Research On Typical Mechanical Behavior Of Single Hole Fiber-metal Laminate Joints

Fiber metal laminate(FML)as a hybrid composite materials is an excellent aviation grade composite material,which has high specific stregth,specific stiffness and fatigue resistance.As the excellent ductility of metal,it overcomes the weakness that the fiber reinforced resin matrix are prone to brittle failure.FML has a lot of advantages,but how to connect a large number of FML or other composite materials in the industry application,while ensuring the structural strength and reliability has always been the bottleneck problem restricting its wide application.In recent years,the researches on FML have become a hot topic,but investigations about their assembly structures are still insufficient.For example,the influence of uncertain paramters on the mechanical properties of FML joints and the interaction mechanism between adhesive layer and fastener in hybrid joints are not clear.The damage onset and evolution law of each component material in FML joints with different geometric parameters is still a difficult point for the experimeteral investigation.There are few studies on the mechanics,damage evolut ion and energy absorption characteristics of FML joints subjected to high strain rates loading.If they could be applied as a tension-absorber,the prospect is wide.The mechanical response of FML under different temperature conditions and long-term service need to be further explored.Based on this,the paper aims to explore the above key problems in the fabrication and service process of FML joints,so as to make up the gaps in the existing researches.Firstly,in order to explore the mechanism of adhesive layer and fastener in the initial deformation stage of the hybrid(bonded/bolted)joints,a novel double-spring model of the hybrid FML joints is established.Monte Carlo sampling and calculation of sensitivity based on the variance are used to calculate the global sensitivity indices of parameters that have the ablility to quantify the raltive importance of assmenbly factors in load distribution between the bolt and adhesive layer,fracture displacement at the initial stage and other mechanical properties.It is found that bolt-hole clearance,adhesive layer stiffness and geometrics of laminate are the top three key factors.Therefore,the precision of parameters should be strictly controlled in assembly of structures.Secondly,this work introduces the spring-dashpot element into the spring model of bolted joints to describe the stiffness relaxation of structures under different ambient temperatures.The proposed model can well describe the relaxation of bearing capacity of joint ed structure at different temperatures.Based on the limited experimental data and time-temperature superposition principle(TTSP),the mechanical responses of bolted FML joints under normal temperature for a long time are predicted,which provides a refer ence for the performance of FML jointed structures under actual working conditions.Then,in order to study the influence of the different geometries of jointed structures on the failure mode and the onset and evolution of damage in FML,the signals emitted by damage events during the experiments are collected and classified by the acoustic mission in situ damage detection technology.Then the damage onset and evolution could be described based on the distribution of cumulative energy.The results show that the bearing capacity of the structure dominated by bearing failure is the strongest,and the bearing damage mainly occurs in the matrix and metal layers.The structure dominated by shear failure mainly has shear failure of matrix and metal and the pull-out of fibers.The ultimate bearing capacity of the structure is too low to be applied in the industry.In the structure dominated by tensile failure,its matrix fracture occurs at the early stage.The destructive failure of fibers and metals all occurs at t he ultimate point,which belongs to brittle fracture with low structural strength.Finally,in the structure with limited space available for buffering energy absorption,it is impossible to set up a large energy absorption device,which gives rise to the new concept of composite tensile energy absorber.The tensile experiments of bolted,hybrid joints and pin joints are carried out under different loading rates.The effects of loading rate,adhesive layers and fiber sequence on structural damage modes and impact energy absorption are investigated.The results show that with the increase of loading rate,shear damage mode of bolted joints at 1m/s is transformed into brittle tensile fracture at 3m/s.However,with the introduction of the adhesive layer,the damage mode of hybrid joint at 1m/s could also be brittle tensile fracture,but whether damage pattern s could change are determined by the bonding quality of adhesive layer.If FML is used as the material of tension-absorber joints,the best sequence of fiber is 0°/90°,which can ensure enough static strength and have good energy absorption efficiency.