裂隙砂岩变形破裂过程中应变场灰度及纹理特征分析

Gray and texture features of strain field for fractured sandstone during failure process

为探究裂隙岩体变形破裂机制以及前兆特征,利用高速水刀切割技术对板状红砂岩试件预制不同倾角的裂隙,对其进行单轴压缩试验,通过数字图像相关技术实现试件加载过程中应变场的非接触式观测,引入均值、标准差、三阶矩、平滑度、一致性和熵6种灰度特征参数以及对比度、相关性、角二阶矩和熵4种纹理特征参数,量化研究了裂隙砂岩试件变形破裂过程中应变场灰度及纹理演化规律。研究结果表明:裂隙砂岩试件在裂纹起裂、扩展及贯通过程中,应变场灰度整体从低灰度级向高灰度级方向移动,灰度直方图形态从“矮胖”型转化为“高瘦”型,峰值频率随之增大。裂隙砂岩试件的应变场灰度/纹理特征参数-时间曲线可划分为波动段、平稳段以及突变段3个阶段;应变场灰度/纹理特征参数突变段的出现与裂纹萌生、扩展有关,对应的变化趋势可划分为增长型、降低型以及变异不明显型3种。绝大部分灰度特征参数与纹理特征参数的皮尔逊相关系数绝对值>0.5,说明这2类特征参数之间存在显著或高度的相关关系。灰度/纹理特征参数的第1次突变,对应于张拉裂纹萌生,且在试件临近破坏时出现,可视为前兆点,作为裂隙岩体即将失稳的预警信号,对应的前兆/峰值应力比为0.83~0.97,前兆/最终破坏时间比为0.85~0.95。相比于完整砂岩试件,灰度/纹理特征参数对裂隙砂岩试件的破裂前兆识别能力更强,这是因为预制裂隙的存在使得岩体的应变场不连续性更为显著。最后,针对完整岩石和裂隙岩体,分别提出了建议监测的灰度/纹理特征参数。

In order to explore the deformation and fracturing mechanism and precursory characteristics of fractured rock mass,the flaws with different inclinations were prefabricated in the red sandstone plate specimens by using the high-pressure water-jet cutting technology.The uniaxial compression experiments were conducted on the fractured sandstone specimens.The digital image correlation method was applied for a non-contact measurement of strain field during loading.Six gray feature parameters including mean,standard deviation,third-order moment,smoothness,consistency and entropy and four texture feature parameters including contrast,correlation,angular second moment and entropy were used to quantify the gray scale and texture evolution of strain field during the deformation and fracturing process of fractured sandstone specimens.The results show that the grayscale of strain field changes from low gray level to high gray level during the loading process.The shape of gray histogram changes from“pyknic”type to“lanky”type,accompanied by an increase in the peak frequency.The gray/texture feature parameter of strain field-time curve of the fracture sandstone specimen can be divided into the fluctuation stage,steady stage and mutation stage.The abrupt changes in the gray/texture feature parameters of strain fields are related to the crack initiation and propagation,and the corresponding variation trend can be grouped into the growth type,attenuation type and little change type.The absolute values of Pearson’s correlation coefficients between the gray feature parameters and texture feature parameters are mostly greater than 0.5,indicating that there exists a significant or high correlation between the two types of feature parameters.The first abrupt change in the gray/texture feature parameters is associated with the initiation of tensile cracks,and appears when the specimen is close to failure.It can be regarded as a precursory point and can be used as an early warning signal of imminent instability of fractured rock mass.The ratio of precursor stress to peak stress is in the range of 0.83-0.97,and the ratio of precursor time to final failure time is in the range of 0.85-0.95.Compared with intact sandstone specimen,the gray/texture feature parameters have a better identification ability of failure precursor for fractured sandstone specimens.This is because the existence of pre-flaw makes the discontinuity of strain field more pronounced.Finally,the recommended monitoring gray/texture feature parameters are proposed for intact rock and fractured rock mass,respectively.