Automated Structural Health Monitoring of Bolted Joints in Railroad Switches

Current switch bolt inspection on rail systems is a labor intensive and sometimes unreliable approach to maintaining the switch integrity. Recent rail accidents in the United Kingdom (Potters Bar in 2002 and Grayrigg in 2007) underscore the need for routine inspections of the switch mechanisms. From the Grayrigg report of 23 February 2007 the main causes of the accident were found to be the loosening and, as a result, the initiation and growth of cracks, and, eventually, rupture of the bolts of the switch bars, especially the one maintaining the switch rails at a correct distance apart. Such findings also resulted from the 2002 crash report but unfortunately frequent visual inspections were not forthcoming. In this paper, an effective method for monitoring the loosening of the switch bolts is described. As the loosening of the bolts further causes the crack formation in the bolted joints, it seems valid to say that the early detection of loosening of bolted joints in railroad switches will be of great importance in eliminating the need for frequent visual inspection by totally automating inspection of the switches’ mechanical condition. The first part of the present paper focuses on the use of smart materials and structures for the health monitoring of bolted joints in railroad switches. It is shown that using the piezoelectric transducers and the impedance-based structural health monitoring technique, the loosening of the bolted joints are detectable. The accuracy in loosening detection is as high as 25 ft-lbs which corresponds to merely 1/10^th of a bolt turn. Being able to detect the loosening of the bolted joints in railroad switches, the concept of self-healing bolted joints is applied in the next part in order to automatically retighten the loosened bolts to their prescribed functional conditions.