Elastic Constants of Compression-Annealed Pyrolytic Graphite

Abstract

The elastic constants of pyrolytic graphite which has been highly ordered by annealing under compressive stress have been determined by ultrasonic, sonic resonance, and static test methods. Ultrasonic tests yielded the elastic constants c₁₁, c₁₂, c₃₃, c₄₄ = 1/s₄₄, and the stress derivative of c₃₃. The moduli 1/s₁₁ and c₄₄ were determined from the free flexural resonant vibrations of bars, and the shear modulus c₄₄ also was determined from the fundamental torsional resonance of the bars and from the resonance of compound torsion oscillators composed of thin graphite disks with steel end pieces. Static tension, compression, and torsion tests on the pyrolytic graphite yielded a complete set of compliances (s₁₁, s₁₂, s₁₃, s₃₃, and s₄₄). The following self‐consistent set of elastic constants (c_ij in units of 10¹¹ dyn/cm²; s_ij in 10⁻¹¹ cm²/dyn) has been deduced from the results: c₁₁ = 106±2, c₁₂ = 18±2, c₃₃ = 3.65±0.10, c₁₃ = 1.5±0.5, c₄₄ = 0.018 to 0.035, 1/s₁₁ = 102±3, 1/s₃₃ = 3.65±0.10, s₁₂ = −0.0016±0.0006, s₁₃ = −0.0033±0.0008, and s₆₆ = 0.023±0.002; with the possible exception of c₄₄, these constants are expected to be typical of the graphite single crystal. The shear strength for shear between basal planes was found to be in the range of 90–250 g/mm².