Investigation of the strain evolution in CFRP coupons subjected to LVI impact and compressive fatigue loads by distributed optical fiber sensors

Composite structures made of CFRPs are increasingly adopted in aerospace and high-performance engineering applications due to their superior strength-to-weight ratio and excellent fatigue resistance. However, their susceptibility to barely visible impact damage (BVID), especially following Low Velocity Impact (LVI) events, remains a critical challenge in structural integrity management. These damages, often invisible on the surface, can lead to internal delaminations that compromise long-term performance under cyclic loading conditions. To address this challenge, Structural Health Monitoring (SHM) systems have been increasingly adopted in aerospace and engineering fields. One of the most promising methods for SHM involves the use of distributed fiber optic sensors (DFOS), in virtue of their low size, light weight, high sensitivity and the ability to detect small strain changes, which makes them particularly effective in monitoring the early stages of damage progression. 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 investigates the fatigue-driven progression of LVI-induced damage using a Brillouin Optical Frequency-Domain Analysis (BOFDA) system capable of distributed strain measurements with 8-mm spatial resolution. The experimental investigation was conducted on multiple CFRP specimens with nominal dimensions of 100 mm × 150 mm × 2.7 mm. All laminates featured a symmetrical stacking sequence of [(45/−45/90/0)]₂s. For DFOS measurements, a polyimide-coated optical fiber with an outer diameter of 145 μm was bonded to the surface of each specimen, as highlighted in orange in Fig. 1(a). Additionally, six PZT transducers were surface-mounted in a 2×3 array near the central area of each panel, to realize ultrasonic guided wave (UGW) investigation along multiple directions. Each specimen was impacted at its planar center with an energy level of 15 J using a StepLab DW 2000 drop tower. Then, a compressive-compressive fatigue loading was applied under cyclic conditions (stress ratio R = 0.1, fatigue load frequency f = 4 Hz), using a StepLab Dynamic UD040. In the pristine state, after the impact and at predetermined interruption points of the fatigue load, damage growth was tracked using BOFDA strain maps for 3 different compression loads (L = 5 kN, L = 10 kN, L = 15 kN), as well as UGW-based signal analysis across all actuator-receiver combinations. As an example, we show in Figure 1(b) the strain profiles acquired via our BOFDA sensor, under a constant compressive load of 5 kN and at various fatigue stages. The acquired profiles show a gradual increase in the compressive strain levels, compatible with the delamination growth and consequent stiffness degradation of the laminate. Some acceleration in the strain increase was observed when passing from 800k to 1000k fatigue cycles, i.e. in proximity of the panel failure occurred after 1010k cycles. Similar results, obtained on other panels and with different compressive loads, indicate that the high-resolution strain field measurements provided by BOFDA sensors are effective in indicating internal damage progression in CFRP panels subjected to cyclic loading.