Damage Monitoring of External Patch Repairs with Guided Ultrasonic Waves

A composite panel with a pre-introduced hole was repaired with an external repair patch and tested under tension-tension fatigue. X-ray radiography and microscopic analysis were employed to assess the developed damage which appeared first in the form of axial splitting at the hole and propagation until the final patch debonding. Piezoelectric transducers bonded on the top and bottom of the tested surface were utilised in-phase and out-of-phase in order to excite separately the symmetric and antisymmetric fundamental Lamb wave modes. The estimation of each mode's arrival time was performed through the Akaike information criterion which indicated that only the excitation of the antisymmetric mode was successful due to the possible corruption of the symmetric mode with the shear horizontal mode. The study investigated the problem of inadequate training data set that resulted in a negative Mahalanobis squared distance after outlier analysis application. The study suggested a pseudoinverse approach for the inverse of the covariance matrix and a reduction in the selected feature through the principal component analysis and the isolation of the most representative principal components. Finally outlier analysis was applied, which successfully represented the condition of the structural integrity.