Effect of the length-to-diameter ratio on rock failure behavior under one-dimensional coupled static–dynamic loading

The length-to-diameter ratio (L/D ratio) effect is an important manifestation of the size effect in rock mechanics. Although this effect has been widely investigated under static and dynamic loading conditions, its influence under one-dimensional coupled static–dynamic loading remains insufficiently understood. In this study, coupled static–dynamic compression tests were conducted on red sandstone specimens using a modified split Hopkinson pressure bar (SHPB) system. The specimens had a fixed diameter of 50 mm and four different lengths of 25, 30, 40, and 50 mm, corresponding to L/D ratios of 0.5, 0.6, 0.8, and 1.0, respectively. The results show that the dynamic peak strength increases with strain rate for all L/D ratios, indicating a clear rate-dependent behavior. However, at a given strain rate, the dynamic peak strength decreases with increasing L/D ratio. This trend is consistent with that observed under static compression but differs from the commonly reported trend under purely dynamic loading. The peak strain also exhibits an L/D ratio effect, although its variation is more complex than that of peak strength. High-speed photography and digital image correlation (DIC) analysis reveal that specimens with larger L/D ratios are more prone to hoop cracking and axial segmentation, thereby reducing the specimen's equivalent stiffness and load-bearing capacity. In contrast, specimens with smaller L/D ratios mainly fail by axial splitting. These findings clarify the role of specimen geometry in rock failure under one-dimensional coupled static–dynamic loading and provide useful insights for evaluating the stability of rock structures subjected to dynamic disturbances.