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.

在这项研究中，利用超声反射的频率依赖性提出了扁平和层滴复合材料的无基线缺陷特征。通过CIVA®模拟和实验研究，研究了超声与复合材料中分层和树脂层相互作用的频率依赖性。为了通过一次测试提取复合材料中超声反射频率相关的定量数据，采用同步小波变换将获取的宽带原始信号分解为窄带频率。定量分析得出结论，复合材料中缺陷的超声反射对频率的依赖性受波长-缺陷比，聚合物粘弹性，频率相关的层间反射和波纹引起的散射的影响。考虑到这些因素，我们的研究发现，体积缺陷反射具有独特的频率依赖性。这允许在不与基线参考进行比较的情况下实现超声B扫描中的体积缺陷识别和表征。所提出的方法为具有不同微观结构的多层复合材料的无损评估提供了一种有前途的解决方案。