PSI - Issue 25

E. Solfiti et al. / Procedia Structural Integrity 25 (2020) 420 – 429 E. Solfiti and F. Berto / Structural Integrity Procedia 00 (2019) 000–000

425

6

3.2. Tensile properties

The tensile strength in the in-plane direction increases linearly with the density, approximately in the range 0.5 1.1 g / cm 3 (see fig.6 and Dowell and Howard (1986)). Conversely Gu et al. (2002) did not observe the same strong correlation in the out-of-plane directions.

(a)

(b)

Fig. 6: (a) Linear relation between density and ultimate tensile strength and (b) e ff ect of flake sizes on tensile strength. Pictures by Dowell and Howard (1986) and Reynolds and Greinke (2001), respectively.

Such linear relation was also investigated by Ionov et al. (2000) that noticed a dependence by the intercalate species both on the absolute values of the strength and on the regression slope against the density. According to Yoshida et al. (1991) it has been argued that some intercalate substances like sulphuric acids result in larger flat ”baloons” and hence larger deformation units (closer to the size of the pristine flakes) in which an eventual pull-out stresses is hardened. The pristine flake size also a ff ects the tensile strength probably because wider flakes gives higher exfoliated volume as explained in section 2. The e ff ect of the exfoliated volume on the tensile strength is clearly visible only when the GIC is ”fully” exfoliated (Leng et al. (1998); Wei et al. (2010); Gu et al. (1985); Reynolds and Greinke (2001)). It was noticed that the tensile strength also depend on the ash content of the pristine graphite: Dowell and Howard (1986) found that increasing ash content means to decrease the potential enhancement leads by the density rise and, afterwards, Savchenko et al. (2012) directly observed a linear relation among the tensile strength and the ash content. Despite of the intercalant species (and the exfoliated volume enhancement), the negative slopes of such a relation are very similar. About the tensile strain along the in-plane direction, it was found to be completely di ff erent between the axis parallel to the rolling direction and perpendicular to it. Rolling process might induce an higher tensile deformation in the rolling direction and a ff ect the maximum strain achievable, despite the ultimate tensile stresses seems una ff ected (Dowell and Howard (1986)). Typical stress-strain curves of tensile tests are reported in Dowell and Howard (1986); Mo et al. (2019); Sykam and Rao (2018) but the only attempt to carry out a constitutive model were carried out using the Jenkins (1962) equation but with no-satisfactory results on fitting beyond a half of the tensile strength. Both elastic and plastic components contribute on the deformation, giving anisotropy in-plane (due to the rolling) and out of-plane (due to the porosity and deformation units preferred orientation). Young‘s tensile modulus values reported from Dowell and Howard (1986) goes from 0.5 to 3 GPa, in accordance with 1.38 GPa shows Xi and Chung (2019) and with commercial data from furnishers ( sito sigraflex - quella parte introvabile ). Typical values of the tensile strength in the in-plane direction can be up to 7 MPa whereas in the out-of-plane direction are lower than two orders of magnitude (Gu et al. (2002)). Higher values up to 17 MPa were carried out by Sykam and Rao (2018). Elongation at break in the in-plane direction ranges in 1-2%, comparable to those reached by carbon fibers. About the fracture mechanism only a few basic notions are known. During rolling or compaction no chemical bonding acts on the locking mechanism resulting in purely mechanical interactions among units that are weaker than the internal bonding of the worms. During tensile tests of notched specimens it was observed by Gu et al. (2002); Leng et al. (1998) that the