The effect of the applied force shedding rate on the da/dN-ΔK relationship near the threshold region was investigated using two types of metals, and it was found that the da/dN-ΔK relationship is independent of the applied force shedding rate for shedding interval of 50 μm and the shedding rate less than 2%. In the crack opening measurement by the compliance method, the crack opening point can be clearly identified and the effective stress intensity factor range ΔK_eff can be automatically determined by averaging the strains in loading and unloading, and using a normalized-strain and applied force relationship, where the normalized is accomplished by strain divided by the linear strain at the same applied force.

This study aimed to gain a comprehensive understanding of the microscopic phenomena in the non-propagating region of fatigue cracks. We conducted tensile tests on pure Fe and an Fe–3%Si alloy with single-phase ferrite using scanning electron microscopy (SEM) and observed the opening and closure behavior of cracks and variations in strain distribution. The Fe–3%Si alloy is available in two types: normal grains with a mean diameter of approximately 100 µm and coarse grains with a mean diameter exceeding 1 mm. Based on the observation results of the crack morphology and crystal orientation, we calculated the resolved shear stresses (RSSs) and discussed the mechanism of crack propagation immediately before non-propagation. In the in-situ SEM observation and digital image correlation in the tensile test of the Fe–3%Si alloy with normal grains, strain concentration regions appeared in two directions from the crack tip. This corresponds to the calculation results of the high RSSs in the two directions. In pure Fe and the Fe–3%Si alloy with coarse grains, the crack tip remained closed, and a strain concentration region appeared in one direction. This corresponds to the calculated results for a high RSS in one direction. Based on observations of the dislocation structures near the crack tips in the Fe–3%Si alloy using the electron channeling contrast imaging method, screw dislocations with Burgers vector b in the same direction as the high-RSS slip directions existed with high frequency. Accordingly, slip systems with high RSSs were activated immediately before non-propagation. Therefore, fatigue cracks immediately before non-propagation could be classified into two crack propagation mechanisms: first, two-direction slip systems are activated alternately, and cracks propagate with the crack tip opening and closing; and second, only one slip system is activated, and cracks propagate with the crack tip remaining closed.

In recent years, some railway accidents including train derailment and serious incident occurred due to the deformation of the bogie frame caused by the fatigue crack. In those accidents, cracks had initiated in the bottom plate of the side beam of the bogie frame and had propagated by cyclic loadings. Thus, the fatigue crack growth prediction under in-service loads in the bottom plate of the side beam is being emphasized. In this study, to examine the fatigue crack growth prediction models under in-service loads acting on the bottom plate of the side beam, crack propagation tests under constant amplitude loading and variable amplitude loading were conducted using the center cracked tension specimens made of the steel commonly used in bogie frames. In addition, the crack growth rates under in-service loads were predicted using some crack growth prediction models based on the equations for estimating crack growth rates. Afterwards, the test results and the prediction results were compared to consider suitable prediction models that match the test results. As a result, it was considered that the fatigue crack growth prediction model applying the modified miner’s law about the effective stress intensity factor range is in good agreement with the crack propagation test results under in-service loads acting on the bottom plate of the side beam.

High entropy alloys (HEAs), which have five or more principal elements with an equiatomic composition, exhibit multiple excellent mechanical properties. Superior fatigue properties would be required to use HEAs as structural components in engineering fields. The purpose of this study is to develop HEAs having high crack propagation resistance via melamine reduction-nitridation process and powder metallurgy. The HEA compact was fabricated by mixed sintering of nitrided and un-nitrided CrMnFeCoNi powders to control the amount of nitrogen addition. Stress intensity factor, K, decreasing tests were conducted for CrMnFeCoNi alloys with nitrogen at force ratios, R, from 0.1 to 0.8 in air at room temperature. Threshold stress intensity range, ΔK_th, of HEA compact with nitrogen was almost the same as the un-nitrided one at R = 0.1, whereas ΔK_th of HEA compact with nitrogen was high in comparison to the un-nitrided one at R = 0.7. This was because the nitrogen addition increased the effective threshold stress intensity range, ΔK_eff,th, of the HEA compact, but the magnitude of crack closure was reduced. The HEA compact with nitrogen shows excellent fatigue crack propagation properties, especially at high R conditions where crack closure does not occur.

In this study, the fracture strength of a three-dimensional octet-truss lattice structure was investigated using finite element simuation and 3-point bending tests. The specimens were made of resin and the fracture toughness was evaluated for brittle materials without plastic deformation and ductile materials with plastic deformation. An octet truss structure was selected as the lattice structure and fracture toughness was calculated using crack opening displacement (COD) and load-displacement diagrams. As in the continuum, the fracture toughness of brittle materials was evaluated at the threshold of the stress intensity factor, K_IC, and that of ductile materials at the threshold of the J integral, J_IC. Furthermore, the fracture toughness value J_IC for ductile materials was calculated using the maximum load values in the experiments. Although there are still issues with the technology to accurately create high-density 3-dimentional cellular structures, it was confirmed that the toughness J_IC can be predicted with a certain degree of accuracy.

The effect of filler material on Mg-4%Al-1%Ca (hereinafter referred to as AX41) non-combustible magnesium alloy TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) weld joint was investigated through plane bending fatigue test and crack propagation test. Two types of filler materials were used in the welding process: one was the same as the base material, AX41, and the other was Mg-6%Al-1%Zn-1%Ca (hereinafter referred to as AZX611), which had a higher Al content compared to AX41. According to the results of plane bending fatigue test, the weld joint that used AZX611 as filler material showed higher fatigue strength properties compare to the joints that used AX41 as filler material. Fatigue crack initiated from welding defects in MIG weld joints, and it initiated from both defect and grain interior in TIG weld joints. In the MIG weld joints, the crack propagation resistance was improved with increase in Al content, which resulted the high fatigue strength properties in MIG weld joint which used AZX611 as filler material. In the TIG weld joints, the joint that used AX41 as filler material showed the similar crack propagation resistance with joint that used AZX611 as filler material. However, the enhanced mechanical properties and decreasing in grain size might increased the crack initiation life, which resulted the longer fatigue life in TIG weld joint that used AZX611 as filler material.

This study aims to reveal the continuous induction welding behavior of CFRTP composites by high-frequency induction heating method. This welding system consists of a high frequency induction heating device (Frequency:1.8 ~ 2.2MHz, maximum output power: 3kW), induction heating coil (double D type coil) and pressing and cooling rollers. This induction heating device has a smaller impedance matching device compared to a self-excited matching device, so it is suitable for continuous induction welding equipment. The material used is woven CF/PPS laminates. The effects of high-frequency power, induction coil height, moving speed of induction coil and pressure of pressing and cooling rollers on continuous induction welding behavior of woven-CF/PPS laminates and single lap tensile shear strength were investigated. The temperature of laminates surface and joining part were measured by K type thin thermocouples. From this experimental result, it was found that the temperature was increased with increasing the high-frequency power, and also the joining part temperature decreased as the induction coil height and moving speed increased. From the result of tensile shear test, it was suggested that the tensile shear strength was influenced by the joining part temperature and the pressure applied by the pressing and cooling roller. Based on the results of this study, it is desired to develop the induction coil that can perform induction heating and pressurization at the same time.

In this study, a comprehensive investigation was conducted to reveal the influences of the base plate temperature on fundamental properties of AlSi10Mg alloy fabricated by laser powder bed fusion (LPBF). Concurrently, a meticulous analysis was undertaken to clarify their anisotropy generated through the formation of microstructural texture. The increase in the base plate temperature decreased the residual stress induced on the surfaces of the LPBF materials, but slightly increased the number of defects with relatively large sizes. On the cross-sections of the LPBF materials, a large number of fan-shaped melt pool traces stacked perpendicular to the building direction were observed. As the base plate temperature increased, the spindle-shaped fine microstructure in the fan-shaped melt pools extended to the upper fan-shaped melt pools, and the reticular microstructure along the arc of the melt pools became continuous. Although the static strength decreased with increasing the base plate temperature, the ductility in the building direction improved due to the microstructural changes. Regardless of the base plate temperature, a characteristic anisotropy in the static strength was found due to the formation of the microstructural texture. The decrease in the tensile strength with increasing the base plate temperature resulted in the decrease in the fatigue strength.

In recent years, there has been active research and development of periodic cell structures with high specific rigidity and strength, and they have been applied to practical products such as shoe insoles and helmet liners. In this study, the aim was to apply periodic cell structures to cushioning products and achieve comfortable products by optimizing the density distribution of Young's modulus. First, we investigated the fabricable range of periodic cell structures using FDM (Fused Deposition Modeling) 3D printing and created a fabricable map. Next, we examined periodic cell structures capable of adjusting Young's modulus by changing the line width. As a result, by using Gyroid as the periodic cell structure and adjusting its line width with an appropriate period, it became possible to vary Young's modulus over a wide range. Furthermore, compression tests were conducted on specimens made of TPU, resulting in stress-strain curves showing a trend similar to that of cushions, and Young's modulus values comparable to those of cushions were obtained.