Effect of hydrolytic ageing on the fracture properties of repaired carbon/polyamide 6 thermoplastic composites

Abstract

This study investigates the repair potential of carbon/polyamide 6 specimens after extensive hydrolytic degradation, using double cantilever beam (DCB) characterization. To evaluate repair effectiveness, DCB test results from repaired specimens were compared with those from aged but unrepaired specimens. The results indicate a reduction in mode I fracture toughness G_IC for most repaired specimens compared to their unrepaired counterparts (from 3.44 kJ/m² down to 0.94 kJ/m² in the non-repaired state compared to 2.44 kJ/m² down to 0.84 kJ/m² in the repaired condition). However, as ageing duration increased, the difference in G_IC values between repaired and unrepaired specimens progressively decreased, eventually converging for the longest ageing duration studied (from −29% in the unaged state down to −10% for the ultimate degradation duration). To understand this behaviour, complementary analyses, including molar mass and crystallinity ratio measurements, X-ray tomography, and SEM observations, were conducted. These investigations suggest that fibre misalignment and a weakened fibre/matrix interface after repair contributed to the observed reduction in G_IC for a given ageing duration. While prior studies have addressed the repair of thermoplastic composites, the effect of hydrolysis on their repairability remains largely unexplored. This work highlights the repair potential of thermoplastic composites even after significant irreversible degradation.