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

Recent advances in the field of nanomaterial have led to the development of ultralight materials of high strength applicable in various fields that demand high strength-to-weight ratio, excellent fatigue resistance and flexible designability, such as: aerospace industry. One such material is CPRF (Carbon fiber reinforced plastics). CPRF is currently being used in primary structures of airplanes, ships, and other vehicles and for applications that demand sustained high reliability during long-term operations. This material allows manufactures to redesign established metallic structures like manipulators in order to decrease their weight and to increase their level of performance. Unfortunately, fiber misalignment, resin-rich layers and porosity may occur during CFRP fabrication. Consequently, in such a scenario, failures such as: delamination, fiber breakage and cracks upon external loading become inevitable. In essence, these defects reduce the strength of the composite material and pose a potential risk for structural damage. As a resolve, on-destructive evaluation (NDE) technologies, such as ultrasonic testing/guided wave testing, X-ray computed tomography (CT) and eddy current, are often employed for quality control. Moreover, recent research has shown ultrasonic testing and guided wave testing to be the most formidable techniques for in situ composites inspection.

Technically, ultrasonic defect evaluation is usually conducted by calculating and comparing the signal-to-noise ratio (SNR) between cross-sections with and without defect. However, this cannot be done for composites inspection since cross sections without defects are typically structurally different from those with defects present. As such, uncertainties of composites’ microstructure introduce difficulties in generating such baseline reference for defect evaluation, especially for complex-shape composites with varied cross-sections. Therefore, there is need to resolve this shortfall. In this regard, Dr. Zhen Zhang and Melody Png, in collaboration with Dr. Menglong Liu  and Dr. Qian Li at Tongji University developed a new novel technology for baseline-free defect characterization of complex-microstructure composites. The proposed technology was based on the unique frequency dependence of defects. Their work is currently published in the research journal, Composites Part A.

In their approach, the new baseline-free volumetric defect evaluation of composite materials was proposed using frequency-dependent ultrasound reflections. Specifically, CIVA® simulation and experimental investigations were conducted to study the influence of inter-ply reflections and polymer viscoelasticity on the frequency dependence of defect reflections in flat composites. Further, to extract quantitative data of frequency dependence of ultrasonic reflections in composites with a single test, synchro squeezed wavelet transform was adopted to decompose the acquired wide-band raw signals into narrow-band frequencies.

Based on the quantitative analysis conducted, the authors reported that the dependence of ultrasound reflections from defects on frequency in composites were affected by wavelength-to-defect ratio, polymer viscoelasticity, frequency-dependent inter-ply reflections and waviness-induced scattering. More so, by considering these factors, the research team found out that volumetric defect reflections had a unique frequency dependence. This was seen to allow volumetric defect identification and characterization in ultrasonic B-scans to be achieved without comparison to a baseline reference.

In summary, the study presented an in-depth investigation of wave propagation in flat and ply-drop composites, by use of numerical simulation and experimental validation. Through the application of this technique, the influence of viscoelasticity attenuation, inter-ply reflections, wavelength-to-defect ratio and fiber waviness on the defect reflections were quantitatively analyzed. Remarkably, the proposed method presented a promising solution for non-destructive evaluation of multi-layer composites with diverse microstructures. In a statement to Advances in Engineering, Dr. Zhang and Dr. Li mentioned that the proposed baseline-free UT defect characterization method has promising application for UT inspection of complex aerospace composites with varied cross-sections.