Residual Stress Quantification Using Strain Gauges

Abstract Strain gauges attached to the metal sheets in an FML can measure the strains that develop during the curing process from the beginning on. This is an advantage compared to measurements in FRP, where the connection between the sensor and substrate is only established during the curing process. The measured strains across various cure cycles show that significant residual stresses evolve in the metal sheets of a CFRP-steel laminate with a metal volume fraction of 23.5%. With modified cure cycles, these residual stresses can be reduced by up to 50%. The strain signal over the curing process further indicates the point in time the CFRP resin possesses a significant shear modulus and can consequently transfer loads between the metal and fibers, which leads to an interaction between the two FML components. The strain gauge measurements are validated with integrated fiber Bragg grating sensors. Furthermore, temperature measurements and the development of a numerical heat transfer model prove the validity of the measurements at only discrete points in the laminate, and the transfer of these results into a residual stress state of the entire laminate. The results further show that assuming the bonding temperature equals the stress-free temperature is only valid for bonding temperatures close to the final curing temperature. When the connection is established at lower temperatures, significantly below the final curing temperature, errors are made when calculating the residual stress state using this assumption.