Study On Service Life Evaluation Of Railway Axle With Flaws Based On Fracture Mechanics

Railway axle is the most important component for high-speed trains. The material elements are severely subjected to complex stress status inside press fitted and geometrical transition region due to cycling fatigued loads and environment. Fatigue crack is initiated and rapidly propagates in such high stress zone under random loading, which leads to the failure of axle and serious accident.Fatigue crack is a strongly discontinuous problem. The stress increase is generated near the crack tip. Unfortunately, the design code cannot evaluate the influence of concentrated stress on fatigue life of critical structures based on cumulative fatigue damage according to S-N curve because the nominal stress method is predominant currently. It is therefore necessary to introduce the fracture mechanics based damage tolerance approach for the assessment on engineering structures with macro flaws. The following points are carried out in this thesis.(1) The assessment scheme and some flaws of railway axles have been thoroughly investigated. Meanwhile, NASGRO law with the stress ratio is adopted to evaluate the damage evolution.(2) To accurately acquire the life evaluation model, static tensile testing, low cycle fatigue, high cycle fatigue, fatigue crack growth rate, the threshold stress intensity factor and fracture toughness have been conducted for the EA4T steel used for railway axle.(3) The stress field near crack tip inside fitted zone has been simulated by using the singular finite element under the dynamics stress spectrum at the speed of 350km/h.(4) By employing the fitted NASGRO equation, fatigue crack rate and life together with the stress intensity factor curve have been obtained for stress relief groove. The cyclic loading numbers of crack propagation is achieved based on measured stress spectrum and thus life mileage is predicted.(5) Researches indicated the predicted threshold stress intensity factor is not constant on account of stress concentration decrement and tip stress ration decrease during on revolution with the growth of fatigue crack.