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The damage tolerance approach enables the design of lightweight composite structures, provided that damage progression remains controlled throughout their operational life. However, the fatigue-driven growth of delaminations resulting from low-velocity impact (LVI) remains poorly understood. The prevailing no-growth design philosophy assumes that delaminations in the high-cycle fatigue regime exhibit an extended plateau phase with negligible propagation. However, recent findings suggest that this plateau phase may stem from the limitations of conventional non-destructive evaluation (NDE) techniques, rather than an actual absence of damage evolution. In particular, C-SCAN ultrasonic inspections, commonly employed for damage assessment, may fail to detect internal delaminations encapsulated within outer ones due to the shadowing effect, potentially leading to an underestimation of fatigue-driven damage progression. This raises concerns about the reliability of traditional inspection methods for monitoring damage in critical composite structures.
As part of the TU-LEARN (Structural Life Extension Enhanced by Artificial Intelligence) project, funded by Unione Europea – Next Generation EU under the PRIN 2022 PNRR – D.D. n. 1409 del 14-09-2022 program, this study aims to enhance the understanding of LVI damage propagation mechanisms under fatigue loading. An experimental campaign was conducted on Carbon Fibre Reinforced Polymer (CFRP) coupons with a [(45,-45,90,0)]₂s stacking sequence, representative of aerospace-grade laminates. The methodology involved LVI tests following the ASTM D7136 standard, performed at an impact energy of 15 J using a 4.567 kg drop mass to induce a representative damage state. The post-impact behavior was assessed through compression after impact (CAI) fatigue tests, in accordance with the ASTM D7137 standard, under compressive-compressive cyclic loading with a stress ratio R = 0.1 and frequency f = 4 Hz.
To monitor damage progression at multiple fatigue stages, an ultrasonic guided wave (UGW)-based Structural Health Monitoring (SHM) system was integrated, complementing conventional C-SCAN inspections. This multi-sensor approach enabled a more detailed assessment of delamination growth, highlighting potential limitations in the conventional no-growth assumption. The UGW-SHM system demonstrated the capability to track internal damage progression, even in regions where C-SCAN proved insufficient.
The findings underscore the need for advanced NDE techniques to ensure a more reliable assessment of fatigue-driven damage evolution in composite structures. The integration of UGW-based SHM enhances the detection capabilities beyond traditional ultrasonic inspections, offering a more comprehensive evaluation of internal damage progression. These results contribute to refining damage tolerance strategies and improving predictive maintenance frameworks for aerospace and other safety-critical applications.
