PSI - Issue 42

Ralf Lach et al. / Procedia Structural Integrity 42 (2022) 3–8 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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curves can be very difficult to recognize on the fracture surfaces of the 3D-printed materials investigated in this study (ABS and PC).

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w e

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60 mm/s, 45°/45° 80 mm/s, 45°/45° 60 mm/s, 0°/90° 80 mm/s, 0°/90°

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 w p

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Work of fracture w f (N/mm)

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 w p (N/mm

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Ligament length l (mm)

Fig. 4. Results of the fracture mechanics investigations using the EWF method at quasi-static loading conditions depending on the processing parameters for ABS: (a) specific work of fracture w f as a function of the ligament length L, (b) resistance against stable crack initiation w e and stable crack propagation  w p . As shown in Fig. 4b both the resistance against stable crack imitation (w e ) and stable crack propagation (  w p ) reveals a qualitatively similar picture compared to the data in Fig. 3a. It is confirmed that also at quasistatic loading conditions the printing speed (but not the building direction) has only little effect on the toughness parameters.

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Fig. 5. Local strain fields close to the crack tip during fracture mechanics investigations of mini-CT specimens of ABS for different orientations: 0°/90° (a) and 45°/45° (b). Independently of the loading rate, the larger toughness parameters for the building direction 45°/45° compared to the building direction 0°/90° can be explained as follows: Even though a macroscopically predefined tensile load is applied, which should result in the crack opening under tension (crack opening mode I), the crack propagation tends to follow the built layers including the building direction. This means that crack propagation occurs under mixed mode I/II (mode II – in-plane shear crack opening) for the building direction 45°/45°, either microscopically (ICIT, EWF concept) or macroscopically (R curve concept). This can be confirmed by the pronounced fracture surface roughness detectable by light microscope and by digital image correlation i.e. the local strain fields in Fig. 5 (especially the shape of plastic zones). It is well-known that fracture mechanics parameters are larger under mode II than under mode I.