Effects Of Process Parameters On Curing Residual Stress And Properties Of Interlaminar Hybrid Composites

Interlaminar hybrid composite is a new type of composite material developed on the basis of advanced composite materials.It is made of two or more fibers as reinforcements and compounded with the same resin matrix.It combines the advantages of two fibers and has strong designability.At present,interlaminar hybrid composites are more and more used in aircraft wing and tail structure and hull structure.However,in the curing process of interlaminar hybrid composites,the mismatch of thermal expansion coefficients between composites with different fiber systems,fibers and resins and the chemical shrinkage of resins lead to the generation of curing residual stress.Large residual stress will cause the deformation of the parts and affect their mechanical properties.The generation of residual stress is inevitable.How to minimize it is a key problem,which requires accurate evaluation of residual stress and corresponding process optimization methods.In this paper,aiming at the problem of residual stress control in the curing process of hot pressing tank of carbon/quartz interlaminar hybrid composites,the evolution law of strain in the curing process of interlaminar hybrid composites is explored by using the sensor combination mode of "fiber bragg grating+thermocouple",and the effects of ply direction,cooling rate and forming pressure on the strain evolution and residual stress in the curing process are studied;The mechanical properties such as bending and tensile were tested for the samples under different process parameters,and the failure mechanism of interlaminar hybrid parts under different process conditions was revealed by multi-scale characterization;On this basis,aiming at the problem that it is difficult to reduce the curing residual stress in the curing process of interlayer hybrid composites,two control methods of curing residual stress are proposed.The research work has certain guiding significance and application value for realizing high-quality and high-precision hot press curing manufacturing of large-scale interlayer composite components.The main research contents and results of this paper are as follows:(1)Using the sensor combination on-line monitoring system of "fiber bragg grating+thermocouple",the effects of ply direction,cooling rate and forming pressure on strain evolution and curing residual stress during curing of interlaminar hybrid composites were studied.The results show that the thermal expansion mismatch between carbon/quartz fiber and resin matrix leads to different inhibition effects of fiber on the thermal expansion,cold contraction and curing shrinkage of resin,resulting in significant differences in the curing residual stresses in different ply directions.The curing residual stresses of six groups of experiments with ply directions of 0°,45° and 90° of the two materials are-9.23 MPa,-26.83 MPa,-40.88 MPa,-19.17 MPa,-17.39 MPa,-37.25 MPa;The final residual stress of composite components is mainly caused by the mismatch of thermal expansion coefficient in the cooling stage.When the cooling rate decreases from 10 ℃/min to 1.5 ℃/min,the slower cooling rate can improve the stress relaxation in the cooling process of composite parts,and the maximum reduction of curing residual stress is 26.6%;When the process temperature is determined,the molding pressure becomes another main factor affecting the curing residual stress of interlaminar hybrid composite parts.A larger molding pressure increases the fluidity of the resin,making the adhesion between the fiber and the resin better,so that the fiber has a stronger inhibition on the resin expansion.When the molding pressure of the parts increases from 0 MPa to 0.6 MPa,the maximum reduction of the curing residual stress is 17.4%.The research results provide important theoretical support for the formulation of low residual stress horizontal curing process scheme of interlaminar hybrid composites.(2)In view of the unclear influence of different process conditions on the mechanical properties of interlaminar hybrid composite parts,the failure mechanism of interlaminar hybrid parts under different process conditions was revealed by means of ODM and SEM.The results show that when the curing residual stress decreases from-30.88 MPa to-24.00 MPa,the maximum increase of bending strength and tensile strength is8% and 6.8%.The process induced residual stress plays a major role in matrix cracking,crack initiation and propagation.The weakening effect of curing residual stress should be considered when evaluating the mechanical properties of parts.When the forming pressure is increased from 0 MPa to 0.6 MPa,the maximum increases of bending strength and tensile strength of the parts are 42.2% and 14.7%.After the forming pressure reaches 0.4 MPa,the defects such as internal pores and delamination of the interlayer hybrid composite parts are greatly improved.(3)Aiming at the problem that it is difficult to reduce the curing residual stress of hybrid composite parts in the hot pressing curing process,this paper puts forward two control methods.One is the smart cure cycle control method with cooling and reheating process,which uses the viscoelastic mutation in the rapid cooling stage before the curing of resin matrix to release the residual stress.On the premise of consistent curing quality,the curing residual stress of parts after molding is reduced from-24 MPa to-16.72 MPa,The reduction range is 30.03%,and the corresponding bending and tensile properties are increased by 5.9% and2.2% respectively;The other is to use the vibration pretreatment process to carry out the research on the regulation of curing residual stress of parts,and use the vibration field to improve the inhibitory effect of fiber on resin expansion.After low-pressure molding,the curing residual stress of parts is reduced from-26.95 MPa to-24.35 MPa,with a reduction range of 9.6%.The performance of parts formed by vibration pretreatment is increased from 356.6 MPa,218.3 MPa and 243.9 MPa without vibration pretreatment to 484.7 MPa,308.4 MPa and 333 MPa,The corresponding range increased by 37.69%,42.23% and 38.62%.