Interlaminar shear strength testing of non-unidirectionally reinforced composites
摘要整理
One of the most detrimental failure types in composite materials is delamination, a fracture that starts \nto propagate between the individual fibre-reinforced layers of the composite. Delamination can be \ncaused by different forces but in this thesis the focus point is in delamination caused by interlaminar \nshear. Interlaminar shear strength, ILSS, is a value that is often used for comparing different composite \nmaterials to each other and basically tells how well the composite can resist interlaminar shear forces \nbefore breaking. \n \nThis thesis primarily focuses on experimental testing of non-unidirectionally reinforced laminates in \norder to find out their interlaminar shear strenght. The testing is done by three point bending according \nto standard ASTM D2344 that defines the rules for the ILSS testing. Two main types of test specimens \nare tested: aramid and glass-fibre-reinforced specimens. The glass-fibre-reinforced laminates have two \nlay-ups, which are being compared to find out how the ILSS behaviour changes with different lay-up. \nTo determine the stresses in different parts of the composite specimen during bending in detail, finite \nelement models of the glass-fibre-reinforced laminates with both lay-ups were created. \n \nBased on the experimental results, the delaminations happened often at the lower interface of the \nlaminates, where the fibre direction changed. This was confirmed with the simulations, which showed \npeak stresses at the interface. The wet testing conditions and the elevated temperature during testing \nboth had a negative effect on the ILSS values as well as the behaviour of the laminates during ILSStesting. \nAgeing in a water-sulphuric-acid immersion decreased the average ILSS values but did not \naffect the ILSS behaviour and the specimens delaminated in an interlaminar way. Simulated critical \nshear stresses were 14-24% lower than the values based on the ASTM standard. The simulated pure \nlay-up-induced decrease in shear stress between the two laminates was 34-58% (depending on the \nfree-edge-effects), which is higher than suggested by the experiments (21-26%).