Interior crack initiation during the very-high-cycle fatigue of railway wheel steel under axial loading and rolling contact loading

In this paper, a comparative study of the very-high-cycle fatigue (VHCF) behavior of railway wheel steel under axial loading and rolling contact loading was conducted. Fatigue tests were performed with an ultrasonic fatigue test machine under axial loading, and the fracture surfaces from the fatigue tests and shattered rims taken from the failed railway wheels were observed. The wheel steel under axial loading presents a VHCF behavior with Mode I crack, and that under rolling contact loading is a VHCF behavior with mix Mode II-III crack. For the VHCF behavior with Mode I crack ,surface and interior crack initiation occurred with equal probability at both low and high stress levels and produced a dual linear S-N curve since the value of fatigue limits for the surface and interior crack initiation are close. For the VHCF behavior with mix Mode II-III crack, cracks were initiated from the interior Al O inclusion and the fatigue life was beyond 10 cycles. Fatigue bands were observed on the fracture surface under rolling contact loading. The ferrite nanograins formed due to the stress state of shear plastic strain with a large compressive stress. The formed nanograins were softer than the matrix caused by the redistribution of the carbon.

Large deflection nonlinear mechanics of curved cantilevers under contact point loading

Presented here is a comprehensive model for hook bending behavior under contact loading conditions, motivated by the relevance of this problem to reusable hook attachment systems in nature and engineering. In this work, a large deflection model that can describe the bending of hooks, taken as precurved cantilevers with uniform initial curvature, was derived and compared with physical testing. Physical testing was performed with stainless-steel and aluminum hooks shaped as semicircular arcs. The force versus displacement behavior exhibited a linear portion for small displacements but at large displacements there was an asymptotic relation where the force approached some limit and remained flat as further displacement occurred. Comparison with testing showed that the model developed in this paper gave good agreement with the physical testing. Surprisingly, in dimensionless form, all parameters to define the hook transform to approximately linear functions of displacement. Using these linear relations, several equations are presented that rapid calculation of the dimensional force versus displacement for a hook.

Вихретоковый контроль усталостной деградации метастабильной аустенитной стали при гигацикловом контактно-усталостном нагружении

The surfaces of parts made of austenitic chromium-nickel steels are subjected to heavy cyclically repeated contact loads, including impact loads, during operation. Therefore, the study of contact endurance and the development of methods for non-destructive testing of fatigue degradation of such steels is a relevant task. The aim of this work is to study the possibility of eddy-current testing of the fatigue degradation of the AISI 321 steel under contact loading. Mechanical testing for gigacycle contact fatigue has been conducted in accordance with the pulsing impact “plane-to-plane” contact scheme with an ultrasonic loading frequency. It has been shown that eddy-current testing of the fatigue degradation under contact loading of the AISI 321 steel is possible, but has certain limitations due to non-monotonic changes in the eddy-current device readings α depending on the number of loading cycles. Along with this, using the ascending branch it is possible to test the degree of failure of the surface layer, taking into account in the control technique the ambiguous character of the dependences of the eddy-current device readings on the number of loading cycles. The testing can be performed by measuring the eddy-current device readings at the excitation frequency of the eddy current probe f = 124 kHz. In this case, mainly surface layers are analyzed, where the processes of the fatigue degradation that affect the physical characteristics of the steel are developed intensively. The greatest influence on the value of α is exerted by the quantitative ratio of austenite to strain-induced martensite in the control zone.