The crack propagation mechanism of thick plates is difficult to describe accurately, and thus, it is difficult to predict the lifetimes of thick plate structural cracks in extreme construction environments. In this paper, a theoretical model of the time-integrated crack propagation that accounts for the thickness effect is established. The constraint factor is introduced to characterize the thickness effect of crack propagation, and the constraint factor formula was obtained by fitting the experimental results. The results of the fatigue crack growth tests and the crack propagation theoretical predictions for Q345D specimens showed the time-integrated crack propagation model predicts the crack propagation more accurately in the initial stage and during the stable expansion stage of crack propagation. These results have guiding significance for the fatigue life prediction of equipment with thick plate structural features.INDEX TERMS Thick plate, crack propagation, crack closure, fatigue test, time integration method.
The nonlinear load of the axle box bearing and the spatially coupled relationship of vehicles have seldom been considered in the dynamic modeling of vehicle-track systems in recent years. Therefore, based on the time-varying nonlinear contact load of an axle box bearing, a vehicle spatially coupled dynamic model is established. Subsequently, the vehicletrack spatially coupled dynamic model is established by the dynamic coupled relationship between the wheel and rail. Finally, the vehicle wheelset testbed is set up to verify the correctness of the theoretical model. Experimental results show that (1) the vibration of an axle box has a strong nonlinear coupled relationship. The axle box vertical amplitude aroused by lateral alternating load is approximately 0.108 mm, which was approximately 22.2% of the lateral vibration and (2) the lateral displacement and acceleration errors of axle box between the simulation and experiment are about 5.7% and 6.9%, respectively, which validates the theoretical model. The vehicle-track spatially coupled dynamic model accurately reflects the vibration characteristics of locomotives under track irregularity and provides theoretical support for the design of key components, such as axle box bearings.
Lubrication failures of axle box bearings can lead to accidents, such as bearing burnout and hot axle cutting. Presently, the modeling of the vehicle-track system dynamics rarely considers the nonlinear contact load of axle box bearings, and this leads to imperfection in the vehicle-track system dynamics calculation. And then, the load distribution and lubrication characteristics of axle box bearings are difficult to obtain. Therefore, in this paper, we fully consider the time-varying nonlinear contact load of bearings and track irregularity in establishing the bearing-wheel-rail system coupling-dynamics model. The dynamic response of axle box bearings is obtained by taking the vertical, strong impact-time-varying load on the carrying saddles as the external excitation. The load-balance equation of dynamic pressure lubrication is then obtained, according to the slicing method of bearing rollers. Finally, the elastohydrodynamic lubrication (EHL) model of axle box bearings is established considering thermal and scale effects. The results show that the central film thickness under thermal EHL was decreased by 13.61% compared with that under isothermal EHL. As the velocity of the contact pair increases, the thickness difference between thermal and isothermal EHL became larger. Thermal effects should be considered in the EHL model, in order to truly reflect the characteristics of EHL under a high speed.
In this paper, the nonlinear dynamics of a single degree of freedom system for the rolling mills with clearance is investigated. The boundary conditions of the self-excited vibration system basically involve discontinuities, and the effects of boundary conditions on the vibrating system are analyzed by using asymptotic method and numerical simulation. Different forms of nonlinear vibrations such as periodic, quasi-periodic and chaotic motions are detected. Influence of the system parameters on the nonlinear vibration behaviors is also examined by applying the Poincare sections, phase portraits, waveforms, the bifurcation diagram and the largest Lyapunov exponent. New phenomena are observed in nonlinear motions of the rolling mill mechanism.
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