脆性岩石之力學行為與力學模式探討

Abstract

本研究整理交大岩力實驗室過去對脆性岩石之力學行為在應力-應變關係方面之研究 交大岩力實驗室曾進行一系列有關脆性岩石在高溫高壓下力學行為的研究，已累積許多寶 貴資料。本文針對實驗室中三軸壓縮試驗之資料加以分析與模擬，以期能建立合適之組合 律模式，可用來描述脆性岩石之力學行為。 由實驗的結果發現，在三軸壓縮試驗中，裂縫閉合的效應明顯影響脆性岩石初始的 應力-應變關係。初始之應力-應變曲線顯示切線勁度隨應力增大逐漸增加，而在裂縫閉合 結束點應變到達材料之彈性勁度。在圍壓對裂縫閉合之影響方面，隨著圍壓的增加，材料 的彈性勁度也隨之上升，但裂縫閉合結束點之應變則下降。至於溫度對裂縫閉合趨勢之影 響，由實驗的結果顯示則仍非完全明確。脆性岩石過尖峰強度的力學行為，明顯呈現第Ⅱ 類岩石之特性。 基於以上實驗結果的考量，本研究提出一個組合律模式來描述脆性岩石之力學行為， 此模式整合了一個裂縫閉合模式和一個連體損壞力學模式，同時進一步運用非線性最佳化 的電腦程式輔助，可由實驗結果中校正適當之材料參數。經過比對模擬結果與實驗結果， 可發現本模式儘管僅利用甚少的材料參數，卻已可合理描述脆性岩石之應力-應變關係。 由參數研究顯示，只要適切地選擇模式中的參數，本模式可以模擬(i)第Ⅰ類和第Ⅱ類尖 峰強度後的行為，及(ii)壓縮過程中體積膨脹的效應。 A series of experimental work were carried out previously in NCTU (National Chiao - Tung University) to investigate the mechanical behavior of brittle rocks under elevated pressures and temperatures. In this study, the experimental data of triaxial compression tests is summarized and re-analyzed. Furthermore , the study develops a constitutive model to describe the mechanical behavior of brittle rocks. The experimental results show that crack-closure siginificantly affect the initial stress - strain relation of a brittle rock during triaxial compression. The initial stress - strain curve reveals that the tangent stiffness gradually increases and reaches an elastic stiffness at a crack- closed strain. As the confining pressure increases, the elastic stiffness raises while the crack-closed strain decreases. The effect of temperature on the elastic stiffness and the crack - closed strain , however, is still not concluded from the experimental data. For the post - peak behavior , the class II behavior is obvious. On the basis of the experimental results, a constitutive model is proposed to d escribe the mechanical behavior of brittle rocks. The presented constitutive model contains a pore/crack closure model and a continuum damage mechanics model. The study also developes a computer program behavior optimiz -ation procedure for calibrating the appropriate material parameters from a set of experimental data. By comparing the simulated and experimental results , it is demonstrated that the proposed model can closely simulate the stress- strain relation of brittle rocks with very limited material parameters. Parametric studies also demonstrate that, with proper combination of parameters,the proposed model is capable of modeling (i) the class I as well as the class II post-peak behavior,and (ii) the dilatancy during a compression test.