| With the continuous increase of composite materials proportion used in aircraft structures,its connection with metal materials such as titanium alloys and aluminumlithium alloys have become increasingly widespread.Compared with the mechanical connection,the bonded structure has the unique advantages of low cost,light weight,and no stress concentration caused by drilling.However,there are differences between metal and composite materials in terms of material surface,stiffness and thermal expansion coefficient.Moreover,bonded structure generates points of weakness in the composite,mostly between composite layers,causing delamination failure,and the failure modes of bonded structure are complicated.Therefore,a systematic research of titanium-to-composites bonded structures failure modes and mechanisms is of great significance for such structures design and application.In this paper,bonded joints with different adherends,overlap lengths,adhesive layer thicknesses are made of titanium alloy and aramid fiber composite materials.Tensile tests are performed at different ambient temperatures.Digital image correlation technology,strain acquisition systems and optical microscopes are used to characterize joints strain and failure modes under tensile load.The dissimilar joints failure process and characteristic are investigated,and the failure mechanism of titanium-to-composite joints is studied.The main research contents and conclusions are summarized as follows:Tensile tests are performed on dissimilar material joints with different connection parameters,and the joints ultimate load under different ambient temperatures are analyzed.The results show that the bearing capacity of dissimilar material joints is weaker than that of similar material joints.When the overlap length is increased from 20 mm to 80 mm,the average limit load of joints is increased from 4.38 kN to 6.36 kN.When the adhesive layer thickness is increased from 0.2mm to 1.2mm,the joints ultimate load is reduced from 6.13 kN to 5.89 kN.At the ambient temperature of-50 ℃ and 100 ℃,the average value of joints ultimate load are 4.82 kN and 0.60 kN,respectively.The effects of joint connection parameters and ambient temperature on joint crosssections are analyzed,and the failure modes of dissimilar material joints are investigated.The results show the failure modes of dissimilar material joint are mainly the composite delamination failure the interfacial failure between the titanium and adhesive layer.With the increase of overlap length,the failure mode of composite delamination increases.With the increase of adhesive layer thickness,the failure mode of interfacial failure between the titanium and adhesive layer increases.Titanium and adhesive layer interfacial failure occurs at high and low temperatures.The surface strain of dissimilar material joints are analyzed at different times,and the micro-morphology and bonding theory of joints are used to find out the failure mechanism and micro-mechanism of dissimilar material joints.The results show that at the beginning of failure process,the composite adherend end is mainly subjected to shearing stress,and the titanium-to-adhesive interface formed by physical connection is not easy to break.After the joint is debonded and damaged,the titanium-to-adhesive interface is sensitive to out-of-plane peeling stress,and the interface is liable to be damaged.The strain fields of joints with different connection parameters are compared to analyze the joint failure characteristics.Reasons for changes in tensile properties and failure modes of joints with different connection parameters are revealed.The results show that the mismatch in stiffness between composite and titanium makes dissimilar material joints have larger lateral offset and bonded area deformation than that of similar material joints.Changes in joint connection parameters alters the stress state at the end of composite material,affecting the bearing capacity and failure mode of joints. |
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