Effect of phase separation in epoxy/PEI matrix on the mixed-mode I/II delamination behaviour of unidirectional glass fibre reinforced composites
The study reported in this thesis investigates the relationships between the morphology of PEI/epoxy blends reinforced with glass fibres and their fracture properties. Hot stage optical microscopy is used to study the phenomenon of phase separation in the thermosetting blends in the presence of glass, carbon and aramid fibres. Phase separation is shown to be unaffected by the presence of aramid and carbon fibres, but is affected by the presence of glass fibres, to a degree which mainly depends on the PEI concentration. Other parameters like cure temperature, the nature of the glass fibre surface and fibre volume fraction are also examined. The most striking feature is the initiation and development of an epoxy-rich layer around the fibres for blends modified with 15 wt % PEI. This concentration corresponds to a co-continuous network of PEI-rich particles embedded in an epoxy-rich matrix. The effects of morphologies formed during phase separation on the fracture properties of glass fibre-reinforced composites are studied using the mixed-mode bending test rig developed by NASA. Extensive scanning electron microscopy (SEM) observations provide qualitative support to the delamination results. They show that different micro-mechanisms of deformation can occur, depending on the matrix microstructure and the fibre/matrix interfacial strength. SEM observations show that cusps are not only present under mode II loading, but also under mixed-mode I/II loading. They reveal that the spatial density and angle of cusps depend on the applied loading mode, the interfacial strength and the nature of the matrix itself. These observations are the base of a model which describes the delamination behaviour of composites from pure mode Ito pure mode II loading.