Defect Detection using Electro-Mechanical Impedance Sensing and Optical Fiber Bragg Gratings – a Comparison

Abstract

Electromechanical impedance (EMI) sensing and strain sensing via optical fiber Bragg grating (OFBG) sensors are both widely researched structural health monitoring (SHM) techniques, but are based on very different approaches. The EMI approach monitors changes in the frequency of high order structural vibration modes that are generally not observable using conventional vibration testing. SHM using OFBG sensors is performed either dynamically, monitoring vibration, or statically, monitoring defect-induced perturbations in the strain field. While the EMI approach, being a semi-active approach, is probably the more versatile and powerful, the aerospace community has for some time looked more favorably, broadly speaking, on OFBG sensors due to the inherent advantages of optical fiber sensors, such as lightweight, multiplexing ability, amenability to be embedded in composites, etc. In other words, the choice of method to be explored was often based on system considerations rather than inherent ability to detect damage. With recent technical developments in microelectronics, including ultra-low power devices, decreased component size and weight, and wireless sensor network protocols, the EMI method can now be considered in applications that were previously not considered possible, and engineers are thus more able to decide on a SHM method based on damage detection ability alone. It is thus important to clearly understand how well the two methods compare in their ability to detect damage, and one way of getting at this understanding is to apply both methods to the same problem. Thus, in this effort both methods are applied to the problem of detecting crack growth in a metal plate. Specifically, the defect to be detected consists of an edge slit in a metal plate, with multiple fasteners bridging the slit. Thus, loosening the fasteners is analogous to increasing the size of the slit. Macro-fiber composite (MFC) piezo-electric sensors were used to observe the EMI signature, and an optical fiber Bragg grating sensor was used to record dynamic strain during natural vibration of the metal plate.