Lamb waves are the most commonly used plate waves for health monitoring of plate-like structures. The propagation of a Lamb wave depends a great deal on the selected frequency and the material thickness. The Lamb-wave-based approach has been successfully applied to the health monitoring of composite structures, metallic structures and steel – reinforced concrete structures.
• Health monitoring of composite structures. Delamination is a major concern for in-service composites. Su et al.  applied four distributed piezoelectric transducers to generate and monitor the ultrasonic Lamb wave with a narrowband frequency in quasi-isotropic carbon fiber/epoxy (CF/EP) laminates. Toyama et al.  investigate the effects of transverse cracking and delamination on the S0 mode velocity in carbon fiber reinforced plastic (CFRP) cross-ply laminates. Experimental results showed that both the stiffness and the velocity decreased as the transverse crack density increased. Metallic structures are common in many areas of engineering, particularly in aerospace, ground and sea transportation.
• Health monitoring of metallic structures. Due to aging, fatigue and erosion of metals, there can be cracks, holes or other types of damage inside or on the surface of a metallic structure. Staszewski et al. [51,52] proposed a laser-based Lamb wave sensing approach for a rectangular metallic plate by using two piezoceramic disks as actuator and sensor. The study showed the potential of the method for simple and rapid detection of the location of damage in a structure. Tua et al.  proposed an approach to locate and determine the extent of linear cracks in homogeneous aluminum plates based on the flight time of Lamb wave by using piezoelectric actuators and sensors. Hiber-Huang transform is used to process the sensor signal to determine accurate flight time and also estimate the orientation of the crack. In situ health monitoring is important in the maintenance of metallic structures. Rajic et al.  proposed an in situ health monitoring system based on a piezoceramic wafer element to the detection of specimen fatigue under cyclic loading condition. Lamb waves propagating through the beam test specimen are sensed using small surface-mounted piezoelectric transducers, then analyzed for indications that relate to the onset of fatigue. The study and the experimental results show the great potential for developing similar automated in situ structural health monitoring systems for application to operational structures such as aircraft.
• Health monitoring of concrete structures. Steel-reinforced concrete (RC) is widely used in civil infrastructure. Steel reinforced concrete (RC) structures usually serve under harsh environments. Wang et al.  studied the Lamb-wave-based health monitoring of both fiber-reinforced composites and steel – reinforced concrete. The piezoelectric sensor network is installed in selected rebars in areas such as the deck, the columns of bridges, and the footing area of columns for the purpose of health monitoring. Experimental results showed that cracks or debonding damage in RC structures can be detected by the proposed built-in active sensing system.
 SAC is a joint venture of the Structural Engineers Association of California (SEAOC), the Applied Technology Council (ATC), and California Universities for Research in Earthquake Engineering (CUREe), formed specifically to address both immediate and long-term needs related to solving the problems of the Welded Steel Moment Frame (WMSF) connection. (October 1997).
 Astaneh-Asl, A. 2002. Seismic Behavior and Design of Composite Steel Plate Shear Walls. Steel TIPS Report, Structural Steel Educational Council, Moraga, CA.