Fracture Toughness And Adhesive Properties Of Epoxy Based Nano-composites

Epoxies are a kind of cross-linked thermoset with excellent mechanical properties,and have been widely used in many different engineering fields especially as matrics for carbon fiber reinforced epoxy composites(CF/EP)for aircrafts.However,their moderate strength and low toughness are limiting their more widespread applications.The poor delamination resistance has seriously limited many practical applications for CF/EP composites.Over the past 2-3 decades,many techniques have been developed to improve the bulk and interlaminar fracture toughness for epoxy matrics and CF/EP composites.A lot of previous results have shown that the toughness of bulk epoxies can be improved by incorporation of various fillers.However,the transfer efficiency from bulk fracture toughness to CF/EP interlaminar fracture toughness was not high.The interlaminar fracture toughness of CF/EP should be further improved.Meanwhile,the widely applications of epoxies are due to its high adhesive strength.The effects of moisture and cyclic loading on the epoxy composites adhesive are still not clear.The main objectives of this thesis are using fillers(SCF,graphene,CNT-SCF)to improve the fracture toughness of bulk epoxy and CF/EP composites,and study the influence of hygro-thermal treatment and cyclic loading on epoxy-based composites adhesive.Using the short carbon fibers(SCFs)and graphene improving the bulk toughness of epoxy matrix,respectively.The functionalized graphene sheets were fabricated by a simple thermal reduction method in air at 700?.The fracture toughness and fracture energy were increased by 142%and 367%by adding 2.5wt.%SCFs into epoxy matrix.The incorporation of 1.0 wt.%of graphene into epoxy was found to significantly improve the matrix fracture energy by?150%and reduce the thermal expansion coefficient by-30%.The work in this chapter is the basis of the latter two chapters.Hierarchical short carbon fibers(SCFs)synthesized with carbon nanotubes(CNTs)were used as CNT-SCF interleaves to increase the mode I delamination fracture energy(GIC)of CF/EP composite laminates,and further improve the toughness transfer efficiency from bulk SCF/epoxy to CF/EP composites.Firstly,the functional CNTs were grafted onto SCFs by flame synthesis method.Then the SCFs with or without grafted CNTs were used as interleaves in CF/EP laminates.By comparing the toughening results,the full toughening effect of SCFs in bulk epoxy cannot be effectively transferred to the laminates during delamination growth.After grafting CNTs on SCFs,even at a relatively low CNT-SCF areal density,1.0 mg/cm2,GIC(1.17 kJ/m2)was increased by 125%compared to the control laminates(0.52 kJ/m2),which is a very high value compared to those results obtained by other interleaving methods in CF/EP laminates.Additionally,positive synergistic toughening effects were found in these CNT-SCF interleaved CF/EP laminates.SEM examination confirmed the synergistic toughening was due to the improved adhesion between SCFs and epoxy as well as CNTs pullout.Graphene/epoxy was used as interleaves in CF/EP composites to achieve a further improvement in the interlaminar fracture resistance.Additionally,the damage sensing capability of the graphene-interleaved CF/EP composites was proven.Partially-cured graphene/epoxy composites were used as interleaves in CF/EP laminates and co-cured.Due to the improved adhesion between carbon fibers and the graphene-interleaved matrix,and failure mechanisms associated with and promoted by the graphene sheets,the graphene/epoxy interleaves increased significantly the delamination resistance by 140%relative to the baseline laminates without interleaves.Moreover,the damage sensing capacity of the graphene interleaved CF/EP composites with a simple electrical response method was demonstrated,where the electrical resistance change increased almost linearly with crack increment.Using the nano-fillers to improve the epoxy adhesive strength,and studying the effect of hygro-thermal treatment and cyclic loading on the adhesive properties.Based on the previous studies on the mechanical properties of bulk epoxy incorporated with nanofillers,in this chapter the effects of three kinds of nanofillers on the adhesive strength of epoxy were studies.The results showed that all the fillers had positive effect on the performance.Additionally,the effects of nano-silica on the adhesive properties of epoxy were systematically studied under quasi-static and cyclic loadings when the adhesive joints before or after hygro-thermal treatment.The results showed that even after hygrothermal treatment,the benefit of having nano-silica in neat epoxy on the adhesive joint strength was retained.In cyclic fatigue,without hygrothermal aging,nano-silica/epoxy adhesives had longer lifetimes than neat epoxy;but after hygrothermal treatment,they had similar lifetimes for given stress amplitudes.