Monitoring of L-Shape Bolted Joint Tightness Using Thermal Contact Resistance M. Jalalpour & J.J. Kim & M.M. Reda Taha Received: 3 September 2012 / Accepted: 6 May 2013 # Society for Experimental Mechanics 2013 Abstract In this paper, we describe the possible use of hermal contact resistance measured under ambient conditions as a non-destructive feature to monitor L-shape bolted joints. We demonstrate that thermal contact resistance can be correlated to the contact pressure of L-shape bolted joints and thus can be used to infer joint tightness. Five different torque levels of bolt tightness in an L-shape bolted joint were measured for contact pressure and thermal contact resistance using Fuji Prescale® pressure-sensitive film and a test setup in ambient conditions. Using probabilistic analysis, probability density functions (PDFs) and intervals developed from the PDFs, the contact pressure and thermal contact resistance were developed and correlated. By examining the intervals of the contact pressure and thermal contact resistance at each torque level, it was concluded that thermal contact resistance measured under am- bient conditions can be used to describe bolted joint tightness. Keywords Structural health monitoring (SHM) . Thermal contact resistance . Contact pressure . L-shape bolted joint . Joint tightness Introduction Bolted joints are critical components in aerospace structures. As human errors during assembly and disassembly can cause significant risk, it is necessary to check that the bolted joint tightness has been achieved to a desirable level before any bolted aerospace structure is put into service. Recent advances in structural health monitoring (SHM) can provide techniques not only to automate this process but also to make it more reliable. In any SHM systems, four steps are considered: data acquisition, damage feature extraction, damage pattern recognition and damage prognosis. These steps relate to the collection of data from sensors in the structure, the identification of a critical measure (feature) that can describe the damage of the system from a baseline healthy performance, the use of the damage feature to quan- tify damage, and the estimation of the remaining service life of the structure [1–3] respectively. Joint tightness monitor- ing requires relating the damage features to physical quan- tities such as the contact pressure at the joint interface to represent the structural reliability of bolted joints. The contact pressure of bolted joints was investigated extensively in the 1960s and 1970s [4, 5]. Ito et al. [6] studied the interface pressure in bolted flanges of various surface topographies using ultrasonic waves. Li el al [7] conducted research on the effects of the fasteners’ clearance fit and friction coefficient on the stress in riveted lap joints. Pau and Baldi [8] applied ultrasonic methods to quantify the pressure at the interface of the bolted joints. Todd et al. [9] presented a method based on nonlinear time series analysis of chaotic waveform to detect loosening of a bolted joint in an aluminum frame structure [9]. Fujifilm Prescale® is a film used to measure contact pressure at the joint interface. The film consists of a micro-encapsulated color-forming layer and a substrate layer. Depending on the magnitude of the pressure applied to the film, microcapsules in the color-forming layer burst and a characteristic pink stain is produced on the substrate layer by a chemical reaction from the liquid in the burst capsules with the liquid in the substrate layer. Deep (i.e. dark) colors are produced by high pressure. Many researchers reported the successful use of the film to measure the contact pressure at the joint interface [10–13]. Aymerich and Pau [10] evaluated their proposed ultrasonic technique to measure contact pressure by comparing the results with those obtained M. Jalalpour Structural Technologies, LLC., Hanover, MD 21076, USA J.J. Kim (*) Green Infrastructure Technology for Climate Change Research Center, Yonsei University, Seoul 120-749, South Korea e-mail: jjkim@unm.edu M.M. Reda Taha Department of Civil Engineering, University of New Mexico, Albuquerque, Albuquerque, NM 87131-0001, USA Experimental Mechanics DOI 10.1007/s11340-013-9759-9