Photoacoustic inspection of CFRP using an optical microphone

Air-coupled ultrasound (ACU) is already an established method for the non-destructive failure inspection of carbon fiber reinforced polymers (CFRP). In the through-transmission setup, plate-like structures are placed between the ultrasound (US) source and the receiver. The ultrasonic wave propagating through the material is observed; deteriorations inside the material such as defects alter the captured signal. Such defects can be delaminations, cracks, thickness changes or porosity. In the measurement setup chosen, conventional piezoelectric transducers and receivers are replaced by laser-based components. On the excitation side a nanosecond laser pulse, illuminating the plate surface, was used to induce ultrasonic waves (thermal regime) directly into the specimen. On the receiver side a laser-based optical microphone was tested. This membrane-free microphone detects the refractive index changes of the air, when the ultrasound propagates through the miniature Fabry-Pérot etalon. Using this new measurement setup, C-scans of CFRP plates were performed containing impact damage, delaminations and blind holes. In comparison to conventional aircoupled testing methods, our method is sensitive over a broader frequency range, has better signal-to-noise ratio (SNR) and a smaller acoustic aperture. This allows obtaining a more detailed image of a specimen including defects.