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Axial stress determination using Impedance-based method and its application on the thermal stresses measurement in continuous welded rail

This paper presents the current investigation in UCSD on the feasibility of using an impedance-based Structural Health Monitoring (SHM) technique in monitoring the Continuous Welded Rail (CWR). Being welded to form uninterrupted rails that are several miles long, the CWR has been widely used in the modern rail industry since 1970s. However, the almost total absence of joints for expansion of CWR would create the potential of buckling with high temperature and breakage in cold environment due to the rail thermal stresses. The objective of this research is to utilize the capability of the impedance method in identifying the neutral temperature or zero-stress state in CWR. The principle of Electromechanical Impedance (EMI) is to utilize high frequency structural vibration through a piezoelectric transducer to detect changes in structural point impedance due to the presence of change of structural integrity or in-situ stress. In practical CWR monitoring, the rail track structure being monitored is undergoing changes due to the effect of thermal stress and the environmental factors. Based on this assumption, three sets of experiments were conducted: the influence of axial stresses on the EMI signature was studied with an axial loading test on a rectangular section of steel milled from a 136lb RE rail; the temperature effect on the proposed method was investigated with heating-cooling cycle test on an unconstrained 136lb RE rail; the third test to simulate the monitoring scenario as expected in the field was conducted with heating-cooling cycle test on constrained 136lb RE rail testbed in UCSD. During the analysis, both the real and imaginary parts of the EM signatures were studied since both the stress and temperature would have different influence on the signatures compared with defect detection. The temperature effects on the piezoelectric materials and structures were investigated.