Contents I. Introduction
Applications of Lamb waves (LW) in nondestructive testing and structural health monitoring (SHM) have been increasing in recent years, due to their inherent advantages such as long-range propagation (especially in metallic structures), and high sensitivity to structural changes along the propagation path [1]-[3]. The former feature enables the monitoring of large areas with a relatively low number of sensors, while the latter allows detecting various types of damage, including delamination in composite structures [4]. Despite their advantages, one of the most important issues is the sensitivity of Lamb waves to the variations of environmental and operative (load) conditions [5]. For example, several studies highlighted substantial changes in Lamb waves amplitude response caused by temperature excursions [6]. This amplitude variation may mask the damage-related information present in the captured signals, when comparing the structure response acquired under different conditions, e.g., actual and baseline configurations, through proper damage indicators [7]. Measuring the actual temperature and strain conditions of the structure under investigation may provide a means to compensate the changes in the detected Lamb waves. Optical fiber sensors represent a valid solution, as they permit to realize multi-parametric (temperature, strain, and vibration) and spatially distributed sensing. Furthermore, optical fiber sensors are small, lightweight, immune to electromagnetic interference, applicable to curved or irregularly shaped structures and capable of working in unfavorable environments such as wet, underwater, and high temperatures.
