Papers by Author: M.J. Liu

Abstract: Critical safety wheel wear size is investigated through considering the fatigue safety effect on RD2 type axle of China railway freight cars. The wheel wear size increase results in a rise of the wheel-rail contact forces, which increase the fatigue stress history of the axle in service. Therefore, the size must be controlled to maintain the axle safety in service before the next overhaul inspection. A field investigation was firstly made on the wear size change regularity. Then, the size related wheel-rail impact forces are solved by a non-linear Hertzian contact theory with a vehicle multi-body dynamic model. In addition, a statistical method is further developed for incorporating the impact forces into the wheelset service load history. Finally, a wear size related critical state equation is established for ensuring the axle safety in service. A stationary solution is statistically obtained for the critical wear sizes with respect to the axle circumferential crack and semi-elliptical crack. It is concluded that the critical safety wear size should be controlled in the range of 100 mm at the statistical level of survival probability 0.99 and confidence 95%.

Abstract: Critical fatigue safety state is investigated on RD2 type axle of China railway freight cars. Attention is paid on the grooves near axle boxes, where more fatigue cracks were early appeared even result in at least six derailed accidents. Load history was obtained by vehicle dynamics simulation combining with on-line inspection. Braking loads and effects of maintenance and off-round of wheels are also incorporated. Semi-elliptical and external circumferential cracks are employed for crack growth simulation. Crack shape change and shear stressing of the semi-elliptical crack are taken into account. New crack growth rate modeling in which covers from cracking threshold to toughness related fracture state is applied for residual life evaluation. Critical fatigue safety state is defined as the crack size from which the axle can be safely operated to next overhaul inspection. Critical crack size is estimated step-by-step with the crack shape change. Results show that the external circumferential crack is more dangerous than the semi-elliptical crack. Application more than two years in production verifies that the present assessment is available and reliable.