Generally, rock material failure is controlled by cracks under specific conditions. The study of rock fracture toughness belongs to the current frequent directions of research in the area of rock failure. The present paper describes the effects of parameters influencing the resultant properties of rock materials (bending rate, rock moisture) during fracture toughness measurement of different kinds of rocks (sandstone, marble, granite). The highest fracture toughness values were found in the marble samples. This is probably due to the inner structure of analysed marble, which is composed of only one mineral (calcite) and also has a lower porosity than the used granite. The lowest fracture toughness values were found in the sandstone sample, and reached c. 17-30 % of the measured fracture toughness values of the analysed granite and marble samples. As in the case of the other mechanical properties of rocks (e.g. uniaxial compressive strength) also in the case of higher sandstone (carboniferous) moisture the fracture toughness values decrease and its deformation ability increases. Preparation of samples for fracture toughness tests and performance of these tests are more complicated than in the case of tensile tests (e.g. the Brazilian test) and therefore this contribution presents a comparison between fracture toughness of analysed rocks and tensile strength values. The measured data in this study considering the fracture toughness tests and Brazilian tests were compared with results published by Zhang (2002)., Leona Vavro and Kamil Souček., and Obsahuje bibliografii
For low-alloyed cast ferritic steel simultaneous effect of temperature and notch root radius on fracture toughness has been investigated. Due to fact that fracture tests were performed on notched specimens (cylindrical tensile specimens with circumferential notch having there notch root radii), the concept of Notch Fracture Mechanics was applied. Volumetric fracture criterion with two parameters, effective stress and effective distance, was developed. The goal of the work was to quantify the influence of notch radius on transition temperature and notch fracture toughness, and, in addition, to present jointly the role of both these parameters in fracture behaviour.
In the case of the studied cast steel with a low yield stress (375 MPa), effective distance is 3 to 4 times longer than the Creager's effective distance (half of notch radius). The transition region and temperatures are shifted to higher values when notch radius decreases. At temperature higher than a temperature called plateau temperature Tp dactile filure appears. The Tp temperature is sensitive to nothe radius being affected by stress triaxiality. It is possible to define a notch sensitivity compensated temperature T* = √ρ/ρc. In the transition regime, critical notch stress intensity factor varies linearly with T*. and Obsahuje seznam literatury