Thermal Stress Measurement in Continuous Welded Rails Using the Hole-Drilling Method

Continuous Welded Rail (CWR) has been widely used in modern railway system for it provides smooth ride, higher freight speed, and less maintenance. A major safety concern with this type of structure is the absence of the expansion joints and the potential of buckling in hot weather. According to the FRA safety statistics, the track alignment irregularity is one of the leading factors responsible for the accidents and the most economic/environmental damages, among all the railway accident causes. However, the thermal stress measurement in the CWR for buckling prevention has been an unresolved problem in railroad maintenance. In this study, a method is introduced to determine the in-situ thermal stress of the in-service CWR by using the Hole-Drilling method. The ASTM Hole-Drilling test procedure, as one type of stress relaxation methods, was originally developed to measure the in-plane residual stresses close to the specimen surfaces. The residual stresses are typically computed based on the relieved strains with the calibration coefficients. Inspired by the stress relaxation philosophy, an investigation on the thermal stress measurement of the CWR using the Hole-Drilling test procedure is conducted in this paper. First, the feasibility of using the Hole-Drilling method of the thermal stress measurement is examined via a 3-D finite element model. The stress relaxation computed from the Hole-Drilling test is compared with the applied uniaxial thermal stress. To facilitate the implementation on the CWR, a new set of calibration coefficients with finer depth increment is computed with a novel three-dimensional finite element model for more realistic simulation. The updated coefficients are experimentally validated with an aluminum column specimen under uniaxial load. For the experimental studies, a roadside prototype is developed and two sets of tests are carried out on free-to-expand rail tracks and on rails subjected to controlled thermal loads at UCSD Powell Laboratories. The relieved stresses are computed using the updated calibration coefficients, and a linear relationship between the axial and vertical residual stresses at the neutral axis is observed for both 136RE and 141RE rails. Furthermore, the in-situ thermal stresses are estimated with the residual stress compensation and the neutral temperatures are predicted according to linear thermal expansion theory. These tests illustrate that the determination of the thermal stresses by the Hole-Drilling method is in principle possible, once ways are developed to compensate for the residual stress relaxation. One such compensation is proposed in this paper. A statistical interpretation on the proposed method is also given to provide a reference for railroad applications.