The effect of intermittent lubrication on the fatigue life of pearlitic rail steel in rolling-sliding contact

Railway flange lubrication can be unreliable, resulting in intermittent rather than continuous lubrication. Twin-disc contact simulation tests were carried out to investigate the influence of intermittent lubrication on rail steel fatigue life, using both a colloidal suspension of molybdenum disulphide in an oil carrier fluid (similar to a commercial flange lubrication product) and water. A brief interruption of the lubricant supply during molybdenum disulphide lubrication produced rapid surface failure of the rail steel, which was followed by rapid contact fatigue crack growth upon reapplication of the lubricant. During water lubrication the interruption of the lubricant (water) supply did not produce such rapid crack growth. The results were found to be in qualitative agreement with the predictions of the three-mechanism model of rolling contact fatigue, which combines the mechanisms of ratchetting (accumulation of unidirectional plastic strain until the critical failure strain of the material is reached) and the fracture mechanics-based mechanisms of shear stress driven and tensile fluid assisted crack growth. Analysis of the test results using this model revealed crack face friction to be an important factor controlling crack growth, which may be responsible for the difference in behaviour between contacts lubricated with the molybdenum disulphide-based lubricant and those lubricated by water.