PSI - Issue 28

P. Santos et al. / Procedia Structural Integrity 28 (2020) 1816–1826 P. Santos/ Structural Integrity Procedia 00 (2019) 000–000

1822

7

For all laminates, it is noticed a decrease of the stress from an initial value (  0 ) to one that is not yet constant, because this study focuses on short-term tests. These tests are an easy, fast and reliable method to predict long-term behaviour (Lim et al. 2004), where this constant value will be expected only for higher stress values. Another evidence conveniently reported in the open literature is the existence of an initial regime in which the stress decreases considerably in relation to the remaining time (Ferreira et al. 1999; Reis et al. 2019). For example, for full carbon composites, there is a decrease of about 2.1% after 40 minutes, while in the remaining time (between 40 and 180 minutes) this value is around 1.2%. The same comparison for full glass fibres composites promotes values around 9.6% and 3.4%, respectively.

1000

Full carbon laminates

900

800

700

600

Bending stress [MPa]

Full glass fibre laminates

500

400

-4

-3

-2

-1

0

1

Log strain rate [s -1 ]

a)

51

30

49

28

Full carbon laminates

47

26

45

24

Bending stiffness [GPa]

Bending stiffness [GPa]

43

22

Full glass fibre laminates

41

20

-4

-3

-2

-1

0

1

Log strain rate [s -1 ]

b) Fig. 5. Effect of the strain rate on the: (a) Bending stress; (b) Bending stiffness.

From Fig. 6 is also clear that full glass fibre composites are more sensitive to stress relaxation than full carbon composites. While for the first laminates, the stress decreased around 12.7% after 180 min, this value is only 3.2% for full carbon composites. According to the open literature, these results are essentially consequence of two mechanisms, matrix phase relaxation and fibre-matrix bond failures (Sreekala et al. 2001). In terms of matrix, the stress relaxation