Dynamic mechanical properties of sandstone–mudstone composite specimens under SHPB tests

The dynamic mechanical response of sandstone–mudstone composite specimens under impact loading is influenced by wave-impedance mismatch and layer-thickness configuration. Based on impact tests conducted on specimens with different sandstone-to-mudstone thickness ratios, the evolution of stress-wave propagation, energy distribution, and failure behavior was systematically analyzed. Results show that increasing sandstone thickness modifies the equivalent wave-propagation response of the composite specimen, thereby reducing wave reflection and promoting energy transmission, and this effect gradually weakens as impact velocity increases. The peak dynamic stress increases markedly with strain rate, while the characteristic double-peak feature in the stress–strain curves gradually diminishes as sandstone thickness increases. Energy analysis reveals that the absorbed energy displays a non-monotonic dependence on thickness ratio, reaching a minimum at equal sandstone and mudstone thicknesses. Failure observations show that, with increasing sandstone thickness, crack initiation shifts from the mudstone interior toward the interface, accompanied by reduced crack deflection during propagation across the interface. Correspondingly, fragmentation characteristics indicate that the sandstone primarily contributes to impact energy transmission, whereas the mudstone dominates energy dissipation, and its fractal features dominate the composite’s fractal behavior. These findings provide a mechanistic basis for assessing dynamic failure and wellbore instability risks during gas drilling in interbedded sandstone–mudstone formations.