In classical fracture mechanics, cracks propagate when the stress intensity factor reaches or exceeds the critical stress intensity factor. If the crack growth can occur in a stress intensity factor lower than the critical state, it is called subcritical crack growth (SCG). The study of subcritical crack growth is of great importance to investigate the long-term behavior of rock under applied loads and as a result to eva‎luate the long-term stability of underground and surface structures in rock masses. Subcritical crack growth is one of the main causes of time-dependent behavior in rocks, which is mainly attributed to stress corrosion cracking. Stress corrosion is a chemical reaction between silica present in the crack tip area under stress and water. However, subcritical crack growth is also influenced by environmental factors. Considerable research has been done to determine the parameters of subcritical crack growth in mode I, but the studies conducted on the parameters of subcritical crack growth in mode II loading are limited, although this loading mode is useful in many rock engineering problems. It has important applications. For example, for the time-dependent stability of underground and surface excavations in rock, the time-dependent growth of shear fractures is often more important than the growth of tensile cracks. The aim of this study is to understand the effect of heat as one of the most important environmental factors on the fracture behavior of hornfels stone, which were heated at 25°C (without heat treatment), 250, 500 and 750°C, respectively. In this study, in order to determine the subcritical crack growth parameters, the constant stress rate test and also one of the available fracture mechanics tests for mode II loading (four-point bending test) have been used. Experiments were carried out at three fixed displacement rates of 0.06 mm/min, 0.6 mm/min and 6 mm/min, and in each case, three experiments were performed to control repeatability. Based on the results, the fracture toughness of hornfels samples increased with increasing temperature up to 250 degrees Celsius and then decreased with increasing heat treatment temperature. As the heat treatment temperature increased from 250°C, the toughness decreased drastically due to the thermal breakdown of the quartz crystal structure and the creation of wider intergranular fractures. With the increase in temperature, the grain size and the average opening of the crack first decreased and then increased, while the reverse trend distortion has the ratio of the grain size and the average opening. In this study, the subcritical crack growth rate was calculated using the subcritical crack growth parameters and the stress intensity factor in mode II loading. The results of the experiment showed that for a certain value of the stress intensity factor (KII), the highest subcritical crack velocity results for a temperature of 750 °C (4.63×10-7-7.87×10+1 m/s) and the lowest subcritical crack velocity occurs for a temperature of 250 °C (2×10-10-6.80×10-4 m/s).

محمود بهنيا

لهراسب فرامرزي

عليرضا باغبانان، حميد هاشم الحسيني