In-situ quantification of manufacturing-induced strains in fiber metal laminates with strain gages

Abstract

The predominant use of FBG sensors to characterize the residual stress state in composite materials to date does not permit absolute strain measurements. The reason for this is the loss of the connection between the sensor and laminate during phase transitions of the resin. Thus, points of significant changes in the measurement signal (e.g. bonding temperature) need to be used for the residual stress evaluation. For fiber metal laminates (FML) however, strain gages applied to the metal layer allow absolute strain measurements since the metal behaves purely elastic over the entire manufacturing process. Hence, residual stresses in the metal layer of an FML are quantified directly. Despite the sensors being applied to the metal layer, it is shown that the cure state of the resin can still be analyzed by changes in the coefficient of thermal expansion. Thus, the effects of different modifications to the cure cycle are assessed in terms of residual stress reduction. It is shown that assuming the bonding temperature to be equal to the stress-free temperature results in a conservative estimation of the residual stress state. The strain gage signal is shown to be in good agreement with FBG sensor data during a combined experiment.