Compressive failure of glass fibre NCF composites

Abstract: Compressive failure of polymer matrix compos ites is very much controlled by material heterogeneity (e.g. fibre waviness and gaps) and out-of-plane loadings (e.g. impact), and to enable design of robust and trustworthy components it is important to understand how different heterogeneity and loadings affect the governing failure mechanisms.In the present thesis the governing failure mechanisms for laminates based on glass fibre non-crimp fabrics (NCFs) and unsaturated polyester have been studied. NCF composite materials are well established within the marine and wind turbine industries due to their very good mechanical properties and cost-efficient manufacturing, and has more recently also started to become very popular within the aerospace industry.Specimens with impact damage, holes, artificial delaminations, and dents have been tested in compression and examined by post-mortem analyses. Based on the results obtained it is clear that the governing failure mechanism for the specimens with impact damage and artificial delaminations was delamination growth, while the governing failure mechanisms for the specimens with holes were fibre microbuckling and kink band formation. A single dent in the specimens did not seem to influence the compressive strength to any great extent, but when the dent was combined with artificial delaminations the compressive strength was significantly reduced and the specimens failed due to delamination growth.As initiation and propagation of delaminations are very much affected by out-of-plane loadings, compression tests were conducted with a low through-the-thickness pressure. A hole was drilled in the middle of the specimens and a bolt and two washers were used to apply a through-the thickness pressure, and as expected, the initiation and propagation of delaminations was postponed and the compressive strength substantially improved.