Mechanical and fracture mechanics investigations of unidirectionally fibre-reinforced thermoplastic polymer tapes

Thin polymer tapes reinforced with unidirectionally oriented (UD) continuous fibres (glass and carbon fibres) provide a basis for innovative components used in light weight construction. Within the scope of different research projects, a UD-tape production line was developed within the laboratory scale. The thermoplastic UD tapes manufactured with this facility were procedurally as well as material-specifically analysed. Based on the reference adjustment previously obtained by optimization processes of the laboratory-scale impregnating tool, the impact of selected processing parameters and/or fibre treatment on the mechanical and fracture mechanics properties of the UD tapes was investigated. Due to the pseudo-ductile stick–slip crack propagation behaviour of the UD tapes cross to the fibre orientation both the essential-work-of-fracture approach and the Vu-Khanh method are comparably useful to quantify the crack initiation toughness. While both methods result in nearly the same material parameters and similar material ranking, the methods cannot be applied in fibre orientation. Irrespective of whether a clear influence of selected processing parameters (all other parameters were the same as a reference tape) and/or fibre treatment on the crack initiation resistance was found. The fibre–matrix adhesion quantifiable by the crack initiation toughness in fibre orientation also affects the crack initiation toughness cross to the fibre orientation, because fibre pull-out is the main energy-dissipative failure mechanism inside the UD tapes. The line roughness determined by profilometer-supported 3D surface scans, especially the highly processing-dependent roughness data in fibre orientation, can be used as a fast and effective instrument for quality assessment of the UD tapes. This roughness data correlates with the crack initiation toughness in fibre orientation and can be used to trigger morphology–property relationships.