Conventional nitriding improves the tribological properties of titanium alloys; however, reduces their fatigue strength owing to grain-coarsening. The purpose of this study is to examine the effect of the nitrided-fine particle peening (N-FPP) on the formation of nitrided layer and fatigue properties of Ti-6Al-4V alloy. This approach forms the nitrided layer on Ti-6Al-4V alloy by bombardment with nitrided commercially pure (CP) titanium fine particles. Plasma nitriding and gas nitriding were performed at 873 or 973 K to form a nitrided layer on the surface of CP-titanium fine particles, and then N-FPP was performed for 1, 10, 30 s in air at room temperature. Nitrided layer with high hardness and compressive residual stress could be found on the N-FPP treated surface of Ti-6Al-4V alloy. Nitrogen concentration in nitrided layer tended to increase with the particle nitriding temperature and N-FPP treatment time. Fatigue life and fatigue limit of the N-FPP treated specimen under four-point bending fatigue tests were higher than those of conventional nitrided one, whereas were lower than those of the un-peened specimen. This was because a fatigue crack in the N-FPP treated specimen was initiated from the surface dent formed by N-FPP due to the stress concentration. In contrast, FPP using un-nitrided CP-titanium fine particles increased the fatigue limit of Ti-6Al-4V alloy.
Fatigue limit is well predicted by tensile strength or hardness, and the relationship is often discussed by the linear regression by the minimum squared approximation. However, the prediction of the number cycles to failure at given stress amplitude, which means the prediction of S-N curve, has not been realized. The present study, therefore, aims to investigate the estimability of the S-N curve by the random forest method based on the data described in NIMS fatigue data sheet. The random forest method is one of the machine learning algorithms and is an ensemble learning algorithm that integrates weak learners of multiple decision tree models to improve generalization ability. It was clarified that the machine learning of multiple decision tree model is excellent in fatigue limit prediction. The S-N curve can be accurately estimated by combining the prediction of fatigue limit and that of the number of cycles to failure at given stress amplitude.
Historically, S-N curve has been expressed in the form of an exponential relationship between applied stress σ and number of cycles to failure Nf regardless of scatter of data. This paper elucidates the essential structure of S-N curve and the causes of scatter of fatigue life and fatigue limit based on the prediction model for materials containing various defects. The validity of the model was verified by testing specimens containing small defects with various sizes and by analyzing the difference of failure lives due to defect size. The model was successfully applied to the data reported by other researchers. It was revealed that the statistical nature of the scatter of fatigue data should be expressed by the statistics of extremes, because the source of the scatter is originated from the largest defect contained in specimens.
The origin of fatigue life scatter was quantitatively analyzed by fatigue test of specimens containing small drill holes with various sizes. The size of the holes ranged from 40 µm to 400 µm. In several specimens, three holes with different sizes were introduced to investigate the behavior of crack growth from holes which were observed at the cycle Nf of fatigue failure from the largest hole. The dependency of fatigue life and its scatter on defect size and stress level was analyzed by the model explained in the 1st report and the factors which cause the scatter of fatigue life for other materials such as cast irons and AM materials were discussed by comparing the fatigue data and the predictions.
Sand erosion is a phenomenon in which the collision of solid particles erodes a material surface. The rate of sand erosion is higher in carbon fiber reinforced plastics (CFRP) than in metallic materials. Therefore, CFRP requires a light and protective coating material. Herein, to improve the erosion resistance of CFRP, five polyurethane coated CFRPs with different glass transition temperatures were investigated at elevated temperatures, and a prediction formula of the erosion rate at the elevated temperatures was established. Furthermore, computational fluid dynamics was used to predict the surface temperature and erosion rate of fan exit guide vane (FEGV) when polyurethane coating was applied, and the coating thickness for FEGV in the erosion environment was estimated based on these predictions.
It has been known that material strength used in corrosive environment will be lowered compared with structural materials used in air due to the effect of corrosion fatigue. However, there are few research data concerning fatigue tests under petrol environment assuming actual usage. In this study, fatigue tests on a type of 440C stainless steel, which has been used as a part of the automobile fuel injection system, under air and petrol environment were conducted by means of a small sized specimen and a small sized fatigue testing machine performable in corrosion and flammable liquid. The testing machine can be also used for fatigue tests under push-pull loading at high frequency region. In addition, the fracture surface was observed by SEM and EDS to confirm the difference of fracture morphology in air and corrosive environment. Additional fatigue tests were also conducted in order to investigate the mechanism of corrosion fatigue. Fatigue lives in air were on the same level as those in regular petrol while fatigue lives in deteriorated petrol focusing on corrosion were dramatically decreased due to the difference of fracture origin.
Wood materials are attracting attention as an alternative material to plastics for solving environmental problems, such as sea microplastic. An effective wood processing method should be developed for producing products with various shapes, which leads to an effective use of the wood. This paper proposed extrusion of the wood powder mixed with a natural binder composed of sucrose and citric acid, which effectively forms products with same cross-sectional shape in the extruding direction. Circular tube was extruded in this experiment. It was important to solidify the wood powder inside an extrusion die for molding the tube without cracks and fracture. Therefore, influence of the extrusion die length, the punch speed and the heating temperature, which concern solidification with cooling, were investigated for improving the dimensional accuracy and the mechanical property of the tube. At first, the fracture of the molded product was prevented by setting an appropriate die length. High punch speed caused the tube expansion due to insufficient cooling before extruding, while low punch speed caused the fracture due to long time heating. Density and strength of the molded product increased with the decrease in the temperature, although the cracks easily occurred on the tube surface under the low temperature of 140°C. The tube surface state became smooth by setting the molding temperature to 160 or 180°C. From the above results, the circular tube, which has good dimensional accuracy and mechanical property, was molded by controlling the extruding condition.
Safety glass, such as general laminated glass and glass/polycarbonate(G/PC) laminated safety glass, is used in the viewing ports of machine tools. G/PC laminated safety glass meets the requirements for being used as a viewing port and has good impact resistance, solvent resistance, abrasion resistance, and transparency; however, its impact fracture behavior is unclear. In this study, we investigated impact fracture behavior of G/PC laminated safety glass experimentally and analytically. Specimens of G/PC laminated safety glass with different interlayer and PC thicknesses were tested under different impact energies using a drop weight test machine. Moreover, finite element analysis (FEA) model of the G/PC laminated safety glass was carried out considering the drop weight test, using explicit FEA code LS-DYNA. Storage modulus measurements obtained from dynamic mechanical analyses was applied to the interlayer in the FEA model. Results showed that the critical impact energy increased when PC thickness increased due to the drop weight test. As the interlayer thickness increased, the critical impact energy increased, at a constant PC thickness. The glass was also damaged radially and circumferentially by the impactor; the damaged state differed depending on the structure of the G/PC laminated safety glass. According to FEA results, damage propagation of the glass changed from circumferential to radial as the PC thickness decreased. FEA results for features of impact fracture behavior of G/PC laminated safety glass were also compared qualitatively with experimental results.
In the manufacturing process of ceramics, bisque firing is performed to make the green body strong. In case the green body has sufficient strength, it could be possible to save energy by eliminating the bisque firing. Aiming at further reduction of CO2 emission for low-temperature sintering porcelain, carboxymethyl cellulose (CMC) was added to reinforce the green body. The amount added, viscosity and degree of substitution (DS) of CMC on the strength of the green body were investigated. Increasing the amount of CMC, the strength of the green body was gradually increased, reaching double to triple value by adding 0.5 % of CMC. The addition of CMC reduced water absorption of the green body, meaning that it increased the bulk density. DIC (Digital Image Correlation) method proofed that the strains inside the green body was suppressed by adding CMC, suggesting that CMC played a role of binder between low-temperature sintering porcelain powders to lead to improve the strength. Low-viscosity CMCs are easy to handle, however, higher-viscosity CMCs demonstrated slightly higher strength. Using CMC with lower DS tended to decrease the strength probably due to lower miscibility to water.