The paper aims to assess the fiber/matrix interface and adhesion quality in commingled GF/PP composites by means of complementary experimental methods. For this purpose, different interfacial qualities were obtained by a modification of the sizing of the fiber reinforcement (polypropylene specific sizing, non-specific sizing and no sizing), and of the matrix (with or without coupling agent). Transverse tensile stress-strain curves have been recorded while monitoring acoustic emissions. An analysis of acoustic emission data associated with fracture surface examinations has helped explain the differences observed in mechanical properties and damage mechanisms, emphasizing the importance of the interphase in the performance of these new composites. Thus it has been shown that the association of three experimental investigation methods (transverse tension, acoustic emission, and fracture surface analysis) makes it possible to characterize both the glass/silane bond (so-called fiber/matrix interface) and the global fiber/matrix adhesion (addition of several different zones and interfaces). Polym. Compos. 25:577-588, 2004.

The second paper in this series aims to identify the relationship between fiber/matrix interface and cracking phenomena occurring in commingled yarn glass fiber/polypropylene (GF/PP) composites. For this purpose, different interfacial qualities were obtained by a modification of the fiber reinforcement sizing and the matrix coupling. In addition to transverse tensile stress‐strain relationships (Part 1 (17)), mode I interlaminar resistance relationships have been recorded while monitoring acoustic emissions. A mode I mechanical analysis points out the influences of the sizing on the mechanisms contributing to energy absorption, which are protection of fibers against break, creation of an interphase and control of adhesion at the glass fiber surface. Sizing appears as a key parameter governing crack initiation and propagation for these commingled thermoplastic composites. Owing to the relationship between global fiber/matrix adhesion and cracking mechanisms, mode I fracture mechanics is a powerful characterization technique of the interfacial quality of GF/PP composites. Polym. Compos. 25:589–600, 2004. © 2004 Society of Plastics Engineers.

This study aims to assess the effects of ageing in boiling water on the transverse tensile mechanical properties of unidirectional commingled GF/PP composites, as well as the influence of the fibre/matrix adhesion on the water absorption and desorption mechanisms. For this purpose, different interfacial qualities were obtained by a modification of the fibre reinforcement sizing (polypropylene specific sizing, or no sizing), and of the matrix (with or without coupling agent). A very good retention of the mechanical properties was observed for those composites which had been treated so as to improve the fibre/matrix adhesion. It is also shown that the better the adhesion, the longer the water remained inside the composite material. This induced effect appears to be the counterpart of the protecting role against moisture of a strong interface. The interfacial interactions act as barriers both during absorption and during desorption. This leads to water molecule entrapment. Hence, the persistence of water trapped at the interfaces in the case of sized glass fibres composites leads us to recommend investigations on the long term effects on longitudinal tension properties, since the effects of zero-stress ageing are known to reduce fibre strength.

This work aims to analyse the influence of the fibre/matrix interface on the acoustic response of commingled yarn thermoplastic composites submitted to transverse tension and mode I fracture mechanics loading. For this purpose, different interfacial qualities were obtained by modification of the fibre reinforcement sizing and the matrix coupling. The acoustic emission rate (number of acoustic events emitted per time unit) has appeared suitable to quantify the damage involved by the mechanical loading according to the interfacial quality. The comparison of the acoustic emission obtained for the two loading modes has shown that, although the amplitude range [59-66dB] is characteristic of interfacial debondings in transverse tension, it is no more the case for a mode I delamination test for these commingled GF/PP composites.