The Interfacial Chopped-fiber Toughening And Fracture Mechanisms Study For Sandwich Structure Composites

Fiber-Reinforced Polymer(FRP)has a high ratio of modulus,ratio strength,excellent fatigue performance and corrosion resistance,so that it has been widely used in aerospace,marine,construction and transportation.Compared with traditional materials,it has the defects of low residual strength and low energy absorption during and after impact.The sandwich structure composite material prepared by placing the foam core material between two layers of the joint plate can solve above problems.Meanwhile,the appropriate selection of skin and core material can give the sandwich structures additional functionality,such as high energy absorption,low thermal conductivity and flame retardant,which makes the sandwich has a broader development prospect.However,the use of thermosetting resin as an adhesive result in brittle surface-core interfaces.Under the external load,the interface prone to local deformation and crack propagation,resulting in interfacial delamination,which in turn makes the sandwich structure fail as a whole.Therefore,this work has developed a series of interlayer toughening methods based on chopped fibers.The toughening effect is described by the characterization of interlayer toughness and overall mechanical properties.On this basis,the failure process of the sandwich is analyzed through the observation of macro,detailed and micro levels,and the action mode and the mechanism of the toughening phase are expounded.The main research work in this paper is as follows:(1)The preparation method of foam sandwich structure composites with chopped fibers toughening interfaceThe traditional sandwich structure was prepared by bonding skin and core together,which is not only complicated and time-consuming,but also introduces the quase-delamination layers.The quase-delamination layers are prone to local bending and deformation in the process of re-service,which results in the early failure of the whole structure.In this work,glass fiber/epoxy composites are selected as skin,and the liquid phase flow extension method to prepare chopped fiber films in advance placed at the surface-core interfaces,the use of a dip,one-curing molding process.Based on the tests of differential scanning calorimetry,viscosity-time curve and gel time,control the resin viscosity of flow molding,improve the device,and prepare foam core sandwich composites with short fibers interfaces.(2)Mechanical behavior of foam sandwich structure compositesTo quantify the effectiveness of the chopped fibers of interfaces,the types of chopped fiber,fiber surface density,fiber dispersion and other factors on the mechanical properties of the overall sandwich structure were studied by using quasi-static three-point bending and pendulum impact test.The experimental results show that the introduction of interfacial fiber can significantly improve the peak load and energy absorption of the sandwich structure,maintain the integrity of the structure and inhibit the delamination of the skin-core interface.By comparing the effects of the parameter changes on the mechanical properties,the conclusion is drawn that the existence of the optimal fiber surface density makes the sandwich structure show the best comprehensive performance.At the same time,when the dispersion of fibers is improved,the value of the optimal surface density is increased.The bending strength,energy absorption during the bending process and impact toughness of the sandwich sample with 20 g/m2 interface chopped glass fiber can reach 65.18 MPa,18.16J,and 226.34 J/m2,respectively,which are increased of 109%,184%and 47%respectively compared with the blank sample.(3)Failure mode and toughening mechanism of interfacial chopped fibersThe fracture toughness of the skin-core interface is characterized by the asymmetric double cantilever beam.The relationship between interfacial toughness and the overall mechanical properties of the sandwich structure is studied.The failure mode of chopped fiber in the interface crack growth process was studied through macro shooting,optical microscope and scanning electron mirror,and the "bridge link" toughening mechanism with stripping,pulling out and the fracture of the original function as the main absorption method was determined.Considering the competitive relationship between various energy absorption forms and the random orientation of fiber,a microscopic analysis model of chopped fiber is established.In addition,the entanglement and mechanical self-locking between chopped fibers can lead to a decrease in the efficiency of the toughening,which puts higher demands on the interfacial chopped fibers to maintain good dispersion during resin flow molding.Through contact angle testing,determining the fiber-resin interface binding ability is the main factor in determining the strength of the effect.(4)Synergy mechanism of interfacial chopped fiber/carbon nanotube(CNT)Based on the conclusions of the above-mentioned study,the core structure surface-core interface is strengthened by using CNT attachment interface chopped fiber.The experimental results show that CNT can not only increase the surface area and surface roughness of short fibers,but also introduce a large number of active groups,which significantly improves the binding strength of short fibers and resins.On the other hand,along with the resin flow in the molding process,CNT diffuses to the entire surface-core interface,forming a CNT/epoxy reinforcement system.The critical energy release rate of crack propagation of the sandwich sample with 1.5 wt%CNT-loaded chopped interfacial glass fiber reach 681.85 J/m2,which are increased by 35%and 402%compared with the sample with interfacial fiber but without CNT and the blank sample,respectively.Microscopic observations show that a small amount of CNT reunion can strengthen the connection effect between short fibers and construct a micron-nano-scale "bridge" effect.