Properties Study Of Carbon Fiber/Epoxy Composites Toughened By Multi-Scale Interleaves

Carbon fiber reinforced polymer composites（CFRP）have been widely used in aerospace,rail transportation,armor protection,new energy and sports due to their high specific strength,high specific modulus and designability.However,the thin resin region between the plies of CFRP laminates leads to its low interlaminar fracture toughness and susceptibility to delamination damage,which seriously limits the further development of CFRP.Therefore,how to improve the interlaminar fracture toughness and impact damage resistance of CFRP has become the focus of research by scholars at home and abroad.The interlaminar toughening technology based on"controlled interlaminar phase"can improve the interlaminar fracture toughness of CFRP without changing its molding process.In recent years,more research has focused on interlaminar toughening at multiple scales to achieve significant improvements in the interlaminar fracture toughness of CFRP.However,the effects on the strength,elastic modulus and glass transition temperature（T_g）of CFRP have been neglected and the synergistic effects of multi-scale hybrid systems are unclear.Natural structural materials,such as bone and nacre,effectively combine high strength and toughness via multiple toughening mechanisms at different scales.The fracture resistance can be divided into an intrinsic（plasticity）toughening mechanism ahead of the crack tip and an extrinsic（shielding）toughening mechanism behind the crack tip.In this paper,inspired by the multi-scale intrinsic and extrinsic toughening mechanism of natural structural materials,a hybrid toughening system is constructed in the interlaminar region of CFRP.The plastic zone around the crack tip is enlarged by cavitation and tensile deformation of nanoscale core-shell rubber（Nano CSR）and microscale core-shell rubber（Micro CSR）to restrain crack initiation and propagation.At the same time,the local stress and strain fields at the crack tip are reduced by bridging and pulling out of carbon nanotubes（CNT）and short carbon fibers（SCF）in the area where the crack passes,which resists the crack propagation and causes the crack-resistance curve（R-curve）to rise.The generated synergistic effects of the above multi-scale hybrid material are aimed at improving the interlaminar fracture toughness of CFRP and increasing its resistance to low-velocity impact damage without losing its strength,elastic modulus and T_g.The main work in this paper is as follows:（1）SCF/Nano CSR multi-scale interlaminar toughened CFRP laminates are prepared by using nanoscale Nano CSR and microscale SCF as raw materials.Firstly,based on the results of fracture toughness,tensile and flexural tests of the Nano CSR modified epoxy resin system,the optimum addition amount of Nano CSR was determined to be 12 wt%.Secondly,short carbon fiber veils（SCFV）are prepared by using cellulose solution dispersion and screen filtration.And the optimal surface density of SCFV is 20 g/m² by double cantilever beam（DCB）test of SCFV interlaminar toughened CFRP.Finally,a 20 g/m² SCFV/Nano CSR-12 hybrid interlayer is inserted between CFRP interlaminar to prepare interlaminar toughened CFRP laminates.The experimental results show that the interlaminar fracture toughness toughening effect of the multi-scale hybrid material system is superior to that of both Nano CSR and SCF for the individual constituents.Compared with the unmodified specimens,the mode I critical strain energy release rate(G_IC-C)and mode II critical strain energy release rate(G_IIC-C)of the multiscale interlaminar toughened CFRP are improved by 127%and 154%,respectively,and the CFRP strength,elastic modulus and T_g are almost unaffected.This is due to the nanoscale intrinsic and microscale extrinsic toughening mechanisms invoked by the Nano CSR and SCF surrounding the crack tip at different scales,respectively,and the interaction of multiple mechanisms shows coupling effects.On the one hand,the SCF promotes the formation of echelon cracks and creates a larger effective surface area of the resin matrix,which increases the pore distribution density of Nano CSR plastic void,indicating the promotion effect of SCF on Nano CSR.On the other hand,the high modulus SCF has a restraining effect on the plastic deformation of the matrix caused by the Nano CSR voids.SCF and Nano CSR play a leading role in different stages of crack propagation,which determines the synergistic toughening efficiency.The synergistic rate of SCF and Nano CSR is gradually stabilized at 10%under mode I fracture loading.When the SCF bridging effect is limited due to the crack opening distance is not large enough,the constraint of SCF on Nano CSR dominates.When the crack propagates steadily,the toughening mechanisms of SCF and Nano CSR promote each other,exerting a synergistic effect.Under mode II fracture loading,both SCF and Nano CSR promote the generation of echelon cracks,making the synergistic effect of SCF and Nano CSR more significant,reaching about 75%.（2）CNT/Micro CSR multiscale interlaminar toughened CFRP laminates are prepared using nanoscale CNT and microscale Micro CSR as raw materials.First,based on the fracture toughness and flexural test results of the CNT/Micro CSR modified epoxy resin system,the optimum addition amounts of CNT and Micro CSR are determined to be 0.5 wt%and 8 wt%.Then,CNT-0.5/Micro CSR-8 is introduced into the CFRP interlaminar region to prepare the interlaminar toughened CFRP laminates.The experimental results show that the interlaminar toughening effect of multi-scale interlaminar toughened CFRP is superior to that of both CNT and Micro CSR for the individual constituents.Compared with the unmodified specimens,G_IC-C and G_IIC-C are improved by 123%and 79%,respectively,and without affecting the flexural properties and T_g of CFRP.Unlike（1）,CNT and Micro CSR surrounding the crack tip trigger nanoscale extrinsic toughening and microscale intrinsic toughening mechanisms,respectively.On the one hand,the introduction of CNT and Micro CSR into the CFRP interlaminar region promotes the generation of interfacial echelon cracks,reflecting the positive promotion effect of CNT on Micro CSR.On the other hand,CNT traverses between the Micro CSR voids to limit the tensile deformation of the voids,reflecting the constraining effect of high modulus CNT on Micro CSR.Under mode I fracture loading,the interaction between CNT and Micro CSR results in a stable synergistic rate of-10%.Under mode II fracture loading,the CNT bridges across the cracks generated by the shear stress,which delays the interlaminar crack propagation.The synergistic rate of CNT and Micro CSR is about 29%,indicating that the synergistic effect between CNT and Micro CSR is more significant under shear stress.（3）Low-velocity impact tests are conducted on 20 g/m² SCFV/Nano CSR-12 and CNT-0.5/Micro CSR-8 interlaminar toughened CFRP laminates and the results are compared with unmodified specimens.The results show that multi-scale interlaminar toughening improves the interlaminar fracture toughness of CFRP and significantly enhances the resistance of CFRP to low-velocity impact damage.The results show that the energy-time curves of 20 g/m²SCFV/Nano CSR-12 and CNT-0.5/Micro CSR-8 specimens have a rebound area when the impact energy is 15 J,and indicate that the specimens have been not penetrated.With energy absorption values of 13.5 J and 12.2 J,respectively,while the unmodified specimens have been penetrated.At the same energy impact energy,the 20 g/m² SCFV/Nano CSR-12 specimens have smaller damage area and shallower crater depth,showing better resistance to low-velocity impact damage.However,the CNT-0.5/Micro CSR-8 specimens have a weaker shear damage resistance and severer local impact damage,which may be related to its lower G_IIC-C.