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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aluminum, main applications can range from satellites where air convection is not available [Gandhi and Pathak (2012)] or smartphones, tablet, LED lightning and electrical vehicles cells cooling where the low weight and volume are of crucial importance [NeoGraf Solutions, as viewed from website 1 ], and going on, resistive heating [Chugh and Chung (2002)], electromagnetic field shielding [Luo et al. (2002) and Sykam and Rao (2018)] and plasma facing [Song et al. (2005)]. In a recent work Xi and Chung (2019) also observed the electret, piezoelectret and piezoeresistive properties for mechanical sensing and electric powering.

2. Microstructure and mechanical properties

In this section some notions about the microstructure and the related mechanical properties are summarized. As introduced in the previous section, increasing the pressure of compaction means to increase the density and the degree of FG anisotropy. The scheme in figure 2 shows a summary of such a relationship. The expanded graphite particles

Density

Graphite (single crystal)

2.26 g/cm 3

Flexible graphite

Graphite compact

0 g /cm 3

0.4 - 0.6 g/cm 3

3

1.7 - 1.8 g/cm

Rolling

Natural flakes

Uniaxial compression

Commercial FG foils

Expansion

Anisotropy

Isotropic

Anisotropic

Anisotropic - transversal isotropic

Fig. 2: Manufacturing process along with density and anisotropy behavior (pictures partially rearranged from Celzard et al. (2005) and explanatory commercial FG from Sigraflex ® .

(before the compression) own a rather organized honeycomb structure with an average thickness of ≈ 60 layers each wall [Chen and Chung (2013)]. An example of experimental testing on a single work-like particle can be found in Gu et al. (2002). Once compressed, the worms are in a deformed state that inherits in an extent some properties from the expanded state. In particular, the sliding among the cell walls is still allowed resulting on outstanding visco-elastic properties. Dissipative and visco-elastic phenomena in low density compacts have been largely studied from Luo and Chung (2000), Chen and Chung (2012), Chung (2014), Chen and Chung (2015) and Xiao and Chung (2016). Deformed worms are also the fundamental structural units that determine the overall mechanical properties like com pression strength and recovery [Toda et al. (2013)]; hereafter they will referred to as structural units. The interlocking happens by mean of the only friction forces among their sharp edges and mandate a preferred orientation in the direc tion parallel to the bedding plane. The alignment in the two directions of the plane is random thus giving a transversal isotropy with the increasing stages of compression [Celzard et al. (2005)]. Therefore properties like elasticity and conductivity are usually observed in the two directions in-plane and out-of-plane. In rolled sheets, some di ff erences can be found also in the plane directions (parallel-to-rolling and perpendicular-to-rolling) as described by Dowell and Howard (1986). Such structural units participate in two ways to the overall strength of the material: first, they under goes bending and twisting due to their stuck edges and second, mostly in compression loading, the trapped air inside

1 https: // neograf.com /