MicroCT analysis of Interlaminar Graded Interface Strength (IGIS) composites based on a thermoplastic matrix

The hybridization approach in composites is used to improve the impact resistance and the damage tolerance of laminates. It is usually based on the use of two different reinforcing fibres (alternatively stacked, commingled or interpenetrated), one having high stiffness and the other having high toughness. Such approach is effective in improving the low velocity impact resistance of the composites but it poses new issues related to the different specific properties of used fibres (coefficient of thermal expansion, interface compatibility, residual stresses after the laminate production to name a few). A new design has been recently proposed for thermoplastic composites based on the gradation of the interlaminar interface strength (IGIS). The interface strength between fibres and matrix is graded through the thickness by alternating woven fibres with compatibilized or not compatibilized polymeric layers. IGIS laminates involving a commercial grade polypropylene (PP) as matrix and a woven glass fabric as the reinforcement have been prepared with different interface strength configurations (symmetrically or asymmetrically with respect to the middle plane) by means of the well-known film stacking technique. In this contribution Micro Computerized Axial Tomography (MicroCT) analysis was used to investigate the actual damage mechanisms occurring in IGIS laminates and showed how the graded interface strength is capable of shifting fibre breakage at higher impact energies and allowing elastic recovery in conditions such to significantly damage the fully compatibilized laminate configuration.