RESIDUAL STRAIN DEVELOPMENT IN CARBON FIBER REINFORCED METAL HYBRID LAMINATS: AN EXPERIMENTAL INVESTIGATION USING FBG STRAIN SENSORS

This study investigates the residual strain development in thermoplastic carbon fiber-reinforced polymer (CFRP) and metal hybrid laminates during manufacturing, long-term conditioning, and thermal cycling. These hybrid materials combine the lightweight and strength properties of CFRP with the established formability and robustness of metals, making them ideal for aerospace and automotive industries. However, residual stresses induced during the manufacturing process, combined with environmental exposure, pose challenges to the structural integrity and longevity of these materials. Addressing these challenges is critical for developing reliable high-performance hybrid systems.
The research employed fiber Bragg grating (FBG) strain sensors to monitor strain evolution during key stages: hot press molding, mid-term environmental conditioning, and thermal cycling tests. A Ten-layer CF/PA6 stack, combined with 2 mm thick stainless steel or aluminum sheets, were fabricated, with embedded FBG sensor and thermocouple. This arrangement allowed monitoring of strain and temperature changes during the processing and testing phases.
FBG sensors were calibrated for strain and temperature to ensure measurement accuracy, following methods outlined in Prussak et al. (2018). Calibration was performed with the sensors embedded in the host materials to account for shear deformation, coating effects, and bonding conditions. Strain sensitivity was found to be 0.8–1.0 pm/με, consistent with the literature and previous findings, including Bhaskar et al. (2021). While calibration coefficients remained valid up to 80°C, deviations were observed due to phase changes in the thermoplastic matrix during processing. These deviations highlight the necessity of comprehensive calibration under varying conditions to maintain data reliability.
After manufacturing, specimens underwent three months of conditioning at 22°C to simulate mid-term operational exposure. Subsequently, the CFRP/metal hybrids were subjected to thermal cycling tests reflecting automotive and aerospace application conditions. During these tests, FBG sensors provided strain data, which were validated against 3D laser scan curvature measurements of the
specimens. This dual validation ensured the accuracy of residual strain analysis and revealed the interplay between processing-induced stresses and subsequent thermal effects.
The results demonstrated that CFRP-metal hybrids exhibited excellent dimensional stability, with minimal strain release during thermal cycling. These findings are in agreement with evaluations from Prussak et al. (2018), which emphasized the stability of FBG-monitored hybrid systems under thermal loads. Additionally, the limited strain release suggests that the hybrids' structural integrity is maintained, making them suitable for demanding applications in aerospace and automotive industries.
This research underscores the value of integrating FBG sensors into hybrid systems for monitoring of strain and temperature during both manufacturing and operational phases. As noted by Bhaskar et al. (2021), advancements in FBG technology have enabled precise and efficient strain and temperature measurements, contributing significantly to structural health monitoring. This study leverages these advancements to optimize the manufacturing processes and ensure the reliability of hybrid materials.
Future work will focus on refining calibration methodologies under dynamic and elevated temperature conditions to reduce measurement uncertainties. Additionally, incorporating numerical modeling and advanced simulation techniques could provide deeper insights into the long-term performance of these hybrid systems. Such efforts will enhance the reliability of hybrid materials and their adoption in high-performance sectors.