Study On The Interface Structure And Properties Of Epoxy-based Composite Co-cured Bonding

Advanced resin matrix composites offer a range of advantages such as high specific strength,high specific stiffness,and high design capability.In the U.S.,composites are listed as the top four structural materials for aircraft,along with titanium alloys,highstrength steel,and aluminum alloys.Years of research and application practice have proven that resin matrix composites have been widely used in the aerospace field for components such as aircraft engines and helicopter tail fins.Composites are severely damaged and weakened when they are cut or machined,and their interlayer shear becomes more fragile.Therefore,the joining method of composite materials is mostly bonding.According to the different molding processes,the form of bonding is generally divided into the following three:(1)secondary gluing(2)co-bonding(3)co-curing.Cocuring bonding generally refers to two or more connected composite parts,in a curing cycle at the same time to complete the curing molding and bonding into a whole part of the process method.It is increasingly studied because of its short molding cycle and high material accuracy.At present,domestic and foreign research on co-curing is more to explore the feasibility of the molding process,as well as the molding process after the bonding strength and deformation.For its existence in the co-curing bonding interface chemical structure evolution,interface defects and other effects on the overall structure of the study is less.To address these problems,we designed a method based on micro-infrared test to study the co-curing bonding interface based on an epoxy resin adhesive bonding system of epoxy-matrix carbon fiber composites.The selected epoxy resin adhesive is composed of bisphenol A epoxy resin,4,4'-diaminodiphenylsulfone(DDS)and dicyandiamide(DICY),and the epoxy resin prepreg is T300 grade epoxy prepreg.The intrinsic properties of the used materials,such as curing kinetic parameters,thermodynamic analysis,and thermogravimetric analysis,were first investigated by differential scanning calorimetry(DSC)and rotational rheology(DHR),dynamic thermodynamic analysis(DMA),thermodynamic analysis and thermogravimetric analysis.The properties such as curing kinetic parameters,thermal properties,and mechanical properties were characterized.Afterward,the chemical and physical structures of the co-curing and cemented interfaces were investigated using micro-infrared(Mirco-IR)in combination with scaning electron microscopy(SEM)and atomic force microscopy(AFM).In combination with the results of the study,the process of the evolution of the interface was investigated by Fourier infrared spectroscopy(IR),combined with numerical simulation analysis of the differences in the base of the heat source within the interface,resulting in differences in the temperature field and differences in the curing degree field.Finally,its mechanical properties at different temperatures and different curing processes were tested as a verification and evaluation method.The final results were obtained by DSC,DHR,and DMA for the epoxy resin adhesive bonding epoxy-based carbon fiber composites in this paper as an example,and the curing temperature of the DDS system was lower than that of the DICY system,but the reaction rate of the DICY system was faster.The sulfone group curing agent in the prepreg was found to migrate and form an interfacial layer during the curing process by Mirco-IR,SEM.In combination with AFM,this migration was found to peak at the submicron stage.By means of a combination of IR and numerical simulation calculations,the following conclusions were found: when the heating temperature was between room temperature and 130°C,the migration of the sulfone curing agent in the prepreg to the adhesive layer occurred due to heating,resin softening,and viscosity reduction;as the temperature continued to increase to 150°C,the migration formed an interfacial layer that underwent a curing reaction while curing significantly faster than the prepreg and adhesive layers.At the same time,due to the fasting curing speed,too fasting gelation leads to interfacial defects and weak interfacial layer,while the local crosslink density in the interfacial region is too large.These defects resulted in poor performance of the composite bonding specimens in the final shear test.In summary,the structural composition of the interface was studied by Micro-IR,SEM and AFM,combined with IR and numerical simulations to investigate the interfacial evolution process,and the mechanical properties were tested to finally establish an analytical characterization method for the interfacial layer of heterogeneous epoxy bonding,which was linked to the interfacial bonding rheology and the final bonding properties of the material.It provides a new research idea for future scholars on the interface composition and interface design of heterogeneous epoxy bonding.