PSI - Issue 68

Carlos Fernandes da Silva et al. / Procedia Structural Integrity 68 (2025) 1252–1258 C. F. das Silva et al. / Structural Integrity Procedia 00 (2024) 000–000

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and Govaert (2005). In the case of the yield stress, the molecules are initially at rest and assume local equilibrium configuration in which the local segments are entangled. After the initial slip, however, the segments acquire a more linear configuration, so the slip can continue at a lower stress level, which is less a ff ected by the local molecular configurations. In other words, the addition of PA is expected to have a larger impact on the yields stress compared with the drawing stress, as observed in the present results. A di ff erent result is obtained for the specific work of fracture (Figure 4). Here the addition of PA leads to the decrease of the specific work of fracture, with an apparent advantage for the PA 66 resin. The dispersion is very large for this variable and the only conclusion that can be drawn is that the addition of PA leads to a decrease in the specific work of fracture.

ligament: 8 mm

100

80

-2 ]

60

J m

3

w f [10

40

20

PET PA 66 PA 6T6I

0

0

5

10

15

20

% PA

Fig. 4. Dependence of the specific work of fracture ( w f ) on the concentration of PA. Error bars correspond to the 95% confidence limit of the average, lines are just guides to the eye.

With the methodology adopted in the present work, with a fixed ligament length, it is not possible to separate the contributions to the build up of the plastic zone and to the propagation of the crack itself, the results however can be used as a measure of the toughness of the films and to compare the e ff ect of the di ff erent PA additions to the blend. The results show that the addition of the two PA do not a ff ect the specific work of fracture to PA contents up to 10 weight %. The values decrease for the 20 weight %, with a small advantage for the PA 6T6I resin, but the large dispersion of this variable allows only to conclude that the specific work of fracture of both films is smaller than that measured for the PET resin. Films of blends of PET and PA were produced with PA contents up to 20 weight %, using two di ff erent PA resins, a standard PA 6.6 grade and a modified PA 6T6I grade. The obtained films were produced in thicknesses between 0.15 and 0.35 mm, and in this range, the mechanical properties were found to be independent of the thickness. Double edge notched tensile tests were conducted with a fixed ligament length of 8 mm and the properties were characterized by the yield strength, the drawing stress and the specific work of fracture. Yield strength in slightly a ff ected by the additions of PA, resulting in an increment of strength in the case of the PA 6T6I resin and a softening in the case of the PA 6.6 resin. The changes are very subtle, and the substitution in the range here investigated can be considered to maintain the same level observed by the pure PET resin. Drawing stress changes are even more subtle, following the same trends as the yield strength (a slight increase for the PA 6T6I resin and a small decrease for the PA 6.6 resin), the changes in the present case are well within the statistical uncertainty of the measurements, showing that the same properties of the pure PET resin are maintained in the blends. The specific work of fracture, on the other hand, shows a clear decrease for the 20 weight % blends, even consid ering the large dispersion observed in this variable. In general, the obtained blends showed the same mechanical properties of the corresponding PET films, except of the 20 weight % PA additions, which resulted in a lower toughness film. 6. Conclusions