Size effect on damage evolution and failure characteristics of cyan sandstone under uniaxial compression

To investigate the influence of specimen size on damage evolution and failure behavior in cyan sandstone, uniaxial compression tests were conducted on cylindrical specimens with varying height-to-diameter (H/D) ratios (50 mm × 50 mm, 50 mm × 75 mm, 50 mm × 100 mm, 50 mm × 125 mm, and 50 mm × 150 mm). Acoustic emission (AE) monitoring was employed throughout loading to capture microcrack activity, while scanning electron microscopy (SEM) was used to analyze post-failure microstructural features. The results reveal a clear size effect: uniaxial compressive strength (UCS) decreases with increasing specimen height, accompanied by a transition in failure mode from axial splitting to shear failure. Smaller specimens exhibited more intense AE activity and energy release during the compaction stage, suggesting that AE ringing counts and energy surges can serve as precursors to failure. Based on the test data, a UCS size effect law was established, and size-dependent damage models and constitutive relationships were proposed. These findings offer valuable insights for engineering applications such as tunnel support design in heterogeneous rock masses.