On the effect of grain size on rock behavior under cyclic loading by distinct element method

It is well-known that the mechanical behavior of rocks under cyclic loading is much different from static loading conditions. In most constructions, the load applied to structures is within dynamic ranges. That’s why a great deal of attention has been paid towards this field in order to identify the dynamic behavior of rocks in more details. Nevertheless, the nature of dynamic failure in rocks has not yet clearly identified, particularly when it comes to cyclic loading The purpose of this study was to investigate the influence of grain size on the mechanical behavior of rocks under cyclic loading using numerical modeling by UDEC. A total of three grain categories with diameter of 1, 2 and 4 mm were modeled in the software. All models were of Brazilian type with 0.54 diameter. Behavioral parameters required for modeling were determined through laboratory studies and the software was adjusted accordingly. The stresses applied to the samples were in two forms of quasi-static and cyclic loading. The result of static loading is that the smaller the grain size, the model will have higher elastic modulus. In other words, the elastic modulus of the grain size is inversely related to the grain dimensions. Analysis of data obtained from cyclic loading showed that the amount of strain in samples with smaller grain sizes was lower than the corresponding strain in samples with larger grain size during the same loading periods. In other words, resistance of samples with smaller grain sizes to deformation under cyclic load was higher compared to those of larger grain size. Comparison of the stress vectors for these samples showed that with a decrease in grain size, stress distribution in the sample became more uniform and inclusive, and the stress concentration declined. Another important result was that the smaller the grain size, the more the axial stress applied to the sample inclined towards one. This indicated that with a decrease in grain dimensions, the sample behavior approached a plastic behavior.