In this study, the influence of residual austenite (γ_ʀ) on fracture toughness and hydrogen embrittlement resistance of high-strength spring steels were investigated. Volume of γ_ʀ in the steel was decreased by annealing and compared with non-annealed steel. The volume ratio of γ_ʀ was around 4～5vol% in non-annealed steel and 1～2vol% in annealed steel.Fracture toughness was investigated by Charpy impact test and hydrogen embrittlement resistance was investigated by four-point bending test under corrosion cycles. The results showed that increased γ_ʀ improved fracture toughness but reduced hydrogen embrittlement resistance. A flat facet was observed at the hydrogen embrittled fracture origin of non-annealed steel. We considered that the flat facet was formed because deformation-induced martensite (α') that was transformed during delayed fracture test was plastically deformed due to enhancement dislocation mobility of hydrogen on {112} plane. Considering suspension springs, γ_ʀ is decreased by manufacturing processes such as coiling, annealing, and shot peening. Therefore, it is suggested that suspension springs have good hydrogen embrittlement resistance, and sufficient fracture toughness for practical use.

Due to increasing awareness of environmental issues, the required characteristics of vehicles are shifting from high output and performance to lightweight design and fuel efficiency. However, as with conventional structure materials, high tensile strength and fatigue strength are still required. Therefore, high-strength steel could be an efficient solution for environmental issues. High-strength springs used in automotive transmissions enable us to achieve weight reduction for a significant contribution to improved fuel economy. This paper describes the development of 1500 MPa-Class-High strength oil tempered wire, aiming for 10 ％ improvement in fatigue strength compared to existent high-strength steel.

The present study has investigated deformation-induced martensitic transformation in fatigue of unnotched SUS304 plates. Martensitic transformation occurred in unnotched SUS304 plate specimens fatigued at room temperature in air. Volume fraction ξ_α' of α' martensite in the unnotched potion of fatigued specimens was measured by ferrite scope. The relations between the maximum value of ξ_α', ξ_α'max, and the number of load cycles N were represented by reverse sigmoidal curves for all the applied stress range Δσ levels tested in this study. For the most portion of fatigue life, the value of ξ_α'max remained almost constant. This value was increased with increase in the applied Δσ value. The spatial distribution of ξ_α' in the specimens varied with N : i.e., continued cycling of stress made α' transformation localized near the central portion of specimens where the ξ_α' value reached as high as 35-40%. This value is more than doubled compared to the highest ξ_α' value found in SUS304 specimens subjected to static tensile stress at room temperature. Invisible edge cracks of 200 μ m in length were found in the high ξ_α' region. These results imply that the measurement of ξ_α' in fatigued SUS304 components may detect crack initiation sites and may predict residual fatigue life.

The present study has investigated deformation-induced martensitic transformation in fatigue of welded SUS304 plates. Martensitic transformation occurred in welded SUS304 plate specimens fatigued at room temperature in air. Ferrite scope reading of the central portion of welded specimens ranged 10 to 15%, indicating the existence of ferromagnetic ferrite phase produced during the welding process. Modified ferrite scope reading ξ, 1.37 times ferrite reading, showed a slight decrease after the application of cyclic stress, then remained almost a constant value of 15 to 17% irrespective of applied stress range Δσ for the most part of fatigue life, and finally showed sudden increase to about 28% to cause fracture. The spatial distribution of ξ in the welded specimens varied with the number of stress cycles N: i.e., continued cycling of stress made α' transformation localized near the central portion of specimens where the ξ value reached as high as 18-21%. This value is higher than the highest ξ_α' value found in the tensile tests of SUS304 at room temperature. Small edge cracks were found in the high ξ value regions of the welded SUS304 specimens. These results imply that the measurement of ξ in fatigued SUS304 weldments can detect crack initiation sites and predict residual fatigue life.

Long column-shaped shape memory alloys (SMAs) shows negative stiffness during buckling deformation. Since SMAs can recover to its original shape, this negative stiffness during buckling deformation can be used repeatedly. This negative stiffness is expected to be applied to passive vibration isolators that use quasi-zero stiffness mechanisms. We have been investigating the buckling characteristics of tape-shaped Cu-Al-Mn (CAM) SMA, for application to passive vibration isolation devices using a quasi-zero stiffness mechanism. As a result, it was found that CAM SMA has buckling characteristics that are suitable for vibration isolation devices. In order to further improve the functionality of CAM SMA materials, we have fabricated a convex tape-shaped CAM SMA specimen with a curvature in the cross-section of the material. In this study, we investigated the buckling fatigue and functional degradation characteristics of a convex tape-shaped CAM SMA specimen due to repeated buckling deformation and compared them with those of a flat tape-shaped CAM SMA specimen. We also investigated the variation in buckling fatigue and functional degradation characteristics due to the addition of curvature to the cross-section using Finite Element Method (FEM) analysis and Scanning Electron Microscope (SEM) observation. As a result, compared to flat tape-shaped specimens, the negative tangential stiffness value of convex tape-shaped specimens increases, but their functional degradation characteristics and fatigue characteristics decrease. This is thought to be due to the stress concentrating at the edge of the center of the convex tape-shaped specimen during buckling deformation, and the crack propagation developing from this stress-concentrated area.

This study presents a Bayesian inference-based nonlinear scaling model to predict the stress-strain relationship of permanent strength. The model focuses on accurately estimating permanent strength, defined as the stress component independent of time and temperature, crucial for understanding material strengthening mechanisms and evaluating spring properties. A scaling coefficient, represented by a logistic function, was introduced to transform observed stress-strain data into permanent strength stress-strain relationships. The Bayesian framework allows integration of minimal experimental data with prior knowledge, enabling precise parameter estimation while accounting for uncertainties. The proposed method was effectively used for a cold-rolled phosphor bronze plate (JIS C5210-H). Results showed that the model effectively reproduces nonlinear behaviors from proportional limits to plastic regions, achieving high consistency with experimental data, including observed residual stress rates. The model successfully represents anisotropic behavior across rolling, diagonal, and transverse directions, reflecting its reliability in diverse conditions. This approach significantly enhances the accuracy of permanent strength evaluation by leveraging a minimal dataset, offering a practical and efficient solution for materials design and performance assessment. The findings suggest that the proposed method's potential is as good as a robust analytical tool for capturing the stress-strain relationships of permanent strength in materials where elastic behavior, such as in spring materials, is critical.

Various types of springs are frequently used as machine parts. The main purpose of springs is to utilize their elasticity, and analysis of the relationship between load and deformation is a fundamental and important issue in industry. Commonly used spring shapes include coil springs, spiral springs, and thin leaf springs. Apart from common shapes, there are many other springs. Flexible and elastic materials such as polymeric materials exhibit unexpectedly large deformations even under small loads. It is impossible to analyze the deformation behavior accurately by using the conventional small linear deformation theory, and a strict analysis by using nonlinear theory is required. In the meantime, it is necessary to create a model suitable to analyze a large deformation for various supporting conditions. Some analyses have been already done on a cantilever, a post-buckled beam, a three-point bending and a four-point bending of simply supported beams. In these analyzed examples, the beams were generally straight. In this paper, the large deformation behavior of C-type springs (e.g., retaining rings and piston rings) with various opening angles and curvatures in the initial state is analyzed by using nonlinear large deformation theory. Analytical solutions based on elliptic integrals are obtained for several typical deformation amounts when compressive or tensile load is applied horizontally to the tip of a spring. Furthermore, in order to confirm the applicability of the derived theoretical solutions, large deformation experiments are carried out and comparisons are made with the theoretical results.

The main purpose of springs is to utilize their elasticity, and accurately understanding the relationship between load and deformation is a fundamental and important issue in industry. In general, springs show unexpectedly large deformations even when subjected to small loads, so it is difficult to accurately analyze such large deformation behavior by using the conventional small deformation linear theory, and precise nonlinear analysis is required. Incidentally, spiral bellows and zigzag springs (also known as S-springs), which are used, for example, as springs to support car seats and sofas, have a complex shape that combines arc sections and straight sections, and the arc sections are basically semicircular (180°). This paper deals with innovations in the deformation characteristics of such semicircular zigzag springs (S-springs) from a mechanical point of view, and in this paper, a new, so-called non-semicircular zigzag spring (S-spring) whose arc sections are not semicircular is assumed with pitch changes in mind. In other words, a new analysis was carried out on the deformation performance of zigzag springs (S-springs) with an arc greater than a semicircle (> 180°) and smaller than a semicircle (< 180°) when the tip is supported in a rotation fixed and a horizontal tensile or compressive load is applied to the supported end. Originally, semicircular zigzag springs (S-springs) exhibited nonlinear large deformation behavior due to loads, but the deformation characteristics of the assumed innovative non-semicircular zigzag springs (S-springs) also have nonlinearity similar to the conventional semicircular zigzag springs. This large deformation behavior has also been newly analyzed by using nonlinear theory, and analytical solutions by using elliptical integrals have been given for some representative deformation quantities. Furthermore, the deformation characteristics of conventional semicircular zigzag springs (S-springs) have mainly been analyzed by using small deformation linear theory, but the authors were the first to perform an actual large deformation analysis. The merit of this research is that it is the first in Japan to attempt an innovation in semicircular zigzag springs (S-springs), and it has clarified the large deformation behavior of the innovative zigzag springs, revealing guidelines for improving the performance and accuracy of zigzag springs. In addition, this paper presents a generalized theory of large deformation analysis for zigzag springs (S-spring) with various circular arc sections. Semicircular zigzag springs (S-spring) (180°) are recognized as a special example.

One needs to consider the optimization for the decrease in the number of effective coils and the increase in the pitch of a spring not only in the design but also the manufacturing because of causing higher design stress due to weight reduction with all automobile parts. In this study, the simulation of coiling process with coiling pin system was carried out by the dynamic explicit method with finite element analysis, LS-DYNA, and revealed the stress and strain distribution caused by the coiling process in the cross section of the spring wire. As a result, the neutral axis of the spring wire moved toward the outside of the coiling because this movement was caused by the axial compression during the process. However, there was some residual tensile stress after the coiling. This suggested that the spring endured in severe plastic deformation during the coiling process, while the residual tensile stress effected the fatigue less compared to the spring manufactured by lathe system coiling, which caused more residual compressive stress due to the axial tension during the process. It should indicate that some post process to any spring made by coiling pin system might be needed for less the stress.

This study investigates a valve lift designed to prevent valve spring surging in a valve train operating at a constant speed, typical of series-type hybrid engines. The paper outlines a method for designing a cam function that mitigates surging in the primary mode, which is the predominant vibration in an undamped linear valve spring, even during discontinuous speed changes when the valve body seats. The proposed cam function is a finite-time settling function using the solution of a two-point boundary value problem composed of half-odd integer trigonometric functions. By varying the parameter p, a countably infinite number of cam trajectories can be generated, and all of which serve as surging preventive cam functions. The cam function can be tailored to different rise times and collision velocities. By setting parameter p to 1 or employing an infinite series, a valve lift with a wave number of 1 and minimal concavities can be achieved. Additionally, the convergence of the coefficients in the infinite series is found to be important for the valve lift design. Utilizing these valve lifts is expected to significantly reduce engine energy loss due to friction between the cam and the tappet.

Refining the grain size improves the strength of steel. This fact is well known, but the effect on the spring fatigue strength and the amount of spring relaxation is unclear. This study investigated the effect of grain size and annealing temperature on fatigue properties and resistance on relaxation of spring that can be manufactured within the normal manufacturing conditions using SWOSC-V oil-tempered wire. As a result, the fatigue life was improved by the refinement of the grain size, but not improved by lowing the low-temperature annealing temperature. Although an increase in yield strength was expected in both cases, this was thought to be because the increase in yield strength due to the refinement of the grains was greater than the increase in yield strength at the low-temperature annealing temperature. On the other hand, refining grain size and lowing the low-temperature annealing temperature reduced the amount of spring relaxation in room temperature when the effect of constant stress setting was taken into account. In both cases, this was thought to be caused by an increase of yield strength and hardness.