Estimation of Rail Bending Stress From Real-Time Vertical Track Deflection Measurement

High traffic volume, heavy axle loads, and high train speed can produce large rail bending stresses which contribute to increased track deterioration. Rail stress problems are further exacerbated by poor support conditions such as abrupt changes in vertical track modulus and poor track geometry. This paper summarizes the development of a measurement technique, based on a system being developed over the past few years at the University of Nebraska and sponsored by the Federal Railroad Administration, to determine the actual bending stress in the rail in real-time from a car moving at revenue speeds. The UNL system measures the rail height relative to the line created by the wheel/rail contact points. The system functions continuously over long distances and in revenue service. The system establishes three points of the rail shape beneath the loaded wheels and over a distance of ten feet. These points include the location of high bending stress below the loaded wheels. This direct measurement of the rail shape can then be mapped into rail stress through the curvature of the rail and beam theory. As verification of the UNL measurement system, results from tests conducted on the Union Pacific Railroad’s Yoder Subdivision are discussed. In these tests, bondable resistance strain gages were mounted to the lower flange of the rail at several locations. The track was then loaded by spotting the measurement car over the strain gages and by moving the car over the gages at various speeds. The loaded and unloaded rail profiles were measured using surveying equipment and the relationship between the UNL deflection measurement and the measured rail stress was explored. These early results suggest the UNL system is capable of measuring real-time bending stress in the rail.