Improved Toughness Of Epoxy And Interfacial-properties Of CFRP At Cryogenic Temperature

As an important component of rocket,fuel tank is the key container for storing cryogenic propellant.Currently,only metal is used as liquid hydrogen and oxygen tanks in a rocket.But the metal is heavy,which restricts the improvement of rocket launch efficiency.Therefore,it is very important to study the lightweight materials of the tank.The weight of the tank can be reduced by more than 30%and the launching cost can be reduced by about 37%by using lightweight and high-strength carbon fiber reinforced composite(CFRP).However,in the cryogenic temperature,liquid hydrogen/oxygen(-183/-253?),CFRP has the problems of poor toughness of matrix and debonding of carbon fiber/interface.In this paper,the mechanism of epoxy embrittlement in cryogenic temperature is analyzed.Using the graphene oxide/nanocellulose to improve the toughness of epoxy at cryogenic temperature.The mechanical properties of epoxy and CFRP unidirectional laminate under cryogenic temperature are characterized,and the change rules of mechanical properties at cryogenic temperature is discussed.The effect is discussed of the linear expansion coefficient and thermals on cryogenic properties of CFRP.The carbon fiber was modified by graphene oxide/attapulgite(GO/ATP)to realize the cryogenic interfacial reinforcement of carbon fiber/epoxy(CF/EP).In order to study the mechanical properties and evolution mechanism of CFRP at cryogenic temperature,the mechanical properties and linear expansion coefficient of epoxy and CFRP at-196?were measured.Fitting equations of strength,modulus and linear expansion coefficient variation with temperature.The unit cell of CFRP was established to predict the linear expansion coefficient and thermal stress.In the wide temperature range from room temperature to-196?,the tensile strength and modulus of epoxy increase and the elongation at break decreases with the decrease of temperature.The linear expansion coefficient of CFRP is consistent with the calculated value of the model,which verifies the rationality of the model.The mechanical properties(including transverse tensile strength,interlaminar shear strength and fracture toughness)of CFRP at cryogenic temperature are increased by39.3%,6.5%and 27.7%respectively than those at room temperature.Graphene oxide grafted nanocellulose(GO-NCC)was prepared by amidation grafting method.The morphology and chemical structure of GO-NCC were characterized by AFM,FTIR,XPS and Raman spectroscopy.The result indicates that the grafting is successful,and imply the existence of amino groups which accelerate the curing reaction of epoxy.The Tyndall effect showed that GO-NCC was well dispersed in epoxy.The linear expansion coefficient of epoxy is reduced by GO-NCC.Compared with the unmodified epoxy,the tensile strength and fracture toughness(K_IC)of 0.3wt%GO-NCC/EP were increased by 68.9%and 244%,respectively.The tensile strength and K_IC of 0.3wt%GO-NCC/EP was 39.3%and104%higher than those at cryogenic temperature.The toughening mechanism of GO-NCC was analyzed and discussed.The interfacial debonding of CF/EP at cryogenic temperature is one of the main failure modes of CFRP.In this paper,microbond test,theoretical analysis and numerical simulation are used to study the cryogenic-interface properties of CF/EP.The results show that the interfacial shear strength(IFSS)at cryogenic temperature is 16.7%higher than that at room temperature.The average strain and stress of CF/EP interface were calculated by simplifying the microbond sample into theoretical model.The thermal stress of CF/EP interface increased and IFSS increased at cryogenic temperature.The thermal stress distribution and debonding process of CF/EP interface were simulated by numerical simulation.The results show that the thermal stress concentrates on the CF/EP interface at cryogenic temperature,and the interfacial debonding first occurs at the meniscus of epoxy micro droplet.The interfacial debonding force is consistent with the measured value.Theoretical analysis and numerical simulation can be used to predict the interfacial properties of CF/EP at cryogenic temperature.Carbon fibers were chemically modified with GO/ATP to improve the wettability of CF/EP interface,increase the nanomechanical meshing between carbon fiber and epoxy,and enhance IFSS of CF/EP at cryogenic.SEM results showed that GO/ATP was grafted onto the surface of carbon fiber(CF-GO-ATP).The calculation of surface energy shows that GO/ATP improves the wettability of carbon fiber.The microbond sample of CF-GO-ATP reinforced epoxy(CF-GO-ATP/EP)was fabricated,and it increased by 53.1%compared with that of unmodified CF/EP.Similarly,the CF-GO-ATP/EP had high IFSS value after immersion liquid nitrogen and cryogenic/room temperature thermal cycling.