This research investigates the interactions of residual strains in Fiber Metal Laminates (FML), consisting of metal and fiber-reinforced plastic (FRP) components, using an integrated Fiber Bragg Grating (FBG) sensor. The study utilized non-destructive methods, primarily FBG sensors and X-Ray Diffraction (XRD), under various testing conditions. FML specimens, comprised of 0.3 mm thick C75 steel sheets and 24 FRP layers, underwent mechanical and thermal testing to analyze intrinsic thermal residual stresses. A unique device applied consistent preload forces to the sensor fibers, ensuring precise strain measurements without undulation.

The adhesion between steel and FRP interfaces was assessed, revealing an average shear strength of approximately 17 MPa. This indicates a robust adhesion capable of sustaining significant residual stresses. The research primarily employed XRD measurements to determine the FML's stress-free state. These findings were pivotal in adjusting the FBG sensors to the correct stress-free state, ensuring the accuracy of residual stress measurements within the FRP. Calculation models, including the Classical Laminate Theory, were used to explore internal residual strains in HYB-Type FMLs. The "stress-free" temperature was estimated to be between 80°C and 90°C, corresponding with a transition in thermoplastic behavior from non-linear to linear. The maximal residual stress was theoretically calculated and corroborated by XRD, establishing a specific stress-free condition.

Discrepancies were noted in the variance between manually set and actual preload forces in FBG measurements. This highlighted complexities in the embedding process of the fiber into the polymer matrix and suggested the need for further investigation and optimization. Future research directions involve comprehensive computational simulations of the hybrid material, correlated with both XRD and FBG data. This integrated approach aims to enhance understanding, contribute to manufacturing optimization, and aid in predicting the material lifespan.