Low-velocity Impact Damage Characteristics Of Three-dimensional Braided Composites With Yarn Reduction

Three-dimensional braided composites are ideal candidate materials for key components of impact resistance of aerospace major equipment due to excellent mechanical properties,designability and near-net forming ability.Usually,aerospace engine nozzle,aero-engine combustion chamber,space shuttle radome and other components are conical rotating bodies,which need to be realized by adding yarn or reducing yarn technology.However,adding /reducing yarn is easy to change the local fiber interlaced structure,resulting in stress concentration under impact load,which induces damage.Meanwhile,it is difficult to match material-structure performance,which seriously threatens the safety of use.At present,the existing research mainly focuses on the ’ material parts ’ with periodic cell structure.The theoretical research on three-dimensional special-shaped braided composites lags far behind the engineering application.Therefore,in this paper,the three-dimensional braided conical body is used as the traction,and the typical yarn reduction characteristics are extracted.The mechanical properties and damage mechanism under impact load are studied by means of experiment and numerical calculation to clarify the mapping of yarn reduction and impact failure,so as to improve the structural efficiency and reliability of three-dimensional braided special-shaped components.The main research contents include :(1)Four-step process method was used to form three-dimensional five-directional(3D5d)braided yarn reduction by adjusting the number of units with T700-12 K carbon fibers.3D5d braided carbon fiber preforms with and without reducing yarns at 20°,30° and 40° braiding angles were designed and prepared.On this basis,3D5d braided carbon fiber / epoxy resin composites with different parameters were prepared by resin transfer molding(RTM)process with E-51 epoxy resin.(2)The impact mechanical properties of 3D5d braided composites with and without reducing yarns of 20°,30° and 40° braiding angles were studied by using INSTRON drop hammer low-velocity impact device under 30 J,65 J and 100 J.The results show that the yarn reduction will lead to significant decrease in the peak load and response time.At the same time,compared with 20° and 30°samples,40° sample shows faster response speed(1.148ms),lower peak load(9.863 k N)and deformation value(6.925 mm).Compared with the impact energy of 30 J and 100 J,the 30° sample has the largest rebound kinetic energy(21.53J)and the highest peak load(12.84 k N)at 65 J impact energy.(3)The failure modes and damage distribution visualizations of 3D5d braided composites with or without yarn reduction under low-velocity impact load were studied based on Micro-CT.A threshold-based damage extraction method was proposed to realize the identification of damage in different impact areas and clarify the spatial evolution mechanism of low-velocity impact damage.The results show that the sample with yarn reduction has smaller damage area and larger damage depth,and the damage is concentrated at the impact center.The synchronization and consistency of the damage are affected,showing certain randomness and asymmetry.At the same time,the low-velocity impact spatial damage of the three-dimensional five-directional braided composites is elliptical,and the dominant damage modes are fiber damage,matrix crack and interface debonding.(4)Based on the Micro-CT scanning results,the multi-scale geometric models of 3D5d braided composites with and without yarn reduction were reconstructed.The constitutive model including the initial damage criterion and damage evolution of the material was established.The UMAT subroutine for unit cell strength calculation and the VUMAT subroutine for macro strength calculation were written respectively.The damage distribution characteristics under low velocity impact load were simulated with ABAQUS software platform.The results show that the model can successfully capture the low-velocity impact damage distribution of 3D5d braided composites with and without yarn reduction.Whether in terms of surface impact damage or three-dimensional damage distribution,the low-velocity impact damage characteristics with yarn reduction are in good agreement with the experimental results.Moreover,the full-field stress distribution of low-velocity impact well reflects the characteristics of fast response and discontinuous plane stress transmission in the sample with yarn reduction.The high agreement between the simulation results and the experimental results verifies the effectiveness of the simulation method.(5)The residual compressive strength of 3D5d braided composites with and without yarn reduction after low-velocity impact was tested by universal testing machine and digital image correlation technology(DIC).The load-displacement curve,residual strength and damage mode were obtained.The relationship between the percentage of residual compressive strength and the size of low-velocity impact damage was discussed.The results show that compared with the sample without yarn reduction,the sample with yarn reduction has higher compressive load after impact,larger percentage of residual compressive strength and smaller fracture displacement.With the increase of the braiding angle,the influence of the yarn reduction structure on the residual compressive strength is gradually weakened.The percentage of residual compressive strength has a linear negative correlation with the length and width of low-velocity impact damage,and has a nonlinear negative correlation with the depth of pit,damage area and damage volume.In terms of damage mode,the compressive damage is initiated at the impact damage and propagates to both sides to produce transverse cracks.The effect of yarn reduction structure on damage mode is not significant.