岩石材料微裂隙演化的CT识别

CT IDENTIFICATION OF MICROCRACKS EVOLUTION FOR ROCK MATERIALS

采用砂浆来模拟岩石材料以研究其裂隙的演化规律。借助CT扫描获得单轴压缩破坏过程中岩石材料内部的密度分布信息和统计特征,利用同位置点的密度变化来识别微裂隙的活动,通过统计密度变化特征和分形指标Rd描述研究微裂隙的演化行为。研究结果表明:(1)CT图像灰度频数统计变化曲线的幅值和形状变化反映了微裂隙的不同演化效应:荷载水平0%～50%时,曲线波峰顶点上移,而频数变化幅值显著增大,曲线呈规则正弦状;50%～70%时,曲线波峰顶点下移,频数变化幅值降低,曲线形状偏离正弦状;70%～90%时,曲线形状、波峰顶点位置和频数变化幅值基本不变,只有少数灰度区间发生频数变化;90%以后,曲线波峰顶点位置发生非常大的上移,曲线回归正弦状。而波谷顶点的位置移动则相反。这种现象体现了整个压缩损伤过程中微裂隙的主导演化作用由加载前期萌生效应转化为中期压缩闭合效应,再转化为后期的扩展、汇集效应。(2)通过对CT图像灰度进行变换及分形指标Rd描述后发现,Rd随应力增加呈先增加→减小→再增加的三段式增长。这与压缩过程中微裂隙的产生效应与闭合效应的相互影响、转化行为有关;加载前期微裂隙的萌生效应活跃,Rd增大;中期萌生效应趋缓,扩展、汇集效应缓慢发展,此时由闭合效应主导,Rd缓慢减小;后期微裂隙的扩展、汇集效应显著增强,Rd急剧增长。(3)提出的微裂隙体积率分形指标Rd描述能较好地刻画压缩破坏过程中微裂隙演化行为。

The mortar is used to simulated the rock material to study its microcracks evolution laws.The distribution properties and statistical characteristics of density in rock materials are investigated by means of CT scanning during the failure process under uniaxial compression;and the activities of microcracks are reflected by the density varieties of same location in mortar.The evolution behavior of microcracks is studied through the statistical characteristics of density changes and the fractal index Rd.The research results show that：（1） The changes of amplitude and shape of grey frequency statistical curve reflect the different microcracks evolution effects.When load level is at 0%-50%,the curve is a sine curve,its wave peak rises and the amplitude increases evidently.When load level is at 50%-70%,the wave peak declines,the frequency amplitude increases,and the curve has a deviation from sinusoid.When load level is at 70%-90%,the curve shape,wave peak location and frequency amplitude do not change basically,and only a few gray interval？s frequencies change.When load level is greater than 90%,the wave peak rises largely and the curve becomes sinusoid again.Meanwhile,the movement of wave trough is opposite to the wave peak.The change laws of the amplitude and curve shape indicate that the dominant evolution action of microcracks is initiation in prophase,metaphase closing and propagation in anaphase during the failure process.（2） Through the gray transformation of CT images and its fractal index description,the relationship between Rd and load presents three-stage increment,which is related to the interaction of generating effect and closing effect of microcracks in compression process.In prophase,Rd increases with microcracks initiation remarkably.At middle stage,because the microcracks initiation becomes slow and its propagation develops slowly as well,the microcracks closing effect is dominant.So,Rd decreases tardily.In anaphase,the microcracks propagation and influx speed up and Rd is increasing rapidly.（3） The proposed fractal index Rd of microcracks volume fraction can characterize the evolution behavior of the microcracks in the compression fracture process.