Issue 77

A. Casaroli et alii, Fracture and Structural Integrity, 77 (2026) 89-106; DOI: 10.3221/IGF-ESIS.77.07

Figure 9: Fracture surface of 2.0-45-1 tensile specimen (2.0 mm thick, 45° angle, first specimen) at increasing magnifications.

Micro-Tomographic investigation: internal defect characterization To overcome the inherent limitations of two-dimensional surface fractography and obtain a volumetric quantification of the material's internal defects, high-resolution micro-tomographic analysis was performed. The analytical procedure utilized the North Star ImagiX tomograph. Thanks to the differential attenuation of X-rays across phases of different densities, this device facilitates complete, non-destructive three-dimensional reconstruction of both the internal and external surfaces of the analysed sample. Microtomography is widely recognized as the preeminent methodology for characterizing the presence of internal voids in composite laminates. Unlike gravimetric or acid digestion techniques, micro-tomography allows for direct visual analysis of the entire sample volume, allowing researchers not only to verify the absolute void percentage with high precision but also to actively evaluate the complex three-dimensional morphology of individual voids. Furthermore, compared to other conventional methods used to calculate void fraction, X-ray microtomography provides results with a remarkably low standard deviation, ensuring a high degree of statistical reliability. The explicit objective of this tomographic investigation was to map the internal structure of the laminate and confirm the hypotheses regarding the specific causes of the significantly reduced failure load, initially formulated during SEM examination of the fracture surfaces. For both thin and thick specimens, the tomographic scan volume was strictly limited to the central measurement region of the tensile bars, encompassing a targeted extension area of 10 x 10 mm (Fig. 10). Detailed analysis of the reconstructed three-dimensional model was achieved by systematically extracting and evaluating a series of internal cross-sections, oriented along specific planes parallel and perpendicular to the longitudinal direction of the fibre laminate. Sequential examination of these internal tomographic sections provided important information on critical process failures that impacted material performance (Figs. 11 and 12). Analysis of virtual planes sectioned parallel to the lamination direction revealed a clear spatial gradient in defect density; it was evident that the total number of dispersed porosities increased significantly in deeper internal sections of the laminate. This empirical observation demonstrates the difficulty associated with the penetration of viscous resin deep into the dense, non-homogeneous core of the non-woven fabric fibre mat.

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