PSI - Issue 28

6

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

1821

Table 1. Bending properties of all hybrid composites (strain rate of 2.84×10 -2 s -1 ).

Bending stress [MPa] Bending stiffness [GPa]

Bending strain [%]

Laminate

Average

Std Dev

Average

Std Dev

Average Std Dev

8C

843.4

33.2

48.4

0.95

2.04

0.06

6C+2G

653.0

27.5

31.2

1.26

2.21

0.13

4C+4G

637.2

12.8

31.9

2.83

2.39

0.07

8G

633.4

29.9

22.6

0.47

3.11

0.26

Figure 4 shows the strain rate effect on the bending properties for full glass fibre laminates. While Fig. 4a shows all curves obtained for 2.84×10 -2 s -1 , evidencing a consistent behaviour and without significant dispersion, Fig. 4b shows typical curves of all strain rates and, regardless of the value, behave similarly to those described in Fig. 2.

 � � ���� � �� � ��

 � � ���� � �� �� ��

800

800

600

600

 � � ���� � �� �� ��

400

400

200

200

Bending stress [MPa]

Bending stress [MPa]

0

0

0

1

2

3

4

0

1

2

3

4

Strain [%]

Strain [%]

b)

a)

Fig. 4. Bending stress-strain curves for glass fibre composites: a) Tested at 2.84 × 10 -2 s -1 ; b) Representative curves of all strain rates.

Both the maximum bending stress and bending stiffness were obtained from the previous stress–strain curves, and Fig. 5 presents these properties against the logarithm of strain rate. In this figure, the symbols represent the average values and the scatter bands, respectively, the maximum and minimum values of the data. Higher strain rates promote higher maximum bending stresses, independently of the material. Considering Fig. 5a, for example, it is possible to observe that the maximum bending stress for full glass fibres laminates increases from 571.3 MPa at 2.84×10 -4 s -1 to 733.4 at 2.84×10 0 s -1 , which represents an increase of 28.4%. In terms of full carbon laminates, the maximum bending stress increases from 719.1 MPa to 881 MPa, which represents an increase of 22.5%. On the other hand, from Fig. 5b it is noticed that the bending modulus increases from 22 GPa at 2.84×10 -4 s -1 to 23.4 GPa at 2.84×10 0 s -1 , which represents an increase of 6.4% for full glass fibre laminates. Regarding the full carbon laminates, this increase is around 2.5%. Finally, similar tendency was observed for hybrid laminates, but with values between those observed for full carbon and glass fibre laminates. In terms of viscoelastic behaviour, Fig. 6 presents the stress relaxation curves for all laminates. This figure plots the average bending stress versus time, where  is the bending stress at any given moment of the test and  0 is the initial bending stress. For this study, a fixed displacement was applied with values corresponding to bending stresses of 422 MPa, 327 MPa, 319 MPa and 317 MPa for 8C, 6C+2G, 4C+4G and 8G laminates, respectively. These values correspond to 50% of the maximum bending stress of each laminate.