**Research on Heat Transfer during Thermoplastic Fiber Placement and Mechanical Properties of Variable Angle Laminates**

Author:Cao Zhong Liang

Supervisor:fu hong ya han zhen yu

Database:Doctor

Degree Year:2019

Download:69

Pages:134Size:5007K

Keyword:heat transfer，interlaminar property，Mechanical properties，thermoplastic fiber，variable-angle trajectory planning

Thermoplastic composite has a promising application for its advantages of good weldability,high impact toughness,recyclability,chemical corrosion resistance,etc.Using the in-situ curing technology,the production efficiency can be significantly increased and the costs can be reduced as well,with the improved quality of composite components.Thermoplastic fiber placement technology has been widely applied not only in various aerospace devices,but also gradually in some other related fields of manufacture.The in-situ curing of thermoplastic fiber by automated fiber placement method will be expected to become an important manufacturing method for the main-load-bearing components of large aircraft in the future.In this dissertation,thermoplastic fiber placement is taken as the research background,this thesis mainly focuses on the thermodynamic behavior involved in the process of thermoplastic fiber placement,the interlaminar performance of composite component,the variable-angle path planning algorithm and the mechanical properties of variable-stiffness laminates.Under different process parameters,the temperature field inside the thermoplastic composite is very complex and directly affects the crystal quality of the resin.Therefore,it is necessary to study the temperature field.Considering the heat transfer behavior of thermoplastic fiber(AS4/PEEK,melting point temperature 343°C)as the research object in the process of thermoplastic fiber placement,based on heat transfer theory,the heat transfer mathematical model of transient two-dimensional in the thermoplastic composite fiber placement is established and the relevant boundary conditions of heat transfer model are then determined.Since the initial temperature of the mold affects the heat transfer efficiency during the placement process,the influence of the placement process parameters and the initial temperature of the mold on the temperature field in the first layer placement is analyzed.The temperature field variation of the layer during the fiber placement process is obtained,the relationships between the heating temperature,the placement velocity,the number of layers and the temperature field variation are investigated respectively,a reasonable range of heating temperatures is obtained.The platform for thermoplastic fiber placement is built and the temperature field variation in the placement process is detected,the correctness of the two-dimensional model is verified.The in-situ curing technology is applied in the thermoplastic fiber placement,so the interlaminar property directly affect the mechanical properties of the thermoplastic composite component after placement.Considering the intimate contact degree,fusion degree and fusion strength as the research objects,the theoretical model for characterizing interlaminar property is established to obtain the influence and correlation of placement process parameters on the interlaminar property.According to the principle of uniform test method,the placement test and interlaminar property test were designed,the data of interlaminar shear strength and porosity were obtained.The cross-sectional laminates are observed by SEM to get the degree of fusion between the fibers and the resin and the fracture morphology.The influences of placement process parameters on the interlaminar shear strength and porosity of the placement component are analyzed.At the same time,the interlaminar shear strength and porosity of the thermoplastic fiber placement component are optimized,the optimal placement process parameters and the corresponding interlaminar shear strength and porosity are obtained.Favorable placement trajectory planning are the key factors for manufacturing thermoplastic composite laminates.In order to expand the design range of the variable-angle trajectory during placement,a method based on quadratic Bezier curve method to construct a variable-angle reference path is proposed in this thesis and the relevant mathematical model is established.By changing different control parameters,the Bezier curve can be controlled independently in segments,so that the degree of freedom has greatly increased.The placement angle calculation of arbitrary point on the variable-angle path and the calculation of the corresponding curvature are given.The variations of the fiber angle and the curvature value at any point with the change of connection point parameter β is studied and the variation of the fiber placement angle is discussed.The quadratic Bezier curve method is compared with the traditional linear variable angle method to analyze the feasibility of these two variable-angle algorithms.Then the influences of curvature changes on these two variable-angle paths are further discussed.The variable-angle layer construction is determined,the variation law of effective width and overlap of tow with connection point parameter β is investigated.In addition,the quadratic Bezier curve variable angle trajectories placement experiments were carried out to verify the correctness of Bezier curve variable-angle trajectory planning method.By changing the placement angle of the placement path,the fiber direction can be controlled and adjusted to change the load distribution results in the plane,and the stress and natural frequency characteristics in the laminate can therefore be changed to finally obtain the desired mechanical properties of the laminate.To carry out the research on mechanical properties of variable angle laminates,the finite element analytical model of variable stiffness laminates is established based on the fiber placement reference path defined by the Bezier curve method to investigate stress strain,buckling characteristics and modalities of the variable angle laminates.The influence of different connection point parameters β on the stress and strain in the plane of variable stiffness laminates under a compressive load is studied.The first-order buckling load used as the criterion is used to observe the influence of different connection point parameters β on the variable angle layer buckling characteristics under the uniform force load.The influence of the percentage of different initial geometric defects on the buckling load of the variable angle laminates is analyzed.The influences and changes of different connecting point parameters on the natural frequencies of the variable angle laminates are also studied.Meanwhile,the first-order buckling load and first-order frequency are taken as the objectives,the optimal connection point parameters β and the corresponding variable angle laminates are obtained.