Residual Strength Analysis Of Metal Thin-Walled Structures Reinforced By Bonded Straps

Lightweight thin-walled structures and the integrated structures arewidely used in aerospace structures, residual strength of such structures withcracks is the key issue of the damage tolerance and structural integrity.Therefore, it is very necessary to assess the damage properties of thin-walledstructures. Selective reinforcement is a new concept, at the beginning ofdesign, bonding straps to the regions where may having problems in theintegrated structures, and playing well on the restrain of the crack initiationand crack growth.Due to the complexity of the damage analysis of straps reinforcedmetallic thin-walled structures, classic analytical solution is limited, so now itbasically depends on the finite element numerical analysis method and so on.On account of mixed damage problems of the substrate metal structurefracture damage and bonded interface delamination, the paper makes studiesof test and finite element analysis.This thesis consists of two main parts:(1) The tensile test was conducted using the quasi-static stable crackextension ASTM-E2472standard test methods. Configuration of the substratewas respectively compact C(T) and middle single crack M(T) sample，andstraps were unidirectional carbon composite laminates. During the test, crackinitiation load and residual strength of the structure were recorded, and useddirectly optical method to measure crack tip opening angle (CTOA) and thecrack length. It tested that the configuration, initial crack size, whether fatigueprefabricated initial crack and bonded straps had same influence on CTOA ornot. The results showed that CTOA tended to be a constant value after a certain stable extension distance. It had much improvement for the crackinitiation load and maximum residual strength of the substrate structure bybonding straps, and an obvious enhance effect.(2) Finite element analysis based on CTOA criteria, plane strain coremodel and cohesive element method to analyze the stable crack extension andresidual strength of reinforced structures. It used ABAQUS software to buildthe finite element model of reinforced C(T) and M(T) structures, improvedthe traditional plane stress model by plane strain core model with the CTOAfracture criteria and interface element method, so as to analyze the damage ofsubstrate metal structure and bonding interface. The predicted results werecompared with the experimental results; it found that both theload-displacement curve and load-crack extension curve in the trend werewell fitted, and crack initiation load and maximum residual strength valueswere agreed. The results show that the CTOA criteria and plane strain coremodel could effectively predict crack growth and residual strength ofreinforced structures, and suitable for different configuration structures.Based on the validated analysis method, it analyzed the detail design ofstraps which had influence on the residual strength of reinforced structures.Main affected parameters were strap width, length, stiffness and position fromthe crack tip. The results show that strap width, length, and stiffness had lessinfluence on the crack initiation, but much on the maximum residual strength.The strap position from the crack tip had much influence on the crackinitiation. The reasonable design details parameters of straps, in the case ofreducing the weight of the structure as much as possible to obtain the bestenhancement. It provides a design and analysis method for the lightweightthin-walled structure and the integrated structure used in large aircraft.