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.
Composites represent the evolution of the material science and technologies. They are obtained by combining two or more materials of different nature with the aim of exploiting any synergies between the characteristic performances of the raw materials. Their properties, in fact, are influenced by those of the starting components but also by the quality of the interface generated between the combined phases as well as by their mutual distribution. The interphase, even if of minimal extension with respect to the main phases constituting the composite, plays a significant role in the control of the damage mechanisms, determines the breaking strength and the stress / deformation behavior of composite materials. In this work we study the effect of low pressure plasma treatment on intrinsically hydrophilic flax fiber fabrics to improve their adhesion to a hydrophobic polypropylene matrix. The fibers are treated using nitrogen (N2) plasma with four different exposure times. The interfacial adhesion actually achieved was indirectly quantified by interlaminar shear strength measurements. After this, the damaged areas were measured with non-destructive techniques, i.e. Electron Speckle Pattern Interferometry and Lock-in thermography.
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