Makoto Akama

Bayesian analysis for the results of fatigue test using full-scale models to obtain the accurate failure probabilities of the Shinkansen vehicle axle

Abstract Bayesian analysis was performed to estimate an appropriate value of the uncertain propagation rate of cracks that can be initiated at the wheelseat of a Shinkansen vehicle axle. In the analysis, fatigue life distribution obtained by numerical simulation that employed the crack propagation rate obtained from small specimens was used as the prior distribution. Then it was modified by the results of the fatigue test of full-scale models as additional information to obtain the posterior distribution. It was indicated that the variances of fatigue life distribution reduced through the analysis. By using the crack propagation rate obtained from the posterior fatigue life distribution, the failure probabilities of the Shinkansen vehicle axle in operation, that were calculated previously by using the crack propagation rate due to the experiment of small specimens were recalculated. The resulting probabilities of failure were almost the same as those that were not modified, but were slightly lower. Although the difference was not so significant, it was thought that more confident values of the failure probability were obtained.

Boundary element analysis of surface initiated rolling contact fatigue cracks in wheel/rail contact systems

This study performed the boundary element analysis of the cycle of stress intensity factors at the surface initiated rolling contact fatigue crack tip under Hertzian contact stress including an accurate model of friction between the faces of the crack and the effect of fluid inside the crack. A two-dimensional model of a rolling contact fatigue crack has been developed. The model includes the effect of Coulomb friction between the faces of the crack. The fluid in the crack was assumed not only to lubricate the crack faces and reduce the crack face friction coefficient but also to generate a pressure. The obtained results are compared with those of other researchers showing good consistency.

Long co-planar mode III fatigue crack growth under non-proportional mixed mode loading in rail steel

Fatigue tests have been performed to obtain the co-planar crack growth rate in rail steel under non-proportional mixed mode I and mode III cycles. In addition, a finite element analysis (FEA) has been performed to investigate the crack growth behaviour under this loading. The experiments showed that a long co-planar crack could be produced under this loading. Based on fracture surface observations obtained using a scanning electron microscope and FEA, the long co-planar crack growth was thought to be driven mainly by mode III loading and the role of mode I was as an assistant; keeping the crack face open. It was also observed that the crack tended to branch when the degree of overlap between the mode I and mode III cycles increased. Comparing the crack growth rate data under loading with that under non-proportional mixed mode I and II cycles, it was found that the co-planar growth rates were similar regardless of whether the main driving force was in-plane shear or out-of-plane shear.

Possibility of Fatigue Crack Extension Under Cyclic Large Compressive Stress by Medium Frequency Induction Hardening

Induction hardened axles have been used since the start of the Shinkansen (Japanese Bullet train) service in 1964. Axles are subjected to cyclic loading, and induction hardened axles are used under cyclic compressive stress conditions because of the large compressive stress state caused by induction hardening along the axle surface. Japanese Railways regularly inspect its axles, and if any crack larger than 0.15 mm in depth is found, the axle is taken out of service. The compressive stress is around −500 MPa, and magnetic particle inspection is conducted at intervals of 2.2×108 and 4.4×108 rotation. The issue of whether or not a crack more than 0.15 mm in depth will propagate under cyclic large compressive stress conditions is of great importance from the viewpoint of railroad safety. From the above mentioned background, a literature survey and experiment of fatigue crack extension under a cyclic large compressive stress condition were conducted. The results show that the crack did extend in the range of around 3.6–5.0×108 cycles under the condition of −500∼0 MPa cycle. Elastic-perfectly plastic stress analysis was also conducted to understand the possibility of tensile residual stress origination, which is considered to be a cause of extension. The analysis of −500∼0 MPa cycle showed that a large tensile stress of about 89.5 MPa originated at the crack tip when unloading. Whereas, the analysis of −560∼−440 MPa cycle for actual maximum stress in the field showed compressive when unloading.Copyright