Key Engineering Materials Vols. 297-300

Abstract: Fatigue damage plays a significant role in fatigue life of structures and mechanical parts. Characterizing crack length as a random process in fatigue life, is a matter of controversy which is the first step of this new stochastic model for prediction of fatigue crack propagation. Minimum uncertainties and capability of using closure load are the model advantages. Generating series of a nonstationary process, effects of load history in immediate last step of crack propagation using Total Probability Theory (TPT) are some assumptions of this model. The model results have been compared with actual test results of Ti-6Al-4V CT specimens.

Abstract: The fatigue life of hexagon head and socket head bolts, attached to vehicle a wheel, is assessed and the estimation of the residual life of existing bolts in vehicle wheel is investigated. Field- measured load histories were applied in this test. Tensile tests and fatigue tests were performed to evaluate the effect of tightening torque and to obtain the basic experimental data. A three-dimensional finite element analysis was also performed to evaluate the local stress fields. Miner’s rule was used to predict the fatigue life of bolts. The results indicate the prediction of fatigue life of the bolts was in good agreement with the real life of vehicle wheel bolts in this test.

Abstract: Time domain approach with S-N approach and local strain approach were used for fatigue life estimation. But these days, using PSD (Power Spectral Density) method is highlighted, because of short amount of time in measurement and analysis. Especially, PSD method is useful for analysis of fatigue failure which is caused by vibration damage, also FRF (Frequency Response Function) is useful for efficient prediction of fatigue life when the same product is employing different motor vehicle or test condition. In order to estimate fatigue life of compressor for air conditioning, time domain analysis and frequency domain analysis were performed and the results were compared. As a result, results of analysis in frequency domain and time domain were similar. With this, there is recognition of decreasing the period of measuring and analysis in PSD analysis. Moreover, in case of FRF pursued of a part, using FRF is applicable at fatigue life prediction in different testing condition. There was investigated an analysis method with curtailed analysis period by FRF.

Abstract: Rubber-modified epoxy resins are used as a matrix material for glass and carbon-fiber composites. Mechanical properties of fiber reinforced composites depend on the interfacial shear strength between the reinforced fiber and the matrix resin. This study is focused on the interfacial shear strength in the reinforced carbon fiber and rubber-modified epoxy resin system. To evaluate interfacial shear strength between the fiber and the resin, pull-out test is performed using a microdroplet method. Based on experimental results, numerical analysis was also simulated. It is concluded that the interfacial shear strength of carbon fiber/unmodified epoxy resin system was higher than that of carbon fiber/modified epoxy resin system. The reason for decreased the interfacial shear strength of rubber-modified system is that contractive forces in neat epoxy resin acting on carbon fiber were less than those in rubber-modified epoxy resin system.

Abstract: It is well-known that the noise and vibration from the torque change of the engine and the inertia change of its accessories (alternator and water pump) have severe effects on both the quality and the life expectancy of the engine belt system. An automatic belt tensioner is generally used not only to reduce the noise and vibration of such belt system, but also to enhance the life cycle. In the present research, measurement of the user running conditions and the data analyses were performed to try to derive an evaluation method of life cycle of such an automatic belt tensioner system. As a part of results, a running condition for the evaluation was suggested from the study of conditions loaded on an automatic belt tensioner. This running condition was divided into two parts, the Structure part and the Pulley one, and suggested separately for the accelerated life-test of the system. As the other part of the results, an accelerated life-test condition was derived using Palmgen-Miner rule for structure part. And a modified accelerated life-test condition, which uses temperature as an acceleration factor, was suggested for pulley part.

Abstract: The hinge, which is used in various folders of electronic communication devices, damages easily due to the repeated impact-like stress that occurs when the folder is opened. The hinge damage not only affects the lifetime of devices, but it may cause to lose the reputation of the brand name of the company. In this paper, the total process for the life evaluation and failure analysis of this part is proposed. For this, the mechanism of the hinge motion was analyzed first to calculate the force acting on the hinge. Also an automatic durability test system was developed to repeatedly open and shut the folder. Then the change in deformation around the hinge was monitored using an Electronic Speckle Pattern Interferometer (ESPI) method, as the repetition number increased for the opening of the folder. ESPI results showed that the deformation was concentrated at the hinge and it increased according to the increase of repetition number of folder opening. Finally, we evaluated the life of the hinge through an accelerated life test (ALT). ALT results showed that the life of hinge was dependent on the force acting on the hinge

Abstract: OLED has many advantages such as possibility to make micro size, tiny driving power, and fast response time with high resolution. However, significant commercial level problems are being recognized resulting from the damage of emitting layer, degradation of organic layers, and generation of dark spot. It is therefore necessary to overcome the problems for the development of high performance products. Deposition characteristic associated with the materials failure in OLED has been determined, firstly, by means of the analysis of the surface morphology and roughness of thin film. Physical vapor deposition technique was employed to develop test specimens, and atomic force microscopy has been used to identify the uniformity of emitting layer. In addition, optimal control set enabling the roughness of thin film surface to be minimized is presented, and the results obtained are compared with the experimental data. Generally good agreement has been achieved. It is known that the luminance decay mechanisms are directly related to the reliability of finished products, and thus further investigation has been carried out using the luminance decay curves. As shown by the results, the luminance decay may be divided into two major components during the failure processes. One is simple exponential during operation and the other is rapid decline in the initial stages of operation. It can be thought that the simple exponential decline is related to a chemical degradation of Hole Transport Layer, whereas the rapid decline is probably due to the generation of an internal electric field.

Abstract: The differences of coefficient of thermal expansion (CTE) of component and FR-4 board connected by solder joint generally cause the dissimilarity in shear strain and failure in solder joint when they are heated. The first order Taylor series expansion of the limit state function (LSF) incorporating with thermal fatigue models is used in order to estimate the failure probability of solder joints under heated condition. Various thermal fatigue models, classified into five categories: categories four such as plastic strain-based, creep strain-based, energy-based, and damage-based except stress-based, are utilized in this study. The effects of random variables such as CTE, distance of the solder joint from neutral point (DNP), temperature variation and height of solder on the failure probability of the solder joint are systematically investigated by using a failure probability model with the first order reliability method (FORM) and thermal fatigue models.

Abstract: In recent years, package downsizing has become one of biggest trends in packaging technologies because of miniaturization and the high integration of electronic devices. As a result, the reliability of fatigue life has been prioritized as an important concern, since the thermal expansion difference between a package and printed circuit board causes thermal fatigue. But, the reliability of thermal fatigue life may be lowered by the dispersion of design factors such as the dimensions, shapes, and material properties of package systems. Also, the fracture modes (fatigue fracture, brittle destruction in the interface, and compound destruction, and so on) will be affected by these dispersion factors. Although this dispersion should be reduced in order to improve the reliability, reducing all dispersion is inefficient and time-consuming. So, the factors that greatly contribute to thermal fatigue life have to be reduced. In this study, evaluations of the influence of various design factors on the reliability of soldered joints of a BGA (Ball Grid Array) were carried out, and got each influence. With the results, design engineers can rate each factor’s effect on reliability and assess the reliability of their design beginning at the concept design stage. Consequently, it will be possible to avoid almost all reliability problems from the beginning. Also, by rating the factors, the design period can be shortened