Spall Fracture of Graphite Materials by Impact of a Small Projectile

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Three types of polycrystalline graphites and two 2D-C/C composites were impacted by a steel projectile of 4-mm diameter with a velocity of 0.7km/s to 1.7km/s. The damage behavior by impact, especially, spallation of the materials were investigated through the measurement of scattered and spalled volumes, formation and propagation of stress wave, shape of scattered fragments (debris), and the observation of debris by a high speed camera. The followings were concluded: (1) Critical velocity of spallation in graphites increased with increasing their static strength and thickness of the specimen, (2) weak-bonded layers in 2D-C/C composite was easily debonded (peel-off) at the interfaces by impact, (3) in C/C composites debonding region near the rear surface was larger in the case a projectile stopped in them than in the case it passed through them, (4) shock waves in graphites could be measured using acoustic emission sensor, (5) the size of the spalled fragments depended on the material properties even among polycrystalline graphites, and (6) a model of spalling process in a polycrystalline graphite was proposed.
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