海冰在单轴压缩下的韧-脆转化机理及破坏模式

The study on Brittle-ductile transition mechanism and failure mode of sea ice under uniaxial compression

海冰的破坏模式在不同加载速率下呈现出韧-脆转化特征.为研究海冰的韧-脆转化机理,本文对渤海东北部海域的平整冰进行了现场单轴压缩试验.该试验采用的海冰为典型的h2型柱状冰,其盐度变化范围为5.5‰–7.4‰,试样温度在–15°C至–18.3°C之间.试验中对几何尺寸为50 mm×50 mm×107 mm的长方体试样施加垂直于生长方向的压缩载荷,由此得到海冰在不同加载速率下的单轴压缩强度.在试验过程中所采用的应变率区间为4.5×10^(–5)–7.5×10^(–3)s^(–1).由于在不同加载速率下海冰应变率的不同,海冰呈现出3种不同的破坏模式:在加载速率较低时,海冰强度随加载速率的提高而增大,海冰中裂纹尖端的应力集中通过蠕变作用释放而抑制了裂纹扩展,最终呈现为韧性破坏模式;当应变率在9×10^(–4)–5×10^(–3)s^(–1)区间时,海冰内部的裂纹能够沿主轴方向稳定生长并产生贯穿整个试样的主裂纹,形成劈裂破坏,海冰则呈现为韧-脆转化破坏;当加载速率较高时,海冰试样的裂纹尖端无法稳定生长而产生斜向裂纹并构成碎裂破坏,海冰则呈现为脆性破坏模式.以上试验结果还表明,每种破坏模式均有与之对应的强度-应变率关系,且海冰在不同应变率下的破坏模式决定了其单轴压缩强度特征,是其发生韧-脆转化的主导因素.此外,本文还对海冰在不同加载速率下等效弹性模量的变化规律进行了讨论分析.

Under various compressive strain rates, sea ice fails in different modes and behaves a brittle-ductile transition. To investigate the mechanism behind the transition, a fieldwork of uniaxial compression tests were performed on the level ice collected from Northeast of the Bohai Sea. The ice owns a typical h2 columnar structure, whose salinity varies from 5.5%0 to 7.4%0 while temperature ranges from -15℃ to -18.3℃. In the experiments, the samples with uniform dimension of 50 mm×50 mm×107 mm were applied compressive load across to the grain columns. The data shows that the maximum failing stress is 4.7MPa appears at 2.8×10^-3 sI while the lowest value is 1.5 MPa at 4.7×10^-5 s^-1. During the compression tests, the samples failed in three different modes over a wide range of strain rate （4.5×10^-5-7.5×10^-3s^-l）. When the failing stress is further categorized by failure mode, the stain rate-stress relation shows a consistent trend in each mode. Under low strain rate, the compressive strength has positive correlation with strain rate. The crack propagation is limited due to the stress concentration at crack tip is relaxed by creep, which leads to a ductile failure. When stain rate is between 9×10^-4 and 5×10^-3 s^-1, the crack was able to develop stably through the specimen along the loading direction and formed a splitting failure. The crack development became unstable and inclined cracks were preferred under high strain rate, where ice failed in a spalling mode. From ductile failure to splitting, the transition is terms of the competition between crack-tip creep and propagation. With sufficient confinement, the spalling dominates the global failure as splitting is suppressed. For both failure mode transitions, the condition is triggered by stain rate. From the measurements, it shows that each failure mode owns an according stress-strain rate relation, which is believed to explain the Brittle-ductile transition. Moreover, the influence of stain rate on the effective elastic modulus is discussed.