Multi-scale analysis of mechanical phenomena in NCF composites

Detta är en avhandling från Luleå : Luleå tekniska universitet

Sammanfattning: Mechanical phenomenon in Non Crimp Composites is studied in this thesis using a multiscale approach. This approach is choosen due to the architecture of the composite which is heterogenic both on micro as well as mesoscale. Homogenization is therefore performed on three different length scales: micro-scale which homogenizes the fiber/matrix structure, meso-scale where the bundle structure is homogenized into layers creating a homogeneous material and macro-scale which considers the representative volume element. Volume average stress and strain relations are used for the development of both the iso-strain and Partial iso-strain method. These methods are approximate analytical methods using a method of dividing the composite into elements in which the stiffness matrices can be determined. The elements consist of a number of subelements were the material is already homogenized. In this case, the iso-strain method only considers stress in the composite whereas the Partial iso-strain model considers both stress in the composite and also the stress and strain developing in different elements in the composite. Out-of-plane orientation of fiber bundles are also analyzed in terms of stiffness. In this case homogenization is performed using a slightly different approach than in the case were the fiber bundles only have in- plane orientation. Instead of using CLT for the determination of each elements stiffness matrix, volume average of the stiffness matrices for the subelements within the elements is performed, using the assumption of iso- strain in the element. Preliminary calculations are performed with the object of determining a knock-down factor for the determination of strain level in the 90º bundles, which are the most possible locus for failure. Parametric studies on 90º bundle strain are also performed, analysing the importance of media surrounding the 90º bundle, the stiffness ratio between the lower sub- elements in two different elements and the volume content of bundle in the layer. The stiffness-damage relationship for a cross-ply NCF composite is also monitored experimentally and modelled. For this purpose, analytical micro- mechanics models used for analysis of conventional laminated composites are modified and tensile tests are performed. Secondly, RVE based models of the composite meso-structure implemented in FEM are developed. RVE analysis is to provide information on stress and strain concentrations caused by the material heterogeneity and, hence, allow for explanations to damage mechanisms observed in tests. A study of the compressive failure initiation in an imperfect element of NCF composite is conducted. FEM based parametric study is performed on the mesoscale to identify the microscale and mesoscale parameters governing failure initiation in an NCF element containing fiber bundle with a certain degree of out-of-plane waviness which is surrounded by matrix and supported by different layers. As two competitive failure mechanisms plastic microbuckling in the bundle and yielding in the matrix are considered. To verify the conclusions "a model NCF composite" with predetermined out-of- plane waviness is designed, tested in compression and analyzed theoretically.