摘要
冰力学性质的研究在冻土工程、冰工程中都占据极其重要的地位,冰中未冻水含量的变化会导致冰整体性质随之改变。目前,在微观分子尺度上针对冰中未冻水含量控制因素方面的研究尚不充分。本研究通过分子动力学方法模拟了5种温度、3种应变速率、4种晶粒尺寸、3种升温速率下单晶冰与多晶冰的单轴拉伸与压缩试验,研究了不同影响因素下冰晶体力学性质及内部的微观结构变化,揭示了分子尺度上未冻水在冰晶体变形过程中的产生过程与变化规律,以及未冻水比例对冰晶体力学性质影响的内在机理。模拟结果显示,单晶冰力学性质与未冻水比例间的关系不显著,与冰晶体六元环结构的破损程度直接相关。此外,力学性质受温度、应变量、应变速率等多因素影响,且在拉伸和压缩过程中单晶冰均表现出明显脆性破坏,其强度随温度降低和应变速率加快而增强。相比之下,多晶冰力学性质与未冻水比例变化密切相关,且未冻水比例主要受温度、晶粒大小及其界面状态控制。而且,多晶冰对温度和应变速率更敏感,说明晶界处的结构变化对其力学性能起重要作用。与弹性变形不同,晶界滑移、晶粒旋转、非晶化和再结晶等过程主导多晶冰的塑性变形。
In permafrost and ice engineering,the study on mechanical properties of ice is of great importance,wherein changes in content of unfrozen water inside ice can lead to overall changes of properties of ice.At present,the study at microscopic molecular scale on control factors of unfrozen water content in ice presents non-adequate.As a relatively special solid material,ice can be classified into single-crystal and polycrystalline ones in terms of their morphological difference,the latter of which usually exists in a polycrystalline structural state,in nature and artificial laboratory environments.And the structural difference between polycrystals and single crystals is mainly that the former have grain boundaries,that is,some relatively disordered or less neat regions in the middle of two regularly arranged lattices.In contrast,the entire structure of a single crystal is a long-range ordered crystal structure.Namely,polycrystals can be viewed as being composed of many single crystal particles.The exploration on mechanical properties of polycrystalline ice basing single-crystal ice can help us to further understand its microstructural changes.In this study,after building single-crystal and polycrystalline ice models,setting up molecular force fields,and initializing the system for models,uniaxial tensile and compressive tests were implemented on single-crystal and polycrystalline ices by molecular dynamics simulations,under five temperatures,three strain rates,four grain sizes and three heating rates.The mechanical properties of ice crystals and internal microstructural changes inside them were then investigated,under different influencing factors,aiming to reveal the underlying influencing mechanism of unfrozen water proportion on mechanical properties of ice crystals.In terms of simulation results,for single-crystal ice,mechanical properties of them appear weak relation to unfrozen water proportion,but significant dependence on breakage degree of six-membered ring structures,as well as they are influenced by temperature,strain and strain rate.And during both tension and compression tests,single crystal ice shows significant brittle damage,i.e.the strength increasing along with temperature decreasing or strain rate increasing.In addition,the compressive strength of it tends to be higher than tensile one,but shows an obvious nonlinear mechanical response after the yield point.By contrast,due to the existence of grain boundaries,mechanical properties of polycrystalline ice are more sensitive to the unfrozen water ratio and the ice crystal strength increases along with decrease of unfrozen water ratio,which is mainly controlled by temperature,grain size and its interfacial state.The smaller the crystal size is and the larger the grain boundary area is,the easier the unfrozen water forms at the grain boundary.And the mechanical properties of polycrystalline ice change drastically under the influence of unfrozen water ratio.Moreover,due to strain-induced amorphization and collective sliding between grain boundaries,polycrystalline ice with nanograins is unstable and the sensitivity of it to temperature and strain rate is more pronounced in comparison of that of single-crystal ice,indicating structural changes between grain boundaries play a non-negligible role in mechanical properties of polycrystalline ice.In addition to elastic deformation,the combination of grain boundary slippage,grain rotation,amorphization and recrystallization dominate the plastic deformation of polycrystalline ice.Under influence of external forces and warming,the crystal structure in polycrystalline ice crystals gradually changes,from stable hexagonal ring to unstable quadruple ring,five element ring,seven element ring and disordered water molecule structures,etc.Along with the increase of proportion of unfrozen water,the ultimate strength of ice crystals decreases.In this study,relying on molecular dynamics simulation,the authors focus on generation and changes of unfrozen water at the molecular scale during process of ice crystal deformation and also the intrinsic mechanism of influencing effect of unfrozen water on mechanical properties of ice crystals,with an ultimate hope of explaining the acting mechanism behind some macroscopic phenomena,from the perspective of molecular structure changes.
作者
陈文婷
张鹏
陈雪萍
马旭
张莲海
吴青柏
CHEN Wenting;ZHANG Peng;CHEN Xueping;MA Xu;ZHANG Lianhai;WU Qingbai(State Key Laboratory of Frozen Soil Engineering,Northwest Institute of Eco-Environment and Resources,Chinese Academy of Sciences,Lanzhou 730000,China;University of Chinese Academy of Sciences,Beijing 100049,China;Joint NMR Laboratory for Freeze-Thaw Processes in Soils,Lanzhou 730000,China)
出处
《冰川冻土》
CSCD
北大核心
2023年第2期624-640,共17页
Journal of Glaciology and Geocryology
基金
甘肃省科技重大专项(22ZD6FA004)
甘肃省中小企业创新基金项目(22CX3JA003)
冻土工程国家重点实验室自主课题(SKLFSE-ZT-202103)资助。
关键词
分子动力学
冰晶体
未冻水
力学性质
控制作用
molecular dynamics
ice crystals
unfrozen water
mechanical properties
control effect
