CT尺度下冻融岩石细观损伤特性分析

为研究冻融环境下岩石细观损伤演化规律,以红砂岩为研究对象,首先进行冻融循环试验,再对经历0次,5次,10次,20次和40次冻融循环的试样进行CT扫描.采用K均值聚类算法实现了扫描图像的二维孔隙识别;通过三维重构再现了红砂岩的冻融破坏模式,确定了试样的有效承载区,在细观层面实现了对岩石冻融损伤的定量分析.结果表明:通过K均值聚类算法确定阈值,可以有效实现CT图像的二值化;三维重构后的模型可以再现红砂岩试样在冻融过程中的损伤模式以及内部孔隙的发育特征;随冻融循环次数的增加,基于有效承载区定义的损伤变量逐渐增大,并与孔隙的发育趋势一致,能有效地反映冻融循环作用下岩石的损伤演化规律,为冻融岩石细观损伤研究提供了一种切实可行的方法.

基于三维重构的冻融岩石细观损伤研究

为了研究冻融作用下岩石的细观损伤劣化机理,以冻融岩石CT扫描试验为基础,红砂岩CT图像为研究对象,借助三维重构技术和分形理论,获取表征岩石细观结构特征的孔隙率和分形维数,定义用分形维数表征的冻融岩石损伤变量,分析冻融环境下红砂岩的细观损伤演化规律。结果表明:通过图像处理和三维重构技术能够获得表征岩石宏观损伤劣化以及细观孔隙结构发育情况的三维数字化模型;损伤演化方式对应着岩体内部孔隙发育情况,随着冻融次数的增加,红砂岩的孔隙率逐渐增大,分形维数则逐渐降低;基于细观孔隙与裂纹扩展形态的分形特征定义的冻融岩石损伤变量,与宏观建立的损伤变量具有较好的一致性,能够很好地从细观结构变化的角度说明岩石的冻融破坏过程。

冻融与荷载耦合作用下岩石损伤模型的研究

针对寒区工程结构的冻融受荷岩石,提出冻融损伤、受荷损伤与总损伤的概念,拓展损伤变量的内涵;以岩石的初始损伤状态为基准状态,充分考虑岩石细观结构的非均匀性,运用损伤力学理论及推广后的应变等价原理,建立冻融受荷岩石损伤模型;通过损伤变量及本构方程来描述岩石材料细观结构的损伤演化及其宏观损伤行为,与岩石实际冻融破坏情况符合较好。研究结果表明:寒区工程结构的受荷岩石,其力学性能由冻融因子、荷载因子及其耦合效应所决定;冻融与荷载的共同作用使岩石总损伤加剧,并表现出明显的非线性特征,而其耦合效应使总损伤有所弱化;岩石的岩性和初始损伤状态确定各影响因素的权重,表现出不同的损伤扩展特性;相比而言,砂岩对冻融循环更敏感,而受荷损伤最终导致页岩破坏。

基于数字图像处理的冻融页岩温度场的数值分析方法

利用数字图像处理技术与有限元软件ANSYS相结合的方法对冻融岩石CT图像进行处理,获得岩石细观结构图像,分析冻融岩石的温度场分布规律。结果表明,数字图像处理技术可以准确地描述岩石损伤的空间分布,最大限度地挖掘利用岩石CT图像所隐含的信息。岩石细观结构图像与有限元法的结合可实现考虑岩石细观介质及其空间分布的冻融温度场的有限元模拟,岩石的细观结构决定了冻融岩石温度场的分布规律,其对岩石冻融损伤破坏过程有重要影响。提出的冻融岩石温度场研究方法可从细观尺度探索寒区岩石工程冻害产生的原因,为解决寒区岩石工程冻胀、融沉问题提供新的方法。

冻融循环作用下含天然损伤砂岩破坏过程的数值试验研究

裂隙、孔隙等天然损伤使得岩石细观结构呈现强烈的非均质性,对其破坏过程及模式产生重要影响。本文根据CT无损识别技术,结合数字图像处理技术和CASRock数值模拟软件,开展了含天然损伤的冻融砂岩劈裂破坏的数值试验研究。对含天然损伤的冻融砂岩劈裂破坏模式、变形局部化及裂纹演化过程等分析,揭示了含天然损伤砂岩在冻融和荷载作用下的破坏机制。研究结果表明:砂岩内原生孔隙(裂纹)的扩展及新生孔隙的生成是冻融岩石破坏演化的主要形式。冻融和荷载作用下含天然损伤岩石的破坏与天然损伤程度及分布有关。次生裂纹的生成多发生于天然损伤密集区域。在加载过程中,局部化损伤区应力远远大于砂岩整体应力,局部化损伤区岩石的破坏与该区域能量释放和应力释放具有同步性。局部化损伤反映岩石中裂纹的演化,有助于预测砂岩裂纹发展的方向。岩石的破坏模式与冻融循环次数有关,冻融循环促使含天然损伤砂岩逐渐由脆性破坏转化为延性破坏,冻融作用对岩石整体强度的改变是逐渐劣化的过程。

NMR research on deterioration characteristics of microscopic structure of sandstones in freeze-thaw cycles

In order to study the deterioration characteristics of the microscopic structure of sandstones in freeze-thaw cycles, tests of180 freeze-thaw cycles were performed on sandstone specimens. The nuclear magnetic resonance （NMR） technique was applied tothe measurement of sandstone specimens and analysis of the magnetic resonance imaging. Then, the fractal theory was employed tocompute the fractal dimension values of pore development of rocks after different freeze-thaw cycles. The results show that the massand porosity of rocks grow with the increase of freeze-thaw cycles. According to the NMR T2 distribution of sandstones, the poresizes of rock specimens increase after 180 freeze-thaw cycles, especially that of the medium-sized and small-sized pores. The spatialdistribution of sandstone pores after freeze-thaw cycles has fractal features within certain range, and the fractal dimension ofsandstones tends to increase gradually.

Microscopic damage and dynamic mechanical properties of rock under freeze-thaw environment

For understanding the rock microscopic damage and dynamic mechanical properties subjected to recurrent freeze-thaw cycles, experiments for five groups of homogeneous sandstone under different freeze-thaw cycles were conducted. After freezethaw, nuclear magnetic resonance(NMR) tests and impact loading tests were carried out, from which microscopic damage characteristics of sandstone and dynamic mechanical parameters were obtained. The results indicate that the porosity increases with the increase of cycle number, the rate of porosity growth descends at the beginning of freeze-thaw, yet accelerates after a certain number of cycles. The proportion of pores with different sizes changes dynamically and the multi-scale distribution of pores tends to develop on pore structure with the continuing impact of freeze-thaw and thawing. Dynamic compressive stress-strain curve of sandstone undergoing freeze-thaw can be divided into four phases, and the phase of compaction is inconspicuous compared with the static curve. Elastic modulus and dynamic peak intensity of sandstone gradually decrease with freeze-thaw cycles, while peak strain increases. The higher the porosity is, the more serious the degradation of dynamic intensity is. The porosity is of a polynomial relationship with the dynamic peak intensity.

Effect of Freeze-Thaw Cycles on Mechanical Properties and Permeability of Red Sandstone under Triaxial Compression

Geological disasters will happen in cold regions because of the effects of freeze-thaw cycles on rocks or soils, so studying the effects of these cycles on the mechanical characteristics and permeability properties of rocks is very important. In this study, red sandstone samples were frozen and thawed with o, 4, 8 and 12 cycles, each cycle including 12 h of freezing and 12 h of thawing. The P-wave velocities of these samples were measured, and the mechanical properties and evolution of the steady-state permeabilities were investigated in a series of uniaxial and triaxial compression tests. Experimental results show that, with the increasing of cyclic freeze-thaw times, the P-wave velocity of the red sandstone decreases. The number of freeze-thaw cycles has a significant influence on the uniaxial compressive strength, elastic modulus, cohesion, and angle of internal friction. The evolution of permeability of the rock samples after cycles of freeze-thaw in a complete stress-strain process under triaxial compression is closely related to the variation of the microstructure in the rock. There is a highly corresponding relationship between volumetric strain and permeability with axial strain in all stages of the stress-strain behaviour.