Repetitive interface frictional anisotropy mobilized by sand and snakeskin-inspired surfaces

Understanding frictional anisotropy, which refers to the variation in frictional resistance based on the shear direction, is crucial for optimizing the friction angle between a bio-inspired structure and the surrounding soil. Previous studies focused on estimating the interface frictional anisotropy mobilized by snakeskin-inspired textured surfaces and sand under monotonic shear loading conditions. However, there is a need to estimate interface frictional anisotropy under repetitive shear loads. In this study, a series of repetitive direct shear (DS) tests are performed with snakeskin-inspired textured surfaces under a constant vertical stress and two shear directions (cranial first half → caudal second half or caudal first half → cranial second half). The results show that (1) mobilized shear stress increases with the number of shearing cycles, (2) cranial shearing (shearing against the scales) consistently produces a higher shear resistance and less contractive behavior than caudal shearing (shearing along the scales), and (3) a higher scale height or smaller scale length of the surface yields a higher interface friction angle across all shearing cycles. Further analysis reveals that the gap between the cranial and caudal shear zones of the interface friction angle as a function of L / H (i.e. the ratio of scale length L to scale height H ) continues to decrease as the number of shearing cycles approaches asymptotic values. The directional frictional resistance (DFR) decreases as the number of shearing cycles increases. Furthermore, the discussion covers the impact of initial relative density, vertical stress, and the number of shearing cycles on interface frictional anisotropy.