| Composite materials have excellent comprehensive properties,which make them widely used in aerospace,vehicles,ships,energy and weapons and other fields.Composite material is a kind of multi-phase material.Because of the non-synergistic nature of the components,it is easy to process-induced internal residual stress in the manufacture.Residual stress is the inherent characteristic of composite materials.Residual stress may directly lead to delamination,warpage,deformation,defect and damage of composites,which seriously affect the mechanical properties and dimensional stability of composites.Predicting and controlling the process deformation in the curing process of composites is of great significance to ensure the performance,manufacturing and assembly accuracy of composites.In addition,there are many sources of residual stress formation,and there is no consensus on the mechanism of residual stress formation in academic circles.Residual stress not only affects the ultimate potential of composite material system,but also acts on the whole process of structural design and analysis.With the development of fiber placement technology,variable stiffness composites have become a research hotspot,but the traditional research on variable stiffness composites mainly focuses on the buckling performance of structures,the cutout reinforcement for structures and other fields,while the research on residual stress and deformation in the curing process of variable stiffness composites is rare,due to the complexity of the fiber trajectory of variable stiffness composites and other reasons.The effect of residual stress on the structural properties of variable stiffness composites is more obvious.Firstly,the background of the research is introduced,the research status of curing deformation and residual stress of resin matrix composites at home and abroad is analyzed,the influencing factors of curing residual stress are summarized,the laying process of composite materials and the basic characteristics of variable stiffness composite materials and the basis for selecting the subject are introduced.The main phenomenological models of resin curing kinetics are introduced:n-order reaction model,autocatalytic model,Kamal model,Lee mechanism model,Springer-Loos model and Cole model.The differential method for solving the three factors of dynamics is deduced.The basic equation of heat transfer,the constitutive equation of resin curing,the prediction formula of thermal conductivity,the prediction formula of resin curing shrinkage and the calculation formula of thermal expansion coefficient are introduced.The preprepreg EM112 was tested by DSC non-isothermal method,and the curing kinetics equation was obtained.One of the most important problems in the design of variable stiffness composite structures is to determine the fiber trajectory profile.The main fiber trajectories are introduced:the linear change of fiber orientation angle,the cubic function profile,the cubic function contour profile,streamline profile,constant curvature profile,quadratic Bessel curve profile,polynomial fiber profile.The advantages and disadvantages of parallel method and translation method for variable stiffness pavement are compared.The advantages of translation method are that the trajectory of fibers can be easily realized by the auto fiber placement,and the products have better buckling resistance.Parallel method does not produce overlap or gaps between fibers.Then the construction method of the balance of variable stiffness laminates is introduced.A linear variation of steered fiber curves and the change rule of the fiber angles were presented to create the mathematical model of variable-stiffness composite laminates.Compared to the straight-fiber laminate,the reference path with linearly changed angles leads to higher mechanical strength and more design freedoms.A novel methodology was developed to predict the distributions of process-induced residual stresses during cure.The modulus of the resin was determined by using the curing hardening instantaneous linear elastic model(CHILE).The curing kinetics of AS4/3501-6 prepreg was simulated by Kamal model.A three-dimensional thermo-chemical model of the curing process was established,and the mechanical response during the curing process was evaluated with the results of thermochemical analysis.The temperature distribution,curing degree distribution and residual stress distribution were obtained by ABAQUS for laminates with 0°±<0°|30°>,0°±<0°|45°>,0°±<0°|60°>,0°±<0°|90°>,respectively.The results show that the residual stress of composite laminates with variable stiffness decreases with the increase of fiber end angle.From t=143 min to 160 min,the degree of cure had a remarkably increase while the cure-induced residual stress barely changed.Compared with the curing simulation of AS4/3501 and EM112 prepreg,EM112 had been basically cured by 144.7 minutes,while AS4/3501 needed 160 minutes to basically cure.In addition,from the curing temperature,the curing temperature of EM112 is 135?,while AS4/3501 is 170?.So EM112 can greatly reduce the curing time and energy consumption.From the level of curing residual stress,the maximum longitudinal residual stress of EM112 decreased by 235%,while the lateral residual stress decreased by 80%.Finally,The 0°±<0°|30°>?0°±<0°|45°>?0°±<0°|60°>?0°±<0°|90°>layers were prepared by fiber placement technology.The curing process was cured in the autoclave.The temperature of curing process was monitored online by FBG optical fiber.The deformation of the product after demoulding was measured.The correctness of finite element simulation results is verified.The parametric study on the curing springback of L-shaped variable stiffness composites was carried out.The mechanical properties and behavior of the laminates were characterized by a self-consistent micro-mechanical model.A three-dimensional model of L-shaped variable stiffness composite structure is established by using the linear change of fiber angle.The influence of the corner radius,the fiber orientation,the thickness and the length of flange on the spring-in of the L-shaped variable-stiffness part was evaluated using ABAQUS.The results show that the spring-in angle increases with increases of the corner radius and the length of flange and decreases as the thickness increases;in addition,the layup of 0°±<0°|60°>2s results in the minimum spring-in angle.the curing residual stress and deformation of composite cylinder with variable stiffness are studied.A three-dimensional model of an variable-stiffness composite cylinder was established using a linear fiber angle variation.The influence of the inner radius,the fiber end angle and the thickness on the stress and deformation of the variable-stiffness cylinder was evaluated using ABAQUS.The results show that the maximum stress increases with increases of the inner radius,the fiber end angle and the thickness.The inner radius of the cylinder have little effect on deformation,the deformation increases as the fiber end angle and the thickness increases. |
PDF Full Text Request