Basic Research On Molding Process Of Thermoplastic Composite Stiffened Structures

Thermoplastic composites have numerous advantages except for high specific strength and modulus as their thermoset counterpart,such as high toughness,excellent impact resistance,high forming efficiency and recyclable,and becoming an important development direction of composite materials.The process route of stamping forming on the basis pre-forming laminates is mostly used for thermoplastic simple structures at present,for the complex stiffened structures(I-stiffened,T-stiffened,Lstiffened,and so on),it is difficult to achieve one-time forming by stamping process,which restricts the engineering application of such components.In this paper,the carbon fiber reinforced polyether ether ketone(CF/PEEK)composite I-stiffened structures commonly used in the aircraft panel were taken as the research object,aiming at the challenge of one-step forming caused by its complex structure,the molding process scheme of remelting bonding compound molding was proposed,through the research methods of molecular dynamics simulation,finite element modeling of molding process,and experiments,the bonding mechanism and performance differences of remelting interfaces with different pre-forming processes,the stress evolution and deformation behavior during the molding process were studied systematically,the high performance and precision collaborative manufacturing of I-stiffened structures were realized.The theoretical and technical support for the high-quality molding of thermoplastic composite complex stiffened components was provided.The main work and innovation achievements are as follows:(1)In view of the difficulty to ensure the uniformity of temperature and pressure field in the molding process of complex stiffened structures,and the process window to achieve its high-quality molding is unclear.The influence of the three processing parameters,molding pressure,molding temperature,and holding time on the interlaminar properties of CF/PEEK composite structures was studied by the combination of molecular dynamics simulation and experiment,the bonding mechanism of interlaminar interface was revealed,the matching scheme of the process parameters for the best interlaminar performance of components was determined.The results showed that a reasonable molding pressure could promote the non-bond interaction between polymer molecules,and an appropriate increase in molding temperature could promote the thermal motion of polymer molecules,the interlaminar bonding performance showed a trend of increasing-stabilizing-decreasing with the increase of molding pressure or temperature;The interlaminar properties were more sensitive to the change of temperature than pressure,the variation amplitude of interfacial diffusion coefficient and bonding energy with temperature was higher than that of pressure,which showed that the diffusion and permeation behavior of polymer molecules played a leading role in the bonding of interlaminar interface.The interlaminar performance could maintained at a high level when the molding pressure range was 1.8-2.4 MPa,molding temperature range was 400-420°C,and the holding time was not less than 20 min.The research results provided a theoretical and experimental basis for the formulation of the molding process of I-stiffened structures.(2)Aiming at the problem that the influence mechanism of the resin crosslinking reaction in the pre-forming stage on the interface bonding strength of subsequent molded components is unknown,and the overall performance of components is difficult to guarantee,the influence of different pre-forming systems on the bonding strength of the remelting bonding interface of subsequent formed parts was studied.According to whether the polymer on both sides of the remelting interface have underwent crosslinking reaction in the pre-forming stage,three interfaces including no crosslinking reaction occurs bilaterally(Non-crosslinking interface),crosslinking reaction occurs unilateral(Unilateral crosslinking interface),and crosslinking reaction occurs bilaterally(Bilateral crosslinking interface)were defined and their properties were analyzed by molecular dynamics simulation and performance tests,the influence of resin crosslinking reaction in the pre-forming stage on the subsequent remelting bonding interface performance was found out.The bonding quality of non-crosslinking interface was higher than that of unilateral crosslinking interface and bilateral crosslinking interface in turn,the interfacial diffusion coefficient was 62.8% and 101.1%,the interfacial bonding energy was 13.0% and 104.6%,and the tensile strength was 16.0%and 50.6% higher than the two,respectively.The influence mechanism of pre-forming process on the remelting interface properties was revealed:The resin will undergo crosslinking reaction and produce macromolecules with higher molecular weight than the original resin due to long time melting at high temperature in the pre-forming stage,this leaded to the resin viscosity was too high during remelting,which affected the molecular diffusion and penetration at the remelting interface,and further leaded to a low interfacial bonding strength.The function relationship of timetemperature conditions under which the resin will not undergo crosslinking reaction in the pre-forming stage was established,obtaining the timetemperature processing conditions of the pre-forming process according to the function,the interlaminar shear strength of the remelted interface generally reached the level of those formed by one-step,the impact of the pre-forming process on the remelting interface was eliminated effectively.A research foundation for the optimization scheme design of the whole molding process of I-stiffened structures was laid.(3)Aiming at the problem that the existing general model of polymer thermophysical properties(Path-dependent model)is difficult to describe the thermophysical property transition characteristics of PEEK resin accurately in the molding process,it is difficult to predict the residual stress and deformation accurately during the forming process,which leads to the difficulty of precise forming and manufacturing,the correlation law between temperature and crystallization characteristic parameters of PEEK resin was got by differential scanning calorimetry(DSC)analysis,taking the influence of crystallinity on material properties into account,a correlation model(Crystallization model)for the relative crystallinity and resin modulus was proposed in combination with the non-isothermal crystallization kinetics theory.The Crystallization model established and the existing Path-dependent model were introduced into the composite micromechanics model through the secondary development of user subroutines and embedded into the ABAQUS software,a finite element model for predicting the stress evolution and deformation behavior of Istiffened structures was established.The results showed that the internal stress of the crystallization model in the cooling process enters the high speed growth stage earlier,which made the residual stress and deformation values higher than that of the Path-dependent model,the corresponding errors of the crystallization model were 9.3% and 5.1% respectively compared with the experimental results,the simulation accuracy was higher than that of the Path-dependent model significantly.A theoretical support for the mold surface compensation design for the precise forming of such structures was provided.(4)The molding process verification experiments of I-stiffened structures was carried out,and the remelting bonding compound molding process scheme was designed and the mold assembly were developed.The results showed that when the pre-forming C-shaped parts were hot pressed at the optimized time-temperature conditions,the thermal damage degree of the material was reduced while ensuring the occurrence of interlaminar slip,and the shaping requirement of pre-forming was met;Taking the loss of molding pressure during transmission into consideration due to the complexity of component and mold assembly,numerical simulation of the pressure division of I-stiffened structures was carried out according to the process scheme,when the external pressure was 3.7 MPa,the pressure value at each position inside the component was within the recommended range of this paper(1.8-2.4 MPa),the unevenness was less than 0.3 MPa;A control-experiment was carried out between the optimized and the nonoptimized pre-forming process of remelting bonding compound molding,the interlaminar shear strength in the web area increased by 12.4%,the flange area increased by 14.4%,the maximum pull-off load increased by24.4% after process optimization,the applicability of the pre-forming process system to the forming of complex structures was proved.After correcting the mold surface according to the reverse compensation algorithm based on the finite element simulation result which introduced the crystallization model,the maximum deformation of the components reduced from 0.78 mm to 0.11 mm.In this paper,the forming method of thermoplastic composites stiffened structures was studied by the combination of theoretical study,modeling and simulation,and component forming experiment,a new way for the shape and performance collaborative integrated manufacturing of thermoplastic composite complex components in the aerospace field was provided.