Damage Crack Development Of 3-D Braided Composite Under Impact Loading

Three-dimensional（3-D）braided composites are the frequently used engineering structure materials for aircraft and high-speed vehicles design.Impact loadings are often occurred in the service life of the structures.Impact damage morphologies of 3-D braided composites are the key contents of characterizing the impact-resistant behaviors.Damage crack initiation and development are one of the main topics in damage morphology investigation.Herein we study the relationship between crack distribution,propagation and the composites mecrostructure,impact loading consitions,after the composites was undergoing impact compression.The finite element method（FEM）was employed to unveil the impact damage mechanisms.The main research work sections are as follows:（1）Three-dimensional four-directional and three-dimensional five-directional braided composites were prepared using vaccum assisted resin transfer molding（VARTM）technique.Dynamic compressive tests were conducted by a split Hopkinson pressure bar device at strain rates from 500 to 800s^-1.Stress-strain curves were obtained to analyze the dynamic compressive mechanical properties of 3-D four-directional and 3-D five-directional braided composites.（2）The dynamic deformation process of 3-D four-directional and 3-D five-directional braided composites was recorded by high-speed camera.Optical microscope was employed to observe the final distribution morphology of cross-sectional cracks.They are used to analyze the influences of the braiding structure and loading conditions on the distribution and development of cracks.（3）The geometric models of 3-D four-directional and 3-D five-dimensional braided composites full-scale mesostructure level were established to calculate the impact loading damage of 3-D braided composites.The mechanisms of crack generation were analyzed by comparing the results from finite element analysis（FEA）with those in experimental.The thesis conclusions are:（1）Impact compressive stress-strain curves:The compressive stress-strain curves of the 3-D four-directional braided composites show the linear behavior in the initial stage,and then change to nonlinear until the stress wave is unloaded.Compressive modulus,maximum stress and strain increase when strain rate increases.The 3-D five-directional braided composites exhibit bilinear in the initial stage of the curve,and then become nonlinear until the stress wave unloads.The maximum stress and strain increase with the increasing strain rate,while the compressive modulus has no obvious change.（2）Dynamic deformation process:Compressive deformation mainly occur in the 3-D four-directional braided composites during the dynamic impact loading.At 60μs,the composites begin to crack.As the loading continues,the number of cross-sectional cracks develop rapidly.Crack length gradually increase to the length of the cross-section of the yarn.The crack propagation was not obvious.Shear deformation mainly occur in the 3-D five-directional braided composites.The cracks begin to generate at 80μs.As the loading goes on,the yarns and resin slip-shifted along the45°direction showing shear deformation.The number and distribution range of cracks gradually increase.（3）Crack distribution:Braided structure and strain rate have a significant effect on the crack distribution in the 3D braided composites.In the 3-D four-directional braided composites,the higher the strain rate,the more serious the interface cracks between yarns and resin.The crack distribution becomes uniform.The shape and length of crack does not change with strain rate.As the strain rate increases,the number of cracks and the width of shear band increases in the 3-D five-directional braided composites.（4）Results of numerical calculation:It is observed that the deformation of yarns is not equal to that of the epoxy resin.The yarns deformation is far less than the resin deformation under relatively high strain rate,which causes the interface cracking between yarns and resin.The bending deformation and stress of braided yarn in 3-D four-directional braided composites is less than that in 3-D five-directional braided composites.The longitudinal stiffness and strength increase as the existence of axial yarns in 3-D five-directional braided composites.The higher axial stiffness makes the braided yarns more easier to bend twords the direction without axial yarns and to form shear deformation.