Quasi-static And Dynamic Damage And Failure Investigation Of CFRP/Ti Interference Fit Bolted Structures

Carbon fiber reinforced polymer composite/ titanium alloy(CFRP/Ti)bolted joining is still unavoidable in composite structure design of aircraft,which is also more and more widely applied in Chinese large aircraft due to the increasing use of CFRP and Ti.Composite joints represent the potential weak points in composite structures,and the optimization of CFRP/Ti bolted joint is crucial to the efficient design of overall composite structures.However,the studies of the CFRP/Ti bolted joint design are not discussed at great length before,especially,the quasi-static progressive damage and dynamic behavior of CFRP/Ti bolted joints.Moreover,the application of interference fit technology in CFRP/Ti bolted joints is restricted due to that the relative study is rare and the reinforced mechanism is not well understood.Hence,to improve the composite structure designs of the large aircraft,the quasi-static and dynamic damage and failure of CFRP/Ti interference fit joints were investigated using theoretical analysis method,numerical and experimental methods.The research includes:(1)Damage investigation of CFRP/Ti bolted joints during interference fit bolt dynamic installation progress The damage mechanism of CFRP/Ti bolted joints was studied experimentally during the dynamic installation of the interference fit bolt,compared with the quasi-static installation method.The installation resistance and damage were discussed.The results showed that the installation resistance exhibited two stages and was mainly caused by the Ti laminate(80% of the peak resistance).The installation resistance showed little sensitivity to installation methods under small interference sizes,while it was obviously lower using dynamic installation method than that using quasi-static installation method under large interference sizes.The damage modes using quasi-static and dynamic installation method were similar,which were dominated by the damage of CFRP mainly due to the increasing of interference sizes.The first ply around the hole was observed to be the most critical region on which the damage occurred first with a damage ring.With the increasing of interference sizes,the damage of the CFRP was observed to propagate from the first ply to the internal ply and propagate from the hole surface deeper into the laminate.Through the quantitative analysis,the dynamic installation method was found to reduce the damage factor of first ply and the relative deflection of hole surface.(2)Progressive damage study of CFRP/Ti interference fit bolted joints under quasi-static loading The CFRP/Ti bolted joints were developed and a physics-based multi-scale progressive damage model was used to predict the progressive ply crack damage and the stiffness degradation of the joints.The influences of bolt pretension and interference sizes on ply crack damage were also discussed.The results of the damage investigation of CFRP/Ti joints demonstrated that only the local damage occurred around the hole of the CFRP laminates causing by local stress concentration.The stiffness degradation of the CFRP laminates initiated first at the junction of the minimum cross section of the CFRP laminate and the hole.Increasing the preload of the joints increased the stiffness and the damage strength of the joints,meanwhile,suppressed the matric crack propagation and decreased the stiffness degradation of the CFRP laminates.The stiffness of the joints increased with interference size,moreover,the interference fit with small interference size can increase the stiffness and the damage strength of the joints,and also suppress the matric crack propagation.However,the joints with large interference size exhibited the lower damage strength and the more serious degradation of stiffness,which is because of the high level of tangential stress around the hole of the CFRP laminates causing by the interference fit.The large interference size also increased the propagation of the matric crack.The optical section of interference sizes is from 0.20% to 0.60% under the study condition of this paper.(3)Development of the dynamic test system of joints loading by electromagnetic load Based on electromagnetic riveting mechanism and one dimensional dynamic response equations,the mathematical model of the electromagnetic loading system was established and the formula of the maximum equivalent impact velocity was deduced.The key parameters of the loading system were studied by simulations and experiments.To meet different dynamic test requirements,two loading methods were developed: the direct loading method was used for high speed loading test with long loading time,which used electromagnetic force to load on the specimen directly;the impact loading method was used for high speed loading test with short loading time,which used electromagnetic impact force to accelerate the driving head and then the driving head impacted the bar producing stress wave to load on the specimen.Finally,based on the above work,a physical test system was manufactured.Compared the traditional dynamic test system,the developed test system have the advantages of small volume,good control capability,high loading repeatability,high loading speed,etc.,which can be used to assess the dynamic behavior of the joints in aircraft and also improve the anti impact design.(4)Dynamic behavior assesssment of CFRP/Ti bolted joints Dynamic tensile tests were conducted on a developed dynamic test platform to understand the dynamic behavior of CFRP/Ti bolted joints.The bearing and failure loads of the joints were evaluated and the dynamic failure mechanism was studied.In order to optimize joint designs,the influence of interference size on the dynamic behavior of joints was also studied.The results showed that load-displacement responses of the joints experienced significant loading rate dependence.With the increase of impact velocity the bearing and failure loads decreased.The failure modes of the joints were dominated by CFRP failure at high impact velocity.The joints failed in the bearing failure mode at the beginning,but ultimately failed in the tearing-out failure mode,which also experienced pin shear deformation,and the hole elongation of titanium alloy plate.In addition,much fiber and matrix fracture was observed and flied off during dynamic loading progress.The stiffness and bearing loads of the joints under dynamic loading exhibited an increasing trend with interference size.However,the failure loads exhibited little sensitivity to interference size.The interference fit joining was observed to reduce the damage of the joints under dynamic loading.