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Thermal residual stresses in bonded composite repairs on cracked metal structures Albat, Andreas Michael


The objective of this research is to determine the thermal residual stresses and strains in bonded composite repairs on cracked metal structures. This work is an essential contribution to a fatigue damage initiation model for bonded composite repair, where knowledge of the initial stress/strain state after an elevated temperature cure is important. Furthermore, this work is an elementary part for the development of a generic certification approach to bonded composite repairs. Accounting properly for thermal residual stresses in test specimens and in real applications will assist in determining the true feasibility of a bonded composite repair. The objective of this work was realized in four stages of research. In the first stage, seven AMRL sandwich type composite bonded repair specimens were manufactured, of which one was instrumented by placing 44 strain gauges at eight planar locations and within five different interfaces. Residual strains at ambient temperature (including both thermal residual strains and other process induced strains) were measured during the manufacturing process. In the second stage, the stress free temperature for the repaired specimen was experimentally determined and the thermal residual strains measured as a function of operating temperature. In the third stage, a theoretical analysis was carried out to estimate the thermal residual stress and strain distributions in various bonded repairs. This analysis also addressed the effect of symmetrical disbonds around the crack. Finally, a finite element analysis was carried out to assess the limitations of the theoretical analysis as well as to provide a more detailed insight into the complex thermal residual stress and strain state of the AMRL sandwich type specimen. During this work it was found that high thermal residual strains (reaching 15% of the yield strain) are present in the bonded repair specimen at ambient temperature. Previous analysis schemes predicted results nearly 60% higher. The thermal residual strain versus temperature measurement showed that only very small changes in thermal residual strains occurred above 90°C leading to a defined effective stress free temperature of 85.8°C for the employed adhesive FM 73M. By utilizing an effective stress free temperature, a linear-elastic approach was used to model thermal residual stresses and strains in composite bonded repairs. Major achievements in the theoretical analysis include a linear-elastic closed form solution for tapered joints and reinforcements without the need for a numerical solution scheme, a stress field prediction ahead of the crack tip for the metal substrate of a bonded repair based on a concise complete solution of the classical fracture mechanics problem of a center crack in an infinite plate and, an extended Rose model for the prediction of the stress intensity factor of a bonded repair with symmetrical disbonds showing the severity of thermal residual stresses especially for partially disbonded composite repairs to cracked metal specimens. The key to precise predictions of thermal residual stresses in bonded composite repairs is the knowledge of the adhesive behaviour at elevated temperatures under thermal residual stress loading. A generic type specimen is presented which allows to investigate the relevant adhesive behaviour.

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