Mechanical performance of NCF composites

Sammanfattning: Composite materials are today in widespread use in the aerospace and marine industries due to their excellent strength and stiffness to weight ratio. In most marine and aerospace applications traditional composites (pre- impregnated tapes) are still the main choice for design of load bearing structures. Nevertheless, due to their high material costs and sensitivity to out-of-plane loads (eg impact), the use of composites manufactured from pre-impregnated tapes (prepregs) has so far been limited. Non-crimp fabric (NCF) based composites address both these concerns, without any significant drop in in-plane performance. In the presented thesis, the mechanical performance of NCF composites is investigated. NCF cross-ply composites have been tested in tension and the effect on mechanical properties due to intralaminar cracks in the 90° layer has been monitored. Effects of damage on mechanical properties are modeled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals that the mechanical degradation is ruled by the crack opening displacement. Furthermore, NCF cross-ply composites response to tensile loading show large effect of the fabric layer stacking sequence: much larger elastic modulus reduction was observed in [0/90/0/90]S than in [90/0/90/0]S case. Since transverse cracks in 90°- bundles may give modulus decrease about 5%, the observed 40% stiffness reduction is attributed to failure and delamination of bundles oriented in the direction of the applied load. Analysis of micrographs shows extensive delaminations and 0°-bundle breaks. Transverse strain in bundles governs transverse cracking in NCF composites and FE analysis reveals that this strain may be significantly lower than the applied macroscopic strain component in the same direction. This feature is important for damage evolution modelling. Analysing by FEM the importance of media surrounding the bundle on average transverse strain it was found that a power law could be used to predict the average strain in bundles. A calculated H-matrix which establishes the relationship between strains in meso-element and RVE strains could then be used to calculate the “effective stiffness” of the bundle. This “effective stiffness” is the main element in simple but exact expressions derived to calculate the stiffness matrix of NCF composites. Failure initiation under compressive loading in NCF composites containing bundles with out-of plane orientation imperfections has been analyzed using FEM in plane stress and linear elastic formulation. The bundle orientation imperfection in a composite unit was described by a sine function. Failure initiation strain was determined comparing failure functions corresponding to two alternative failure mechanisms: a) plastic microbuckling in the bundle due to mixed compressive and shear load; b) plastic matrix yielding according to von Mises criterion. Parameters for compressive failure initiation analysis were bundle misalignment angle, fiber volume fraction inside the bundle and bundle volume fraction inside the composite unit. Mechanical performance of NCF composites is very dependent on their internal meso- and micro- structure which is defined by the manufacturing process of the fabric and composite processing conditions. This thesis identifies the most important parameters which control mechanical properties of these materials. The identification is based on experimental observations and available theoretical findings. Characteristics of the internal structure of NCF composites are analyzed in context of their significance for in-plane elastic and failure properties. A methodology for the determination of the most typical geometrical parameters of composites using optical observations of cross-sections of manufactured laminates is described.