2chapter2physicalandmechanicsofrockandtest要点.ppt

4) Loading rate See P70 中文书. 单轴压缩试验：施加荷载的速度对岩石的变形性质 和强度指标有明显影响。 加载速率快，测得的弹性模量越大，获得的强度 指标越高。加荷速率越慢，弹性模量越小。 ISRM 建议的加载速率为：0.5-1MPa/S ， 一般从开始试验直至破坏的时间为5-10min. 5) Environmental effects Other factors which affect rock behaviour, in particular moisture content, time and temperature, can be of importance in engineering a) Moisture content. The moisture content is known to influence the complete stress-strain curve because of its effect, in certain rocks, on the deformability, the compressive strength and post-peak behaviour. For this reason, it is recommended, for example by the ISRM, that the moisture content be measured as an integral part of the compressive strength determination of rocks. b) Temperature effects. The effect of temperature on the complete stress-strain curve c) Time-dependent effects There are four main time-dependent effects which are discussed here. strain rate-the total form of the complete stress- strain curve is a function of the applied strain rate; (2) creep-a material continues to strain when the applied stress is held constant; (3) relaxation-there is a decrease in stress within the material when the applied strain is held constant; (4) fatigue-there is an increase in strain due to cyclical changes in stress. Time-dependent effects and the complete stress-strain curve. Time-dependent effects and the complete stress-strain curve. 中文书P52-54 Mohr-Coulomb failure criterion. 岩石的莫尔强度包洛线是直线型 τ =c+σtanφ τ轴的截距称为岩石的粘结力c(MPa),与σ轴的夹角称为岩石的内摩擦角φ(o).与库仑强度准则相对应. 岩石的莫尔强度包洛线一般是抛物线型即曲线型.将包洛线与τ轴相交点的包洛线外切线与σ轴的夹角定义为岩石的内摩擦角φ(o),包洛线与τ轴的截距定为岩石的粘结力c(MPa). 莫尔强度曲线绘制： （由单拉、 单压、三压强 度实验得到） 特点： 曲线左侧闭合，向由侧开放（耐压、不耐拉）； 曲线的斜率各处不同（内摩擦角、似内聚力变化，与所受应力有关）； 曲线对称于正应力轴（破坏面成对出现，形成 X 型节理）； 不同岩石其强度曲线不同（不同岩石具有不同的强度性）。 莫尔包络线的三种形式： （不同岩石具有不同的强度性质， 其强度曲线可分为三个类型） a) 直线型： （与库仑准则相同） 可进行强度计算： 单直线型 双直线型 b) 二次抛物线型： 表达式： 式中： —单向抗拉强度 —待定系数 由图：N 点坐标及NM半径为