Interface strength gradation in thermoplastic composites: A new approach to increase the damage tolerance

A new design has been developed for thermoplastic composites based on the gradation of the interlaminar interface strength (IGIS). IGIS laminates have been prepared by properly alternating layers of a woven fabric with layers of compatibilized and not compatibilized polypropylene films configured symmetrically or asymmetrically with respect to the middle plane by means of the well-known film stacking technique. Maleated PP has been used to compatibilize the polypropylene with glass fibres and thus to manage the interface strength layer by layer. The flexural and low-velocity impact characterizations showed that the presence of the coupling agent, through the strengthening of the matrix/fibre interface, improved the quasi-static flexural properties in conventional composite structures (prepared with fully compatibilized polymeric layers) but considerably lowered the low velocity impact resistance of the composite, in terms of maximum load before fibre breakage and recovered energy after the impact. The use of the IGIS design, which grades the interface strength through the laminate thickness, allowed to prepare composites with a favourable combination of flexural properties and impact resistance. Moreover, the IGIS design proved to be more effective in preserving the integrity of the composite after low velocity impacts. The post-impact flexural characterization performed on samples impacted at 6 J and 25 J, showed that some of the IGIS laminates have better damage tolerance with respect to the fully compatibilized laminate as the flexural response decreased to a minor extent. Acoustic emission (AE) was applied to obtain more information about the level of damage and the failure modes occurred at the different impact energies. The AE signals were recorded in real time during flexural tests on both non-impacted and impacted specimens. The several configurations exhibited different behaviours in terms of acoustic emission activity and AE signal features, such as amplitude and duration, which allowed the identification of the mechanisms responsible of impact energy absorption. The role played by the compression side of the samples was pointed out along with the effective role of IGIS design in tailoring the strength where it is needed thus providing an overall better damage tolerance.