Key Engineering Materials Vols. 462-463

Abstract: In this paper, fatigue crack growth rate (FCGR) analyses were conducted on compact specimens of an AISI 4340 alloy to study the behavior over a range in load ratios (0.1 ≤ R ≤ 0.95) and constant maximum stress intensity factor (Kmax) condition. Previous study had indicated that high R > 0.7 and constant Kmax test conditions near threshold conditions were suspected to be free of crack-closure and that any differences were caused by Kmax effects, from threshold to near fracture conditions. Cracks in high-cycle fatigue (HCF) components spend a large portion of their fatigue life near threshold conditions. In order to characterize the evolution of damage and crack propagation during these conditions, fatigue crack growth rate (FCGR) data at threshold and near-threshold conditions are essential in predicting service life and in determining the proper inspection intervals. Fatigue crack growth model, namely the Forman model were examined, this model implicit the effect of R ratio and ease of curve fitting to measured data. The Forman model may be suggested for use in critical applications in studying fatigue crack growth for different load ratios.

Abstract: Understanding effective parameters in fatigue crack growth (FCG) model under variable amplitude loading (VAL) is of eminent importance theoretically as well as experimentally. In response to this necessity, a systematic study of different analytical concepts and loading sequences in order to gain a practical framework has been proposed. The theoretical background related to the fatigue life prediction by using FCG model has been presented. This has shown the rationale of why we need to calculate local stress-strain in the crack tip in developing FCG models which is the main subject of this research.

Abstract: In order to clarify the effects of a great amount of static pre-strain (1/4 cycle) on fatigue life of AISI 316L stainless steel, strain controlled low cycle fatigue tests were carried out. Hardness and fracture ductility of specimens were measured. Results showed that in the case of a specimen with p=0.01, static pre-strain of 0.16 noticeably reduced a fatigue life. To clarify the reasons for the decrease in fatigue life with pre-strain, the maximum stress amplitude of the hysteresis loop was measured. In the case of the specimen with p=0.01, work softening was observed in the early stage of fatigue life. On the other hand, when the cyclic plastic strain ranges were p=0.10, no softening was observed. These results suggest that work softening is significant in determining the fatigue life of the specimens at a relatively low level of applied cyclic strain.

Abstract: Dissolution properties of Cu in molten Sn-Ag-Cu-Ni-Ge alloys have been investigated. In particular, the effect of the Ni content in the alloys on the dissolution properties has been examined. Moreover, the dissolution properties have been compared with those of Sn-Ag and Sn-Ag-Cu alloys. To investigate the dissolution rate of Cu in molten alloys, Cu wires were dipped in molten alloys heated at 250, 270 and 290°C. Dissolution thickness of Cu wire is proportional to dipping time regardless of alloy type. The dissolution rates of Cu follow the order Sn-Ag > Sn-Ag-Cu > Sn-Ag-Cu-Ni-Ge. In Sn-Ag-Cu-Ni-Ge alloys, the dissolution rate of Cu decreases with increasing the Ni content. In cases of Sn-Ag and Sn-Ag-Cu alloys, a thin Cu-Sn compounds layer forms at the interface between Cu and the alloy and dissolution of Cu does not proceed uniformly. On the contrary, a thick reaction layer, which consists of granular Cu-Ni-Sn compounds, forms at the interface between Cu and the Sn-Ag-Cu-Ni-Ge alloy. Since the reaction layer inhibits dissolution of Cu in molten alloy, the dissolution rate slows down and dissolution of Cu proceeds uniformly in the Sn-Ag-Cu-Ni-Ge alloys.

Abstract: The aim of this study is to investigate the relationship between the inelastic strain range and the thermal fatigue lives of chip size package solder joints with Sn-Pb and Sn-Ag-Cu solder balls. The inelastic strain range was examined by finite element analysis. In both solder joints, the exponential terms in the inelastic strain range term in the Coffin-Manson equation were evaluated as 1.8 - 2.3. These values are very close to the conventional one used in Sn-Pb and the Pb containing solders. The inelastic strain range is almost proportional to the temperature range in the Sn-Pb solder joint. On the contrary, the inelastic strain range is proportional to approximately the square root of the temperature range in the Sn-Ag-Cu solder joint. The deference depends on the change of the stress-strain hysteresis curve in the thermal cycle.

Abstract: The tensile properties of Sn-x(x=1, 2, 3)Ag-0.5Cu-0.05Ni-0.005Ge (mass%) alloys were investigated. In addition, the ball shear force was investigated with solder balls and two types electrodes, Cu and electroless Ni/Au plated Cu, to examine joint reliability under heat exposure conditions. Tensile strength of the alloy decreases with decreasing the Ag content. On the contrary, elongation increases with decreasing the Ag content. When the Ag content reduces, primary β-Sn phases are coarsened and eutectic microstructures diminish. The decrease of the eutectic microstructures causes a reduction of the tensile strength. In as-soldered joints with Sn-Ag-Cu-Ni-Ge solder balls and Cu electrodes, the ball shear force increases with increasing the Ag content. However, the ball shear force decreases with increasing heat exposure time. After heat exposure treatment at 423 K for 500 h, the ball shear force is relatively stable at lower values regardless of the Ag content. In the joints with electroless Ni/Au plated Cu electrodes, the ball shear force slightly increases with increasing heat exposure time. Even after heat exposure treatment at 423 K for 500 h, hierarchy of the ball shear force is maintained. The ball shear force becomes high with increasing the Ag content.

Abstract: Thin cylindrical shells are widely used in modern structures. When the structures are under axial compression, inflectional destruction happens early. In order to design reasonable and reliable shell structures, stiffened cylindrical shells are applied in the dissertation, ANSYS, an valid finite element analysis software, is employed to redevelop and set up parameter calculation model, subjected to volume and variables value range restriction, the structure’s critical buckling load is the objective, and the serial linear programming optimization procedure is executed as well as the optimized thickness of shell and the size of stiffeners are gained accordingly. The critical buckling load of the structure is obviously increased after optimization, and the feasibility of this method is validated due to the comparison with the numerical and theoretical result.

Abstract: In this paper, fatigue tests and finite element analyses are carried out on spot welded joints of mild steel (270MPa class) and ultra-high strength steel (980MPa class) in order to investigate the influence of strength level of base steels on fatigue strength and fracture morphology of spot welded joints. From the fatigue tests the following results are obtained: (1) Fatigue limit of spot welded joints is almost the same in both steels. (2) Fatigue fracture morphology of spot welded joints depends on the load level in the ultra-high strength steel, but not in the mild steel. From discussion based on the finite element analyses the following results are obtained: (3) The fatigue limit of spot welded joints can be predicted by stress intensity factors for a nugget edge, fracture criterion for a mixed mode crack and threshold value for fatigue crack growth in base steel. (4) Plastic deformation around a nugget in spot welded joints strongly affects the fatigue fracture morphology.

Abstract: This paper deals with fabrication and strength evaluation of biocompatible composites consisting of partially stabilized zirconia (PSZ) and pure titanium (Ti). The biocompatible composites of PSZ-Ti were fabricated by a hot pressing method of powder metallurgy. A volume ratio of PSZ and Ti was changed. Four-point bending tests and Vickers hardness tests of the PSZ-Ti composite were performed to determine the Young's modulus, bending strength, Vickers hardness and fracture toughness. These properties were characterized as a function of Ti volume fraction. The Young's modulus and Vickers hardness were higher than the prediction of the rule of mixture. The bending strength and fracture toughness were decreased with increasing Ti content. To discuss these results from a viewpoint of reaction products, the components of raw powders and sintered composites were investigated by X-ray diffraction analysis. It is concluded that oxide of titanium and other reaction products were created after sintering and they affected the mechanical performances of the composites.

Abstract: Most measurement by experiment is about a structure surface and it is dramatically difficult to measure the stress state inside a structure. Although the stress analysis of a big structure is possible by the 3-D finite element method, great time, labor, and expense are needed for modeling and calculation. Then, we developed the 3-D local hybrid method which analyzes only the local model of a structure and can evaluate the stress field inside it from surface data obtained by easy measurement. In previous studies, the good result was obtained with the surface cracked specimen subjected to uniform tension and a cantilever subjected to bending. However, with an actual structure, it is not thought that simple bending deformation which is produced under an experiment environment arises. In this study we examined whether the 3-D local hybrid method would be able to apply to the structure subjected to bending and torsion by using the displacement on front surface and the displacement of upper and lower sides of a local model. It was clarified that the 3-D local hybrid method is very useful to the 3-D stress analysis of the structure subjected to the complicated loading (uniform, bending and torsion, etc.).