Experimental method for in-situ real-time measurement of mixed mode fibre/matrix interface debonding and comparison of fibre sizings in single short fibre composites

In short fibre composites, off-axis fibres are potentially subject to mixed-mode loading, which results in interactive debonding growth in opening and sliding modes. This synergy might differ for strong and weak fibre/matrix bondings inducing discrete debonding patterns. This study employed an optical visualization technique to investigate, for the first time, the mixed-mode debonding growth for short fibres with different sizings and off-axis angles, through in-situ real-time measurements. For this purpose, embedded single short-fibre specimens were fabricated using E-glass fibres and an epoxy matrix, with a novel and more efficient manufacturing method than done so far in literature. E-glass fibres with two different sizings were utilized, including a polypropylene-compatible (weak) sizing and an epoxy-compatible (strong) sizing. While there was a relatively rapid debonding growth for the transverse fibres with the weak sizing, the strong sizing experienced a gradual debonding growth independent of the fibre angle. For the weak sizing, partial debonding cracks were initiated at different locations along the length of the fibres, where they coalesced into dominant debonding cracks. For the strong sizing, debonding was always initiated and localized at the fibre tip, and there was a clear synchronous propagation of debonding in the arc and length of the fibre.