and application conditions are discussed. The analysis helps explain why lead-free solder joints may not be more reliable in certain application conditions than the current Sn-Pb eutectic solder joints.     

(Al2O3)f/Al复合材料疲劳损伤模式的SEM研究

通过扫描电子显微镜(SEM)对(Al2O3)f／Al复合材料的疲劳断口和纵截面组织结构进行了观察，研究了复合材料的疲劳损伤模式。研究结果表明：该复合材料兼有钛基和树脂基纤维复合材料疲劳损伤的特点，高应力下由单个裂纹的起源和生长导致复合材料的失效；低应力下，疲劳损伤模式包括纤维劈裂、众多基体裂纹和单个基体裂纹的横向扩展，其中纤维劈裂是主控机制。其更高的疲劳极限可归因于低应力下纤维的纵向劈裂。     

Fatigue failure criteria and degradation rules for composites under multiaxial loadings

To develop design rules for dynamically loaded composite structures, extensive static and cyclic tension/compression-torsion tests were carried out on carbon-fibre-reinforced composites, in which especially the important influence of multiaxial loading conditions on their fatigue behaviour was investigated. Physically based failure criteria for static loadings are modified for multiaxial cyclic loadings, and a good agreement with experiments is achieved. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 5, pp. 631–641, September–October, 2006.     

The effects of annealing on fatigue crack propagation in polyethylene

Fatigue crack propagation tests on annealed and quenched medium-density polyethylene showed the annealed specimens to have much lower resistance to crack initiation and subsequent propagation. Although the same fracture mechanism, in which the brittle crack gradually becomes more ductile, prevailed in both cases, the voided and fibrillated crack tip root craze in the annealed material was much weaker that the nonfibrillated quenched root craze. Microstructural analyses indicate that the annealed material had separate crystallite populations, whereas the quenched material had a more homogeneous morphology. The highest melting fraction of the annealed material was composed of lamellae that were about 270 Å thick, and the quenched lamellae were estimated to be 160 Å thick. The reduced fatigue crack propagation resistance of the annealed material was suggested to be a result of a lower concentration of tie molecules and its reduced damping capability, compared to the quenched material. © 1995 John Wiley & Sons, Inc.     

Monitoring of structures or mechanical components by an acoustoultrasonic signature technique

This paper describes a methodology for adapting the acoustoultrasonic technique for use in monitoring structures or mechanical components. As a practical example, the adaptation of the technique for monitoring fatigue tests on front suspension arms is considered. From this particular example, it will be shown that the procedure is suitable for application to more general cases. The results of a number of fatigue tests carried out at the Fiat Research Center on a typical component test rig are also presented.     

Effects of cooling rate on mechanical properties of near-eutectic tin-lead solder joints

This paper reports the results of a study on the effect of the cooling rate during solidification on the shear creep and low cycle shear fatigue behavior of 60 Sn/40 Pb solder joints, and on bulk solder tensile properties. Solder joints were made with three different initial microstructures by quenching, air-cooling and furnace-cooling. They have similar steady-state strain rates under creep at relatively high shear stresses (i.e. in the matrix creep region) but creep at quite different strain rates at lower shear stresses (i.e. in the grain boundary creep region). These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate. Tensile tests on bulk solders that were cold-worked, quenched and furnace-cooled show that a faster cooling rate decreases the ultimate strength and increases the ductility at low strain rates. The fatigue life of quenched solder joints is shown to be longer than that of the furnace-cooled joints.     

焊接接头热影响区表面裂纹疲劳扩展速率

给出了数据处理的方法，通过对试验数据的分析，给出了裂纹扩展速率与ΔK的关系，为油田的安全生产提供了依据。     

The mechanism of fatigue failure in a polyethylene copolymer

The fatigue behavior of an ethylene-hexene copolymer was investigated. The effects of R, frequency, relative times under the maximum and minimum stress, and waveform were measured. The phenomenological aspects were related to the microscopic aspects of the failure process. The maximum stress produces damage by disentangling the molecules in the fibrils of the craze and the minimum stress produces damage by bending the fibrils. The net damage, which is a product of these two damage processes, has been represented by a simple equation which accounts for the phenomenological observations.