Key Engineering Materials Vols. 353-358

Abstract: In order to investigate fatigue characteristics of vulcanized natural rubber (NR), fatigue tests are carried out under various stress ratios R (R = minimum stress / maximum stress). It was considered that the fatigue cracks were initiated from flaws in very early stage of total life. The fatigue damage process was almost the fatigue crack propagation process and it is independent of R. The crack growth rate was proportional to the crack length to about the first power, when the crack length was defined as the length of the direction perpendicular to the loading direction. Miner’s rule was examined to observe the fatigue crack behavior and checked by using two-step loading fatigue tests experimentally. It seems Miner’s rule has a possibility to predict fatigue lives.

Abstract: This paper describes the state of the art in design codes and guidance using fracture mechanics. In Germany, a railroad accident occurred in June, 1998, which resulted in about 100 passenger deaths due to a wheel fracture. In September, 1999, a water leak accident due to a crack in a pipe happened at the Tsuruga nuclear plant in Japan. Such serious accidents are the result of fracture events. Fracture mechanics is thought to be a tool to avoid such catastrophic fracture accidents. The state of the art in designing mechanical components or structures applying fracture mechanics was reviewed. The American Society of Mechanical Engineer, the Japanese Society of Mechanical Engineers, the Japanese Welding Society, and the Japanese Industrial Standard were mainly surveyed. This report suggests that a more consideration of fracture mechanics in the design codes and guidance is needed for avoiding fracture accidents in components or structures.

Abstract: A fuel channel, which is the major structure of a nuclear reactor, is excited by the flow of cooling water during the operation of the nuclear reactor. This flow of cooling water can cause excessive vibration of the structure by the resonance. So, in the design process of a nuclear structure, the exact evaluation of the effect of water to modal characteristics and stiffness characteristics is very important to generate an exact analysis model. In this study, the effect of water height for modal characterisics of the fuel channel is investigated and the stiffness evaluation of that is conducted using the modal test results.

Abstract: The law of low-cycle fatigue with hold time at elevated temperature is investigated in this paper. A new life prediction model for the situation of fatigue and creep interaction is developed, based on the damage due to fatigue and creep. In order to verify the prediction model, strain-controlled low-cycle fatigue tests at temperature 693K, 823K and 873K and fatigue tests with various hold time at temperature 823K and 873K for 316L austenitic stainless steel were carried out. Good agreement is found between the predictions and experimental results.

Abstract: A fatigue analysis method was developed to predict the fatigue behavior of the T300/QY8911 multidirectional laminated composites. The finite element analysis of the interlaminar stresses and the average of them localized to the free edges of the specimens were made for predicting the delamination onsets and lives. The residual lives to failure after delamination was estimated afterwards. The predicted fatigue lives agree well with the test results.

Abstract: Laser shot peen-forming of sheet metal(or LasershotSM Peening) is a new plastic forming technique for metallic materials, which uses high-power pulsed laser replacing the tiny balls to peen the surface of sheet metal. When the pressure of shock waves induced by laser impresses an inhomogeneous residual stresses distribution in a given depth on the surface of sheet, it responds to the stress by elongating at the peened surface and effectively bending the sheet. In order to investigate the mechanism of laser shot peen-forming, the narrow strip peen-forming experimental of aluminum alloy 6061-T6 was carried out by using a pulsed Nd:glass laser with 0.5Hz repetition-rate. Here, under some given laser energy, laser pulse width, laser beam diameter and pulse repetition frequency and so on, the influence of shot strip interval and shot times on surface residual stresses and the deformation of the sheet is analyzed. The results show that the bending forming of the sheet metal can be found, and the peened surface of sheet metal becomes convex. That the bending increases with shot strip interval increase is not obvious, but it increases with the shot times increase in a proper range of shot times. Besides, because laser shot peen-forming generates compressive residual stresses on the surface, it offers many desirable characteristics in shaped metals and is a valuable technique for producing components for a range of industries.

Abstract: The isothermal low cycle fatigue (LCF) behaviors of a directionally solidified (DS) nickel base superalloy, coated with a NiCrAlYSi coating were studied. The study concerned NiCrAlYSi coating formed by an arc-discharged physical vapor deposition (PVD) process for protection against high-temperature corrosion and oxidation of gas turbine blades. The effect of protective coating on LCF life of coating/substrate system was investigated at high temperatures and compared with uncoated alloy. The test results show that coating has no or less effect on LCF life under high strain range and the LCF life is governed by the fatigue behavior of substrate at different temperatures. However, when strain range is smaller, crack initiation and propagation are observably affected by temperature, which leads to a shorter LCF life of coating/substrate system at 500°C and a longer LCF life at 760°C or 980°C.

Abstract: We applied the fracture mechanics approach and the nonlinear finite element method to the analysis of interfacial failure in cylinder rubber bush bonded on their outer and inner curves surfaces to rigid metal cylinders. The fracture parameter, tearing energy for rubber bushes was calculated using global energy change. The contact of interfacial crack during deformation was considered. The analyses for initially small cracks and relatively large cracks growing downwards disagree with the previous linear solutions. The variation tearing energy for cracks growing upwards is consistent with the existed results except a small range of crack length at the bottom end, where negative tearing energies exist which indicates a small crack is unexpected to initiate.

Abstract: In order to clarify the effects of the hybrid surface modification process; a combination of Fine Particle Bombardment (FPB) treatment and nitriding, on the fatigue properties of AISI 4135 steel (stress concentration factor: α=2.36), high cycle fatigue tests were carried out with a rotational bending machine at room temperature. Observations of fracture surfaces and measurements of hardness and residual stress distributions were carried out to investigate the fracture mechanism and fatigue strength. It was revealed that treating process sequence did affect residual stress distributions. Compressive residual stress generated at the surface of FPB treated specimen after nitriding was higher than that of the one FPB treated before nitriding. It was clarified that the higher the specimen hardness was, the higher compressive residual stress was generated at the surface. Therefore, FPB treatment after nitriding increased the fatigue strength of steel.