Microstructure Mechanisms On Transverse Impact Deformation And Damage Of 3-D Braided Composites

Three-dimensional(3-D)braided composites have the geometrical structures features of preform structure integrity and can design the different distributions of braided yarn trajectories.The braided composites have widely applied to manufacture the impact-resistant engineering structures,especially complex-cross-section structures.Nowadays the braided composites have successfully used to in aircraft,high-speed vehicles.How to establish the relationship among microstructure,constituent material properties and impact resistance behaviors is an important issue for the braided composite application and the engineering structures optimization.Herein we studied the relationship between transverse impact deformation,damage characteristics and composites microstructure.The transverse impact deformation process and failure mechanisms were unveiled from finite element analyses(FEA).We focused on:(1)The testing of transverse impact behaviors of the 3-D braided composites with modified split Hopkinson pressure bar(SHPB)apparatus.The effects of impact gas pressures,braiding angle and axial yarns on transverse impact mechanical properties would be analyzed.(2)Observations on the transverse impact deformation and damage evolution.The impact deformation development was recorded with high-speed camera.(3)Establishment of a unit-cell geometrical model of the 3-D braided composites.The impact deformation and damage developments would be numerically calculated with FEM and compared with those in experimental.The damage mechanisms were unveiled from the finite element analyses results.We found that:(1)The multiple transverse impact phenomena induced by multiple incidence and reflection of the stress wave in the input bar.During the first three transverse impacts,the peak load decreased gradually.The impact energy was absorbed from the deformation and damage of the braided composites coupons.The energy absorption would lead to the decrease of the peak value.(2)The axial yarns in five-directional braided composites will increase the stiffness,strength and impact-resistance.Compared with four-directional braided composites,the five-directional braided composites’ failure modes are resin cracks and interface debonding.For the effects of braided angle,the damage modes are yarn’s shear breakage with brittle behaviors for lower braided angle composites,and transverse breakages with ductile behaviors for higher angles.(3)Finite element analyses(FEAs)at the unit-cell level manifest that the braided angles and braided yarns’ spatial distributions are the key parameters influencing the impact behaviors.To adjust the axial yarns and the braided angles will change the impact behaviors significantly.