Rock fatigue denotes the progressive degradation in rock strength due to cyclic loading, as opposed to static failure which results from a constant load. Understanding the interaction between fatigue loading and freeze-thaw cycling is crucial for improving design and preventing premature failure in various rock engineering projects, such as thermal energy storage, geothermal wells, underground excavation stability, and open-pit mining subjected to diverse climatic conditions. The bidirectional interaction between tension and compression in rocks remains under-researched, rendering it a significant topic of interest.The present study examines the fatigue behavior of three rock types through stress-life tests conducted under different freeze-thaw conditions: 10, 30, and 50 cycles at temperatures of -5°C and -20°C, followed by thawing at 20°C, 40°C, and 60°C, and subjected to reversed cyclic loading. The properties of these rocks were then compared to reference specimens using an innovative rotating rock beam fatigue testing apparatus, inspired by metallic rotating beam bending tests. The objective of this study is to develop a mathematical model for quantifying and predicting damage, eva‎luating its impact on fatigue strength parameters, and investigating the relationship between physical properties (porosity and P-wave velocity) and fatigue behavior in three carbonate rock types. The results indicated that travertine, marble, and green onyx possess defined endurance limits ranging from 0.49 to 0.66 of their flexural tensile strength, revealing a strong correlation between endurance limits and tensile strength. Experimental results demonstrated that factors such as the number and temperatures of freeze-thaw cycles and initial porosity significantly affect fatigue strength. These factors resulted in reductions in fatigue strength ranging from 1.33% to 60.77% at low stress levels and from 1.18% to 51.85% at high stress levels, with an increase of up to 51.96% in the slope difference between the fitted and reference curves. Specimens subjected to thermal cycling exhibited more ductile behavior and reduced brittleness compared to control specimens, leading to lower slopes in their corresponding curves. This altered behavior is attributed to the combined influence of temperature and the number of cycles, rather than either factor alone. In 60% of cases, the reduction in values at high stress levels was less pronounced than at low stress levels, suggesting that the effects of freeze-thaw cycles become more significant as fatigue life increases, resulting in a greater reduction in endurance limits. Freeze-thaw cycles have a substantial impact on the physical properties of rocks. Repeated cycles induce changes in physical properties that are dependent on the rock type. In travertine, porosity increases with repeated cycles, followed by a deceleration in the rate of increase, which parallels a decrease in wave velocity. The other two rock types did not exhibit a specific pattern, with porosity increasing and wave velocity decreasing rapidly due to the number of F-T periods and the wider temperature range. Overall, porosity increased from a minimum of 0.74% to a maximum of 86.16%, while wave speed decreased from a minimum of 1% to a maximum of 22.07%. Analysis of fracture surfaces of fatigue-damaged specimens subjected to F-T cycles showed that the characteristics of these surfaces are influenced by the number of freeze-thaw cycles, temperature range, and porosity. Fatigue cracks were not evident, and there was distortion due to crushing and loss of intergranular bonding, possibly from volumetric expansion and water intrusion. Travertine and marble exhibited rough fracture surfaces, whereas green onyx showed no crushing or visible alteration, indicating substantial differences in fracture behavior among rock types under freeze-thaw conditions.

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حميد هاشم الحسيني

ابراهيم قاسمي ورنوسفادراني , امين ازهري