Non-destructive evaluation of damage detection in hybrid basalt/flax fibres-polypropylene composites

In the last decades, fibre-reinforced composites have attracted outstanding interest especially for weigh-sensitive applications as well as for their specific mechanical properties as strength and stiffness. However, with the awareness that, like most materials, fibre-reinforced composites also exhibit the so-called strength versus toughness dilemma and, over the years, many different strategies have been proposed to improve their damage resistance. One of the most accounted strategies consider the use of at least two different reinforcing fibers distributed in the same matrix with typical configurations best known as interlayer, intralayer or intrayarn, depending on whether the fibres of different nature are arranged on as many laminate of the composite, in the same lamina or side-by-side in the strand making up the reinforcing phase. In this frame, the research was focused on polypropylene-based composite laminates manufactured by film-stacking and hot-pressing steps and reinforced by a commercial hybrid fabric obtained by weaving flax and basalt fibres. Specimens, consisting of 6 plies and 3.0 mm laminate thickness, were cut from the prepared plates along the direction of both flax and basalt fibres and subjected to Quasi Static Indentation (QSI) tests. The tested specimens were investigated by combining Optical microscopy (OP) and Electronic Speckle Pattern Interferometry (ESPI) to analyze the complex damage which can be generated as a result of stress on this kind of fibre-reinforced composites.