Modelling and Monitoring the Response of Bonded Composite/Metallic Structures During Manufacturing Process and Damage Evolution

The paper proposes experimental and numerical approaches to investigate and characterize delamination phenomena at the interfaces between carbon fibre reinforced composite laminates and layers of titanium alloy, obtained through co-cured and co-bonding process. The build-up of thermal residual stress between the different materials is controlled by properly designing specimens suited to be adopted in fracture mechanics experiments, such as Double Cantilever Beam (DCB) or End Notched Fracture tests (ENF). Manufacturing process is monitored through the insertion of strain sensors carried by optical fibres, which are subsequently used to monitor the strain evolution during DCB tests, performed with two different types of specimens. Numerical models are developed for the estimation of thermal stress during the cooling phase of manufacturing process and of the mechanical response in fracture tests. The results confirm that the proposed specimens can control the development of distortions due to the mismatch of coefficients of thermal expansion and point out capabilities and limits of simplified linear numerical approaches to capture the development of strains and stress during the cooling phase of manufacturing process. The optical fibre based monitoring system successfully monitored both the manufacturing process and the evolution of internal strains during the development of interface damage. A complete numerical approach is developed, by using multistep explicit analyses to carry out simulation of fracture propagation after an evaluation of thermal stress in the structure. The approach is validated in terms of overall force vs. displacement responses and local strain evolutions at the locations monitored by internal strain sensors.