Study On The Construction Methods Of Damage Model Of Composite Laminates And Their Application

Advanced composite materials are being increasingly used in many engineering fields dueto their excellent mechanical properties. In order to use composite materials fully, a largenumber of material damage models have been proposed by researchers. All these damagemodels can be used to solve many engineering problems. For example, they can be used topredict the damage process and residual strength of composite materials. However, they alsobrought a lot of confusions. In order to understand the development process of the damagemodels and the relations among them, it is necessary to study the construction methods of theexisting damage model. In this thesis, the main contents are as follows:(1) The construction methods of the damage model used to describe intra-ply failuremodes of composite material, failure criteria of delamination modeled in intra-ply area ofcomposite laminates, and the cohesive model modeled in inter-ply are firstly given, which arebased on experiments, theory and experience. The damage model used to describe intra-plyfailure modes contains failure criteria, constitutive relations, damage variables and damagestiffness matrix, and the cohesive models contains damage onset criteria, damage evolutioncriteria and constitutive relations, among which the number of the construction methods offailure criteria is the most, and its connotation is also the most plentiful. Meanwhile，the finiteelement techniques used to simulate delamination are investigated, and their constructionmethods are also proposed. Having a better knowledge of the construction methods can notonly make the thinking hidden in the literatures clear, deepen the understanding of the essenceof the building process of damage models, can also improve the contents of damage model,propose new damage models, and provide the foundation for further development of damagemodels.(2) Based on the construction methods, some new failure criteria, damage variables anddamage stiffness matrix of the damage model used to describe intra-ply failure modes ofcomposite material，and new damage onset criteria, damage evolution criteria and constitutiverelations of the cohesive models are proposed here. Finally, the building process of thedamage model and the relationship among the construction methods are discussed, and aunified expression of the damage evolution criteria is also proposed.(3) Based on the synthetic method metioned in Section II, a new two-dimensional failurecriterion with four main parameters of composite materials and a new three-dimensionalfailure criterion with five main parameters of composite materials are proposed in this study.The two-dimensional failure criterion of composite materials mainly includes four failure modes: fiber failure in tension and compression, and matrix failure in tension andcompression. Delamination is also included in the three-dimensional failure criterion ofcomposite materials. Meanwhile, based on some related work, the matrix failure criterion incompression is modified. The characteristic of the failure criteria are presented. The processof identifying parameters of the new failure criteria and their meaning are also presented. Thecoefficients of many existing criteria are still all experiential, and any engineering materialneeds to get the coefficients of the failure criteria by experiments.(4) Based on Tsai-Wu criteria, a new continuum damage model simulating damage ofcomposite laminates under static loading and a continuum damage model describing theimpact damage of composite laminates are proposed firstly. Numerical analysis is conductedfor two panels, one with damage in the center under tensile loading, and the other under lowvelocity impact loading. The numerical results verify that different definitions of damagevariables and the coefficients of failure criteria will affect the damage description ofcomposite laminates under impact loading.(5) A field-variable analysis method is proposed to solve the problem that compositelaminates may be multiply loaded during the time of manufacture, transportation and service.The numerical approach uses a set of field-variables to describe the damage process ofstructures. This approach is verified by simulating the DCB model and a composite plateunder loading, respectively. The numerical results verify that impact damage will cause thechange of the nature frequencies and higher modes of composite laminates, and the reductionof the nature frequencies is mainly produced by matrix damage.Aiming at the construction method of damage model for composite laminates and thedevelopment of the damage models, some new damage models, such as failure criteria,damage variable, stiffness matrix, two continuum damage models, and a field-variableanalysis method are proposed. The work will help to build new models and find therelationship among different damage models easily, and provide the foundation for furtherdevelopment of damage models.