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Microstructures of investigated specimens before test: (a) graphite (Gr)-free phenolic resin (PF) matrix; (b) 10 vol.% Gr/PF; (c) 20 vol.% Gr/PF; (d) 25 vol.% Gr/PF; (e) 30 vol.% Gr/PF, (f) column diagram of Gr granules, micro-fissures and gaps’ cross section area.
