The Evolution Mechanism Of Mechanical Properties Of Composites Interference Fit Bolted Joints In Marine Environment

Carbon Fiber Reinforced Polymer(CFRP)composites have been widely used in marine aviation equipment in recent years due to their excellent physical/mechanical properties,such as light weight,high specific strength and modulus,excellent fatigue resistance,and unique material designability,which have realized the transition from non-load-bearing parts and sub-load-bearing parts to main load-bearing parts.With the improvement of performance of marine aviation equipment,the strength and fatigue life of composite structure are put forward higher design requirements.The existing research shows that the interference fit technology can effectively improve the strength and fatigue life of the joint of composite structure,so the application of CFRP/CFRP interference fit jointing structure in naval equipment is increasing gradually.However,when marine aviation equipment serve in marine environment with high temperature,high humidity and high salt fog for a long time,the aging damage will inevitably occur in composite materials and their structures,which will lead to the change of physical/mechanical properties.In addition,in the process of taking off,flying and landing,not only the high-low temperature alternating but also the alternating loads and impacting loads will further accelerate the expansion of material aging damage and seriously threaten the service safety and reliability of marine aviation equipment.At present,the research on the evolution mechanism of CFRP/CFRP interference fit joints in marine environment is relatively lag,which seriously hinders the application of composites interference fit joints in marine aviation equipment.Therefore,in this paper,the evolution mechanism of mechanical performance of CFRP/CFRP interference fit joints in marine environment is studied.The main research contents and results are as follows:(1)The mechanical behavior of composites interference fit bolt inserting process was analyzed,and the mechanism of interfacial damage was revealed.The hole and bolt diameters at the current production technology level were analyzed statistically,and four intervals of interference fit size were determined considering the hole drilling and bolt manufacturing errors.The quasi-static bolt installation experiments were carried out,and the influence of interference fit size on installation load was analyzed.Then,the difference of bolt installation load-displacement curves under different support stiffness at exit was compared.Finally,the bolt installation damage was characterized by the parameters of fiber bending angle and interphase thickness by means of microscopic observation technology.The results show that the bigger the interference fit size is,the more obvious the M-shape characteristics of the inserting load curve is,and the larger the bending angle of the fiber at the exit,and the easier to produce delamination.(2)The load-bearing response characteristics of CFRP/CFRP interference fit bolted joints in marine environment were studied.The load-displacement curves of CFRP/CFRP bolted joints under different seawater aging conditions were analyzed,and the evolution models of load-bearing strength with environmental factors were established.At the same time,the bearing strain of the key points around the hole was analyzed,and the evolution mechanism of the bearing capacity of joints was revealed.Finally,the failure morphology of the joints was observed by means of micro-observation technology,and the failure mechanism of the joints was revealed.The results show that the environmental factors of seawater aging all have negative effects on the mechanical behavior of CFRP/CFRP bolted joints,among which temperature has the greatest effect,followed by ageing time,and salt concentration is the smallest.The interference fit can effectively inhibit the aging of joints in seawater,of which1.15%is the best.The failure mechanism is mainly characterized by fiber squeezing and shearing fracture,and matrix shearing fracture.(3)The fatigue response characteristics of CFRP/CFRP interference fit bolted joints after seawater aging were analyzed.The fatigue loading tests were designed and the load-displacement curves of the joints during cyclic loading were analyzed.Three key points around the hole were selected and the evolution law of strain with loading cycles was analyzed.Through quasi-static loading experiments,the comprehensive effects of seawater aging and fatigue loading on the bearing strength of joints were explored.The results show that with the increase of loading cycles,the strain amplitude and mean strain at the squeezing side all decreased.However,at the non-squeezing side,the strain amplitude decreased,but the mean strain increased.After fatigue loads treating,the load-carrying capacity of the joints has been greatly improved.However,such gain effect decreases with the increase of aging time and ambient temperature,increases with the increase of salt concentration in seawater,and increases first and then decreases with the increase of interference fit size.(4)The physical/chemical damage mechanism of composites in marine aging environment was revealed.First,the hygroscopic properties of composites were analyzed,and the anisotropic diffusion coefficients under different environmental factors were determined.Then,the finite element model of hygroscopic stress of composites was established.The moisture and stress distribution in laminates were simulated and analyzed.Finally,the micro-physical damage of composite materials was analyzed by means of micro-observation technology,and the chemical structure of the composites was examined by Fourier transform infrared spectroscopy(FTIR).The results show that the moisture stress at 0~o layer was the largest in composite laminates,followed by±45~o layers,and 90~o layer was the smallest.The physical damage of composites is mainly matrix shedding,matrix cracking,pitting,fiber corrosion,interfacial debonding,delamination and so on,which are aggravated by the increasing of aging time,ambient temperature and seawater concentration.In addition,the chemical damage is mainly related to the change of O-H and C=O functional groups.(5)The evolution law of physical/mechanical properties of composites was explored.On the basis of accelerated seawater aging experiments,the dynamic mechanical properties of the specimens were tested,and the physical/mechanical properties such as storage modulus,loss modulus,damping and glass transition temperature were analyzed.Further,the effects of seawater aging on the tensile and interlaminar shear properties of materials were analyzed by tensile and short beam shear tests,and the prediction models of mechanical properties were established.Finally,the damage and failure mechanisms of materials was revealed by means of micro-observation technology.The results show that the glass transition temperature,tensile strength and interlaminar shear strength of the composites all increased exponentially and tended to be stable with aging time,and deteriorated linearly with the increase of ambient temperature.The degradation of mechanical properties of composites is caused by the increase of damping value and the decrease of glass transition temperature.The bearing performance of CFRP/CFRP bolted joints is mainly depended on the interlaminar shear strength of composites.