# Multimode Guided Wave Detection for Various Composite Damage Types  
[PERSON]  
1 Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA; [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.F.H. ); [EMAIL_ADDRESS] (V.G. )1  
[PERSON]  
1 Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA; [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.F.H. ); [EMAIL_ADDRESS] (V.G. )1  
[PERSON]  
1 Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA; [EMAIL_ADDRESS] (R.J.); [EMAIL_ADDRESS] (M.F.H. ); [EMAIL_ADDRESS] (V.G. )1  
[PERSON]  
1 Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA; [EMAIL_ADDRESS] (R.J.); haiderm@email. However, these conventional NDT methods are often expensive, time-consuming, and labor-intensive, and they depend heavily on the skill and experience of the operator. Structural health monitoring (SHM) technologies offer a promising alternative for damage detection in composite structures. In recent decades, many SHM techniques have been developed for damage detection, such as acoustic emissions (AEs) [9], electromechanical impedance (EMI) [10], and guided wave-based methods [11; 12; 13; 14; 15; 16; 17; 18]. Among them, guided wave-based SHM technologies have been used to detect various damage types in composite structures, including delamination [12; 13], debonding [14], and impact damage [15; 16; 17; 18]. Guided waves have the advantage of long-distance propagation in complex structures and low energy loss, and they have been widely used in the structural health monitoring of composite structures. However, the problem for a typical guided wave is complicated by the existence of at least two wave modes at any given frequency and by the inherent dispersive nature of the guided wave modes that exist in thin-wall structures, which make the results difficult to interpret. To solve the problem, researchers have been using the tuning effect to find the gist of a suitable frequency that can excite a single-mode guided wave [19]. A technique involving double piezoelectric transducers with symmetric bonding has also been tried for the selective excitation of a single-mode wave [20]. In recent years, an angle beam transducer (ABT) with a wedge has also been utilized to achieve pure-mode guided wave excitation based on Snell's law [21; 22; 23]. [PERSON] et al. [21] used an ABT pair, one transmitter and one receiver, to detect the disbond in composite bonded joints using fundamental symmetric mode (S0) at 300 kHz. The ABT pair has also been utilized to detect the delamination simulated by a Teflon insert in a crossply composite plate, with the pure S0 mode being used [22]. [PERSON] and [PERSON] [23] used an ABT and an acoustic emission (AE) transducer to detect impact damage in a crossply composite plate using S0 mode. These studies have facilitated our understanding of guided wave-based SHM applications using ABTs. Compared to the widely used S0 mode, the fundamental shear horizontal (SH0) wave is relatively simple, but has been investigated less often, probably due to the traditional notion that SH waves are usually excited by electromagnetic acoustic transducers (EMATs) [24]. However, EMAT methods are not appropriate for composites. It is well known that SH0 mode is more sensitive to delamination in composites [25]. The significance of SH waves has increased in recent years due to the development of new techniques to overcome difficulties associated with SH wave generation. SH wave piezoelectric transducers based on d\({}_{24}\), d\({}_{36}\), d\({}_{35}\), and d\({}_{15}\) piezoelectric wafers or strips have been developed for the generation of SH waves [26; 27]. However, these SH wave transducers have to be permanently bonded onto a structure to be effective. The pure-mode SH0 wave excitation and its NDT applications for composites are very limited.