Experimental and numerical study on determination of indirect (splitting) tensile strength of rocks under various load apparatus

The indirect tensile strengths (ITSs) of more than 200 disc specimens for 19 different rock and rock-like materials were determined by considering the concept of fracture toughness under different loading conditions. Steel loading arcs with various contact angles and flat platens were used in experiments to investigate whether they can be used as alternatives for the standard Brazilian jaw. Many of the disc specimens tested under the standard jaw did not fail with ideal tensile cracking at the centre, whereas simple splitting cracks were obtained along the vertical diameter of most of the discs tested under the flat platens. However, flat platens caused cracks to initiate in the compressive zone just under the loading points. This is an important outcome for criticizing the validity of the test because tensile failure is expected to occur along the vertical diameter of the disc. Friction conditions between the disc and jaw have important roles in the failing load. Moreover, fracture toughness is an additional parameter for investigating the ITSs of rock materials. According to the experimental results, fracture toughness values were found to increase with an increase in contact angle. In addition to the experimental study, finite element analyses were performed on several types of rock discs under various loading conditions. As a result, a comparison between all loading conditions was made to determine the best ITSs of the rocks. Both experimental and numerical analyses concluded that deciding the best loading type depends on the rock type. Therefore, the standard jaw is not recommended to use for determination of the ITS values of all rock types. Key words: tensile strength, indirect tensile strength test, splitting method, Brazilian test, loading apparatus, finite element analyses. On a determine la resistance a la traction indirecte (RTI) de plus de 200 specimens discoidaux composes de 19 types differents de materiaux rocheux ou d'aspect quasi rocheux en prenant en compte le concept de << tenacite >> dans differentes conditions de chargement. Des experiences ont ete realisees sur des arches de chargement en acier presentant differents angles de contact et sur des plateaux a surface plane pour savoir si ces arches et plateaux pouvaient remplacer le dispositif standard de compression par fendage. Un grand nombre des specimens discoidaux mis a l'essai a l'aide du dispositif standard de compression n'ont pas cede et comportaient une fissuration ideale de traction en leur centre, tandis qu'une simple fissuration le long du diametre vertical a ete observee sur la plupart des disques mis a l'essai a l'aide des plateaux a surface plane. On a constate que, lorsqu'on utilisait des plateaux a surface plane, les fissures prenaient naissance dans la zone de compression, situee juste en dessous des points d'application du chargement. Il s'agit d'une observation importante qui permet de critiquer la validite de l'essai realise, car on s'attend a ce que la rupture en traction se produise le long du diametre vertical du disque. Les frottements entre le disque et le dispositif de compression influent beaucoup sur la charge de rupture. De plus, la tenacite constitue un parametre supplementaire a prendre en compte lors de l'etude de la RTI de materiaux rocheux. D'apres les resultats experimentaux, on a observe que les valeurs de tenacite augmentaient avec l'angle de contact. Outre l'etude experimentale realisee, des analyses par la methode des elements finis ont ete effectuees sur plusieurs types de disques rocheux differents dans differentes conditions de chargement. On a pu ainsi comparer entre elles les differentes conditions de chargement afin de determiner la RTI optimale de chaque specimen. Des analyses experimentales et numeriques ont permis de conclure que le choix d'un type de chargement optimal dependait du type de roche mis a l'essai. Par consequent, il n'est pas recommande d'utiliser un dispositif standard de compression pour determiner la RTI, quel que soit le type de roche etudie. [Traduit par la Redaction] Mots-cles : resistance a la traction, essai de resistance a la traction indirecte, type de fissuration, essai de traction par fendage, dispositif de charge, analyses par la methode des elements finis.
Introduction The Brazilian test is the most popular indirect tensile strength (ITS) test method because it can be used with conventional compressive test equipment (Hobbs 1964; Barla and Innaurato 1973). [...]