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It is necessary to make sure that the physical structures such as buildings, bridges are structurally safe and sound for the safety of human beings. There are chances that the structure of an instrument is affected by some internal damage, even if it appears healthy from the outside. Accordingly, periodic structural monitoring of complex configurations such as bridge, high-rise structure, and aircraft is necessary (Boukabache et al., 2012). By using only human intervention to find the damage in structural level, high-consequences decisions cannot be made (Mascarenas et al., 2013). So, researchers have proposed many approaches for SHM where limited or no human interventions are required.
SHM is a process where nondestructive evaluations techniques are used to detect location and extent of damage, calculate the remaining life, and predict a potent accident. Different types of sensors such as ultrasonic, piezoelectric, etc. can be used for SHM to generate signals that can travel through solid configurations. The electromagnetic acoustic transducer can generate horizontal mode guided wave, but can only be used in case of magnetic materials. Laser-based ultrasonic instruments are one other method but are expensive and have limited application in engineering (Dai & He, 2014). Guided waves such as Lamb wave can be transmitted and received in solids using piezoelectric transducers (PZT) and these waves can be used for SHM (Boukabache et al., 2012; Park et al., 2010; Giurgiutiu, 2010; Almeida et al., 2014). These Lamb waves are not only reliable due to their sensitivity to changes in structures, but can be much more cost effective as well (Yu et al., 2004; Ciampa et al., 2010). If there is a damage in a structure, the guided waves will be reflected or scattered by the damage. To determine the damage, the difference signal is acquired and compared with a damage free signal. For structural damage localization, irrespective of the geometrical or imaging method used, the key to this process is the acquired time of flight and amplitude of the response to the signal. These factors directly determine the precision of the localization of the damage. The time of flight of these guided waves is linear and is directly dependent on the properties of the material, such as its modulus of elasticity and modulus of rigidity.