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Measurement of the anisotropic strength and toughness of Pitch-55 carbon ribbons

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Abstract

Composites with long reinforcing fibres can be effectively toughened by controlled crack deflection at interfaces of such reinforcements. An important element in achieving this end is the measurement of the strength and toughness of the reinforcing fibres. Here we present results of such experiments. Since measurements of these properties are difficult in fibres of micron dimensions, specially prepared Pitch-55 ribbons of 600 μm width and 35 μm thickness were obtained for doing the tests. As with carbon fibres, these ribbons show strong anisotropy in their elastic and inelastic properties. Hence, the work of fracture along and across the graphitic planes were determined. The average longitudinal tensile strength of these ribbons was found to be 0.6 GPa. Due to inhomogeneities and a possible associated size effect this value is considerably lower for ribbons than that for fibres. The average specific work of fracture of the ribbons across the graphitic planes was determined to be 3.5 J m−2. Unusually high values of works of fracture of 166 J m−2 were obtained for cracks propagatin along the longitudinal graphitic lamellae. These high values were attributed to a profuse kinking type of plasticity on the graphitic lamellae lying parallel to the crack plane. An asymptotic elastic-plastic analysis for single crystals due to Rice [12], was used to estimate the level of inelastic energy dissipation. These estimates were close to the experimentally observed values. The calculation of the dissipated inelastic energy requires knowledge of the interlaminar shear strength across the graphitic lamellae. A torsion experiment was designed to measure the interlaminar shear strength across the weak graphitic planes, giving a value of 83 MPa. An in-plane longitudinal shear modulus of 65 GPa was obtained from the linear portion of the quasi-static torque twist curve of the ribbon. Another measure of this modulus was determined independently from torsional vibration tests, giving an average value of 51 GPa.

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Gupta, V., Argon, A.S. Measurement of the anisotropic strength and toughness of Pitch-55 carbon ribbons. J Mater Sci 27, 777–785 (1992). https://doi.org/10.1007/BF02403896

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