Baseline-free defect evaluation of complex-microstructure composites using frequency-dependent ultrasound reflections

In this study, baseline-free defect characterization for both flat and ply-drop composites were proposed using frequency dependence of ultrasound reflections. This frequency dependence of ultrasound interaction with delamination and resin layers in composites were investigated through CIVA® simulation and experimental investigation. To extract quantitative data of frequency dependence of ultrasonic reflections in composites with a single test, synchrosqueezed wavelet transform was adopted to decompose the acquired wide-band raw signals into narrow-band frequencies. Quantitative analysis concluded that dependence of ultrasound reflections from defects on frequency in composites are affected by wavelength-to-defect ratio, polymer viscoelasticity, frequency-dependent inter-ply reflections and waviness-induced scattering. Considering these factors, our study found that volumetric defect reflections have a unique frequency dependence. This allows volumetric defect identification and characterization in ultrasonic B-scans to be achieved without comparison to a baseline reference. The proposed method shows a promising solution for non-destructive evaluation of multi-layer composites with diverse microstructures.