Residual Stress Estimation Using Hole Drilling and Contour Methods in Rail Welds Treated with and without Ultrasonic Impact Treatment

Abstract

MxV Rail has explored new methods of improving the longevity of both rail and thermite welds. As a part of this research, MxV Rail explored ultrasonic impact treatment (UIT) as a method for increasing thermite weld performance and performed a residual stress investigation to estimate the distribution of the compressive and tensile stresses along the weld cross sections. UIT uses a needle to impart very small and rapid localized impacts to plastically deform the material it is applied to. Weld material reshaping and compressive residual stresses all result from UIT, which produce fatigue life improvements that benefit the life of the weld. A residual stress investigation using latest methods was also executed on both new rails and rails with different amounts of wear to analyze the distribution of estimated stresses. Fatigue defects grow quickly under tensile stresses and slowly under compressive stresses. Since UIT introduces compressive stresses on the surfaces of the weld material, an increase in the life of thermite welds can be expected due to retarded crack growth in localized areas of compressive stresses. In order to estimate the change in stress conditions caused by UIT, MxV Rail analyzed residual stresses in three thermite welds: (1) a UIT-treated new weld, (2) a UIT-treated weld removed from track after 150 million gross tons, and (3) an untreated new thermite weld. The residual stress estimations were accomplished by using hole drilling (HD) and contour methods. While the HD method provides estimated strains along two axes (longitudinal and transverse) at very shallow depths in particular locations, the contour method involves wire electric discharge machining and advanced finite element analysis to estimate the overall longitudinal strains (ε_zz) across the entire cross section of the weld or rail. The contour method was also applied to the rails to better understand the influence of residual stresses due to wheel–rail interaction on the head along with rail wear and how the modified residual stress distribution in worn rail compares to the initial residual stress distribution in a new rail that has been rolled but not laid in track.