The permanent strengths of cold-rolled austenitic stainless steel sheets for springs and subsequent annealing are systematically investigated. The athermal, time-independent permanent strength corresponds to the basic ability to withstand stresses induced by external forces. Stress relaxation tests of sufficiently long duration (24 hours) were performed and the experimental stress-relaxation time relationship was extrapolated to estimate the permanent strength remaining after an infinite time. Permanent strength was dominant at 0.2% proof stress at the general strain rate (έ = 1×10^-2/s). The percentage of temporal strength was not always high in existing cold- worked structural materials. Also, the 0.2% proof stress and the permanent strength were generally equal so that no problems occurred during design without considering the permanent strength. The proportion of permanent strength included in the 0.2% proof stress at the general strain rate (έ = 1×10^-2/s) was about 85% for the delivered material and about 90% for the annealed material, while the increase in permanent strength was about 200 MPa. It was suggested that low-temperature annealing in the specimen not only increased the permanent strength but also increased its residual stress ratio.

In this study, we evaluated spring constants of pearlitic steel tension coil springs heat-treated at various temperatures. We measured the spring constants quasi-statically at room temperature or dynamically at elevated temperature multiple times. We found that the spring constant gradually increased with the heat treatment temperature above A₀ transformation point. In addition, it exhibited unstable behavior with rapid increases and recovery in the sample heat-treated near the A₀ transformation point. These experimental results suggested that the 2nd order magnetic transition at the A₀ transformation point may have some effect on the spring constant. These phenomena can be explained by the decrease in the pinning width of dislocations on the ferrite phase {110} slip plane oriented in the longitudinal direction of the steel wire caused by gradual recovery and weak pinning at the domain wall for the dislocations.

In this study, we proposed an anticorrosion thin film treatment on smooth surfaces to enhance the corrosion resistance and corrosion fatigue life of the TiNi shape memory alloy wire. As a result, the anticorrosion thin film treatment was found to be effective in improving corrosion resistance and corrosion fatigue life. The main achievements obtained in this study are as follows: (1) In the fabrication process proposed in this study, the anticorrosion thin film treatment does not affect the transformation and deformation characteristics of the TiNi shape memory alloy. (2) The anticorrosion thin film treatment results in the formation of a Ni-free layer within the top 20 nm of the material surface. Simultaneously, a TiO_x layer, primarily composed of TiO₂ with a thickness of approximately 60 nm, is generated. (3) The TiO_x layer produced by the anticorrosion thin film treatment serves as a passive film, reducing the current density in the anodic polarization reaction. (4) The TiNi shape memory alloy with a smooth surface having R_a = 0.018 mm achieved through mechanical polishing exhibits an improved fatigue life in air under the rotating bending loading when compared to the as-received material with a thick oxide film. (5) In a corrosive environment, specifically 10% NaCl aqueous solution, the passive film formed by the anticorrosion thin film treatment remains intact even under mechanical loading during rotating bending fatigue tests. As a result, for instance, at a strain amplitude of 1.9%, the fatigue life in a corrosive environment was approximately three times higher compared to TiNi SMA wires subjected only to mechanical polishing without forming a passive film.

There are various types of springs that are frequently used as mechanical parts. The main purpose of springs is to utilize their elasticity, and an analysis of the relationship between load and deformation is a basic and important issue. Generally, the spring shapes that are often used include coil springs, spiral springs, and thin leaf springs, but there are many springs with other shapes, such as clips for fastening paper, and industrially, zigzag-springs that support automobile seats. In general, spring materials show unexpectedly large deformation even with a small loading, so it is difficult to analyze the deformation behavior accurately by the conventional small-deformation theory, and a strict nonlinear analysis is required. In the analysis of such large deformation behaviors, cases when the beam is straight have already been analyzed. However, in the initial state, the zigzag-spring has a complicated shape, such as a combination of an arc member and a straight member. Deformation of zigzag-springs has been previously analyzed based on a small deformation linear theory, but large deformation has not been elucidated. For a large deformation analysis, a new modeling is required. Therefore, in this paper, when the tip of the zigzag-spring is fixed by the rotation-fixed end, the large deformation behavior when a compression or tension load is applied in the horizontal direction at the support end is analyzed non-linearly. An analytical solutions using elliptic integrals are given to the typical quantity of deformation, and an experiment is performed to confirm the adaptability of the analytical theory. Finally a comparison with the theoretical calculation is carried out.

In the previous paper, we performed the deformation analysis of the arbitrary curves without restriction the Archimedean curves. However, the scope of the analysis was limited to the non-contact state. This paper, the scope of analysis was expanded to the domain of the contact. The Lagrange multiplier method is used as the method to solve the contact problem. We conducted an experiment verification of the deformed shape in the contact state. The simulation results were in good agreement with the experimental results.

The large deformation behavior of flexible thin wires when subjected to axial compressive load has already been analyzed and applied to Young's modulus measuring method for thin wires. However, from the viewpoint of material testing, it is necessary to analyze the effect of offset compression on Young's modulus measurement. Therefore, this paper deals with a nonlinear analysis on large deformation when axial compression is applied to an offset beam hinged at both ends. Using the elliptic integral, relationships between arc length s, horizontal distance x, deflection y, curvature 1/R, bending moment M and bending stress δ were derived. Furthermore, the large deformation state was theoretically calculated using the analytical theory, and the effect of the offset amount on the buckling load was clarified. In addition, in order to confirm the effectiveness of the analytical theory, a large deformation experiment using a PVC (polyvinyl chloride) thin plate beam was carried out. As a result, the analytical theory's prediction and the large deformation experiment result were in good agreement and it became clear that if the offset amount was less than 10% of the beam length, it would have almost no effect on Young's modulus measurement. On the other hand, some typical deformation states are represented graphically so that the large deformation can be easily understood visually. In these considerations of issue, the offset beam is defined as the state in which their compression fulcrums are not on the same straight line.

This paper has applied the new in-situ remaining creep life prediction methods previously proposed by the present author to the weld joint of modified 9Cr-1Mo steel. The methods used in the present study were based on two types of the modified theta method and the combination of the modified theta and modified omega methods and were applied to creep deformation data during creep tests made at different applied stress levels at 823 and 923 K in air. Remaining creep lives were predicted by the proposed three methods at arbitrary time in the continuous measurement of deformation during the creep tests. The results show that the combination of the modified theta and modified omega methods gives the most accurate prediction of creep rupture time at the two levels of test temperature for different applied stress levels. These results were found the same as the modified 9Cr-1Mo base material.

The objective of this study was to find the shot peening (SP) conditions for to increase the size of defects that can be is rendered harmless in vacuum-carburized steel. After introducing semi-circular slits of 0.1, 0.2, and 0.3 mm depth into vacuum-carburized SCM420H steel, SP was performed under two different conditions to vary the depth of compressive residual stress (crossing point). One SP condition is a common condition for carburized steel and is called CSP. The other condition provided a deeper crossing point and is referred to as DSP. The crossing points for CSP and DSP were 0.20 mm and 0.25 mm, respectively. These specimens were subjected to plane bending fatigue tests at a stress ratio of R = -1. For CSP and DSP, the depth of slit that could be rendered the defect harmless was 0.10 mm and 0.20 mm, respectively. These results indicate that deep compressive residual stresses can increase the harmless defect size that can be rendered. The fracture mechanics study enabled the prediction of differences in the defect sizes that could be rendered become harmless under different SP conditions.

The “Research Committee on Fatigue Diagrams of Helical Coil Springs”, which ended its activities in 2015, manufactured various springs and conducted fatigue tests with the aim of investigating the validity of the JIS fatigue diagrams. As a result, it was confirmed that the JIS diagram for fatigue strength after 10⁷ cycles was considerably on the safe side. However, due to the small amount of data on the short lifespan side such as 10⁵ and 10⁶ cycles, it was not possible to reach a conclusion from a statistical analysis perspective. Therefore, this committee set test conditions and acquired fatigue data on the low-life side, which the above-mentioned committee had not been able to obtain sufficiently. In addition, statistical analysis was conducted on the test results and the results were compared with the JIS diagram on the low-life side. As a result, it was confirmed that the 50% breakage probability line for this experiment at 10⁵ and 10⁶ times corresponds to a JIS diagram with a safety factor of 1.4 to 1.8.

In recent years, the automotive industry has emphasized not only improving fuel efficiency but also reducing overall environmental impact, aligning with the Sustainable Development Goals (SDGs). Concurrently, the electrification of vehicles has progressed, leading to an increase in vehicle weight. Suspension coil springs that make up the undercarriage of automobiles are required to be even lighter, more durable, and safer. To meet these needs, the stress level acting on the spring must be increased. On the other hand, as the applied stress and material strength increase, the risk of delayed fracture increases. Therefore, it is extremely important to accurately evaluate the delayed fracture characteristics of springs. Regarding delayed fracture evaluation methods, a survey conducted by our society's“Delayed Fracture Test Methods Research Committee”revealed that there is a strong desire in the industry to unify and standardize test methods. Delayed fracture tests using springs have issues in terms of safety and convenience, so the“Research Committee on Delayed Fracture of Springs (Phases I and II)”conducted evaluations by replacing springs with test pieces. To this end, the committee has been considering establishing evaluation methods such as test piece design, hydrogen addition method, and loading method. Based on the results of previous committees, this committee conducted confirmation tests and verified remaining issues regarding test pieces, loading methods, hydrogen addition methods, etc. necessary for standardization of delayed fracture tests. Although not covered in this report, we also investigated the organizations and processes necessary for standardization. These results will be passed on to the Spring Delayed Fracture Test Standardization Committee, which aims to standardize spring delayed fracture tests.