PSI - Issue 26

E. Solfiti et al. / Procedia Structural Integrity 26 (2020) 187–198 E. Solfiti and F. Berto / Structural Integrity Procedia 00 (2019) 000–000

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4

the expanded compressed closed regions gives a significant contribution to unloading recovery, as reported in Toda et al. (2013). FG mechanical properties not only depend on the degree of worms interlocking but also on prior process parameters such as original flake sizes, ash content, chemical intercalant species and exfoliated volume. Larger flakes for example can result on higher exfoliated volume and therefore bigger structural units in which an eventual pull-out stress would be hindered [Yoshida et al. (1991)]. In figure 3a such e ff ect is outlined. Fully exfoliated worms (up to 300

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 1 2 3 4 5 6 7 8

10

8

6

4

2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0

(a)

(b)

Fig. 3: (a) E ff ect on tensile strength (in-plane) due to the average flake size: 1. Leng et al. (1998), 2. Dowell and Howard (1986), 3. Wei et al. (2010), 4. Gu et al. (2002); and (b) e ff ect on tensile strength (in-plane) due to the density: 1. Leng et al. (1998), 5. Reynolds (1965). The intercalant species used are H 2 SO 4 ,HNO 3 and HClO 4 .

times the original flake thickness) assure a tensile strength increase [Leng et al. (1998)]. Pure carbon content without any residual metal oxides could both avoid the burn-out at high working temperature and also improve the overall strength [Dowell and Howard (1986), Savchenko et al. (2010)]. Finally, a linear relation was found among density and in-plane tensile strength as in figure 3b. An increasing trend is valid also for compression values against density increment [Leng et al. (1998)]. Scarce data are available to properly define the main mechanical properties, also due to a large variation of these against density, di ff erent manufacturing technique and parameters: a summary of values for tensile and compression strength are reported in table 1.

Reference

Density [g / cm 3 ]

Tensile strength [MPa]

Young’s modulus [GPa]

Compressive strength [MPa]

Compressive modulus [GPa]

Poisson’s ratio

Seldin (1966) (polycr. graph.) Dowell and Howard (1986) Leng et al. (1998) Gu et al. (2002) Toda et al. (2013) Xi and Chung (2019)

1.72 - 1.94

13.44 - 30.33 8.96 - 17.7

0.07 - 0.22

1

≈ 4 ≈ 4

2.29

150

0.366 - 0.597

1.1

1 1 1 1

≈ 3.5

0.04

1.335

Sigraflex ® Grafoil ®

> 4

1.12

4.5 - 5.2

165

0.190

Table 1: Range of main mechanical properties (tensile properties in-plane, compression properties out-of-plane)