Experimental study of the influence of the relative humidity of leaves and their link to adhesion losses in the wheel-rail contact

Every autumn, low levels of wheel-rail contact adhesion caused by leaves on the railhead surface are reported by train drivers. These incidents cause problems for the safety and regularity of rail traffic and occur predominantly at the beginning and end of the day when high humidity levels are achieved. At those moments the air temperature reaches the dew point temperature creating moisture on the railhead and oxides which react chemically with dead leaves found on the railhead surface. This article presents the results of a test campaign performed on a ring-on-disc tribometer with the aim of reproducing and studying the development of a loss of adhesion at an imposed slip rate. This type of tribometer enables a realistic approximation of the tribological circuit of a rail-wheel contact. Several samples of pollution responsible for adhesion losses were collected on the French railway network during fall and reproduced in a laboratory on a tribometer under imposed relative humidity and slip conditions. The results indicate the presence of reproducible phenomena modulated by variations in hygrometry and imposed slip rates. The decrease in hygrometry of the external third bodies introduced into the contact is at the origin of the generation of debris from the first bodies in contact. This presence of wear debris is reflected by an increase of the power dissipated by friction. Conversely, increasing the hygrometry decreases the ability of the third body to raise adhesion levels on contact and reduces the generation of debris from the first bodies. The results obtained from the analysis of the force and power quantities related to the contact reproduced in the laboratory allow the authors to confirm that the presence of water has a significant effect on the mechanical and physico-chemical evolution of the adhesion of a rail-wheel contact polluted by a third external (leaf-like) body.