Notch effect on the fracture of several rocks: Application of the Theory of Critical Distances

The paper analyses the fracture behaviour of several rocks, namely a sandstone, a limestone and two marbles, one of them being a Carrara marble. The experimental program comprises in total 216 fracture specimens, tested in 4-point bending conditions and including specimens with notch radii varying from 0.15 mm up to 15 mm. The notch effect is analysed through the evolution of the apparent fracture toughness and the application of the Theory of Critical Distances. The present study aims to generalize a previous study on a granite and a limestone to a broader range of rocks. The point and line methods of the Theory of the Critical Distances successfully explain the notch effect on the fracture specimens. The value of the critical distance of these rocks is of the order of mm. Finally, the results show a correlation between the microstructural features of the rocks, specifically the grain size, and their critical distances.The authors of this work would like to express their gratitude to the Spanish Ministry of Economy and Competitiveness for financing the National Plan Project (Ref.: BIA2015-67479-R) under the name of ‘La Distancia Crítica en la Fractura de Rocas’ (The Critical Distance in Rock Fracture)

Notch effect and fracture load predictions of rock beams at different temperatures using the Theory of Critical Distances

ABSTRACT: This work aims to analyse the fracture behaviour of rocks with U-shaped notches subjected to mode I loading and to different temperature conditions. To this end, the so called Theory of Critical Distances (TCD) is applied and four different types of isotropic rocks are studied: a Floresta sandstone, a Moleano limestone, a Macael marble and a Carrara marble. This study attempts to extend a previous work of the authors where the TCD was successfully applied to U-notched components subjected to mode I loading conditions at room temperature. In this case, the effect of temperature is considered as a new variable. The research comprises, in total, more than 790 four-point bending tests and 144 tensile splitting (Brazilian) tests. The latter include 6 disc-shaped specimens for each rock and temperature (6 different temperatures), while the four-point bending tests consist of at least 6 SENB specimens for each rock, notch radius (8 different notch radii varying from 0.15 mm to 15 mm) and temperature (4 different temperatures) combination. The temperatures considered in this study vary from room temperature up to 250ºC, which is a common range in geothermal applications. Temperature has proven to be a significant parameter when analysing the fracture behaviour of the four selected rocks. Its influence on the tensile strength and fracture toughness of the rocks is clear and reveals common patterns. However, no apparent tendencies are shown on the influence of temperature on the critical distance (L). Likewise, the application of the TCD has led to relatively accurate fracture predictions and notch effect analyses at different temperature conditions.The authors of this work would like to express their gratitude to the Spanish Ministry of Economy and Competitiveness and to the European Regional Development Fund (ERDF) for financing the National Plan Project (Ref.: BIA2015-67479-R) under the name of “The Critical Distance in Rock Fracture”

Analysis of notch effect on the fracture behaviour of granite and limestone: An approach from the Theory of Critical Distances

This paper presents the analysis of the notch effect on granite and limestone fracture specimens. The research is based on the results obtained in an experimental programme composed of 84 fracture specimens, combining the two materials and 7 different notch radii varying from 0.15 mm up to 10 mm. The notch effect is analysed through the evolution of the apparent fracture toughness and the application of the Theory of the Critical Distances. The results reveal a significant notch effect in the limestone, whereas the notch effect in the granite is negligible for the range of notch radii analysed. Both observations are justified by the corresponding critical distance of the material