Recent advances in the ultrasonic polar scan method for characterizing (degraded) fiber reinforced plastics

The ultrasonic polar scan (UPS) technique originated in the 1980's as a sophisticated method for inspecting composites. However, it is only in recent times that the true capabilities and strengths of the UPS methodology have been evidenced through experiment and simulation. Nowadays, the UPS method exists in different versions which led to several novel applications in the field of material inspection and characterization. This contribution gives an overview of our recent advances

Multimodal Imaging for Characterisation and Testing of Composite Materials

Carbon fibre reinforced polymers (CFRP) are widely used across several industries, including aerospace, as they are lightweight and offer superior mechanical properties. Barely Visible Impact Damage (BVID), including cracks, delaminations, fibre debonding, as well as manufacturing defects such as porosity, are detrimental to CFRP structural integrity and detection of such faults is important. Different non-destructive evaluation (NDE) methods exist, including ultrasound, X-ray computed tomography (X-ray CT), infrared, and liquid penetrant testing. Edge Illumination X-ray Phase Contrast imaging (EI XPCi) was benchmarked as a viable NDE method for damage detection in CFRP, as it offers additional information through multimodal imaging. With the acquisition of at least three images, EI XPCi allows for the retrieval of the attenuation, differential phase, and dark field signals, using a pair of apertured masks. EI XPCi CT was compared with ultrasonic immersion C-scan imaging and high-resolution X-ray CT for the detection of severe impact damage in a composite plate (visible indent damage on surface of plate and protrusion on the back). The full extent and scale of the different defects were observed in the phase-based signals to a better standard than ultrasonic immersion imaging, with observations confirmed using high resolution X-ray CT. Planar EI XPCi was then compared to contrast agent X-ray imaging and ultrasonic immersion C-scan imaging on a different, less damaged specimen (only small crack visible on surface), showing that planar EI XPCi can detect a network of cracks across the specimen and overcame some of the limitations of contrast agent X-ray imaging. However, in the planar imaging, delamination damage was only detected by the ultrasonic measurement, showing the necessity of using both ultrasonic imaging and EI XPCi for a complete understanding of the damage in the plate. EI XPCi was used for the quantification of porosity for woven composite plates with varying porosity (0.7% to 10.7%), compared to ultrasonic through transmission imaging and destructive matrix digestion. The introduction of the standard deviation of the differential phase (STDP) showed excellent correlation with the porosity calculated from matrix digestion. The STDP signal quantifies the variation of the distribution of inhomogeneities for features of a scale equal to or above the system resolution (in this case, 12µm along the direction of phase sensitivity), which was advantageous for the investigated set of specimens with larger porosity

Recent Development of Reused Carbon Fiber Reinforced Composite Oriented Strand Boards

There is a growing interest for the reused composite oriented strand board (COSB) for stiffness-critical and contoured applications. COSBs are made of rectangular shape prepreg strands that are randomly oriented within the structure. Development of this product form could markedly reduce the scrap generated during aerospace manufacturing processes. COSBs retain high modulus and drapability during processing and manufacturing. However, before any material can be deployed in industrial applications, its various properties must be well understood so that proper design analysis can be performed. Nondestructive testing (NDT) is widely used in research and industry to evaluate the quality of a variety of materials including composite materials and structures. NDT, as the name indicates, has the benefit that it does not alter or destroy the sample like other techniques, such as cross-sectional imaging. In this chapter, two nondestructive techniques, ultrasound and micro-computed tomography (micro-CT), were used to characterize carbon fiber epoxy composites, particularly comparing conventional laminates and reused COSB. The void content and morphology of samples cured using a range of materials and process parameters were determined using NDT and conventional microscopic analysis of cross sections. The mass distribution of fiber and resin within each sample was also determined. The manufacturing and NDT of COSB were introduced, and provided most detailed information on composite microstructure, including void size, void morphology, void distribution, and overall void content. Conventional micro-CT was determined to be ill-suited to scan large samples because of long scan times and large file sizes. To enhance the capabilities of micro-CT for evaluation of composite materials and structures, a micro-CT postprocessing method using stitching computer programming algorithms was developed. The method presented markedly increases the resolution that micro-CT can achieve, as well as the maximum feasible sample size, thus overcoming some of the primary drawbacks to conventional micro-CT. The primary objective of this work was to evaluate the feasibility of NDT methods in the assessment of both conventional composite laminates and the reused COSB fabricated from prepreg scrap. To this end, the advantages and limitations of ultrasound and micro-CT were discussed. The results showed that with stitching up postprocessing, micro-CT can be used to detect global void morphology structure wide, making the technique competitive with ultrasound, yet with greater resolution and equivalent scan size