In these days, high-strength coil springs for construction machines are very much in demand. It is necessary for the coil spring steel to have high hardness and high toughness to meet this needs. Newly developed spring steel has higher Charpy impact value than SUP11A due to the addition of V and the reduction of C. Generally speaking, springs for construction machines are bigger than those for automobiles. This study supported the theory that when the parts size becomes bigger, Charpy impact value decreases since cooling rate becomes slow in the quenching operation. However, newly developed steel has higher Charpy impact value than SUP11A even if the cooling rate is slow.
The surface refining method is a useful technique to improve the mechanical properties such as the fatigue strength of automobile parts. Double Shot Peening (DSP) method, especially its advanced technique of using extensively small shots (less than φ 100μm) at the secondary peening, can increase hardness and compressive residual stress under the surface of the parts. Although the air direct pressure type Shot Peening (ADP-SP) equipments are used as this method, if DSP becomes possible with the centrifugal force type Shot Peening (CF-SP) equipments, the application of DSP to the big size parts such as Ring Gear for commercial vehicle will be expanded. Therefore, an experiment using two kinds of DSP specimens treated by CF-SP and ADP-SP equipments was conducted, and their surface characteristics and bending fatigue strength were evaluated. As the result, it was found that the surface characteristics and bending fatigue limit of the DSP specimen treated by CF-SP equipments and the ADP-SP equipments are almost the same.
Effect of shot peening (SP) on bending fatigue limit of high strength steel (SUP9A) containing a semi-elliptical surface slit was investigated. SP was conducted on the specimens containing a semi-elliptical surface slit with aspect ratio, a/c = 0.4, where a was the slit depth (a=0.1, 0.2 and 0.3mm), and c was the half of surface length of slit. Bending fatigue tests were carried out under stress ratio, R = 0. The result showed that the fatigue limit of the shot-peened specimens having the slit under 0.2 mm in depth was almost same as that of the shot-peened smooth specimens. Meanwhile, some of the specimens fractured from the surface other than the slit. Thus, the maximum depth of slit which could be rendered harmless by SP was 0.2 mm. The maximum depth of crack with various aspect ratios which can be rendered harmless by SP was predicted, assuming that the crack was arrested when apparent stress intensity factor at the slit tip was less than threshold stress intensity factor of the material. The estimated values were in good agreement with experimental values. According to the estimation method, the harmless crack assessment diagram was proposed.
It has been clarified that surface fracture and fracture accompanying a fish-eye on its fracture surface occurred under short and long fatigue life regions, respectively, in the stepwise S-N curve of an austenitic stainless steel obtained at 700℃. In order to clarify the effect of oxidation on fracture morphology, fracture surfaces obtained already for SUS321-B steel were examined in detail by EPMA. After a small fish-eye, of which origin was located at the subsurface of the specimen, had reached the specimen surface, two types of fracture processes occurred. One type was that the fish-eye became a small surface crack and propagated as a StageⅡ crack. The other type was that fracture surfaces of the fish-eye were covered with oxides and the fish-eye became a non-propagating occluded crack. In the latter case, initiation of a new crack at the specimen surface was induced by the occluded crack and then fracture occurred. Oxidized semi-circular area was formed near the fish-eye in both types, which was brought by behavior of oxide-induced crack closure. After a large fish-eye, of which origin was located at the interior of the specimen, had reached the specimen surface, the fish-eye became a large surface crack and propagated as a StageⅡ crack. In this case, oxidized semi-circular area was not formed near the fish-eye. The depth of fish-eye origin from the specimen surface and stress intensity factor at the tip of fish-eye when it reached the specimen surface were pointed out as factors controlling which type of fracture processes occurred.
In recent years, flexible materials with very high performance are widely used and large deformation analyses of these materials have attracted considerable attention. However, there is not sufficient analysis for problems of practical interest such as large deformation of a cable within a channel (or a pipe), or of a drill within a hole in rock engineering and petroleum production, or of a catheter within a blood vessel, and so on. When a lightweight, flexible material must be pushed through the narrow space, it is possible for the materials to buckle since the leading edge of the material strikes an obstacle (or a barrier) and cannot proceed any further. The postbuckled configuration, namely large deformation, will be thus of great concern in handling these flexible materials. This paper deals with the postbuckling behavior of a flexible elastic beam contained within rigid, parallel walls with friction under the action of axial compressive forces at each end. Different contact stages between the beam and the walls are modelled using the nonlinear deformation theory. The theory of the elastica is applied and exact analytical solutions for large deformation are derived in terms of elliptic integrals, and then nondimensionalized. As an example of such results, the sequence of large deformation are illustrated through which the postbuckled beam passes. In this study, some experiments of flexible elastic beams were presented and the experimental results were compared with the theoretical formulas. As a result, the relation between the applied axial force and the beam length, obtained from the experiments, is similar to that predicted by the analytical theory. Moreover, it is made clear that the existence of friction between the beam and the wall causes a nonuniform buckled configuration. The analysis presented in this paper would have application to the design of machinery for handling flexible elastic materials such as sheets, tapes, films, papers, cloths and so on. In addition, designers of machines such as drill or tubing strings, sewing machines and medical instruments should find the results useful and easy to apply.
The Votta’s equation is usually used for constant-force spiral spring. But the calculated result is not consistent with experiment. One of the main factor comes from the value of Young’s modulus. Generally, Young’s modulus measured by tensile test, does not quite correspond to the value by bending test. We measure the Young’s modulus by bending once more to verify the validity of equations. Furthermore, we point out some logical mistakes in Votta’s equation.We formulate the rigorous equation considering variable load, and then simplify this assuming constant load for practical design.
Height reduction of compressive coil springs (height loss) is considered to be the effect of creep phenomenon. Height loss of coil springs made of SWOSC-V behaves 3 different temperature dependency in elevated temperature zone, namely lower region (100-200℃), mid region (250-300℃), and higher region (350-400℃). These behavior are quite different from much elevated temperature region. In lower region, Cottrell effect seems to be the governing factor of creep speed. On the other hand, IS effect has the governing role in higher region. In mid region, the creep speed is governed by a transition mechanism from Cottrell to IS effect.