Sparse guided wave imaging in highly anisotropic plates with phase skewing and amplitude focusing compensation

This paper introduces a sparse beamforming algorithm designed to compensate for guided wave dispersion while addressing phase skewing and energy focusing in highly anisotropic composite plates. Developed for structural health monitoring, this algorithm targets impact-induced defects, such as barely visible impact damages (BVIDs), using a limited number of arbitrarily positioned transducers. Validation is first performed on finite element simulated data with the A0 mode in an eight-ply unidirectional composite with significant anisotropy and a delamination defect, highlighting the critical role of phase skewing compensation. Additionally, energy focusing compensation strategies are introduced, improving image contrast in highly anisotropic materials. Experimental validation is carried out on a real BVID caused by a 6.3 J impact on an aerospace-grade composite plate and compared to a simulation with a similar size delamination. Results show good agreement between experiments and simulations, with significant enhancements in imaging resolution when considering dispersion compensation compared to the commonly used delay-and-sum algorithm. The proposed algorithm improves image dynamics and reduces focal spot area by a factor of more than 100. Finally, the study quantifies errors introduced by neglecting phase skewing in dispersion compensation, demonstrating coherence with observed imaging results for both composite materials studied in this paper.