SH-mode guided-wave impact damage detection in thick quasi-isotropic composites

Due to the extensive use of composite materials in aerospace structures, they are highly susceptible to various types of damage including barely visible impact damage (BVID) due to low-velocity impact events such as bird strikes and runway debris. Implementation of guided-wave based structural health monitoring (SHM) for BVID detection in composites has been extremely challenging due to the anisotropic nature of composites and the complex wave-damage interaction with impact damage. This paper presents a new methodology for the detection of BVID in composites using selectively pure shear horizontal (SH) guided wave mode generated by angle beam transducers (ABT) and received by phased array transducers (PAT). In this methodology, pure SH0 wave excitation was achieved using the angle beam transducers and variable angle wedges and after propagation in the composite it was received by the phased array transducer. First, the semi-analytical finite element (SAFE) method was used to generate the guided wave dispersion curves and the corresponding tuning angle for the SH0 mode excitation was obtained based on Snell's law. Then, pitch-catch experiments using the ABT-PAT transducer pair were conducted to validate the pure SH0 mode excitation. After this, impact damage experiments were conducted on multiple quasi-isotropic carbon fiber reinforced polymer (CFRP) composite plates having different thicknesses by conforming to the ASTM D7136 standard to generate controlled impact damage size of 1 ''. Next, impact damage detection experiments using pure SH0 mode at 500 kHz, were conducted using the ABT-PAT transducer pair. It was observed that pure SH0 mode was sensitive to the impact damage as a significant decrease in the signal amplitude and mode conversion was observed. With our experiments, we demonstrated the validity and usefulness of this technique for impact damage detection in composite structures. An invention disclosure describing the use of angle beam transducers for SH wave excitation method has been filed and is in the process of becoming a provisional patent.