| The advanced composites are increasingly used as structural materials in the aerospace industry for the purpose of weight reduction.However,how to ensure safety and reliability while reducing the weight of the structures is the primary problem for the designers.The aircraft structures may encounter impact threat from foreign objects such as bird,ice,sand and so on,as well as the bird impact and casing containment for the compressor blades.Therefore,the sufficient impact resistance of composite structures is required to guarantee the security of flight.The braided/textile composites exhibit excellent impact damage tolerance than traditional laminate composites since they overcome the disadvantage of easy to delamination while subject to impact loadings.And another outstanding advantage of the braided composite is their suitability for integrated molding of large-scale structures,which reduces the manufacturing cost and fabrication time.These features make the braided composites been widely used as the structural materials to resist impact loadings.However,the complex architecture of braided composites in the micro/mesoscale is a combination of the mechanical response and failure process,which further increases the complexity in characterization,simulation,and design.The main purpose of this study is developing a multi-scale analysis method for the braided composites,which takes the typical two-dimension triaxially braided composite(2DTBC)as an object to elaborate the complete study process,including the investigation of damage and failure mechanism of the braided composite under quasi-static,dynamic and high-speed impact loading conditions.Based on this study,the methodology for the impact design of braided composite structures can eventually be established.Some research contents and relevant conclusions are briefly summarized as follows:(1)The deformation and failure mechanism of the two-dimension triaxially braided composite are investigated by conducting the quasi-static experiments under different loading conditions combined with the meso-scale finite element simulations.As a contrast,tensile failure behavior of the angle-ply composite samples made by the same component materials and molding process is also studied experimentally.According to the test results,the breakage of axial fiber tows dominates the failure process of 2DTBC under axial tensile load and lead to the brittle fracture features of the samples.For the transverse tensile condition,the stress-strain curve shows nonlinearity because the load is mainly transferred by the matrix and interface.Besides that,the free-edge effect makes the damage initiate from the unloading edges and further lead to the shear-type failure of the straight-sides coupon specimen.Inelastic response of matrix material generates the nonlinear response of axial and transverse compression of 2DTBC,the micro-buckling of axial tows is the immediate cause of failure during axial compression,and the free-edge effect also influences the failure behavior of 2DTBC under transverse compression.In addition,the numerical parametric studies declare that the interface properties have a great influence in both the transverse tension and transverse compression responses.The ultimate strength increase with an increase in interface strength,meanwhile,the free-edge effect-induced deformation can be reduced.It can be inferred that the free-edge warping of 2DTBC during transverse loadings is a kind of elastic deformation,which is related to the braided architecture and dimension of the unit cell but is independent of the properties of the component materials or interface.In addition,the failure behavior of angle-ply composite is much different from the braided composite.The damage concentrates on intralaminar or interlaminar,which is difficult to propagate along the thickness direction of the specimen.(2)The failure mechanism of 2DTBC under axial and transverse dynamic compression loadings are studied through carrying out the bidirectional symmetric dynamic compression tests by using the electromagnetic Hopkinson Bar system,as well as conducting the quasi-static compression tests under two different strain rates.From the results,we can found that both the axial and transverse compression strength are strain rate sensitivity.The axial compression strength is consistent under two different quasi-static strain rates 10-4/s and 10-2/s,but increasing significantly while the strain rate goes up to 200/s?300/s.However,the transverse strength increases linearly with the increase in strain rates.Apart from these,the failure mechanism is much different between axial and transverse compression under dynamic loadings.The failure during axial compression results directly from the micro-buckling and further breakage of axial tows,after which the material will lose its bearing capacity immediately.On the contrary,no significant fiber breakage is observed during the transverse compression,instead of this,the accumulation of interface failure between fiber tows and matrix lead to the final shear type failure through the thickness of the specimen.(3)The high-speed impact experiments for the 2DTBC and angle-ply composite panels are conducted by using the single-stage gas gun.Meanwhile,the high-speed cameras are utilized to obtain the deformation process by adopting the 3D digital image correlation(DIC)technology,and the flight speed and impact attitude of the loaded projectile also can be monitored.After that,the nondestructive testing(NDT)include ultrasonic C-scan and X-ray computed tomographic are employed to further inspect the failure modes of composite panels after impact.The test results reveal that the 2DTBC panel has excellent impact resistance and characteristic of crack arrest.The NDT results illustrate that the fiber tows around the impact region on impact face are cut off with the projectile,the cracks in fiber tows and matrix propagate along the axial and transverse direction of the impact face.On the back side,the major failure modes include tensile fracture and matrix cracking in fiber tows,as well as the crushing failure of the matrix material.The cracks initiated from the impact region and extend along axial and bias directions.In addition,the destruction areas concentrate around the impact region,and no global deformation of the 2DTBC panel is found.But for the angle-ply composite,the impact of projectile caused the extensive deformation and delamination.Matrix cracks on the back side of target propagate along fiber direction up to the constrained region.The primary failure modes in the target panel are fiber shear fracture and delamination between each layer.(4)A multi-scale modeling framework for the impact damage simulation of2DTBC is established,which contains a micromechanical model,meso-scale finite element model,and macro-scale subcell model.At the same time,a meso-macro homogenization approach is proposed to precisely obtain the equivalent performance of each component in the subcell model.The comparison between numerical simulation results and high-speed impact experimental results fully illustrate the feasibility and accuracy of this multi-scale numerical modeling approach.The further investigation of the failure behavior of 2DTBC panel under impact loadings reveals that the whole impact process can be divided into three different stages.In stage?,the projectile slowed down rapidly with the initial deformation of the target panel.The damage accumulation in the composite panel and a large proportion of the energy transformation have accomplished in this stage.The maximum deformation of the target panel appears in stage II,and the projectile will penetrate the panel or stopped by the panel and turn back.In stage III,the projectile perforates the panel or rebound with the recovery deformation of the panel.The kinetic energy of projectile and internal energy of the target panel are going to stabilization,the impact process is finished with the end of this stage.Furthermore,the influence of the yaw angle of projectile before the first contact with the target is discussed with the help of the fully validated macro-scale subcell model.It can be concluded that the probability of penetration increases with the increase in yaw angle of the projectile under consistent initial speed,and stepped influences of increased yaw angle on impact process are observed.However,the impact process will not be significantly affected while the yaw angle is less than 2°. |
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