Structural health monitoring and damage prognosis in composite repaired structures through the excitation of guided ultrasonic waves

The interest in composite repair technologies has been recently increased following the wide applications of com- posite materials in aerospace industry. Bonded repair patch technology provides an alternative to mechanically fastened repairs with signi cantly higher performance. The work here demonstrates two case studies: a woven composite panel repaired with an external composite patch and a woven composite panel with a scarf repair that minimises discontinuities and provides smoother surface. In the rst case, piezoelectric transducers were utilized in order to excite low frequency Lamb waves while the panel was tested under static tensile loading. In the sec- ond case, piezoelectric (PZT) transducers were surface bonded and the propagation of the rst anti-symmetric Lamb waves mode was investigated while the panel was subjected to fatigue loading. Along with Lamb wave testing, additional techniques were used to monitor the extent of the developed damage such as 3-dimensional Digital Image Correlation, X-ray radiography and optical microscopic analysis. All signals in both cases were further processed with outlier analysis and linear and non linear principal component analysis in order to develop an appropriate damage prognosis strategy. The current work identi es the problems of the traditionally used novelty detection approaches and proposes solutions through the concept of principal curves and appropriate feature extraction techniques.