A B S T R A C TThe work outlined in this paper is a part of a wider effort aimed at the development of a built-in system for real-time damage detection in aircraft structures. The objective of this study was to show the feasibility of applying low-frequency phased-array transducers for damage detection in metallic structures with bonded repairs and, also, in structures made of composite materials. This paper is divided into two main portions: a component dedicated to the monitoring of crack growth under a simulated composite repair and a part describing an introductory study done for composite panels monitoring. The data acquisition system is also described. The results of the study described in this paper indicate that a low-frequency, built-in phased-array system can successfully estimate the length of a crack growing under a bonded repair. Satisfactory results of monitoring simulated defects in composite structures are presented, as well.Keywords non-destructive damage evaluation; phased array; smart materials. N O M E N C L A T U R EA = signal amplitude C = wave propagation velocity (m/s) F = frequency M = piezoelement number R = distance (m) T = waveform pulse duration (s) δ = time delay (s) φ = orientation ( • )