Effects of brine and gas inclusions on the macro‑ and microscopic mechanical behaviour of sea ice under bending

Abstract. The macroscopic mechanical properties of sea ice in flexure are controlled by underlying microstructural damage. In this study, the macro‑ and microscopic mechanical behaviours of sea ice under four‑point bending were investigated using columnar‑ice specimens machined from a sea ice block collected in the summer Arctic. Due to brine drainage, the gas content of the specimens exceeded the brine content. A total of 83 bending tests were performed at different temperatures to measure the flexural strength and effective elastic modulus. At each temperature, one specimen was monitored by four acoustic emission (AE) sensors to trace the evolution of microcracking. The flexural strength ranged from 124.6 to 1394.7 kPa, and the effective elastic modulus varied from 0.5 to 4.2 GPa. Both strength and modulus decreased with increasing temperature and gas volume fraction. To extend the range of brine and gas contents, the present data were combined with previous studies, and a two‑variable empirical equation was established to estimate sea ice flexural strength using brine and gas volume fractions, unlike conventional formulae based solely on brine content. AE monitoring showed that microcracking activity remained weak before main crack initiation, with low ring-down count and energy. A high-energy AE event triggered unstable propagation of the main crack, leading to the catastrophic failure of ice specimens, rather than a progressive accumulation of microcrack density. Fast Fourier Transform of the AE waveforms identified two typical frequency modes centred at 38 and 150 kHz, corresponding to secondary cracking activity and main crack nucleation, respectively. Compared with freshwater ice, sea ice exhibited more active secondary cracking with larger size, which stems from the inherent brine inclusions that promote discontinuous crack propagation, frictional sliding of crack faces, and tortuous crack paths. These results provide a multiscale understanding of sea ice failure under bending and highlight the influence of sea ice inclusions from a mesoscopic perspective.