Research On The Meso-fatigue Failure Behaviors Of Thermally Treated Beishan Granite

Thermal energy released during the disposal of high-level radioactive waste causes the temperature of the surrounding rocks of the repository to rise.Therefore,the influence of temperature must be considered when studying the mechanical properties of Beishan granite.During the long period of construction and operation of the underground radioactive waste repository,the effects of thermal stress changes caused by periodic blasting excavations,traffic loads,and temperature fluctuations on Beishan granite can be regarded as fatigue loads.The researches on fatigue fracture mechanism are of great significance to the long-term stability evaluation of high-level radioactive waste repositories in China and to prevent the underground migration of nuclide.In this thesis,the Beishan granite after 50?-600?thermal treatments is taken as the research object.Using scanning electron microscope equipment with a loading device,meso-fracture experiments under quasi-static and fatigue loading conditions are carried out.The main research contents of the paper are as follows:1.Effect of thermal treatments on mesostructure of Beishan granite and its mechanismThermal treatments changed the microstructure of Beishan granite,including changes in the roughness of mineral particles(without cracks)and thermal cracking,both of which are related to evaporation and escape of water and thermal expansion of mineral particles.The temperature required to change the surface roughness of mineral particles is relatively low.It is observed in the temperature range of 50?to 300?and occurs in a small local area.When the thermal treatment temperature exceeds 100?,the thermal cracking behavior appears.There are two types of thermal cracking behaviors:one is newly formed cracks and the other is further extension of the original cracks.The change of mesostructure of Beishan granite after thermal treatments can be described by the line crack density.The change of crack density of Beishan granite surface with temperature conforms to the exponential function relationship.The crack density increases significantly when the heat treatment temperature exceeds 300?.2.Meso fatigue crack behaviors and fracture mechanism of Beishan granite after different thermal treatments(1)According to the fatigue crack behaviors in-situ observed,the thermal treatment temperature is divided into three groups:low temperature(50?-150?),intermediate temperature(200?-300?)and high temperature(400?-600?).In the experiments at 50?-150?,the number of initial cracks was small,fatigue cracks tended to propagate towards the boundaries of mineral particles and a small number of initial crack tips.In this temperature range,there are two mechanisms for transgraunlar cracks.In the 200?-300?experiments,by connecting the cracks along the particles and the initial cracks generated during the cyclic loading process,the rock bridge plays an important role in the fatigue crack propagation process in this temperature range.Most of the transgranular cracks are formed by the further expansion of the initial intragranular cracks and the intergranular cracks are mainly formed by decohesion of mineral grains.In the400?-600?experiments,many cracks were formed by thermal treatments and the rock bridges between the cracks were very short and easy to be broken.The new crack initiation phenomenon becomes less and less as the temperature increases,and the initial crack opening gradually becomes the main crack behaviors.(2)Comparing the meso-fracture process of Beishan granite under quasi-static and fatigue loading conditions,the differences between crack behaviors under the two loading conditions are as follows:Firstly,the initial crack propagation direction.Under fatigue loading,the initial crack will expand in two directions along its own length,especially when the crack direction is approximately parallel to the loading direction.This phenomenon is rarely observed under quasi-static loading conditions.Secondly,the rock bridge linking plays a more significant role under fatigue loading conditions.Thirdly,the critical crack opening under fatigue loading conditions is much smaller than that under quasi-static loading conditions.Fourthly,regardless of the thermal treatment temperature,the fatigue failure shows sudden and no-precursor characteristics.While under quasi-static loading conditions,the specimens under high temperature treatment are more slowly destroyed.(3)The boundary collocation method and finite element method have been used to calibrate the stress intensity factor of a short-span three-point bending test specimen with an edge crack and the expression of stress intensity factor is obtained:K_IBW^3/2/M=(?).The expression is concise and easy to use,which enriches the existing research results on stress intensity factors and provides a new theoretical basis for calculating the fracture toughness of Beishan granite.(4),Based on the crack length and stress intensity factor expressions,the author uses the Hartman-Schijve equation to quantitatively describe the fatigue crack growth rate of Beishan granite in the range of 50?-250?.The experimental results are in good agreement with the theoretical values.The fatigue crack growth rate da/d N obtained in this study is between 10^-6m/cycle and 10^-11m/cycle,and most of them are between 10^-6-10^-8m/cycle.It is found that the parameters of the H-S equation are more sensitive to the initial crack density.The exponential parameter?of the equation increases approximately exponentially with the increase of the initial crack density.This is a new finding about the exponential parameters of the fatigue crack growth rate equation.In addition,the theoretical results show that as the initial crack density increases,fatigue cracks start to develop more easily from natural defects,which is consistent with the experimental observation.3.Damage evolution law and fatigue life of Beishan granite after different thermal treatments(1)The degradation process of specimen stiffness during fatigue loading is analyzed.The damage factor is defined based on the stiffness,and the damage evolution law is analyzed.According to the damage evolution process,a new three-parameter equation is proposed to describe the fatigue damage evolution process:(?).The three parameters of the equation control the deceleration phase,uniform speed phase and acceleration phase of the fatigue damage evolution curve respectively.When the acceleration phase is short,these three parameters can be used to approximate the critical damage.(2)With the increase of thermal treatment temperature,the fatigue life of Beishan granite increases firstly and then decreases,which is attributed to changes in initial crack density.With the increase of initial crack density,more and more cracks in Beishan granite are repeatedly opened and closed during fatigue loading,and the work done by the external force on the specimen is consumed at more and more crack tips.And when the initial crack density is lower than 1.75mm^-1,the number of initial cracks crossing each other is small,and the rock bridge between the crack tips is less sensitive to the increase of the fatigue load level and difficult to break.At the same time,the rock debris generated during heating and fatigue loading fill the cracks,increasing the anti-sliding ability between crack faces,requiring higher cyclic loading levels and more cycles to make the crack propagate forward.Therefore,when the initial crack density is lower than 1.75mm^-1,the fatigue life increases with the increase of the initial crack density.When the initial crack density is higher than 1.75mm^-1,crack density results in more initial cracks intersecting each other,and the bridge between the cracks is shorter and easier to break.Therefore,the crack is easy to expand when the load level increases and during the same level fatigue loading process,so the fatigue life is gradually reduced.4.Mesomorphic features of beishan granite fatigue fractureThe meso-fatigue fracture morphology features of Beishan granite are systematically displayed,including mirror,velocity hackle,twist hackle,wake hackle,arrest line,fatigue striation,etc.The differences between similar morphologies are identified and the formation mechanism of morphology is discussed.According to the observation,fatigue striations are summarized into three categories:arc-shaped,straight-lined,and step-shaped,among which straight-lined fatigue striations include straight transgranular fatigue striations,straight intergranular fatigue striation with spire,and straight intergranular faituge striation with domes.Step shaped fatigue striations include arc-stepped and straight-stepped fatigue striations.Statistical analysis was performed on the mesomorphic dimensions of some features.It was found that the mirror radius and length and density of velocity hackle meet the log-normal distribution,and the twist hackle length,width and aspect ratio satisfy the three-parameter Weibull distribution.