PSI - Issue 56

Alexandru Isaincu et al. / Procedia Structural Integrity 56 (2024) 167–175 Alexandru Isaincu / Structural Integrity Procedia 00 (2019) 000 – 000

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The breaking forces are increasing with the orientation angle. Lower forces are obtained when the crack is parallel with the fibers (0° orientation). An increase in force can be observed, in all orientations, if a higher thickness is considered. A more constant transition can be seen in terms of force for a higher thickness. This aspect is due to a higher randomized fiber orientation in the specimens that will provide a higher isotropic behavior. The fibers are strongly orientated for the 2.0 mm case and will lead to a higher orthotropic behavior. This is especially visible for PPS GF40 with 2.0 mm thickness. A substantial change in force obtained for 0° and 45° to 90° orientation can be distinguished. All previously mentioned trends can be noticed for both materials. Overall, higher forces can be seen for PPA GF33, in comparison with PPS GF40. The Stress Intensity Factors (SIF’s) solution in mode I (K I ) are defined as follows: = √ ∙ ( ⁄ ) (1) In formula (1), σ is the applied stress and F(a/b) is a geometrical function that depends on notch length/depth (a) and the width of SEC specimen (b). The maximum breaking force was used to compute the applied stress. For the geometrical function, many variations can be observed in the literature. In this study, the following formula was used: ( ⁄ ) = √2 ⁄ ∙ ( 2 ⁄ ) ∙ 0.752 + 2.020 ∙ ( ⁄ ) + 0.370 ∙ (1 − ( 2 ⁄ )) 3 ( 2 ⁄ ) (2) The formula references derive from Tada, Paris and Irwin (2000), they suggested that relation (2) applies also for anisotropic materials in order to estimate the stress intensity factors. Based on the breaking force, the fracture toughness ( K Ic ) was computed for different orientations. The obtained results are presented in Fig. 5 for both materials. The influence of fiber orientation relative to crack orientation can be noticed for both materials and thicknesses. A decrease in fracture toughness with the decrease in orientation angle can be distinguished. This outcome is due to the higher degree of energy required by the crack to develop at 90° in order to fracture, split and pull the fibers. For the 0°, the crack develops through the matrix and the fibers have a lower contribution. The amplitude of the orientation decrease is dependent on the material thickness. This situation is explained by the level of isotropy in the specimens. The 3.2 mm thick specimens are closer to an isotropic behavior than the 2.0 mm specimens. This aspect occurs due to the injection molding process that will align the fibers in a more uniform direction if the thickness is smaller, Amjadi and Fatemi (2020).

8 10 12 14 16 18 20

PPA_GF33 2.0mm

PPA_GF33 3.2mm

PPS_GF40 2.0mm

PPS_GF40 3.2mm

0 2 4 6

Fracture Toughness [MPa√m]

-15°

0°

15°

30°

45°

60°

75°

90°

Orientation (fiber orientation vs. crack) [ ° ]

Fig. 5. Fracture toughness function of orientation and thickness.