Damage tolerance assessment of the interface strength gradation in thermoplastic composites

The newly developed hybridization approach for thermoplastic composites, based on the gradation of the interlaminar interface strength (IGIS design), was used to prepare polypropylene/glass fibre composites by properly alternating layers of reinforcing fabric with compatibilized and not compatibilized polymeric films. Maleated polypropylene was used to improve the matrix/fibre interface strength. The flexural and low-velocity impact characterizations showed that the use of the coupling agent improved the quasi-static flexural properties through the strengthening of the matrix/fibre interface, but considerably lowered the low velocity impact resistance of the laminate. The use of the IGIS design, which grades the interface strength through the laminate thickness, enabled the fabrication of composites with a favourable combination of flexural properties and impact resistance and proved to be effective in better preserving the integrity of the fibres. The damage after impact has been assessed by means of micro-computed tomography, which elucidated the improving effect of the matrix hybridization on the impact resistance, and acoustic emission analysis, which allowed the identification of the mechanisms responsible for impact energy absorption. The role played by the matrix hybridization sequence was pointed out along with the effective role of IGIS design in the overall mechanical performance of laminates.