Stresses and curvature relationships in wheel-rail contacts for heavy axle loads

This study presents a systematic investigation of wheel-rail contact stresses, geometries and rail steel behavior with a view to improve the capability of rail to carry heavy axle loads. To put the rail head profile design on sound engineering basis, the lower and upper bounds of contact stresses were investigated. The lower bound of contact stress was derived by using 2-D Hertzian contact analysis of fully worn wheel and rail profiles and was found to be about 98 ksi. On the IIT GMEMD wheel/rail testing facility, experiments were conducted to determine wear rates for free rolling 125-ton car wheel load using Hertzian simulation for different contact stresses by change of laboratory wheel profiles. The upper bound, called the threshold stress, was derived in terms of surface wear rates. Under free rolling contact sharply increased wear rates were found to relate to plasticity spreading to the surface to indicate internal rail damage as a initial condition phenomenon. The threshold stress for rail steel was found to be about 200 ksi, and corroborated the earlier finding at the IIT Railroad Engineering laboratory. Contact movement geometric constraints were investigated in terms of wheelset lateral oscillations, rail tilting and worn wheel and rail profiles. It was determined that wheelset oscillations did not put a constraint on the choice of practical radius of rail crown radius. On the other hand, contact tilting analysis suggested a provision of accommodation of contact tilt capacity of larger than 5 degrees (defined as beta angle) to be used such that wheel tread contact could be constrained on the rail crown. Parametric contact stress study using lower and upper bounds to study relative wheel/rail curvatures suggested using 15-20 inch range for rail crown radius and 20-30 inch for wheel tread profile curvature. Improved rail head and wheel tread profiles using these findings were designed. Ellipse was found to best suit the rail head profile improvements in light of lowering the contact stress and simultaneously providing large beta angle. These profiles were determined for 132-RE and 136-RE rails, and it was found that with small grinding these profiles could be instituted on these rails. Wheel tread profile curvature of 20 to 25 inches to provide a contact tilt accommodation (wheel beta angle) of 3 to 6 degrees is suggested for new wheel profile. A preliminary wheel profile illustrating these process of developing new wheel profiles based on certain design criteria is given.