卸荷条件下冻结砂岩力学性质及损伤本构模型研究

Study on mechanical properties and damage constitutive model of frozen sandstone under unloading condition

为探究低温软岩卸荷力学性质和损伤特性,采用GCTS低温岩石三轴仪对冻结砂岩(温度-5℃,-10℃和-15℃)进行不同初始围压和不同卸荷速率下的加卸载试验,并与常规加载试验相对比,通过获取全过程应力–应变曲线分析卸荷冻结砂岩的力学性能与变形特性,以及岩样的破坏特征。结果表明:(1)卸荷冻结砂岩呈剪切破坏和劈裂破坏组合破坏模式,且以劈裂破坏为主;高初始围压和高卸荷速率下,岩样脆性特征明显,破坏更加剧烈。(2)卸荷砂岩的峰值强度普遍低于(约降低1/4)常规砂岩的,残余强度则恰好相反,而二者强度值均与初始围压呈线性正相关;卸荷砂岩的弹性模量约为常规砂岩的1.5倍,泊松比为2~4倍。(3)卸荷使冻结砂岩的变形明显增大,扩容现象显著,应力–应变曲线峰前应力强化效应和峰后应力弱化效应减弱,峰后应力出现水平段。(4)卸荷主要通过降低黏聚力、提高内摩擦角来改变冻结砂岩的力学强度;卸荷速率的升高、温度的降低,使得冻结砂岩的峰值强度和弹性模量增大,变形、泊松比和残余强度减小。基于损伤力学等应变假设和Weibull分布,建立了适用于冻结砂岩的卸荷损伤本构模型,能够切实地反映冻结砂岩的侧向卸荷力学特性。研究成果可为低温软岩力学特性研究及其工程应用提供重要参考。

In order to research the mechanical properties and damage characteristics of low-temperature soft rock under unloading,triaxial loading and unloading tests of frozen sandstone(temperature-5℃,-10℃and-15℃)at different initial confining pressures and unloading rates were carried out by using GCTS low-temperature rock triaxial machine,which was compared with conventional loading tests.By obtaining the stress-strain curve during the whole experimental process,the mechanical properties and deformation characteristics of unloaded frozen sandstone were analyzed.The failure characteristics of sandstone were studied.The results show that:(1)The failure mode of unloaded frozen sandstone is a combination of shear failure and splitting failure,and the main failure mode is splitting failure.At high initial confining pressure and high unloading rate,the brittle characteristics of the rock sample are obvious,and the failure is more severe.(2)The peak strength of unloaded sandstone is generally lower(about 1/4 times)than that of conventional sandstone,but the residual strength is exactly the opposite,and both strength values are linearly positive correlated with the initial confining pressure.The elastic modulus of unloaded sandstone is about 1.5 times that of conventional sandstone,while the Poisson¢s ratio is 2–4 times.(3)The deformation and dilatation of frozen sandstone are obviously increased by unloading.The pre-peak stress strengthening effect and post-peak stress weakening effect of stress-strain curve are weakened,and the post-peak stress appears horizontal section.(4)Unloading affects the mechanical strength of frozen sandstone mainly by reducing cohesion and increasing internal friction angle.With the increase of unloading rate and the decrease of temperature,the peak strength and elastic modulus of frozen sandstone increase,while the deformation,Poisson¢s ratio and residual strength decrease.Based on the equal strain hypothesis of damage mechanics and Weibull distribution,the unloading damage constitutive model suitable for frozen sandstone is established,which can effectively reflect the lateral unloading mechanical characteristics.The research results can provide important reference for the study of mechanical properties of low temperature soft rock and its engineering application.