Research On Hole Expansion Strengthing Technology Of Composites And Their Stacks Joint Structures

Lightweight and durability have become the common goals of design and manufacturing engineers for both advanced fighter and modern commercial aircraft.Due to their low weights and high mechanical properties,nearly for a decade,composites materials have been more widely used in aeronautical and aerospace structures for the fuselage,wing as well as other structural parts.Mechanical connection is the main connection way for aircraft parts.Bolted joints are still the dominant fastening mechanisms used in jointing of primary structural parts.However,the presence of connection holes compromises the structure integrity,and causes a local stress or strain rise in aircraft parts.The stress concentration and damage around holes reduce the connection strength,and decrease the fatigue life of aircraft parts under the action of alternating loads.Fatigue failure becomes one of the most important failure modes in aircraft structures.In order to improve the fatigue life of aircraft structure,the hole expansion for mechanical connection structure of carbon fiber reinforced polymer composite materials(CFRP)and their stacks were conducted in this paper.The main creative contents in this paper are as follows:(1)Aim to the situation that stress concentration was occurred around the center hole and end hole of two kinds of typical aircraft structure components,the calculation methods and influencing factors of stress concentration coefficient were discussed for them.Two hole expansion processes including seamless sleeve of laminated composite plate and slotted sleeve of titanium plate were developed respectively.The technical requirements and evaluation index of the hole expansion processes were proposed.3D finite element simulation model of hole expansion processes were constructed,and the effect of hole expanded to stress concentration was explored.The feasibility of hole expansion process for improving the fatigue life of mechanical connection structure of CFRP and their stacks was pointed out.(2)Experimental apparatus for hole expansion of CFRP was designed.The seamless sleeves which is suitable for hole expansion of CFRP were developed from TC4 titanium.The deformation and stress distribution around expanded holes were studied by finite element simulation.Hole expansion process of CFRP was carried out,and the expanded quality was evaluated.Based on the hole expansion experiment and fatigue test,the effects of hole expansion process on the fatigue gain of CFRP were discussed.The hole expansion process to strengthening technology which can control shape and property was achieved.(3)Hole expansion experiment with slotted sleeve were carried out in TC4 titanium alloy plate.The surface integrity of hole wall including micro hardness and microstructure were researched.Residual stress field generate by hole expansion and the distribution law of stress in the hole of loaded specimens were discussed.Fatigue test of TC4 titanium alloy under different expanded degrees was accomplished.The influence mechanism of hole expansion process to the macro and micro morphology is revealed in the fatigue fracture surface.The relationship between the expanded degrees and fatigue gain was found by experiments and analysis.(4)Tensile and fatigue tests were carried out for two kinds of typical stacks connection structures including composite to composite and composite to titanium alloy.The influence of hole expansion process on the deformation law and the stress distribution of the connecting hole in stacks structure was discussed.The bearing capability and failure mode of the stacks structure were obtained in different conditions.The fatigue gain effect of hole expansion process was verified,and the intrinsic relationship between the hole expanded degrees and the fatigue gain was established.