Characterizing and Inspecting for Progressive Epoxy Debonding in Bonded Insulated Rail Joints

costs and train operations. The impact is sometimes magnified by imperfect or poorly understood techniques for predicting and detecting IJ failures. A common factor in many failures of bonded IJs is a tendency for the epoxy to come debonded from the rail or joint bar near the center of the joint (2). The debonded region starts small but grows with time and traffic, spreading outward toward the ends of the joint bars. As the debonded region grows, the joint becomes weaker and loses stiffness. Eventually, the remaining epoxy bond does not have sufficient strength to resist longitudinal loads and the bond ruptures, leaving a joint that is held together only by the mechanical action of the bolts. For clarity, the authors define two terms to refer to the two phases of epoxy degradation: progressive epoxy debonding (or debonding) is used to describe the gradual spread of a debonded region; the sudden failure of the remaining bond is referred to as complete epoxy failure. This paper primarily covers progressive epoxy debonding in joints that have not yet experienced complete epoxy failure. Understanding this progressive deterioration is necessary both for designing improved joints and joint materials and for developing better maintenance practices to deal with deteriorating joints. The research focuses on understanding the effects of progressive debonding on IJ performance and on developing and evaluating techniques for assessing the extent of debonding in an in-track joint (3). In this paper, the authors develop a rigorous approach to characterizing and quantifying the extent of epoxy debonding in a degrading IJ. The ability to measure debonding does not itself affect the rate at which IJs deteriorate. However, condition assessment is a necessary (if not sufficient) component of any maintenance program. Furthermore, any empirical research program into the functional consequences of epoxy debonding requires that the input variable—debonding—be characterized in a consistent, complete manner.