Sinto (Japan)

In order to investigate the effect of laser peening treatment on high cycle fatigue properties, rotating bending fatigue tests were carried out for laser peened gas-carburized SCM420H steel. In this study, two types of pulse laser oscillators such as conventional high power laser equipment and hand-held compact laser equipment were prepared to generate different residual stress distributions in specimens and the effectiveness of the hand-held laser oscillator was verified as a laser peening equipment. Fatigue test results showed that laser peening with the hand-held laser oscillator improved fatigue strength to the same extent as that with the high power laser oscillator. Since fatigue strength was correlated with hardness distribution and residual stress distribution, the fatigue limit was predicted using both distributions. The attenuation behavior of the residual stress was predicted from the hardness and used to estimate the fatigue limit. Although the hand-held laser peening imparted compressive residual stress only near the material surface, the fatigue limit was significantly improved. Furthermore, no fatigue failure occurred inside the specimens despite the application of stresses exceeding the local fatigue limit. This suggests that there is an unknown factor in the hand-held laser peening that suppresses fatigue crack initiation.

The double rotary inverted pendulum (DRIP) system belongs to the class of under‐actuated mechanical systems, and it is a highly nonlinear, unstable, and benchmark system to test the different control techniques. This paper successfully designed the nonlinear hierarchical sliding mode control (HSMC) techniques to stabilize the DRIP system. We compare the performance of these techniques numerically with each other and with the previously designed control technique. We propose an aggregated HSMC technique as it has a much shorter stabilization time than other designed techniques.

In order to investigate the effect of residual stress distribution pattern in the specimen on the fatigue crack initiation behavior and fatigue strength at 2×107 cycles, rotating bending fatigue tests were conducted for carburized SCM420H steels treated with several shot peening conditions. In the case of a specimen having a S-type residual stress distribution in which a peak residual stress exists inside the specimen and the surface residual stress is smaller than the peak residual stress, the specimen fractures in surface fracture type under high stress load and in internal fracture type under low stress load. The boundary stress between surface and internal fracture type is the lower limit stress at which the surface crack can proceed through the depth where the peak of the residual stress exists. On the other hand, most of the SP-treated specimen fractures in internal fracture type and clear fish-eye pattern was observed on the fracture surface. And the crack initiation site was at the location of transgranular cracking that occurred in a region deeper than the hardened layer. It was predicted that the fatigue strength at 2×107 cycles of SP treated specimen corresponds to the crack generation limit when the residual stress peak position is relatively shallow. Furthermore, it was predicted that the fatigue strength at 2×107 cycles corresponds to the crack propagation limit when the residual stress peak position is relatively deep.

This study aimed to improve the bending strength and reliability of ceramics using laser peening (LP). In the experiment, LP without coating (LPwC) and with coating (LPC) were applied to silicon nitride (Si3N4) under various conditions. The surface roughness, residual stress, and bending strength were then measured for the non-LP, LPwC, and LPC specimens. The results show that the LPwC specimen had a greater surface roughness but introduced larger and deeper compressive residual stress when compared with the non-LP and LPC specimens. In addition, the bending strength of the LPwC specimen was higher and scatter in bending strength was less compared with the non-LP and LPC specimens. This may be attributed to the transition of the fracture initiation point from the surface to the interior of the LPwC specimen because of the compressive residual stress introduced near the surface. Thus, it was demonstrated that the application of LP is effective in improving the strength and reliability of ceramics.

Rotating bending fatigue tests were conducted to investigate the effect of shot peening on fatigue properties of maraging steels made by direct metal laser sintering process which was one of additive manufacturing processes. In order to investigate the effect of lamination direction of material, two types of specimen changed modeling pattern as horizontal modeling and vertical modeling were prepared. As the result of the fatigue test, non peened specimen was fractured from initial defect near the specimen surface, but the shot peened specimen was fractured from initial defect inside specimen where located 600µm deeper from surface of specimen. The size of initial defect differs according to the difference in modeling pattern and its size of the vertical modeling material was larger than the horizontal modeling material ones. However, threshold stress intensity factor at which specimen fracture of both materials were the same levels. The cause of this, fatigue limits of the vertical modeling material was slightly smaller than the horizontal modeling material ones. And then, since the final dimensions of the fish eye of both materials become the same, fatigue life of vertical modeling material shorter than horizontal modeling material.

ABSTRACTFatigue properties of a transformation-induced plasticity-aided martensitic steel subjected to vacuum carburisation under carbon potentials ranging from 0.70 to 0.80 mass% and subsequent fine-particle peening were investigated for the fabrication of precision transmission gears. The fatigue limits of smooth specimens increased by 34–41% with increasing carbon potential, compared with that of heat-treated and fine-particle peened steel. The notched fatigue limits increased to a lesser degree except in case of carbon potential of 0.70 mass%. The increased smooth fatigue limits were associated with the high Vickers hardness and compressive residual stress via severe plastic deformation and the strain-induced martensitic transformation during fine-particle peening, as well as a 50% reduction of retained austenite fraction during fatigue deformation.

The effects of laser peening (LP) on the bending fatigue limit of an austenitic stainless steel JIS-SUS316 welded joint containing an artificial semi-circular slit on the weld toe were investigated. Three-point bending fatigue tests were carried out at a stress ratio of R = 0.05 for both LP treated welded specimens with the slit on the weld toe and those without the slit. The fatigue limits of all specimens increased by 23-64% due to LP. LP treated specimens with a slit depth of a = 0.2 mm had high fatigue limits that were nearly equal to those of non-slit, LP treated specimens. Therefore, a semi-circular slit of less than a = 0.2 mm could be rendered harmless due to LP. The improvement of fatigue limit due to LP was mainly achieved by compressive residual stress.