Surface defects, such as surface cracks and non-metallic inclusions, are decreased the fatigue strength of vehicle parts. However, the study on overcoming the surface defects by using surface refining methods is hardly reported. The cavitation peening (CP) is a high possibility as a new surface refining method, which is a shot-less peening method and increases compressive residual stress. On studying the effects of the CP to the defects, an experiment using the two types of specimens with an artificial semi-circular slit, whose slit depth (a)=0.04, 0.075 and 0.10mm on the surface of specimens, was conducted and their refined surface characteristics and fatigue limits were evaluated. Consequently, it is concluded that by using CP, the acceptable defect size (slit depth: a) with the smooth and that with notched specimens can be estimated less than 0.04mm and 0.075 mm, respectively.
Torsional fatigue tests were conducted for a spring steel, SUP7, with Vickers hardness of 480. The effect of shot peening on the torsional fatigue strength of smooth and artificially notched specimens in the corrosive environment and air was studied. Following results were obtained. The increase of the fatigue strength of smooth specimens in air by shot peening was relatively small, and the surface roughness due to peening did not reduced the fatigue strength. On the other hand, the fatigue strength of notched specimens increased greatly by shot peening and the notch sensitivity was reduced. In corrosion fatigue, the fatigue strength increased by shot peening in the medium life range, around 106 cycles. The strength of shot-peened specimens was reduced in the long life range around 107 cycles and was approached to that of not-peened specimens. A similar improvement was observed for notched specimens in the medium life range, and the notch sensitivity was reduced by shot peening. The compressive residual stress layer made by shot peening extended about 0.3 mm below the surface, and relaxed by fatigue loading. The residual stress distribution was used to interpret the effect of shot peening on the fatigue strength of spring steel by modified Goodman diagram.
In order to overcome difficulties that have hindered precision casting of shape memory alloys, we made ingots created with self-propagating high temperature synthetic method (SHS) which eventually led us to the success of precision casting utilizing lost wax process. Also we succeeded in creating functional product that was treated with the ultrasonic shot peening process, improving the recovery elasticity against deformation. Although the common casting products are created using many different kinds of metals and alloys, casting products utilizing shape memory alloy have not been widely adopted due to the lack of successfully manufactured products. The reason for this is that there was much difficulty in developing equal property structures that is free of gravity segregation of Ti-Ni. Also its essential characteristics (being the shape memory and super elasticity), the makeup of component had to overcome the structural weaknesses in order to withstand from possible deformation.
For environmental and economical reasons, most vehicles need weight reductions. A useful method to make lighter vehicles is to increase the fatigue limits of vehicle parts. On the previous paper, to prevent the decrease in the fatigue limits of carburized or carbonitrided gears for vehicles by accidental temperature rise while using the gears, a new method, which is treated with the middle temperature tempering before shot peening, was proposed. Based on this consideration, the authors conducted a study by using carburized and carbonitrided specimens on the new method, and evaluated their surface characteristics and bending fatigue limits. The result of the study showed that the fatigue limits of the carburized and carbonitrided specimens processed by the new method achieved 30% and 49% better than those of the specimens assumed accidental temperature rise respectively.
The strength of spring steels has been increasing in order to cope with the demands of lightweightness in automobile applications. However, high strength steels, typically consisting of tempered martensite structures, accompanied by large reduction of toughness are susceptible to failure of spring properties such as fatigue resistance in corrosive conditions, because of increase in notch sensitivity. Since bainite often exhibits high strength and toughness in comparison with tempered martensite, we have studied microstructures and mechanical properties of an austempered spring steel (JIS-SUP12) by means of scanning electron microscopy and tensile tests in order to examine the potential application of bainitic spring steels. The result shows that carbide-free and lath-like bainitic ferrite was formed at austempering temperatures higher than 400°C, while plate-like bainitic ferrite with fine carbide precipitations was formed at temperatures lower than 300°C. The fraction of the bainitic ferrite increased whereas that of the martensite decreased with increased holding time in austempering. The amount of the retained austenite increased during the early stage of transformation and then decreased with further holding. At optimum austempering condition, the bainite with high strength, high elongation and reduction of area superior to tempered martensite was obtained.
The relationship of the aging treatment conditions, hardness, and torsional fatigue strength of the β type titanium alloy for the springs was examined, in addition observation of micro-structure change due to aging was conducted. As a result, the maximum hardness occurs at the aging treatment temperature of 450℃, while beyond this temperature it decreases. On the other hand, the torsional fatigue strength rises above 450℃, and reaches maximum at 540℃ aging treatment temperature. The notch sensitivity factor, β of the specimen for 600μm diameter-drilled hole is 1.29 at 480℃ and 1.24 at 540℃ aging treatment temperature, and the notch sensitivity factor, β of the specimen treated at 540℃ shows a lower value. Furthermore,the increases in the amount of α-phase precipitation due to aging increase the torsional fatigue, which becomes maximum when α-phase precipitation amount reaches 35%.
In application of flexible multi-layered materials it is very important to evaluate mechanical properties of these materials in both analytical and technological interests. This paper deals with a new method (Cantilever Method) for measuring each Young's modulus in a flexible multi-layered material. The method is based on a nonlinear theory that takes into account large deformation behaviors of a multi-layered material. By means of measuring the horizontal displacement, the vertical displacement and the deflection angle at the free end of the cantilever, each Young's modulus can be easily obtained for thin and long multi-layered materials. Measurements were carried out to confirm the applicability of the new method on a two-layered material (PVC: a high-polymer material and SUS: a stainless steel material). The results confirm that the new method is suitable for thin flexible multi-layered materials. In the meantime, the new method proposed in this paper can be applied widely to the measurement of Young's modulus in a thin layer formed by PVD, CVD, Electrodeposition, Coating, Paint, and others.
Shot peening coverage is conventionally and typically measured by visual comparison with the standard image of a peened almen strip surface under 20 to 30 power magnification. There, however, are problems that indented terrain on an actual peened surface may not always give a clear enough image to compare, and that visual comparison may contain individual-oriented variability in measurement results. This article shows that probability density distributions of the peened almen strip surface profile agree with Gaussian mixture distributions, and that separation of the Gaussian mixture distributions enables coverage computation, and thereafter suggests a quantitative coverage measurement method that is independent of individual variability.