Study of the delamination behaviour of epoxy matrix composite laminates reinforced with electrospun polymer nanofibers

Among various failure modes in composite laminate materials, the delamination is one of the most common. Out-of-plane stresses, repeated cyclic stresses, impact, and so on, can cause layers to separate, forming a mica-like structure of separate layers, with significant loss of mechanical strength. Delamination is widely investigated, and many techniques are developed in order to improve the material strength. The aim of the present study is to investigate the infuence of electrospun nanofibers on the mechanical behaviour of composite laminate, and in particular on the delamination strength. Nanofibers, with respect to micrometric fibres commonly used in composites, are characterized by high surface to volume and aspect ratios. Moreover they also have higher strength with respect to microfbers of the same nature due to alignment of polymer chains along the fiber axis as well as to a low density of flaws and defects. The usage of nanofibers to enhance the mechanical performances of composite has been recently investigated by researchers and examples of the use of electrospun polymer nanofibers to improve the mechanical behaviour of laminates are available in the literature. The basic idea of this work is to use electrospun polymeric nanofibers as an interleaving filler placed between adjacent plies of a laminate composite material and to study the behaviour of the specimen under two specific loading conditions. Three different kinds of Nylon 6,6 are considered, and two different types of tests are performed: the Double Cantilever Beam test and the End Notch Flexure test. The specimens are manufactured by using carbon fiber/epoxy matrix laminate prepreg. The nano-modified composites are obtained by electrospinning the nanofibers directly on the ply surfaces before lay-up, thus generating a layer of fibers with a negligible thickness compared to the ply thickness. For the specified quasi-static test, one nanofiber layer is placed in the middle of the specimens. The behaviour of nanoreinforced composites is compared with the non reinforced ones. The experiments are carried out according to the ASTM standards. For the DCB tests the trend of GIC is studied depending on the thickness of nanolayer placed in the interlay. The study is repeated for the GIIC in the ENF tests. The optimum thickness of the nanofiber interlay is determined for each test. It is observed that a mat of nanofiber interleaved between the most critical plies of the composite significantly influences the delamination resistance of the same laminate. The behaviour of the laminates is different in the four tests considered, the nanofiber toughening effect being different in each one of them. By performing DMTA analysis of the resin-nanofiber system, damping effect and tensile/bending behaviours are determined in order to built up an analytical model capable to account the effect of nanofiber sheet placed in the laminates. The model is based on the lamination theory and the stiffness contributions of nanolayers are superimposed to all the other plies contributions. A new matrix formulation is presented and considerations regarding the membrane and bending behaviour are developed. In order to account the randomness of nanofibre orientation which influences the mechanical properties of composite nano-interleaved materials a probabilistic variational model is presented and discussed.