| Adhesively bonded composite repair, a novel, efficient and cost-effective method to extend the service life of the damaged aircraft component, has been widely used in the military and commercial aircrafts. Reinforcing the cracked component by single-sided composite patching technology is gaining more and more concern when it is difficult or not possible to access both sides of a component, such as the aircraft fuselage or wing skin. Composite repair method has been shown to be very promising owing to the light weight, high strength and stiffness of the composites. While significant thermal residual stresses, resulting from the mismatch in the thermal expansion coefficients between the composite patch and the metallic substrate, arise in the single-sided composite patched components after cured at an elevated temperature. The single-sided composite patched specimen (SSCPS) may experience a considerable out-of-plane bending induced from thermal residual stresses or external loadings due to the load-path eccentricity. Therefore, thermal residual stress and bending are key features for the design of single-sided composite patching. Interactions between thermal residual stress and bending present a great challenge in the study of the mechanical behaviors of the SSCPS.In this paper, studies on the properties of the cracked thin aluminum alloy plate with a single-sided repair by high-performance carbon/epoxy composite patch is carried out experimentally and theoretically. Measurement of the thermal residual strains in the SSCPS is focused on, and the thermal residual stresses/strains in the SSCPS are predicted by analytical and numerical methods, respectively. Effects of the interactions between thermal residual stress and bending on the quasi-static tensile properties and fatigue behaviors of the SSCPS are also discussed.(1) Experimental and theoretical studies on the thermal residual stress/strain in the SSCPSEffects of cure cycle and loading history on the stress free temperature (SFT) of the SSCPS is discussed, and a prediction model for the SFT of the SSCPS after multi-step cure cycle, based on the adhesive cure kinetics and Maxwell viscoelastic model, is developed. The results show that the difference between the stress free temperature and the cure temperature rises as the cure temperature increases, and loading history slightly affects the SFT. The contributions of the loading history to the decrease of the SFT are mainly within the early stage of the fatigue loading. When predicting the SFT of the SSCPS prepared under the two-step cure cycle, the SFT prediction model can control the error within 7.4%, compared with the experimental data.Effects of some important parameters, including cure cycle, patch design, crack length and interface disbond, on the specimen deflcction and the thermal residual stress/strain are comprehensively investigated, experimentally and numerically. Experimental data and three-dimensional finite element calculation results indicate that significant thermal residual stresses/strains are introduced into the SSCPS by the temperature difference between the SFT and ambient temperature. Two-step cure cycle and patch design are efficient ways to decrease the thermal residual stress/strain. Thermal residual stress/strain concentration can be reduced with the increase of patch width, therefore a full-width composite repair is recommended. Crack in the aluminum substrate greatly affects the stress/strain field around the crack and its tip, while the stress/strain field in the other region of the SSCPS is less sensitive to the crack. Interface disbond can remarkably decrease the thermal residual stress/strain in the aluminum substrate near the adhesion interface within the disbond region. It is also found that the thermal residual stress can be decreased with the increase of the adhesive layer thickness or the patch to substrate stiffness ratio.Hart-Smith double-lap joint model is improved to describe the thermal residual stress distribution in the SSCPS, and the improved Hart-Smith model and the bi-metallic strip model are used to predict the thermal residual stress in the SSCPS. The comparison between the predicted results and the experimental data shows a relatively large difference. While the analytical models are efficient to predict the variational tendency of thermal residual stress distributions at various cure cycles.(2) Effects of the interactions between thermal residual stress and bending on the mechanical properties of the SSCPSRose model and Wang-Rose model are adopted to investigate the stress intensity factors (SIF) in the single-sided composite patched semi-infinite and finite-width aluminum plate. The calculated results show that single-sided composite patching can significantly reduce the SIF at the crack tip.Wang-Rose model is modified to involve the effects of thermal residual stress and bending on the SIF. It's found that thermal residual stress makes no contributions to the SIF when the SFT reaches 36.2℃, however the effect of thermal residual stress on the SIF diminishes with the increasing external loadings when the SFT exceeds 36.2℃. The quasi-static tensile strength of the cracked aluminum plate can recover over 90% of the fracture strength of the perfect aluminum plate after the single-sided reinforcement by unidirectional carbon/epoxy composite patch. The tensile strength of the repaired specimen was decreased by the thermal residual stress.Equivalent modulus, which perfects the evaluation criteria for the composite repaired components, is defined to characterise the stiffness-recovery capability of the SSCPS. During the early stage of the tensile loading, the strains on the aluminum plate surface within the bonded region increase rapidly with the increase of the loading due to the recovery of the specimen warpage. With the further increase of external loading, the equivalent modulus of the unpatched cracked aluminum decreases, however the equivalent modulus of the composite patch reinforced region increases. It is also found that equivalent modulus is efficient to describe the effects of cure cycle on the stiffness of the SSCPS.The material constants of Paris law, C and m, are determined, and they are 2.55×10-10 and 2.85, respectively.Single-sided composite patching can significantly improve the fatigue properties of the cracked aluminum plate, and the thermal-mechanical coupling shows adverse influence on the fatigue life of the SSCPS. The experimental results indicate that single-sided composite patching can extend the fatigue life of the cracked aluminum plate over 11 times. Modified Wang-Rose model and the Paris law are efficient to investigate the effects of thermal residual stress on the fatigue life. It is found that the theoretical predictions of the fatigue life agree well with the experimental measurement results. The fatigue life can be significantly shortened due to the thermal residual stress when the stress ratio is low. Effects of the thermal residual stress on the fatigue life of the SSCPS become small when the stress ratio is high.
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