Mechanical Properties Characterization Of Needled Composites And Process Aided Design Of Preforms

The aerospace industry has made extensive use of three-dimensional needled composites due to its numerous good qualities,including their quick preparation time,adaptability to unique shapes,and design-friendly design process.Needling processes introduce the in-plane fibers to the thickness direction.As a result,the needled preforms develop a three-dimensional network structure with fiber distribution.The in-plane fibers are damaged by needling processes.The fiber geometry and the mechanical properties of the needled composites are uncertain.Therefore,higher standards are proposed for the stability and quality of needled composite products.Needled preforms include a wide range of movable parameters.It is very important to determine the relationship between the needling process parameters and the material properties.To fulfill the requirements for the structural material properties,the design method of needling processes should be developed.In the current study,the micro structures of needled composites are examined,and the needling processes are simulated.It is established that there is a correlation between the mechanical properties and the needling process parameters.The impact of needling processes on mechanical characteristics is investigated in conjunction with composites mechanical studies using various needling processes.Furthermore,the needling processes are designed to meet the needs of the properties of needled composites.This study offers a theoretical framework for the preparation of needled preforms.First,the micro structures of carbon fiber needled preforms and their composites were examined utilizing microscopic observation technologies.The macro mechanics experiments of three-dimensional needled composites,such as in-plane tensile test,out-of-plane tensile test,in-plane shear test and interlayer shear test were carried out.The effects of needling processes on the mechanical properties of the composites were analyzed.Failure causes for various loading conditions with combined fracture morphology analysis were identified.Second,the impact of pinhole spacing on the fiber shape in the needled area was investigated using the micro structure characteristics.Four different types of typical areas may be seen in the fiber structure.The method for calculating the stiffness parameters of four typical areas was established.Therefore,a model for predicting the stiffness properties of needled composites was put out.According to the micro structure of composites,the out-of-plane tensile modulus prediction model was adjusted.Then,the fiber volume reduction coefficient was added in combination with the local load sharing model to create the theoretical analytical model for predicting the in-plane tensile strength of needled composites.The strength prediction models were also created in other loading states,such as out-of-plane tensile load,in-plane shear load,and interlayer shear load,based on the failure analysis of the micro structure.The strength of the composites under various loads was predicted after analyzing the failure causes.Furthermore,the effect of needling processes on the strength of needled composites was analyzed.Finally,a database of the characteristics of needled composites created using various needling processes was created.The needling process parameters were designed in accordance with specific mechanical properties of needled composites using machine learning techniques like neural networks and genetic algorithms.In addition,the changing local fiber content of the variable density needled preform was investigated.The influence of needling processes and compaction processes on the fiber content of the preform was obtained.The fiber content of the needled preform was therefore subject to macro control.As a result,the auxiliary design of the needled preform processes was accomplished.This thesis reveals the connection between the macro mechanical properties of needled composites and their micro structure.The models for predicting mechanical properties are developed,and the effects of various needling processes on mechanical properties are investigated.The preparation of needled preform is designed using a machine learning approach.The study is significant for the preparation and use of needled composites in engineering.