PSI - Issue 25

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

371

2

Karataş et al. 2018) . On the other hand, air pollution, global warming and the scarcity of fossil fuels are putting enormous pressure on industry, especially in the transport sector, which is responsible for 25% of greenhouse gas emissions in Europe. In this context, weight reduction is the main priority for reducing fuel consumption and pollutants emitted. Therefore, the use of polymer composites reinforced by natural fibres have been increasing due to their low density, abundance, low cost, renewable, non-abrasiveness during processing and biodegradable (Reis et al. 2011). Regarding the intrinsic properties, they have a specific weight that is about half the weight of glass fibres and a tensile modulus very similar to aramid fibres, but it should be noted that all mechanical properties are dependent on the cellulose content in the fibre, degree of polymerisation of the cellulose and microfibril angle (Jayaraman 2003). Flax fibres, for example, are the strongest natural fibres. Table 1 compares their properties with those of glass fibres. On the other hand, and in the same context, green resins are an alternative to petroleum-based resins to manufacture eco-friendly and biodegradable composites due to their mechanical performances and desirable functional properties that allow chemical modifications. Table 2 compares physical and mechanical properties of a typical synthetic epoxy resin and a green epoxy resin.

Table 1. Physical and mechanical properties of glass fibre and flax fibre (Campilho 2015)

Table 2. Physical and mechanical properties of a synthetic epoxy resin and a greepoxy resin.

Fiber

E-glass 2.5-2.59

Flax

SR GreenPoxy 56 / SD Surf Clear

SR 8100 / SD 8822

Matrix

Density Length Diameter

[g/cm 3 ] [mm] [µm] [MPa] [GPa]

1.4-1.5

Viscosity (@ 20 ºC) Modulus of elasticity

[mPa×s] 390 ± 80

900

-

5-900

<17

12-600

[N/mm 2 ] 3390

3300

Tensile strength Tensile modulus Specific modulus

2000-3500 343-2000

70-76

27.6-103

[N/mm 2 ] 115

114

Maximum resistance

[approx.] 29

45

Elongation at max. load Elongation at break Glass transition / DCC Water absorption 48 h / 70 °C Resilience

[%]

3.9

4.5

Elongation Cellulose

1.8-4.8

1.2-3.3

[%]

[wt.%] [wt.%] [wt.%] [wt.%] [wt.%]

- - - - -

62-72

[%]

5.8

4.7

Hemicellulose

18.6-20.6

Lignin Pectin Waxes

2-5 2.3

[kJ/m 2 ]

19 66

16 65

[ºC]

1.5-1.7

Micro-fibrillar angle [degrees] -

5-10 8-12

[%]

1.2

-

Moisture content

[wt.%]

-

Finally, as a consequence of the inherent viscoelasticity of the matrix phase, polymer composites are prone to creep and stress relaxation, making it a great challenge when they are used in long-term applications. A better understanding of these properties is required to predict the dimensional stability of load-bearing structures. Therefore, the main goal of this work is to study the stress relaxation and creep behaviour of flax composite laminates and the hybridization effect on the viscoelastic properties of such composites. For this purpose, composites with the same lay-up but with different fibres (flax and glass fibres) were manufactured with a green epoxy resin. The bending mode was selected because it is the most sensitive for this type of analysis and one of the most popular on structural loading. Finally, the Kohlrausch-Williams-Watts and Findley models will be used to model the viscoelastic response and to predict the long-term structural behaviour. 2. Materials and experimental procedure Flax bi-directional woven fabric (taffeta with 195 g/m 2 ) and glass fibre woven bi-directional fabric (taffeta with 195 g/m 2 ) with a SR Greenpoxy 56 resin and a SD Clean hardener were used to prepare different composite laminates.