Key Engineering Materials Vols. 297-300

Abstract: Anisotropic conductive film (ACF) is commonly used as underfill for flip chip assembly. The present study focuses on elastic recovery and stress distribution along the interfaces of particle-pads and underfill-pads associated with heat or a mechanical loading. In the same manner as the experimental process for ACF assemblies, ACF interconnection is simulated using FEM. Firstly, the properties of the nickel were determined by fitting FEM to the experimental results. After that, the nickel properties are used for ACF interconnection analysis. We found that delamination may also occur at a three-joint interface of a particle, a pad and an underfill at the lowest temperature during a heat cycle.

Abstract: Hold-down spring screw fractures due to primary water stress corrosion cracking were observed in nuclear fuel assemblies. The screw fastens hold-down springs that are required to maintain the nuclear fuel assembly in contact with upper core plate and permit thermal and irradiation-induced length changes. In order to investigate the primary causes of the screw fractures, the finite element stress analysis and fracture mechanics analysis were performed on the hold-down spring assembly. The elastic-plastic finite element analysis showed that the local stresses at the critical regions of head-shank fillet and thread root significantly exceeded the yield strength of the screw material, resulting in local plastic deformation. Preloading on the screw applied for tightening had beneficial effects on the screw strength by reducing the stress level at the critical regions, compared to the screw without preload. Calculated deflections and strains at the hold-down springs using the finite element analysis were in very close agreements with the experimentally measured deflections and strains. Primary water stress corrosion cracking (PWSCC) life of the Inconel 600 screw was predicted by integrating the Scott’s model and resulted in a life of 1.42years, which was fairly close to the field experience. Cracks were expected to originate at the threaded region of the screw and propagated to the opposite side of the spring, which was confirmed by the fractographic analysis of the fractured screws.

Abstract: A combined axial-torsional low cycle fatigue test was carried out to predict the fatigue life under in-phase and out-out-phase loading conditions for CF8M cast stainless steels. The Fatemi-Socie (FS) parameter which is based on the critical plane approach is not only one of the many methods but also the best method that can predict the fatigue life under a biaxial loading condition. But the result showed that, a biaxial fatigue life prediction by using the FS parameter with several different parameters for the CF8M cast stainless steels is not conservative enough but at the same time it was the best result so far. So in this present research, we proposed a new fatigue life prediction parameter (Park-Kwon parameter) by considering effective the shear stress instead of the FS parameter which considers the maximum normal stress acting on the maximum shear strain and its effectiveness was verified.

Abstract: For many applications, the understanding of very long life fatigue in materials becomes extremely important. In this study, the fatigue behavior of bearing steel GCr15 (conforming to AISI 52100) at very high number of cycles has been examined. Experiments on hourglass specimens were conducted in air at room temperature, for fully reversed loading condition (R=-1), using a piezoelectric fatigue testing machine operating at a frequency of 20kHz. The results indicate that the S-N data does not reach a horizontal asymptote (signifying the fatigue limit) at 107 cycles, as conventionally believed, and that the material can fracture up to 109 cycles. Therefore, to quote a fatigue limit at 107 cycles may not hold good for the material studied. The influence of defects (such as inclusions) on the crack initiation and fracture was analyzed by scanning electron microscopy.

Abstract: A stochastic approach has been presented for superplastic deformation of Ti-6Al-4V alloy, and probability functions are used to model the heterogeneous phase distributions. Experimentally observed spatial correlation functions are developed, and microstructural evolutions together with superplastic deformation behavior have been investigated by means of the probability functions. The strain-rate dependent failure strain can be correctly predicted by the model. As shown by the results the probability varies approximately linearly with separation distance, and significant deformation enhanced probability changes occur during the process. Since an initial microstructure is the most crucial factor that determines the properties of final microstructure, Monte Carlo simulation has been used coupled with the probability functions for the reconstruction of microstructures. By imposing the precisely optimized distributions of phase on the test specimens, therefore finite element implementation shows better agreement with experimental data of the failure strain.

Abstract: As the operation time of a power plant increases, the degradation and the cracks inside of the structure exposed to high temperature will increase gradually. Therefore, degradation rate, crack growth rate and fracture life of the structure can be evaluated according to the level of degradation and the growth of crack length. We performed creep rupture test and crack growth test with stress and temperature changes to evaluate the degradation rate, crack growth rate and fracture life. Degradation rate was evaluated using micro-cavities. The area fraction of the cavities increased with the increasing temperature and life fraction (t/tf). da/dt, the crack growth rate against Ct estimated from the relationship between load line displacement rate and cavity increase rate, was in good agreement with the result of da/dt vs Ct acquired from the test. It shows that the creep crack growth rate can be evaluated by the increase of cavity area fraction. It was also found that the predicted life calculated with the cavity growth rate was in good agreement with experimental results.

Abstract: Proposed in this paper is an analyzing model for the slope reliability taking the random effects of the seepage field of groundwater and the dependent factors into consideration. Taking the Circle Slice Method as its base of mechanical analyses for the stability of a slope, the model can analyze the movement of groundwater by using BEM and calculate the slope reliability index, β ,and probability of failure, P, by means of Monte Carlo Method (FOSM). The way sampling on the failure surface adopted in the paper incorporates the strong points of Monte Carlo Method and First Order Second Moment Method, and its calculating efficiency is remarkably higher than that of traditional Monte Carlo Method. Employing the model proposed, the reliability indexes and probabilities of failure of a real slope are estimated. The tendencies and extents of influence of rainfall supply and the uncertainty of anisotropic character of media in permeability on the slope reliability are studied for the first time.

Abstract: There are several types of life test method for hose assemblies. The two major tests used for hose assemblies are impulse test and burst test. And magnification adjustment of impulse pressure, heating of testing oil and repetitive motions of bending and straightening of testing hose are also performed for accelerating the life. According to the manufacture process of hose and swaging process of fitting, there is a difference in the life of hose assemblies from minimum 7 times to maximum 40 times during the life test in the same functioning condition. Like this, the life test of hose which has a wide scope of life distribution gives a problem that observation should take a long time to find out the existence of the bursting from the beginning of the test to the completion of bursting of hose assemblies. Therefore, this research proposes a process of concentrating on the defective section of hose assemblies and maximizing the life acceleration by giving ‘Knockdown stress’ to hose assemblies just until before the hose assemblies get out of order.

Abstract: Engineering safety diagnosis of collided subway electric multiple units (EMUs) was conducted for safety assessment. Several advanced engineering analysis techniques including nondestructive evaluation (NDE) techniques and stress and structural analyses programs, were performed for better understandings and exploration of failure analysis and safety concerns. NDE techniques such as ultrasonic testing and magnetic particle testing, were used to detect manufactureinduced and/or in-service defects and collision-induced flaws, and measure the dimensions of deformed and non-deformed parts on damaged EMUs due to a rear-end collision. Moreover, stress and structural analyses using commercial I-DEAS software provided important information on stress distribution and load transfer mechanisms as well as the amount of damages during the crash. A good agreement has been found between structural analysis results and the results of actual damages in EMUs during crash. In this investigation, various advanced engineering analysis techniques for the safety analysis of subway EMUs have been introduced and the analysis results have been used to rovide the critical information for the safety assessment of collided EMUs.

Abstract: In practical design applications, most design variables such as thickness, diameter and material properties are not deterministic but stochastic numbers that can be represented by their mean values with variances because of various uncertainties. When the uncertainties related with design variables and manufacturing process are considered in engineering design, the specified reliability of the design can be achieved by using the so-called reliability based design optimization. Reliability based design optimization takes into account the uncertainties in the design in order to meet the user requirement of the specified reliability while seeking optimal solution. Reliability based design optimization of a real system becomes now an emerging technique to achieve reliability, robustness and safety of the design. It is, however, well known that reliability based design optimization can often have so multiple local optima that it cannot converge into the specified reliability. To overcome this difficulty, barrier function approach in reliability based design optimization is proposed in this research and feasible solution with specified reliability index is always provided if a feasible solution is available. To illustrate the proposed formulation, reliability based design optimization of a bracket design is performed. Advanced mean value method and first order reliability method are employed for reliability analysis and their optimization results are compared with reliability index approach based on the accuracy and efficiency.