Smart cure cycles for fiber metal laminates using embedded fiber Bragg grating sensors

Abstract

In this paper, a smart cure cycle with an additional cooling step for co-cure bonded fiber metal laminates (FML) is developed to reduce process related thermal residual stresses. An online measuring technique with fiber Bragg grating (FBG) sensors is used to gain a clear understanding of the formation process of residual stresses during cure, as well as to determine the resulting residual stress level. Fiber optic sensors are integrated in carbon fiber reinforced plastic (CFRP) – steel laminates. The recorded processing strain curves enable the characterization of the mechanical interaction of the metal and CFRP-layers. By introducing an additional cooling-step, the moment of the connection between the individual components is being influenced. The conducted experiments show a significant reduction of the resulting stress-free temperature.A variety of temperature cycle modifications, like different cooling temperatures, as well as modified cooling and heating rates, are investigated. The impact of these parameters on the process-induced strains and stresses are detected by the fiber optic strain sensors. The thermal residual stresses are reduced by up to 23% for the specimens in focus, when the developed cure cycles with FBG sensors for a monitored cooling and reheating are used.