Relevance of Manufacturing-Induced Residual Stress States in Fiber Metal Laminates for Structural Health Monitoring

Structural health monitoring (SHM) is a promising technology for monitoring the structural integrity of fiber metal laminates (FMLs) during operation. SHM systems using guided ultrasonic waves (GUWs) enable the detection of damage located inside a laminate and are, therefore, particularly suitable for FMLs due to their complex damage characteristics. This dissertation explores how manufacturing-induced residual stress states in FMLs affect such an SHM system. Experimental methods for quantifying residual stresses in FMLs are developed and evaluated. This work shows that process strains can be measured throughout manufacturing with sensors integrated into the laminate. The strain measurements verify different modified cure cycles to change residual stress states in FMLs consisting of carbon-fiber-reinforced polymer (CFRP) and steel. To avoid complex sensor technology in industrial manufacturing processes, a robust method is additionally presented that derives a process-specific parameter, the stress-free temperature, from asymmetrical reference laminates. The stress-free temperature can quantify residual stress states of other laminates with a symmetrical layer structure produced in the same manufacturing process. The quantified residual stress states in various CFRP-steel laminates allow the experimental investigation of the influence of stress on the propagation velocity of fundamental GUWs. With laser-scanning-vibrometry, dispersion diagrams for various FMLs over a wide frequency range are experimentally determined. The results show no significant influence of the residual stress states on the phase velocity of the fundamental symmetric and asymmetric GUW modes for the investigated FMLs. In contrast, comparative measurements under external tensile prestress show a considerable increase in phase velocity with increasing prestress for both fundamental GUW modes over a wide frequency range. Finally, fatigue tests are performed on CFRP-steel laminates to investigate the influence of different residual stress states on the residual life prediction capabilities of an SHM system. The results show that reducing residual stresses through modified cure cycles delays crack initiation, reduces crack propagation velocity, and, consequently, extends the fatigue life of the investigated FMLs. Therefore, the results indicate that the fatigue strength of CFRP-steel laminates under cyclic loading can be improved using modified cure cycles. The findings of this dissertation lead to two main consequences in the context of SHM systems for FMLs. First, since wave propagation velocity is important for damage localization using GUWs, the influence of external loads must be considered in contrast to the influence of manufacturing-induced residual stress states. Second, for remaining service life predictions after damage detection, the knowledge of the residual stress state in FMLs helps improve the prediction quality. The results contribute to advancing the technological maturity of SHM systems for FMLs by demonstrating fundamental relationships between residual stress states and relevant variables of an SHM system and by quantifying the effects.