Mesomechanical Study Of Interlaminar Deformation And Damage In Thermoplastic Composite Laminates

In the aerospace industry, thermoplastic AS4/PEEK composite material is nowwidely used. Compared with thermoset composite materials, more pronouncedplasticity in the AS4/PEEK composite is due to the increased ductility of the PEEKresin. Therefore, it is not only difficult but also necessary for the design andapplication of this material to analyze accurately its behaviors of mechanical anddamage under complex stress state.In present work, using mesomechanical analysis including the representativevolume element (RVE), the macro and meso-FE models of the thermoplasticcomposite laminates under uniaxial tensile loading are developed by means ofsubmodeling technique in the finite element method, and the interlaminar stresses oflaminates are analyzed using global model in laminar scale;especially in fiber scale,the mesomechanical elastic-plastic submodel is developed using RVE with two phasesmicrostructure of fiber and matrix. In the calculation of this submodeling, the fiber iselastic with actual diameter about 7μm and volume content 63%;and PEEK matrix haselastic-plastic behavior which can be characterized using the stress-strain curve thattested by its producer.Firstly, the stress-strain curves of the AS4/PEEK lamina under tension,compression and shear are respectively predicted by the meso-FE models with RVE,Comparing these curves with experimental ones, it is shown that the meso-mechanicalbehaviors from meso-model match the experimental facts. Secondly, the boundaryconditions (BC) of the meso-submodel with two phase materials are given by theresults of its global model. This kind of boundary conditions is more actual than thecommonly periodic BC. Lastly, the interpolation precision of displacements betweenthe global model and its meso-submodel has been improved by means of multi-plysubmodeling, in order to ensure the numerical precision of the meso-submodel.Based on these methods above, using elastic-plastic meso-mechanicalsubmodeling in the fiber scale, the elastic-plastic stress and strain fields near the areasof fiber-matrix interface and interlamination are obtained for several kinds ofAS4/PEEK laminates, such as [0/90]2s, [0/± 45/90]2s and [± 25]s4 and so on, underuniaxial tensile loading. And the influences of the PEEK matrix ductility and strengthof the fiber-matrix interface on sub-critical damage behaviours are studied, includinginterlaminar stresses, fiber-matrix debonding, matrix crack and delamination. Aboveexperimental damage behaviours can be explained by present numerical results forAS4/PEEK laminates. For instance, both matrix crack and delamination in[0/± 45/90]2s laminates come from fiber-matrix debonding, and the fiber-end cracknear the area of interlamination is the cause of delamination for [± 25]s4 laminate. Sothe method in this paper is important for optimize design of thermoplastic compositelaminates, especially for the design of fibre-matrix interface.Using multi-ply submodeling of FEM, the influences of the size of growingmatrix crack on interlaminar stresses in [0/ ± 45/90]2s laminate under uniaxial tensionloading are also discussed.