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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the mean value of the difference of maximum and minimum value in five equal measuring sections. According to particular circumstances, R z is also a measure of the waviness.

a

b

2 mm

250 µm

Fig. 3. (a) Cross-section of a PA6/CF-based UD tape; (b) profilometer-supported 3D surface scan of a PA6/CF-based UD tape.

The values of R a are 13 ± 4 µm (PP/GF) and 3 ± 1 µm (PA6/CF) to be generally in the order of the diameter of the used glass fibres (ca. 10 µm) and carbon fibres (ca. 5 µm) and the R z values cross to the fibre direction are only hardly dependent on the processing parameters and/or the fibre treatment ( R z = 136 ± 11 µm for PP/GF and 31 ± 6 µm for PA6/CF), whereas the R z values in fibre direction highly depend on processing conditions and/or fibre treatment ( R z = 49...204 µm for PP/GF and 10…31 µm for PA6/CF). In the present case, high R z values of PP/GF-based UD tape 172 indicated a pulsating production process. 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. 3. Fracture performance and structure‒property correlations

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 (EWF) approach and the method of Vu-Khanh (for more information see Tillner et al., 2020; Lach and Teuscher, 2023) are comparably useful to quantify the crack initiation toughness. While both methods provide nearly the same material parameters and similar material ranking (see Fig. 4a,c), these methods cannot be applied in fibre orientation. For the fracture mechanics investigations, the specimens were loaded using a universal testing machine Z020 (ZwickRoell; traverse speed: 1 mm/min, standard climate) at tension up to break and the load–displacement diagrams were recorded. To characterise the stable crack initiation and propagation of the UD tapes cross to the fibre orientation, the EWF concept (Clutton, 2021) has been adapted to the specific requirements of UD tapes. Double-edge notched tension specimens (DENT; specimen width W = 20 mm, varied ligament length L = W – a = 5...15 mm, a – depth of a sharp notch) were used for the experiments. The EWF concept can be applied if full plasticizing of the ligament is finished before crack initiation and if the load–displacement diagrams are self-similar (Lach et al., 2005). In this case, the total work w f for fracture can be split into two components, a component related to the energy dissipated inside the plastic zone and a second component comprising the work for crack propagation inside the process zone. In Equation 1: w f = w e + b w p • L (1) w e , the essential work of fracture (EWF), is a measure of the resistance against stable crack initiation and b w p is a measure of the resistance against stable crack propagation ( b depends on the shape of the plastic zone). For the UD tapes in fibre orientation, single-edge notched tension (SENT) specimens were used ( W = 20 mm, a / W = 0.5) to calculate the critical energy release rate G Ic and the critical stress intensity factor K Ic (see also Tillner