Investigation of propagation mechanisms under compressive fatigue loads of LVI damage in CFRP coupons using ultrasonic guided waves

The damage tolerance approach enables the design of lightweight composite structures, provided that damage progression remains controlled throughout their operational life. However, delamination growth under fatigue loads, particularly for low-velocity impact (LVI)-induced damage, remains poorly understood. The no-growth design philosophy assumes that delaminations in the high-cycle fatigue regime exhibit an extended plateau phase, during which no significant propagation is observed. Nevertheless, recent studies suggest that this plateau phase may result from the limitations of conventional inspection techniques (i.e. C-SCAN), rather than from an actual absence of damage evolution. In particular, C-SCAN-based ultrasonic inspections, typically used for the scope, may fail to detect internal delaminations encapsulated within outer ones due to the shadowing effect, leading to an underestimation of fatigue-driven damage progression. This raises concerns regarding the reliability of traditional non-destructive evaluation (NDE) 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 stacking sequence of [(45,-45,90,0)]2s, representative of aerospace-grade laminates. The test protocol included LVI tests in accordance with ASTM D7136 standard, followed by compression after impact (CAI) fatigue tests as per ASTM D7137 standard, to assess the residual strength and progressive damage evolution under cyclic loading. Impact tests were performed using an energy level of 15 J, with a drop mass of 4.567 kg, ensuring a representative damage state. Subsequently, fatigue tests were conducted under compressive-compressive loading with a stress ratio of R = 0.1 and a fatigue load frequency of f = 4 Hz. To monitor damage evolution at multiple fatigue stages, an ultrasonic guided wave (UGW)-based Structural Health Monitoring (SHM) system was integrated into the test campaign, complementing C-SCAN ultrasonic inspections. UGW signals were acquired in a pitch–catch configurationusing surface-bonded piezoelectric (PZT) transducers: at each inspection stop, a repeatable tone-burst excitation was applied to the actuator, and the received waveforms were recorded on selected sensing paths (with signal averaging to improve the signal-to-noise ratio). Damage Indexes (DIs) were then computed by comparing each acquired signal to an undamaged baseline to quantify damage-induced changes. This approach provided a more detailed assessment of delamination growth throughout the fatigue life, revealing potential limitations in the conventional no-growth assumption, Figure 1. The use of UGW-SHM appeared capable of tracking the internal damage progression, even in regions where C-SCAN technique was insufficient.