A Laboratory Investigation of Threshold Stress in Wheel/Rail Interaction for Heavy Axle Loads

Abstract

This study is a continuation of earlier investigations to determine acceptable upper bound of contact stresses (threshold stress) for new wheel and rail under heavy axle load conditions. Using sharply increased wear rate (as initial maximum contact stress increases) under free rolling conditions as an indicator of threshold stress, laboratory investigations of wheel-rail rolling contact wear, simulating the 125-ton car wheel load, were conducted. Tangent track conditions with clean and dry surfaces were tested on two facilities; one of one-quarter scale and the other of one-fifteenth scale wheel-rail simulation. The earlier experiments conducted in the IIT Railroad Engineering Laboratory to investigate threshold stress used a varying load with the same wheel-rail profiles, the present study used constant load with varying wheel-rail profiles to determine whether the above increase in wear rate phenomenon will still be observed. While the results of the small (1/15th) scale facility were not considered quantitatively conclusive because of small size, qualitatively they indicated the presence of threshold stress phenomenon. The results of the one-quarter scale test facility very clearly confirmed that there is indeed a threshold stress above which the rolling contact wear rates increased sharply. The value of this stress, found to be approximately the same as reported in the earlier study, was nearly 200 ksi (for standard carbon steel rail). From the earlier and the present investigations of threshold stress, it is clearly concluded that the increased wear rates are primarily due to the increased initial contact stresses and 200 ksi is the value of threshold stress for standard carbon steel rail. Initial wheel-rail contact stresses for new wheel-rail should not be allowed to exceed this value to reduce damage to rail for heavy axle loads. It is recommended that new wheel-rail profiles should be developed to achieve stresses at or below this threshold stress level. Benefits of improved rail life with this approach are also discussed.