| Fiber metal laminates(FMLs)are a kind of super hybrid composites composed of fiber reinforced plastics(FRPs)and metal sheets.Due to its unique advantages such as high specific strength and stiffness,good fatigue resistance and excellent impact resistance,FMLs have a wide application prospect in aerospace,automobile and other fields.However,residual stress may be produced due to the physical property differences between metal sheet and composite layer,which could reduce the interlaminar fracture toughness and then affect the overall mechanical properties of FMLs.In this paper,a synergistic toughening technology,combining interleaf and metal surface treatment,was developed to improve the interlaminar mechanical properties of FMLs.Also the toughing mechanisms were systematically investigated by the experiments and numerical simulation,which are beneficial to the development of other novel FMLs.In this paper,the interlaminar mechanical properties of glass fiber reinforced plastics(GFRP)/ aluminum alloy super hybrid laminates were studied.Aluminum sheets were treated with alkaline etching.Meanwhile,a graphene oxide(GO)interleaf was introduced between the aluminum sheet and the glass fiber reinforced epoxy composite.Finally,the FMLs were successfully prepared by hot pressing process.Synergistic effects of the interleaves and metal surface treatments on the interlaminar mechanical properties of the FMLs were investigated based on the double cantilever beam and end notched flexure tests.The obtained results show that the toughening efficiency of the interleaf are dependent on the aluminum surface characteristics as well as the GO loading.Further comparison reveals that the highest Mode I and Mode II fracture toughnesses are obtained in the specimens with alkali etching treatment and addition of GO interleaf with 0.5 wt.% of GO loading,which are 510% and 381%higher in comparison to those of the plain specimen.To uncover the interlaminar toughing mechanism,scanning electron microscope and other instruments were employed to characterize the physicochemical properties of aluminum alloy and the fracture surfaces of the tested laminates.The results revealed that the aluminum alloy surface treated by alkali etching becomes rougher,which can effectively improve the mechanical interlocking between aluminum alloy and resin matrix.Meanwhile,more hydroxyl groups were produced on the surface of aluminum plate and the surface energy of the Al alloy has been changed by the alkali etching,which are favor to improve the wettability of resin matrix and then enhance the bonding strength between the resin matrix and aluminum alloy.The fracture surfaces of the tested laminates revealed the failure characteristics are different.For the specimens of synergistic toughening,cohesive failure occurred under Mode I loading,and mixed failure under Mode II loading.For the specimens without surface treatment,the failure modes were mainly interface failure.In addition,a more irregular and rougher fracture morphology can be observed for the specimens with addition of GO,which will create a larger fracture area,and require a higher driving force and energy,and then improve the interlaminar fracture toughness of super hybrid laminates.Based on the cohesive model,Mode I and Mode II delamination behaviors of super hybrid composites were simulated by finite element method.The simulation results were in good agreement with the force vs.displacement curves obtained by the experiments,which verified the validity of the cohesive model.Based on the inversion method,the interfacial strength between composite layer and aluminum alloy was predicted by matching the initial stiffness and interlaminar fracture toughness,which further demonstrated the toughening effects of the proposed improvement strategy. |
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