PSI - Issue 25

P. Santos et al. / Procedia Structural Integrity 25 (2020) 370–377 P. Santos./ Structural Integrity Procedia 00 (2019) 000–000

373

4

For flax laminates, the bending stress is around 128.6 MPa and when are added glass fibres on the outside of the laminate ([2GF+8FF+2GF]) this value increases 66.9% (214.6 MPa) but with more glass fibres, in the middle section of the laminate [2GF+4FF+2GF+4FF+2GF], this value reaches values 83% higher (235.3 MPa).

100 150 200 250 300

100 150 200 250 300

Flax 1xhybrid 2xhybrid

0 50

0 50

Flexural stress [MPa]

Flexural stress [MPa]

Flax

1 x hybrid 2 x hybrid

0

1

2

3

4

Strain [%]

Material

b) Fig. 2. (a) Flexural stress strain representative curves of the laminates; (b) Bending strength for all laminates.

a)

In terms of stress relaxation, Fig. 3 shows typical curves obtained for flax laminates and hybrid 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. These results are representative of the stress relaxation behaviour for all conditions analysed.

1,00

1,00

0.93

0,95

 /  0

0.92

 /  0

0,98

2xhybrid

0.91

0.91

1xhybrid

0,90

0.90

0.92

0,95

0.93 0.89

0.91

50 MPa

0.90

0,85

0,93

0.92

Flax

0.91

118 MPa

0.84

0,80

177 MPa

0,90

0.83

0.90

0.82

0.89

0,75

0.88

0,88

0

50

100

150

200

0

50 100 150 200

Time [min]

Time [min]

a)

b) Fig. 3. Stress relaxation curves for: a) different laminates and strain correspondent to a bending stress of 50 MPa; b) 2× hybrid laminates and strain correspondent to different stress values; In detail, from Fig.3a) it is possible to observe that, regardless of the degree of hybridization, the stress decreases with the time, but laminates with higher number of glass fibres are less prone to stress relaxation. For example, after 180 min and for flax laminates this decrease is around 17.4%, while for 1xhybrid laminates is about 8.7% and for 2xhybrid laminates is 8.1% (values around twice less than for flax laminates). Therefore, increasing the hybridization degree decreases the sensitive to the stress relaxation. On the other hand, Fig. 3b) shows the sensitivity of the 2xhybrid