| The bonded patch repair method is an advanced rapid repair technique for damaged aluminum structures, which has been successfully applied in repairing thin aluminum plates of military and civil aircraft. However, overseas researches are still at the laboratory stage, lacking necessary experimental data, especially the data about the fatigue properties of repaired thick aluminum plates. Compared with carbon fiber reinforced epoxy (Cf / epoxy) composite, boron fiber reinforced epoxy (Bf / epoxy) composite is characterized by high-modulus and bigger coefficient of thermal expansion, which is more suitable for repairing the damaged thick aluminum plates. In China, even fewer studies about Bf / epoxy composite and its application in repairing thick aluminum plates have been carried out.This dissertation studies resin matrix properties of Bf / epoxy composite, interfacial properties of interface between fiber and the matrix, mechanical properties of the composite, then determines the formula of the resin matrix and the preparation technology of the composite to make composite patch according to bonded repair requirements. The study took the Bf / epoxy and Cf / epoxy composite patches to repair center-cracked aluminum plates. The cracked plates were made of LY12CZ aluminum plate with thickness of 1.76 mm and LC52CS aluminum plates with thicknesses of 5.20 mm and 10.20 mm respectively. It aimed to explore the effects on the repair efficiency caused by residual thermal stress, plate thickness, patch material, ply stacking, patch geometry, repair technology, and so on. Study focuses were on the residual thermal stress, quasi-static tensile and fatigue properties of the specimen.Firstly, epoxy resin E-51 was modified by liquid nitrile rubber (LNBR) to explore the thermal properties, stress properties and curing reaction kinetics properties. Contact angle method and developed single fiber pull-out tests were applied to study the interfacial properties of interface between fiber and the matrix, and the effects of boron fiber surface processing method and epoxy resin system on interfacial shear strength were investigated. Based on this experiment, it prepared the Bf / epoxy composite, and studied its mechanical properties and tensile failure mode. The results show that when the boron fiber surface is treated by boiling ethanol, the unidirectional Bf / epoxy composite prepared by epoxy resin of E-51/10 wt% LNBR / 4, 4'-diamino- diphenyl methane (DDM) has good mechanical properties. The indexes of the mechanical properties are as follows: tensile strength is 1257MPa, tensile modulus 187GPa, elongation to fracture 0.80%, bending strength 1606MPa, bending modulus 187GPa, and the interlaminar shear strength 77.5MPa.Secondly, the study analyzed the fracture toughness and the stress state of two types of LC52CS aluminum plates with thicknesses of 5.20mm and 10.20 mm respectively. At the room temperature, the fracture toughness for the plate with thicknesses of 5.20mm and 10.20mm is 45.2MPa·m1/2 and 42.7MPa·m1/2 respectively. Theoretical analysis shows that the plane stress state plays a dominant role in the aluminum plate with thickness of 5.20mm, while the plane strain state plays a dominant role in the aluminum plate with thickness of 10.20mm. Then, three-dimensional finite element model was adopted to calculate Stress Intensity Factor (SIF) of the repaired thick specimens, and the effects of the residual thermal stress, plate thickness, patch material, and ply stacking and patch geometry on the SIF were investigated. It is found that after the bonded repair by Bf / epoxy patch, the SIF of the crack tip of repaired specimens is significantly decreased and the value increases with the increase of the plate thickness. Residual thermal stress of the repaired specimen decreases the repair efficiency. The repair efficiency of Bf / epoxy patch is better than that of Cf / epoxy patch, and there exists better patch length, width, and thickness.Thirdly, the study analyzed residual thermal stress and thermal strain of the single-side repaired thick specimen. The results show that center bending deflection of the repaired specimen, due to mismatching of the coefficient of thermal expansion between the composite patch and aluminum plate, decreases with the increase of the plate thickness. The residual thermal stress and thermal strain near the crack tip of the unrepaired surface are bigger than those around other areas, and they increase with the increase of the plate thickness. The thermal deformation matching ability of single-side repaired specimen of the unidirectional Bf / epoxy patch is better than that of the unidirectional Cf / epoxy patch.Fourthly, quasi-static tensile properties of the repaired specimens were analyzed. The results show that the loading capacity reservations (η), the loading capacity resumption (χ) and the critical crack opening displacement (δc) increment of the repaired specimens decrease with the increase of the plate thickness. The maximum tensile loading for the single-side repaired specimen with thickness of 10.20 mm is 85.80% of the undamaged specimen. Itsηis of 8.05% smaller than the single-side repaired specimen with thickness of 1.76mm. But theχis only of 3.71% smaller. The increase amount of the maximum tensile loading increases with the increase of the plate thickness. The equivalent stiffness (Estiff) of the repaired area increases with the increase of the plate thickness, and both the value of them are bigger than the stiffness of the undamaged plate of the same size. The repair efficiency of single-side repaired specimen of the unidirectional Bf / epoxy patch is better than that of the unidirectional Cf / epoxy patch.The results also show that under the research condition of this study, using anodizing treatment by phosphoric acid to process aluminum plate surface, or taking unidirectional stacking, or double-side repair method could increase the loading capacity of the repaired thick specimen under the unidirectional loading. The repair efficiency of the repaired specimen with thickness of 5.20 mm is the best when the patch width is equal to the plate width, patch length is 100 mm and the number of patch plies is 10.Lastly, it studied the fatigue properties of the repaired thick aluminum plate. The results show that when the initial crack length is 12mm, the maximum stress is 80MPa and the stress ration is 0.1, then the fatigue life of the repaired specimens with thicknesses of 1.76mm, 5.20mm and 10.20mm is 22.30 times, 12.84 times and 8.40 times of those unrepaired specimens respectively. When the unidirectional Bf / epoxy gets single-side repair, the crack growth rate of the aluminum plate is 1 order of magnitude smaller than its growth rate before the repair. That is, bonded patch repair method can greatly delay the fatigue crack growth of aluminum plate, and the crack growth rate of both the repaired specimen and unrepaired specimen increases with the increase of the plate thickness. The normalized crack length difference (Δa ) of the single-side repaired specimen increases with the increase of the plate thickness, which demonstrates that the restriction of patch to the unbonded surface crack growth in single-side repaired specimen decreases with the increase of the plate thickness. The fatigue life of single-side repaired specimen of the unidirectional Bf / epoxy patch is longer than that of the unidirectional Cf / epoxy patch.The dissertation also studied the effect of fatigue failure mode on the repair efficiency. The results show that when the fatigue failure of the repaired specimen with thickness of 1.76mm is controlled by adhesive interfacial debonding, then no matter how long the initial crack of aluminum plate is, the single-side repair of the composite patch can largely increase the quasi-static tensile and fatigue properties. The maximum tensile loading of the single-side repaired specimen can be recovered to 90% of the undamaged specimen, and even the repaired specimen in which the aluminum plate was completely failed can also be recovered up to 60% of the undamaged specimen. The fatigue life, with which the maximum stress is 80 MPa and stress ratio is 0.1, could arrive at 21 times of the repaired specimen with initial crack length of 12mm. However, when the fatigue failure of the repaired specimen with thicknesses of 5.20mm and 10.20mm is controlled by crack growth of aluminum plate, the quasi-static tensile and fatigue properties decrease with the increase of the initial crack length.The dissertation also took Paris law to predict the fatigue life of the single-side repaired thick specimen. The result shows that by matching the experimental data it gets two material constants C and m of the Paris law. The constants in this study have a dependence on the plate thickness. The Paris law can better predict fatigue life of the repaired specimen in which the fatigue failure is controlled by crack growth of aluminum plate, but it is not effective for predicting the repaired specimen controlled by adhesive interfacial debonding. It has been suggested that amplitude formula of interfacial debonding ratio and strain energy release rate range could be used to predict the fatigue life. |
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