“…For damage detection in an SHM system, a PZT (lead zirconate titanate) transducer is usually utilized as an actuator to generate ultrasonic waves along a structure, and other PZT transducers are used as sensors to detect the changes in both environmental and operational conditions [ 8 , 9 , 10 , 11 , 12 ] and structural damages [ 13 , 14 , 15 , 16 , 17 , 18 ]. The damage-induced changes in properties such as electromechanical impedance [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ], ultrasonic energy [ 29 , 30 , 31 , 32 , 33 , 34 ], nonlinear characteristics of Lamb waves [ 35 , 36 , 37 , 38 ], and other ultrasonic parameters [ 39 , 40 ], are further employed to estimate the health state of the structure [ 41 , 42 , 43 , 44 , 45 , 46 ] or locate the damage [ 47 , 48 , 49 , 50 , 51 ]. However, previous studies of damage detection mainly focus on how to make use of these structural changes to estimate or locate the damage, but do not consider the ultrasonic changes which are brought about by structural damage.…”