Optical and acoustic damage detection in laminated CFRP composite materials

Composite materials would gain substantial added value if it were possible to equip them with a system that could continuously monitor their damage state. In this approach, fibre-optic sensors could offer an alternative to the robust piezoelectric transducers used for acoustic emission (AE) monitoring. An intensity-modulated sensor based on the microbending concept was built and used to detect damage in laminated CFRP composite materials. Pencil lead break tests and tensile tests have been performed. The short-time Fourier Transform (STFT) has been computed and noise-reduction algorithms (adaptive filtering and spectral subtraction filtering) have also been used. The specimen final fracture can clearly be seen but transient signals were also detected before this final event. They can be correlated with the acoustic emission signals, analysed by a classical AE parameter study and with a modal acoustic emission (MAE) system. It is thus shown that the optical signal contains information on the elastic energy released whenever damage is being introduced in the host composite. Hints are that time-frequency analysis could be used to characterise this damage in a way similar to what is done for MAE.