岩石材料的三维各向异性损伤破坏模型与数值模拟

基于复合材料以及连续介质损伤理论,给出了岩石材料的各向异性损伤破坏模型。通过引入与岩石材料单轴加载行为相对应的特征模态构成的四阶对称损伤张量,描述了岩石材料的损伤演化过程,其中对不同主应变方向采用不同的损伤变量,而对同一主应变方向拉压时的损伤则采用不同的损伤变量来描述。在数值模拟岩石破坏过程的程序中,采用了张量分解的方法。将该模型编写用户材料子程序,并嵌入到大型有限元分析程序ABAQUS中,通过ABAQUS/EXPLICIT SOLVER的显式有限元算法求解。利用此程序对岩石材料的单轴压缩进行了数值模拟。     

基于应力空间变换的修正剑桥模型改进

从岩土类材料极限应力状态线所致的各向异性出发,提出了应力空间变换的思想。以修正剑桥模型屈服面的中心为映射中心,给出了重塑土的应力空间变换、应力增量变换的公式, 并考虑了应力洛德角的影响。在变换应力空间中对修正剑桥模型进行了重新表述与改进,给出了基于应力空间变换的改进模型的具体应力应变计算式。通过4种应力路径的基于应力空间变换的改进模型与修正剑桥模型预测结果的对比,揭示了改进后模型的优越性：可以较好的反映各向异性的影响;可以很好地反映三轴伸长等应力路径的应力应变特性;可以反映 等路径所致的软化现象。     

Predicting the initiation of static liquefaction of cross‐anisotropic sands under multiaxial stress conditions

Experimental evidence has shown that the liquefaction instability of sands can be affected by its material density, stress state, and inherent anisotropy. In order to predict the initiation of the static liquefaction of inherent cross‐anisotropic sands under multidimensional stress conditions, a rational constitutive model is needed. An elastoplasticity model able to capture the influences of intermediate principal stress ratio (b  = (σ ₂ ? σ ₃)/(σ ₁ ? σ ₃)) and loading direction on stress–strain relationships and volumetric properties was proposed. The yield function was formulated to be controlled by Lode angle, loading direction, and material state; the stress–dilatancy was a material state‐dependent function. After using the existing drained hollow cylinder tests to validate the proposed model, this model was used to simulate the existing undrained hollow cylinder tests. During this simulation, the second‐order work criterion was used to determine the initiation of static liquefaction. The results showed that an increase in both the intermediate principal stress ratio and the loading angle induces a decrease in the second‐order work. Static liquefaction is initiated more easily at a stress state with a large intermediate principal stress ratio and a large loading angle, and the mobilized friction angle at the instability points decreases with the intermediate principal stress ratio and the loading angle. Copyright © 2017 John Wiley & Sons, Ltd.     

SANISAND-FN: An evolving fabric-based sand model accounting for stress principal axes rotation

SANISAND is the name of a family of bounding surface plasticity constitutive models for sand within the framework of critical state theory, which have been able to realistically simulate the sand behavior under conventional monotonic and cyclic loading paths. In order to incorporate the important role of evolving fabric anisotropy, one such model was modified within the framework of the new anisotropic critical state theory and named SANISAND-F model. Yet the response under continuous stress principal axes rotation requires further modification to account for the effect of ensuing noncoaxiality on the dilatancy and plastic modulus. This modification is simpler than what is often proposed in the literature, since it does not incorporate an additional plastic loading mechanism and/or multiple dilatancy and plastic modulus expressions. The new model named SANISAND-FN is presented herein and is validated against published data for loading that includes drained stress principal axes rotation on Toyoura sand.     

考虑地层各向异性井壁稳定性研究进展

考虑地层各向异性井壁稳定性研究对地质特征复杂的非常规油气储层开发意义重大,在对国内外近10年相关研究成果调研基础上对其主要进展进行了综述。影响井壁稳定性的地层各向异性因素包括:区域地应力大小、区域应力性质、岩石物理力学参数差异、井身结构、水化膨胀、微裂缝及弱层理盘角度等。详细梳理了各单因素对井壁稳定性的影响机制,地层应力系统及岩石强度对井壁稳定性的影响最大。对于具弱层理盘地层,沿不同钻井方向岩石强度差异较大;沿弱层理面地层容易发生水化膨胀,且往往微裂缝发育,井壁容易失稳,因此应采用高抑制钻井液体系或油基钻井液。在复杂地层钻井过程中,以上各单因素相伴而生,进行各单因素耦合条件下井壁稳定性分析对合理制定应对策略具有重要意义。加强考虑地层各向异性井壁稳定性基础理论、数值模拟及基于三维可视化建模的近实时井壁稳定动态模拟预测研究是该研究在未来的主要发展方向。     

Directional properties of small strain shear stiffness in soils

A compiled database of shear wave velocity measurements in a variety of clays, silts and sands shows directional hierarchies between downhole (V_sVH), standard crosshole (V_sHV), and rotary crosshole (V_sHH) tests. The special in situ database has been collected from 33 well-documented geotechnical test sites. Expressions relating the small-strain shear modulus in terms of effective confining stress level, stress history and void ratio are explored for each of these three modes of directional shear wave velocity. The relationships are separated initially into soil groups (intact clays, fissured clays, sands and silts), and then generalised to consider all soil types together.     

2017年武隆5.0级地震序列剪切波分裂研究

利用流动台及原有固定台记录的数字波形资料,采用SAM分析方法对武隆5.0级地震序列进行剪切波分裂特征研究,得到震区5个台站共94对剪切波分裂参数。研究结果表明,武隆震区快剪切波偏振方向总体呈NW向,受区域主压应力场的控制作用较为明显,而第一优势偏振方向由NW向转为NNW向可能是受该区域多条NE向断裂及复杂构造的综合影响;震中东、西两侧台站的快波偏振优势方向呈现一定的空间分布变化;快波偏振方向在震后早期较为离散,后期呈现逐步收敛的趋势,显示震后应力场逐步趋于稳定;在较强余震发生前,时间延迟具有上升趋势,其中部分较强余震前的短时间内还观测到时间延迟减小的现象,这可能反映震前应力场增强和临震应力释放;慢波时间延迟与震源深度的关系不明显。