Experimental Study On Impact Properties Of Carbon-aramid Hybrid Two-dimensional Braided Composites

In order to make full use of the advantages of the 2D braided fabric,such as high production efficiency,less fiber twist,less fiber damage and flexible tailor-ability,carbon-aramid hybrid structures were designed,trying to improve the impact resistance for composite material.The carbon and aramid 2D braided hybrid layer composite materials are first as the research object.This project mainly did experimental studies on impact behaviors and compression behavior after impact of 2D braided hybrid laminated composites.The low-velocity impact response of hybrid laminated composites based on the 2D braided fabrics were investigated experimentally.The following three different types of carbon-aramid/epoxy hybrid laminates were produced and tested,(a)interply hybrid,(b)sandwich-like hybrid and(c)unsymmetric hybrid.Non-hybrid carbon laminates were also tested for comparison.The effects of the hybrid structure on the impact properties such as the peak load,the ductility index(DI)and internal damage were discussed.The internal damage and failure mechanisms were analyzed from the C-scan imaging and B-scan imaging.We can preliminary conclude the impact resistance of composites.The non-hybrid carbon laminates has the highest impact peak load,while the DI values of hybrid laminates are higher than those of non-hybrid carbon laminates.For those three different hybrid laminates,the interply hybrid laminate has the highest peak load and the highest DI value.The internal damage and failure mechanisms were analyzed from the C-scan imaging and B-scan imaging.The carbon laminates show the largest depth of damage.The damage depth of interply hybrid laminates is lower than that of carbon laminates,while the damage in impacted surface is more than that of carbon laminates.There are severe delamination cracks in sandwich-like hybrid laminates and internal delamination damage occurs in unsymmetric hybrid laminates.It can be concluded that placing of high stiffness carbon fabric at highly stressed regions as reinforcement would result in enhanced properties,and the damage tolerance performance of composites with interply hybrid structure are better than those of other hybrid composites.Further,compressive test and compressive after impact test were carried out.And digital image correlation(DIC)was used to measure the deformations and strains on the samples.By comparing before and after impact compressive properties,we can conclude better specimens with optimum impact resistance properties.Compression results show that though the all specimens are subjected to only compressive loads,but both compressive and rensile strains are present in the all specimens.This is due to the occurrence of large out-of-plane displacement at higher load levels as it is not possible to perfectly maintain the pure compressive loads at higher deformations.At higher axial deformations,the applied load will be eccentric and produces direct strain as well as bending strain.The strain distribution of carbon laminate is mainly bend tensile strain,however,the strain distribution of other three kinds of hybrid laminates are mainly compressive strain.It is indicated that the contribution of aramid fiber in resisting bending strains.And fiber hybrid could toughen composite effectively.By comparing before and after impact compressive properties,it is found that the carbon laminate has the best compression after impact behavior and is most insensitive to the low velocity impact damage,exhibiting the higher impact damage tolerance.In the three kinds of hybrid laminates,the interply hybrid laminate has the lest loss rate of compressive strength,bending tensile strain.It is indicated that interply hybrid structure could provide better resistance to elastic deformation and failure resistance.