Research On The Prediction Method Of Equivalent Mechanical Properties Of Non-crimp Fabric Composites

Multiaxial multiply warp-knit fabric,also known as warp-knitted non-crimp fabric(NCF),is constituted by a large amount of fairly straight fiber tows that are placed side by side and bounded by warp-knitting.Compared with the unidirectional(UD)pre-preg composites,NCF composites have many advantages,such as lower production cost,higher out-of-plane damage tolerance and fracture toughness.Thus,it is becoming more popular in the manufacture of complex and thicker parts than UD pre-preg composites.In the aeronautic sector,NCF composites as a new material,the study of its mechanical properties and analytical methods have been widely concerned and studied.However,the mechanical properties of NCF composites are influenced not only by the composition of materials but also by the knitting process parameters,which increases the difficulty of modeling and calculation in the mechanical analysis of such composites.In the engineering research,an analysis model with simple modeling and high computational efficiency is needed to evaluate the mechanical properties of NCF composites.For this purpose,from the NCF composites manufacturing processes,the prediction methods of NCF composite material equivalent mechanical properties are carried out in the aspects of mesostructure characteristics,and prediction method of mechanical properties,and three dimensional stress calculation method for composite materials,and effective layer stiffness and strength prediction model,and specific engineering application case.The research work can be used to create an equivalent continuum model for the composites at macroscale level.The main contents and results are as follows:(1)Comprehensive study on the mesostructure of NCF CompositesThe mesostructure of NCF Composites is one of the main input factors to predict equivalent mechanical properties.The existing research do not have a unified understanding of the internal geometric structure of NCF.Therefore,firstly,the category of NCF in forming technology of fiber ensembles is defined.Then,the manufacturing process characteristics of NCF and the multi-scale material characteristics of NCF composites are studied.It is pointed out that the NCF is divided into open structure and continuous plies according to the difference of coursewise weft insertion process in fabric manufacturing process.The open structure NCF composite have shown that fibre bundles have rather large out-of-plane waviness and the rich resin region has a linear channel shape.In the continuous plies NCF composites,the waviness of the fiber bundles occurs in-plane,forming a linear channel and a long rhomboidal shape rich resin zone.(2)Study on the mechanical properties acquisition strategy of the effective layer of NCF composites.In order to predict the equivalent mechanical properties of NCF composites,it is necessary to determine the specific form of the effective layer.In this connection,the characteristics of NCF composites warp-knitted layer and a single fiber direction ply are analyzed,and a single fiber direction ply is defined as an "effective layer",which makes the "equivalent layer" assumption of NCF composites.Then,in order to obtain the mechanical properties of the equivalent layer,a new modeling strategy for the meso-scale features of QNCF is presented.The idea of this modeling approach is derives from the testing method of mechanical properties of a single NCF lamina and engineering models based on semi-laminar considerations.Finally,the process of predicting the mechanical properties of NCF composites lamina is clarified,and NCF composites lamina model with large cracks,small cracks and channels is established for the multiscale prediction of NCF composites equivalent lamina stiffness and strength.(3)Three dimensional stress analysis of composites and the calculation method of interlaminar stress based on interface modelThree dimensional stress analysis exists in various scales of multiscale analysis of NCF composites.In addition,due to the characteristics of the NCF multiply and low preform,NCF is often used in the manufacture of thick composite parts,and the mechanical properties of the thickness direction can not be ignored in the analysis of thick laminate.Considering that the composites interlaminar strength is low,in the process of three-dimensional stress analysis,the interlaminar stresses should be focused.Therefore,in this paper,the three-dimensional stress analysis of composite materials and the calculation method of interlaminar stresses are carried out.In order to efficiently evaluate interlaminar stresses under the complex boundary condition,a calculation strategy that using zero-thickness cohesive interface element is presented and validated.Based on this method,the interlaminar stress concentration in a laminate located at the outer edge of the bolt head is found,while the traditional understanding of interlaminar stress concentration only occurs at the free edge of the laminate.(4)Estimation of lamina stiffness and strength of NCF composite based on lamina decomposition modeling method.After the establishment of NCF composites lamina model using the decomposition modeling method,a specific mechanical calculation model is needed to predict the lamina mechanical properties and output the equivalent lamina homogenization stiffness and strength properties.The equivalent lamina stiffness of NCF composite is evaluated by numerical homogenization model.It is found that the effect of in-plane fiber distortion on the lamina stiffness of continuous plies NCF composites is not significant.In order to evaluate the equivalent lamina strength,the Tsai-Wu failure criterion was used to predict the transverse isotropic fiber failure.Different plastic deformation and yield models were used to describe the deformation behavior of pure resin under different loads.A damage analysis model with non-linear shear behaviour is proposed.According to the analysis,it is found that the in-plane fiber distortion will reduce the longitudinal strength by more than 10%.(5)Analyzing thick-section composites drag brace lug with hole bonded sleeveTo study the failure characteristics of the composite drag brace lug,the hole structure of ultra thick laminate lug is designed according to the scheme of F-16 fighter and B787 passenger landing gear drag brace.Aiming at the complexity of finite element modeling caused by the large number of plies in ultra thick laminates,a modeling strategy with interface element and sublaminates is proposed,and based on the equivalent mechanical properties of NCF composites prediction method,the equivalent lamina stiffness and strength are determined.Then,a three dimensional finite element model of thick-section composite lug is established.The influence of the sleeve stiffness on the stress distribution and the failure mode of the lug is discussed.The results show that the failure stress and damage position are mainly concentrated in the no contact region of the hole-axis,stiffer metal sleeves reduce the interlaminar stress concentraitons of hole edge,but will reduce the carrying capacity of sleeve adhesive.According to these phenomena,a special failure mode with sleeve composite material is presented.