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

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3.1. Compression and recovery properties

Compression and recovery are the fundamental properties for sealing performances and they are a straightforward e ff ect of the inherent resilience of FG. Dowell and Howard (1986) found a non-linear relation among stress and strain during compression until a value of density equal to 1.7 g / cm 3 . Beyond this value, irreversible compression is not easy to perform anymore and stress-strain behavior turns into a linear trend. Moreover some critical points were highlighted: the sti ff ness in compression increases when the misalignment of the basal planes (which in this case probably are equivalent the aforementioned deformation units) increases, the reversible work of compression is more important than irreversible work at higher densities and the micro-mechanism of carrying the load is entirely attributes to relative bending of the basal planes. However, about the latter mechanism, Leng et al. (1998) supposes a contribute even of the trapped-air among the layers and Toda et al. (2013) finally confirmed such an hypothesis: the mechanism is assumed to include both bending of the units and trapped molecules, in di ff erent manners in compression and recovery. Leng et al. (1998) also investigated such quantities in the out-of-plane axis, within the commercial range of densities and the natural flakes sizes (fig.5), underlining that the ideal working range for sealing fall in 1 − 1 . 4 g / cm 3 where both assume the best values. The size of the natural flakes do not seem to have a remarkable influence on compression / recovery.

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Fig. 5: (a) E ff ect of density and (b) e ff ect of flake sizes on comprimibility / recovery. Pictures by Leng et al. (1998).

Recent applications of nanoindentation techniques in Khelifa et al. (2018); Chen and Chung (2015) allowed to extrapolate local Young‘s modulus (to be compared to elastic compression modulus) and yield stress values from the unloading curves: for 0.86 g / cm 3 densit, the modulus does not exceed 1.7 MPa (Chen and Chung (2015)) whereas for 1.1 g / cm 3 , it overcomes 200 MPa Khelifa et al. (2018). Even though they must be confirmed, these values are significantly smaller than those owned by any other types of graphite. As an example, a single crystal of graphite has a modulus of 36.5 GPa whereas highly oriented pyrolytic graphite (HOPG) ranges in 15-20 GPa (Xiao et al. (2013)). Yield stress carried out by Khelifa et al. (2018) is equal to 1.9 MPa whereas Dowell and Howard (1986) reports 100-150 MPa. The latter seems more consistent with data from furnishers of commercial FG (approximately from 70 to above 200 MPa). ( come faccio a citare un datasheet? ). It has to be account for that the nanoindentation is a local technique in which the defects content of the sample is minimized, therefore local values and overall values are probably not comparable.