| Fiber-reinforced composite materials are extensively used in the fields of aerospace and automobile manufacturing and other fields due to the excellent mechanical properties.Hybrid composites are broadly defined as a composite material with the same resin matrix reinforced by two or more continuous fibers and characterized by giving full play to the mechanical properties of hybrid materials and lowering the cost of reinforcement materials.Besides,the performance of hybrid composites is closely related with the hybrid structure.In this paper,based on the advantages of flexible designability of warp knitted unidirectional fabrics,the in-plane carbon/glass hybrid structure is developed,and the warp knitted unidirectional intralayer carbon/glass hybrid fabric is designed by changing the carbon fiber content and hybrid structure.Moreover,the interlayer carbon/glass hybrid structure is realized by designing the stacking structure of warp-knitted unidirectional non-hybrid fabrics,the hybrid composite laminates are made by vacuum assisted resin transfer molding process.On this basis,the impact resistance and damage mechanism of intralyer and interlayer hybrid composites were systematically studied by experiments amd simulation.The impact resistance of intralayer hybrid composites with a variety of hybrid structures and hybrid ratio were studied through conducting the impact test.It was found that the hybrid ratio has a remarkable impact on the impact resistance in comparison to the hybrid structure.Besides,quasistatic indentation and acoustic emission experiments demonstrate that fiber fracture is the main factor leading to the failure of laminates,and the intarlayer hybrid structure can contribute to reducing fiber damage.Moreover,the delamination area is directly proportional to the glass fiber content for the three intralayer hybrid composites with various hybrid ratios.More precisely,when the carbon/ glass hybrid ratio is 1:1,the lowest damage degree and the highest load will be witnessed.When the hybrid ratio was 1: 2,the failure of the composite was mainly included the fracture failure of carbon fiber,which has the highest absorbed energy.When the hybrid ratio is 1:4,the impact stress is mainly carried by glass fiber,presenting high ultimate load.In addition,by analyzing the results of the high-speed impact test performed on the intralayer hybrid composites,it can be observed that the intralayer hybrid composites have large deformation during the projectile penetration,indicating more absorbed energy than the carbon fiber composites.When the impact velocity is higher than 50% of the ballistic limit velocity,the internal damage of the composites are manifested as fiber shear failure,fiber bundle peeling off the back face of laminates,and the intralayer hybrid synergistic effect is not obviousThe impact resistance of interlayer hybrid composites were studied.The results showed that,under low velocity impact load,the ultimate load of interlayer hybrid composites was higher,and the sandwich hybrid structure had greater energy absorption.Meanwhile,the interlayer hybrid composites were damaged due to the failure of the top fiber and the bottom delamination,and the sandwich hybrid composites experienced delamination damage of internal layers.Compared with carbon fiber composites,the damage degree was decreased,which showed a positive hybrid effect.And the penetration process of the interlayer hybrid composite shown a larger deformation than carbon fiber composites,facilitating energy absorption.The interlayer hybrid composites with carbon fiber on the outer surface had higher ballistic limit velocity and energy absorption per unit area.When the impact velocity was higher than 50% of the ballistic limit velocity,the hybrid structure had limited influence on the damage mode.By analyzing the damage of composites,it is found that the interlayer hybrid structure that is designed with the placement of higher-strength fiber on the outer surface of the impact,and the higher toughness fiber on the middle layer present a satisfactory hybrid synergistic effect,whereas the sandwich hybrid structure designed with the placement of higher toughness fibers on the surface layer and higher strength fibers on the sandwich layer has a favorable hybrid synergistic effect.Since the intralayer hybrid composites have both in-plane and longitudinal hybrid interfaces,the in-plane hybrid interfaces are beneficial to inhibit damage diffusion.In particular,the maximum inhibitory effect on damage propagation were witnessed when the carbon/glass hybrid ratio is 1:1.Besides,the longitudinal hybrid interface can be deemed as the interlayer carbon/ glass hybrid interface,which leading to a better impact resistance of intralayer composites.Finally,the impact damage model were established for hybrid composites.To be specific,a macroscopic interlayer hybrid composite model and a mesoscopic intralayer hybrid composite model were established respectively with considerations in calculation efficiency and in-layer hybrid structure.Geometric parameters of the mesoscopic model were established through the observation of the cross-sectional morphology of the intralayer hybrid composites using a metallurgical microscope,whilst a representative volume element(RVE)model was constructed for the fiber bundle to calculate macro-mechanical parameters of the mesoscopic model.Meanwhile,a progressive damage constitutive model was defined.Considering the stress change drastically during the impact process,this paper uses the strain form Hashin failure criterion to define the damage initiation of composites,and the exponential form damage factors were defined to establish the constitutive relationship between the damaged material and the intact material.In this paper,the cohesive element was added between the back fiber bundles,which accurately reflects the tearing and peeling damage of the back fiber bundles of composites during high speed impact.The finite element simulation results showed that the failure process and stress distribution of each fiber can be observed in mesoscopic model of intralayer hybrid composites,which can help analyze the damage process and failure mechanism of the hybrid fiber.The simulation results fit well with the experimental results.By analyzing the simulation results,it is found that the synergistic effect of carbon/glass hybrid under impact load are presented as follows: Carbon fiber showed the brittle failure under impact load,and glass fiber can delay the failure process of carbon fiber.The brittle damage feature of the hybrid composite under the impact load is decreased in comparison with that of carbon fiber composite.Residual stress generated from the failure of carbon fiber were converted into delamination damage through glass fiber’s deformation,which reduced the failure ratio of fiber damage inside of composites.Moreover,failed carbon fibers still contributed to the impact resistance because of wrapped by glass fiber.As carbon fibers and glass fibers have different various deformation under the impact load,the carbon/ glass interval distribution have effect to buffer the impact stress through triggering delamination damage.Also,the intralayer carbon/ glass hybrid interface functioned as an inhibitor of damage crack propagation can postpone the failure of the composite.Precisely,the closer the in-layer hybrid interface,the more beneficial to stress transfer and consumption between hybrid fibers,and the stronger the resistance of damage propagation will be.Hence,respective mechanical properties of carbon fiber and glass fiber can be fully employed to achieve a strong hybrid synergistic effect and a high impact resistance through reasonably designing the interlayer hybrid structure and the intralayer hybrid structure. |
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