Investigation On The VAT Reinforcing Technique For Open-hole CFRP Laminates

The research and development of novel lightweight materials have drawn extensive attention from all over the world because of the energy crisis and environmental degradation.Carbon fiber reinforced polymer,as one of the most promising materials in the 21^st century,is getting more and more broadly used in areas such as aeronautics and astronautics?transportation?oceanographic and sports equipments for its advantages such as high specific strength,high specific stiffness,strong fatigue resistance,and strong corrosion resistance.However,to meet the requirements of structural connection,weight reduction and visualization requirements,carbon fiber composite components often need to be perforated,where the stress concentration and local damage introduced by open hole may degrade the mechanical properties of the composite laminate accordingly limiting the applications of the composite material.Therefore,it is of great significance to develop a simple but efficient open-hole reinforcing method to improve the mechanical properties of the composite laminates and promote the large-scale application of composites in engineering projects.In this research,the variable angle tow(VAT)technique is applied to reinforce the open-hole composite laminates.By combining theoretical analysis,multi-scale numerical simulation and experimental research,the quasi-static mechanical properties(tensile,compressive and bending properties)of VAT reinforced open-hole composite laminates influenced by distinct fiber placement trajectories and reinforcing methods are studied.Dynamic properties(impact and fatigue properties)of the open-hole composite laminates are further investigated to analyze the reinforcing mechanism VAT.The main contents are organized as follow:(1)The multi-scale modeling and simulation of VAT reinforced open-hole composite laminates.Parameterized microscopic unit cell model that consists of fiber,interface and matrix is established to analyze the representative volume element of carbon fiber reinforced composites,and to study the relationship between strength/stiffness and fiber content of the composites.Afterwards,the parameterized gradient-property model of the VAT reinforced open-hole composite laminates is established where the constitutive relationship and failure strength vary from element to element.By transferring“Fiber volume content-material properties”relationship into the gradient-property model,the displacement-load relationship and progressive damage behavior of VAT reinforced open-hole composite laminates under different loading conditions can be studied.(2)Elliptical VAT reinforcements with different eccentricities are applied to reinforce the open-hole composite laminates in both bonding and co-curing ways.Simulation and experimental studies are carried out to analyze the quasi-static tensile,compressive and bending properties of the reinforced open-hole laminates.The results of the research indicate that VAT reinforced open-hole laminates with co-curing reinforcement perform better than the bonded ones.The mechanical properties of the VAT reinforced open-hole laminates with co-cured method vary with the eccentricities of the VAT reinforcements.When the eccentricity of the VAT reinforcement is 0.36,the tensile strength and compressive strength of the VAT reinforced open-hole laminates with co-cured method reach the maximum:the tensile and compressive strength increase rate reaches 38%and 27%respectively.The ultimate bending load of the VAT reinforced open-hole laminates with co-curing method reaches its maximum when the eccentricity of the VAT reinforcement grows to 0.43,and the enhancement rate of the ultimate bending load is 75.3%.The failure modes of the open-hole laminates with VAT reinforcements are successfully predicted by the FE model and fit well with the experimental result.The simulation deviations of the tensile strength,compressive strength and maximum bending load are 10.8%,15.2%and 11.6%,respectively.The simulation and experimental results indicate that the quasi-static mechanical properties of VAT reinforced open-hole composite laminates depend on both reinforcing method and fiber path.On the one hand,the strength of co-curing interface is higher than that of bonding interface,and co-curing method can better reinforce the open-hole laminate;On the other hand,the appropriate fiber path of VAT reinforcement varies when loading condition changes,and right fiber path can significantly improve the property of VAT reinforce laminate.(3)Research on dynamic mechanical properties of the co-curing type VAT reinforced open-hole laminates is carried out since the co-curing type laminates perform well in quasi-static tests.The research results indicate that the VAT co-curing reinforcing method is able to improve the impact properties of the open-hole laminates effectively,and the reinforcement successfully increase the in-plane impact strength by35%.However,the VAT reinforcement method has insignificant effects on the increase of the fatigue properties of the open-hole laminates:when the eccentricity of the VAT reinforcement is 0.36,the VAT reinforcement can only improve the fatigue limit of the open-hole composite laminates by 3.5%.The FE model of the open-hole composite laminates with co-curing VAT reinforcement can well predict the failure modes of the laminates under impact or fatigue loading conditions.The deviations of the simulated impact strength and fatigue limit are 18.8%and 16.7%,respectively.It can be concluded from the simulation and experimental results that for VAT reinforced open-hole laminate with co-curing method,delamination happens at the interface when suffering impact load or fatigue load.The interface strength limits the dynamic mechanical properties of VAT reinforced open-hole laminates.In this paper,the fiber trajectories and connecting method between the VAT reinforcement and open-hole composite laminates are investigated.Both experimental and multi-scale simulation methods are used to study the mechanical properties of the reinforced open-hole laminates.This paper may assist to solve the performance reduction problem of the open-hole composites and offer a new technique of reinforcing the open-hole composite laminates.