PSI - Issue 56

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

173

7

With the increase of material thickness, a smoothing effect of the fracture toughness along the orientation (the variation in between 0° and 90° orientation is smaller for the 3.20 mm thick specimens) can be distinguished. The difference between the 0° and 90° orientation will decrease and will tend to converge to a single value (similar to an isotropic material), if the thickness increases further. For both materials, the average fracture toughness (computed using the 0°, 45° and 90° orientation) has similar values regardless of the thickness (PPA GF33: for 2.0 mm thickness, the value is 13.04 MPa √ m and for 3.2 mm thickness, the value is 12.57 MPa √ m; PPS GF40: for 2.0 mm thickness, the value is 7.11 MPa √ m and for 3.2 mm thickness, the value is 7.97 MPa √ m). Based on these values, it can be concluded that the fracture toughness tends to flatten out around the average value obtained using the 0°, 45° and 90° orientation with the increase of material thickness. The orientation effect is much stronger for higher fracture of glass fiber content. This can be seen especially for the PPS GF40, when considering the 2.0 mm thickness. The values obtained for the 0° and 45°orientation are much closer. A significant increase for the 90° orientation can be noticed afterwards. For both materials, the predictions arising from 2.0 mm thick specimens do not coincide with the ones arising from 3.2 mm. For PPA GF33, an overlap at 45° orientation can be noticed. This statement is not valid for PPS GF40, the value at 3.2 mm is approximately two times higher, compared to the value at 2.0 mm (8.3 MPa √ m, compared to 4.3 MPa √ m). The fracture toughness will be higher for 3.2 mm thick specimens, at 0° and 45° orientation and lower for 90° orientation, in comparison with 2.0 mm thick specimens. Equation (3) was used to predict and compare the fracture toughness at 45° orientation, based on the 0° and 90° values. The formula is raised in (Kfouri 1996). 1 ( ) 2 = cos 2 (0 ° ) 2 + 2 (90 ° ) 2 (3) The results are presented in Table 2. Different results for the two materials were obtained. In the case of PPA GF33, the results match precisely with the values obtained using equation (1). Differences of 2% - 3% can be seen for both thicknesses. For PPS GF40, the prediction is not reliable. For 2.0 mm thickness, an overestimation of 23% can be distinguished. For 3.2 mm thickness, an underestimation of 15% can be distinguished.

Table 2. Fracture toughness comparison for 45° orientation.

Eq. (1) [MPa√m]

Eq. (3) [MPa√m]

Difference [%]

Material

Thickness [mm]

PPA GF33

2.0 3.2 2.0 3.2

11.9 12.2

12.1 12.6

2% 3%

PPS GF40

4.3 8.3

5.3 7.1

23% -15%

4. Conclusions and discussions The current paper investigated the effects of thickness and orientation of two short fiber reinforced polymers (PPA GF33 and PPS GF40) on fracture toughness. In this direction, a set of tensile tests on SEC specimens were performed to determine the breaking force. An analytical approach was used to determine the fracture toughness of the materials based on the test results. The main conclusions that can be deduced from this study are the following: • The results in terms of breaking force have a relatively high spread. The notch was manually cut using a vertical bandsaw machine, that led to an inconsistent and irregular crack on all specimens. Nevertheless, both effects of thickness and orientation can be noticed in terms of reaction forces for both materials. • Increasing the material thickness of the specimen will lead to uneven change in fracture toughness at different orientations. For 0° and 45° orientations, an increase in fracture toughness can be identified, opposite to the case for 90° orientation, where a decrease trend can be observed. This phenomenon is similar for both