纳米铜晶胞体积弹性模量变化规律的分子动力学模拟

用分子动力学的方法详细模拟纳米铜晶胞随温度与压强变化的规律,得到了铜晶胞体积弹性模量突变的敏感压强点.模拟结果表明:单晶铜的在压强小于75GPa时体积弹性模量随温度升高而降低,随压强增大而增大;在压强大于75GPa时,体积弹性模量随温度升高而增大,随压强增大而减小.     

基于RUSLE模型的湟水流域土壤侵蚀时空变化

研究黄河上游土壤侵蚀的时空变化对于维持黄河上游生态系统服务功能、保护黄河上游水塔具有重要意义。以黄河上游典型区域湟水流域为研究区,采用RUSLE模型定量评估了该流域2000—2015年土壤侵蚀的时空变化特征,并分析了有无梯田措施下土壤侵蚀的空间变化,从而量化了梯田建设对防治坡面土壤侵蚀的影响。结果表明:2000—2015年,湟水流域的土壤侵蚀强度整体呈现减小趋势,侵蚀模数由1 183 t/(km~2·a)降低至940 t/(km~2·a),减少幅度为20.54%。不同土地利用类型以及不同坡度下的土壤侵蚀强度均有所降低,其中耕地上的减幅最大为20.58%。15°～20°坡度区间的侵蚀模数减幅最显著,为23.11%。通过有无梯田措施情景模拟发现,湟水流域2015年土壤侵蚀模数由940 t/(km~2·a)降低至有梯田的837 t/(km~2·a),减少11.00%。研究结果可为流域的水土流失防治和生态环境保护提供科学依据。     

黄河中游河龙区间侵蚀产沙对景观特征的非线性响应

为明确退耕还林（草）工程期间景观特征变化对侵蚀产沙过程的非线性影响机制，以2006—2016年黄河流域河龙区间33个水文站控制子流域为研究对象，采用增强回归树方法分析了河龙区间子流域产沙模数时空差异的景观驱动因子，定量评价了各景观特征与产沙模数的非线性关系。结果表明，林地和耕地面积比例对产沙模数变异贡献最大，分别贡献22.0%和16.9%的变异；林地面积的增加和耕地面积的减少可以减少产沙模数，但当林地面积>15%，耕地面积<20%时，对产沙模数的影响减弱；增加林地和草地的植被覆盖度对水土流失的减缓作用也存在阈值，草地和林地植被覆盖度的阈值分别为0.5和0.3，当超过该阈值后，其对侵蚀产沙过程的减缓作用趋于弱化；适当减少边界密度数量有利于控制流域土壤侵蚀，当边界密度<60 m/100 hm²时，边界密度对产沙模数影响不大，当超过阈值后，产沙模数随着边界密度的增加急剧增大。当边界密度维持在60 m/100 hm²之内时，可以达到适宜的水土流失防治效果。研究结果揭示了河龙区间流域尺度上景观因子对侵蚀产沙过程的阈值效应，可以为黄河流域植被合理恢复提供重要参考。     

加速度计质量和安装位置对木材材料常数测试值的影响分析北大核心

为考察加速计质量和安装位置对木材测试频率的影响,设计了用自由梁和悬臂梁测试一阶弯曲频率以及用自由板测试一阶扭转频率的试验方案。根据能量法和振型推导出适用于计入加速计质量和安装位置影响的弹性模量和自由梁、悬臂梁的一阶弯曲频率的修正关系式以及剪切模量和自由板一阶扭转频率的修正关系式。应用加速度计为传感器实测了西加云杉梁、板和定向刨花板(OSB)沿其长度方向的一阶弯曲频率和一阶扭转频率,验证了导出的修正关系式应用于测试材料弹性模量和剪切模量的有效性,并以加速度计质量m和试件质量M之比m/M为参数,探讨了上述导出的修正关系式的适用范围。     

Root tensile strength of terrace hedgerow plants in the karst trough valleys of SW China: Relation with root morphology and fiber content

Root tensile strength is commonly affected by root morphology and fiber content, which combinedly affect the effectiveness of terrace hedgerow on controlling soil erosion in sloping farmland. However, the relationships between these root characteristics are still elusive. This study aimed to compare the root tensile strength, root morphologies, and root fiber contents, and quantify their differences among different plant species. Complete root systems of three terrace hedgerow plant species, i.e., O. bodinieri, V. villosa, and D. lablab L. were sampled for detecting root morphologies and fiber contents at different slope positions on a representative hillslope in karst trough valley. Single root specimens were collected to measure root tensile properties for these three plant species. Results showed that most root morphological parameters were the highest in O. bodinieri, followed by D. lablab L. Three plant species presented the same dominant diameter class as < 1.0 mm, and root volumes as < 1.50 cm³. Cellulose with contents closing to 50% was the main fiber type, and increased significantly linearly with the increasing root diameter in both O. bodinieri and V. villosa. The highest tensile strength and Young's modulus were found in D. lablab L., while the best ultimate tensile force and ultimate elongation were displayed in O. bodinieri. Root diameter showed negative power relationships with tensile strength and Young's modulus, a positive power relationship with ultimate tensile force, and a positive linear relationship with ultimate elongation. Our results will deepen the understanding of the terrace hedgerow functions for controlling soil erosion worldwide.