Effect of specimen geometry and testing method on mixed mode I-II fracturetoughness of a limestone rock from Saudi Arabia

The effect of testing method and specimen geometry such as diameter, thickn ess, and crack length and type on measured fracture toughness was investiga ted using specimens collected from a limestone rock formation outcropping i n the Central Province of Saudi Arabia. Straight Edge Cracked Round Bar Ben d (SECRBB), semicircular disk specimens under three point bending, and Braz ilian disk specimens under diametrical compression were used in this invest igation. SECRBB specimens were used for the Mode-I study, and notched Brazi lian disk and semicircular specimens were used for the mixed Mode I-II stud y. The results show that specimen diameter and crack type have a substantia l influence on the measured fracture toughness; however, loading rate, crac k size, and specimen thickness seem to have a negligible effect on the frac ture toughness. Mode-I fracture toughness is significantly influenced by sp ecimen diameter and crack type, while their effects on Mode-II fracture tou ghness are generally negligible. The different specimens (SECRBB, Brazilian disk, and semicircular) can give comparable results only when the proper s pan to diameter ratio is used. The Brazilian disk with a straight notch was found to be the most convenient geometry to use for fracture toughness det ermination. A simple method of making a precise notch inside the disk is pr esented, using the combination of a drilling machine and a wire saw.