A finite-element model and experimental investigation of the influence of pre-straining of wire on the sensitivity of binary crack sensors

Steel girder bridges make up a significant percentage of all bridges. Many of the steel girder bridges are aging and approaching their designed service lives. Crack formation and its propagation over time is one of the main deficiencies of aging steel girders and may result in unusable or unsafe service conditions. The existing distributed crack detection methods such as fiber optics sensors are costly to deploy and maintain. A new cost effective binary sensor has been developed, has the potential to be installed on steel girder bridges at a fraction of the cost, and is sensitive enough to detect the presence of a crack opening with a width of 0.2 mm. The crack sensor is a closed electrical circuit and comprised of copper wire and epoxy. When a crack forms in the steel girder, the strain will be transferred to the wire through the adhesive. As the crack on the girder widens over time, strains in the wire increase until it reaches its ultimate tensile strain. The wire then fractures and creates an open circuit. This can be detected by monitoring the electrical continuity of the sensor. One of the main challenges in developing the binary sensor is to select appropriate materials for both wire and adhesive. The final tensile strain as well as the bonding stiffness between the wire and the epoxy have important impacts on the performance of the sensor. In this work, pre-straining the sensor wire was found to be effective in minimizing the width of the detected crack opening. The average was reduced from 0.36 to 0.13 mm and the standard deviation reduced from 0.16 to 0.03. In addition, microbond test was carried on to estimate the interfacial-bonding stiffness between wire and epoxy. The interfacial stress was found to be approximately 2.0 MPa. These parameters were used in a finite-element model of the sensor to predict the behaviour of the binary sensor and, consequently, to optimize the installation position of the sensor on a girder of a bridge.