基于核磁共振T2谱图的裂隙砂岩疲劳损伤分析

Fatigue Damage Analysis of Fractured Sandstone Based on Nuclear Magnetic Resonance T2 Spectrum

岩石工程中岩体经常处于周期性的力学行为作用下,研究裂隙岩体在疲劳加载下的损伤变化具有重要的工程意义。选取了5种裂隙倾角的黄砂岩试样进行疲劳试验,采用核磁共振仪器分别测得各倾角裂隙砂岩在不同疲劳上限应力作用下30个疲劳循环前后其内部的孔隙变化情况,发现大孔隙数量随疲劳上限应力的增加呈指数增长,是造成裂隙砂岩疲劳加载后孔隙度增大的主要因素。定义了2种基于大孔隙占总孔隙比值的损伤变量来研究疲劳上限应力对各裂隙倾角砂岩的疲劳损伤影响,均能较好地反映损伤变量随疲劳上限应力的变化情况,得出损伤变化趋于平缓的裂隙岩样最终破坏时的强度大于损伤变量陡增的裂隙岩样。最后从理论角度分析得出孔隙与轴向应变之间存在正相关关系,即用孔隙反映砂岩疲劳损伤是合理的。

In the process of underground mining and tunnel construction,the rock is mostly in the process of repeated loading and unloading,and the macro-cracks and micro-defects in the rock mass itself will continue to expand under fatigue loading and eventually lead to many accidents. Therefore,it is of great engineeringsignificance to study the micro-defects of fractured rock masses under fatigue loading.In the previous studies,acoustic emission and CT scanning technology were mainly used to reflect the change of microscopic damagethrough AE number or CT number,which obtained good results.The porosity measured by nuclear magneticresonance technology can also be used to show the damage of rock samples,but it is currently mainly used inuniaxial compression loading. In this experiment,yellow sandstone specimens with five fracture inclinationangles(0° ,15° ,30° ,45° ,60°)were selected. Firstly,uniaxial compression experiments were performed tomeasure the uniaxial compressive strength of fractured sandstone at various angles. Then carry out fatigueloading experiments,and select the upper limit fatigue loading stresses based on the measured uniaxialcompressive strength to be 19.1 MPa,25.6 MPa and 30.3 MPa,respectively. The internal porosity of eachfractured sandstone before and after 30 fatigue cycles under each upper limit stress was measured with nuclearmagnetic resonance instruments. Combined with the analysis of T2 spectrum measured by nuclear magneticresonance instrument,it is found that with the increase of the upper limit stress,the total area of T2 spectrum andthe accumulation of porosity in fractured sandstone have increased significantly. This shows that its internaldamage increases with the increase of the upper limit stress,and then combined with the T2 spectral area forquantitative analysis,it was found that the spectral area of the small pores in the T2 spectrum changed onlyslightly with the increase of the upper limit stress,while the spectral area of large pores increases exponentiallywith the increase of the upper limit stress.No matter from the angle of the change of the ratio of large pores(tothe total pores)to the fatigue upper limit stresses under different inclination angles,or from the angle of thechange of the ratio of large pores under different fatigue upper limit stresses to the angle of fracture inclination,similar rules are obtained.Therefore,it can be concluded that the change of large pores is the main factor thatcauses the total porosity of fractured sandstones to increase after fatigue loading,and then the curve of the ratioof the proportion of large holes to the upper limit fatigue stress ratio after fatigue loading at various inclinationcracks is fitted.The fitting coefficients of sandstones at various inclination angles are very high,which showsthat the change law of internal damage can be well displayed.Considering the effect of initial damage of rocksamples on fatigue loading,two types of damage variables based on the ratio of large pores to total pores weredefined to study the impact of fatigue upper limit stress on the fatigue damage of sandstones with differentfracture inclination angles.According to the relationship curves between the two damage variables,the upperlimit stress ratio and the upper limit stress,the change of the defined damage variable can be well projected.Further analysis can show the intensity change of the sandstones with different fracture inclination angles,thatis,the damage intensity of the fractured rock sample that tends to be gentle will eventually be greater than thatof the fractured rock sample with a sharp increase in damage. Finally,combining the probability densityfunction of Weibull distribution and generalized Hook’s law,it is deduced from the theoretical formula that it isreasonable to use pores to study sandstone fatigue damage.Combining the results of previous studies and thisexperiment,it was found that the porosity can well reflect the microscopic damage of rock samples in bothuniaxial compression experiments and fatigue loading experiments.