Fatigue Damage Characteristics And Constitutive Model Of Gypsum Rock Under Cyclic Loading

As a widely used engineering material,rocks are not only subjected to static load,but also explosion,impact,vibration and other dramatic load during working process,for example,tunnel rock in the work in addition to the surrounding rock stress and other static load,but also by the vehicle dynamic load.Cyclic load,as the simplest and th e most common dramatic load,maybe the key and basic steps to the research on the r ock mechanical characteristics under complicated dramatic load.The thesis take the gy psum rock which takes from one tunnel located in Hubei province as the research obj ects.By using MTS815 Rock Mechanics Test System to test the fatigue properties of gypsum under cyclic load and the SAEU2 S multi-channel Acoustic Emission detecting system is used to record the acoustic emission characteristics during the cyclic load,u se the Scanning Electron Microscope to observe the microstructure of the gypsum sam ple which was before and after cyclic load experiment.With the base of fracture mech anics,thermodynamics,continuum damage mechanics and Other mathematical and mecha nical theories,with the help of some research achievements in rock creep to do a serie s of research,the main results of the thesis are as follows:1.The typical process of stress-strain curve of gypsum under cyclic load beha ves "sparse-dense-sparse",and the fatigue process can be divided into three stages:initial cycle,cyclic stability and cyclic acceleration.The results of acoustic emission can match with macroscopic stress and strain:both may reflect the evolution of microcracks within the gypsum evolution.2.The cyclic loading condition has a significant effect on the fatigue damage ch aracteristics of gypsum rock,as the cyclic stress level is higher,the upper limit stress ra tio is larger and the cyclic frequency is lower,the hysteresis loop area is larger,the d amage rate is faster and the plastic strain is larger in one cycle,the fatigue life of the sample is relatively small,the damage pattern of the specimen is mainly composed o f a small amount of "big crack" at the macroscopic level,and the microscopic layer i s mainly composed of microcracks.With the higher cyclic frequency,the greater of th e deformation modulus of the sample,the stronger the anti-fatigue failure ability of th e sample.3.By using the concept of statistics,we assume the micro-unit strength of the ro ck obeys the Weibull distribution,and the damage,actually is the proportion of the da maged micro-element,and reference the concept of the effective stress principle,thusconsidered that the upper stress is borne by the damaged area and the undamaged are a to establish the constitutive model of rock under monotonic loading.4.Recognized the similarity of cyclic loading and creep,the mechanics model of rock under cyclic loading is established by referring to the correlation results of rece ntly researched in rock creep and introducing the concept of "equivalent stress".Corre cted the method to calculate the damage variable,put forward a way to calculate res idual fatigue life.5.The evolution model of damage is established by using the relevant parameter s of acoustic emission,which is consistent with the damage evolution model based on macroscopic stress and strain.Based on the conclusion,the probability of acoustic e mission event is investigated.