Study On Dynamic Mechanical Properties Of Single Lap Joints Reinforced By Z-pin

Inserting Z-pin could enhance the static bonding performance of composite single lap joint,however composite single-lap joint structure would suffer the effect of dynamic loads such as impact and fatigue loading in the service process,except the static loads like tensile and flexural load.Thus,there are few researches about damage mechanism and influencing rule of Z-pinned single-lap joint under dynamic load.Dynamic mechanical properties of Z-pinned composites single-lap joint are researched deeply in this article based on previous static properties researches systematically.The effect of different loading rates on the pullout and shear performance of Z-pin is studied by bridge test and shear test.The impact damage delamination of Z-pinned single lap joint is analyzed by finite element modelling.The influence of impact properties and effect of tension-tension fatigue of Z-pin parameters on Z-pinned single-lap joint are studied through designing impact and tension-tension experiments,respectively.(1)Z-pin pullout test and shear test are conducted to study the influence of loading rate on the maximum bridging force and shear force.Researches have shown that: due to the viscoelasticity of interfacial resin,the pullout strength of Z-pin would be influenced by the shear rate of interface between Z-pin and laminates.The maximum bridging force increases linearly with loading rate.Z-pin shear force decreases rapidly with the increase of loading rate.When the loading rate reaches 100mm/min,trend becomes slower.(2)The finite element model is established to analyze the damage condition of each layer of lap joint based on Hashin and B-K energy failure criterion.Researches have shown that: crack would initiate in the area with stress concentration and propagate under the impact load.The damage of 1st to 10 th layers close to the impact surface are relatively severe,while the layers of 1st to 10 th opposite the impact surface show almost no damage deformation.For the cohesive layer the damage areas of 1st-9th layer close to impact surface are larger than those of 1st-9th layer opposite to impact surface.The maximum damage area is existed in the lap surface,and is utilized to reflect the impact damage delamination area of joint.The impact damage area of joint increases with the increase of Z-pin volume content and decrease of the Z-pin diameter.(3)The impact experiment combines with ultrasonic C-scan method is utilized to evaluate the delamination damage area of joint and the experimental results are compared with those of finite element simulation.The residual Z-pin pullout strength and lap joint shear strength are studied experimentally.Researches have shown that:the results of finite element modeling are consistent with the results of ultrasonic C-scan method.The delamination damage area increases with the increase of Z-pin volume fraction and the decrease of Z-pin diameter.The damage area of joint reinforced by 3.0% Z-pin decreases by 46.9% compared with that reinforced by 0.5% Z-pin.The residual strength of joint after impact is improved by Z-pin significantly.When the diameter of Z-pin increases from 0.3mm to 0.7mm,the damage area decreases by 32.7%.The maximum retention rate of lap joint's pullout strength and shear strength are 58.0% and 86.6% with the maximum impact energy(4.230J/(kg?m-3)),respectively.The residual shear strength increases firstly and then decreases with increase of volume fraction of Z-pin,and decreases with the increase of Z-pin diameter.(4)The tension-tension fatigue experiment is conducted to study the fatigue performance of Z-pinned single-lap joint.Researches have shown that: fatigue life of lap joint decreases with the increase of fatigue stress level,and increases firstly and then decreases with Z-pin volume fraction increase.Fatigue life increases with the increase of Z-pin diameter.Residual fatigue shear strength of joint shows a tendency of increasing firstly and then decreasing with Z-pin volume fraction,and then increases with the diameter of Z-pin.The maximum retention rate of shear strength can reach up to 88.1% when diameter of Z-pin is 0.7mm.Z-pin could prevent the propagation of fatigue crack effectively and the rate of crack propagation increases with the increase of volume fraction and decrease of diameter of Z-pin.Morphology analysis of fatigue failure indicates that the failure of Z-pin is shear failure and slightly splitting.When the volume fraction and diameter of Z-pin are higher,the fracture surface of Z-pin is rougher due to the more splitting.