In order to investigate the brittle-striation formation mechanism of a low carbon steel JIS S10C fatigued in a hydrogen gas environment, fractographic observations of the visualized fracture phenomena during some processes of brittle-striation formation were conducted. The following results were obtained. A striation line is formed during the loading part of the cycle as a trace of blunting by slip. A stable ductile crack then starts growing. These processes are similar to those during the normal ductile fracture from a crack; that is, a ductile tearing process in tension. Based on the experimental results, a brittle-striation formation model, in which hydrogen only enhances the microscopic ductile tearing process just ahead of a crack tip, was proposed. The model rationally explains the peculiar load-frequency effect in the quasi-cleavage range on the fatigue crack growth which reveals a lower growth rate in spite of lowering the load-frequency.

View full abstract

New coating methods were proposed to improve the adherence strength between diamond grains and electrolytic nickel bonds on electroplating diamond tools. Diamond grains covered by nested carbon nanotubes (CNTs) were constructed using self-assembly techniques. The grains were first amino-functionalized by silane coupling or photochemical UV treatment. Acid-treated CNTs adsorbed automatically onto amino-terminated grains in N,N-dimethylformamide (DMF) solution by amino-CNT interactions. Subsequent drying and readsorption deposited CNT coatings onto the grains by CNT-CNT interactions due to van der Waals forces. Scanning electron microscope observations indicated that the thickness of CNT coatings on photochemical UV-treated grains was more uniform than that of coatings on grains treated by silane coupling. The adherence strength between the grain and electrolytic nickel bonds was evaluated by shear tests. The adherence strength of the CNT-coated grains was 200-250% of that of the uncoated grains. The photochemical UV treatment was more effective than silane coupling for improving adherence strength. It was surmised that this was due to the uniform thickness of the CNT coatings and the absence of high molecular weight layers contained in the silane coupling agents between CNTs and diamond grains.

View full abstract

For bolted joints failures usually happen at the root of the first bolt thread where the maximum stress concentration occurs. Previously several methods were proposed to reduce the stress concentration through improving bolts and nuts profile. Among those methods, tapering threads have been widely used in mechanical structures. Those types of threads are called CD bolts, which were proposed by Nishida as an effective method for stress reduction. Several experimental studies indicated that CD bolts have higher fatigue strength although little FEM analyses are available. In this study, therefore, the bolted joints with tapered threads are analyzed with the finite element method, and stress reduction effect of CD bolts is discussed with varying geometrical conditions. The reduction of the stress concentration is realized by tapering bolt threads. Then, it is found that the stress reduction is notable when the height of bolt threads is reduced significantly near the bolt head and the nut is closer to the bolt head. According to those results, optimum conditions for stress reduction are discussed. Then, it is shown that the maximum stress can be decreased by 20% compared with the cases of standard bolts and nuts.

View full abstract

The non-combustible Mg alloy is useful as a structural material because of its high specific strength and high ignition point, but its suitable welding conditions have not been established for manufacturing floors. In this study, a simple evaluation method for the fatigue limit characteristics of a welded joint was proposed using the edge shape of the welding bead geometry, the internal defect size and the material characteristics. Moreover, the welding condition parameter ranges to insure a stable fatigue limit for the TIG butt joint of a non-combustible Mg alloy were proposed using this method.

View full abstract

In this study, a fracture probability under a thermal stress distribution is calculated with a dangerous volume quantification method that was proposed in the former report. The fracture strength due to the thermal stress is calculated, based on the finite element method with the fracture temperature and the aluminum creep properties. The fracture strengths predicted with the proposed method are appropriate for the experimental results obtained by heat tests. The calculated dangerous volume intended for 90% of product in alumina spray coating on an aluminum layer is the part where a stress of 96% more than the maximum stress has been received. For the dangerous volume on thermal stress distribution, it is sufficient to consider an experienced dangerous volume in which the dangerous volume is a volume where a stress of 90% more than the maximum stress is loaded. It is shown that the fracture origin is located in the dangerous volume calculated by the present method and that the fracture strength predicted with a fracture origin size is almost equal to the fracture strength predicted by this study.

View full abstract