Study on Tensile Strength and Its Characteristics of Domestic Graphite Based on Brazilian Disc Test

Tensile strength of graphite is a crucial criterion in the design and structural integrity evaluation of graphite components in reactors. To investigate the tensile strength of domestic graphite, as well as the effect of specimen size, grain size and microstructure on tensile strength, three kinds of domestic graphites including coarse-grained, fine-grained and ultrafine-grained graphite that may be used in reactor were selected. The Brazilian disc tests of three graphites were performed on Φ6 mm×3 mm, Φ12.7 mm×6.35 mm, Φ24 mm×12 mm and Φ40 mm×20 mm Brazilian disc specimens in the WDW-50 tensile testing machine at a speed of 0.1 mm/min. Force sensor was used to measure the force in specimen. Photoelectric encoder controller was used to determine the displacement of specimen. The tensile failure process of specimens was observed by high speed camera test system. The failure morphologies were observed by SEM (scanning electron microscopy) to study crack propagation behaviors of specimens. The tensile strength and its distribution of graphites were compared, and the effects of specimen size, particle size, microstructure and other factors on tensile strength of graphite were studied. The results indicate that the splitting test of graphite meets the validity requirements of the Brazilian disc test. The larger the specimen size and particle size of graphite, the easier it is to induce secondary cracks and local compression failure. The tensile strength of graphite usually improves as specimen size increases, but the effect of specimen size and particle size should be considered comprehensively when specimen size is small. The specimen size effect of tensile strength becomes more conspicuous as graphite density decreases. Coarsegrained graphite’s tensile strength is the most sensitive to specimen size, finegrained graphite is the second and ultrafinegrained graphite is the weakest. The tensile strength of graphite decreases exponentially with the increase of particle size, showing significant particle size effect. The tensile strength of coarse-grained, fine graind and ultrafine-grained graphite increases sharply with the decrease of particle size. The dispersion of graphite’s tensile strength is noticeable, and it has an obvious correlation with graphite’s particle size. According to statistical analysis by using Weibull distribution model, the tensile strength dispersion of finegrained graphite is substantially better than that of coarsegrained graphite, and ultra-fine-grained graphite is better than fine-grained graphite. The microstructure of graphite has a significant effect on the tensile strength. There are many defects such as large primary pores in coarsegrained graphite, so its tensile strength is the smallest, followed by finegrained graphite, and ultrafinegrained graphite is the largest.