Literature offers a quantitative number of diagnostic methods that can continuously provide detailed information of the
material defects and damages in aerospace and civil engineering applications. Indeed, low velocity impact damages can
considerably degrade the integrity of structural components and, if not detected, they can result in catastrophic failure
conditions. This paper presents a nonlinear Structural Health Monitoring (SHM) method, based on ultrasonic guided
waves (GW), for the detection of the nonlinear signature in a damaged composite structure. The proposed technique,
based on a bispectral analysis of ultrasonic input waveforms, allows for the evaluation of the nonlinear response due to
the presence of cracks and delaminations. Indeed, such a methodology was used to characterize the nonlinear behaviour
of the structure, by exploiting the frequency mixing of the original waveform acquired from a sparse array of sensors.
The robustness of bispectral analysis was experimentally demonstrated on a damaged carbon fibre reinforce plastic
(CFRP) composite panel, and the nonlinear source was retrieved with a high level of accuracy. Unlike other linear and
nonlinear ultrasonic methods for damage detection, this methodology does not require any baseline with the undamaged
structure for the evaluation of the nonlinear source, nor a priori knowledge of the mechanical properties of the specimen.
Moreover, bispectral analysis can be considered as a nonlinear elastic wave spectroscopy (NEWS) technique for
materials showing either classical or non-classical nonlinear behaviour.