PSI - Issue 68

R. Lach et al. / Procedia Structural Integrity 68 (2025) 1337–1342 R. Lach et al. / Structural Integrity Procedia 00 (2025) 000–000

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et al., 2020; Lach and Teuscher, 2023) for crack initiation ( U max – work up to maximum load F max , B – specimen thickness, f( a / W ) – geometry function) (Anderson, 2005). !" = # !"# $∙('()) and !" = + !"# ∙) $/& $∙'∙,( '" ) (2) First, the results of the PP/GF-based UD tapes are discussed (Fig. 4a,b). Irrespective of whether a clear influence of selected processing parameters (all other parameters were the same as a reference tape) on the crack initiation resistance was found. Cross to fibre orientation, especially an increase of the line speed affects the fracture toughness negatively, whereas an increase of the pull-off force, a reduction of opening of the mould or an increase of the pull-off force at higher drive shaft have a comparatively low influence on toughness. Due to higher matrix fraction, an increase of the pull-off force at higher drive shaft (UD tape 172 in Fig. 4b) results in a much higher resistance against crack initiation in fibre orientation compared to the reference. Despite the overall good fracture mechanics performance, however, an increase of the pull-off force at higher drive shaft cannot be recommended due to reduction of fibre fraction and a pulsating production process. The lower fibre fraction is combined with reduced elastic modulus to be far from the aim of development. All other UD tapes show much lower toughness in fibre orientation than the reference, which may be caused by a highly lower fibre–matrix adhesion.

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Fig. 4. Fracture mechanics values cross to the fibre orientation (a, c) and in fibre orientation (b, d) for PP/GF (a, b) and PA6/CF (c, d).

Secondly, the results of the PA6/CF-based UD tapes are discussed (Fig. 4c,d). UD tapes with treated fibres show generally higher toughness values both in and cross to the fibre orientation compared to that of the UD tape with untreated fibres (reference, UD tape 343 in Fig. 4c,d). The very high toughness in fibre direction of the UD tape 350 (Fig. 4d) results from instable processing causing higher matrix fraction and thus lower elastic modulus. This behaviour is far from the aim of development similar to the PP/GF-based UD tape 172 in Fig. 4b. The crack initiation toughness G Ic in fibre orientation is much lower (ca. 0.2–1.2 kJ/m 2 for PP/GF and ca. 0.6–5.2 kJ/m 2 for PA6/CF, compare Fig. 4b,d) than that cross to the fibre orientation (ca. 200–400 kJ/m 2 for PP/GF and ca. 1300‒2300 kJ/m 2 for PA6/CF, compare Fig. 4a,c). For practical applications of UD tapes compared to conventional composites, the high toughness (due to highly energy dissipative processes such as fibre breakage and fibre delamination) in fibre orientation has a clear effect on the very advantageous stiffness–toughness ratio (Tillner and