Fatigue Damage And Dilatancy Properties For Salt Rock Under Discontinuous Cyclic Loading

Non-occupied underground salt caverns which formed during exploiting the salt resource have been posing lots of threatens on the environmental protection and surface safety.Nevertheless,they could serve as the ideally safe and stable store sites for gas or liquid hydrocarbon resource(such as natural gas,oil,compressed air),owing to its proper burial depth,low permeability and strong rheological behavior as well as the cracks inside could self-heal.The salt cavern storage of natural gas or compressed air plays a critical role in ensuring the energy supply and adjusting the seasonal imbalance.According to the change of market demand for energy,the storages produce energy by using the stored natural gas or compressed air at peak demand and was injected with resource at low demand.Consequently,the pressure inside the storages changes seasonally with the market demand.Under the repeated loading/unloading effect,the storage volume would progressively shrink with the evolution of fatigue damage caused by cyclic loading.As the fatigue damage accumulates,the volume loss of storage will increase,even the instability or gas leakage occurs.Therefore,conducting the research in the fatigue of salt under cyclic loading is of great importance to the safe operation of underground salt caverns storage.With respect to the special characteristics of China salt formation(including the buried depth,formation thickness,ground temperature,mezzanine occurrence,etc.),the thesis perform the uniaxial tests,triaxial tests,conventional fatigue tests and interval fatigue tests,in attempt to investigate mechanical response and involved mechanism for damage forming,reveal the evolution of plasticity,dilantancy and damage under different stress state and establish the constitution model for future numerical experimentation.This thesis' s research content includes the following aspects:(1)Conduct the uniaxial and triaxial tests,to provide the basic data(for example,the compressive strength,elastic parameters)for the sequent cyclic loading tests;reveal the evolution of inelastic/plastic deformation and damage;develop the constitutive relationship for salt rock under monotonic continuous loading.(2)Considering the geological conditions,underground temperature and speed of gas filling/recovery,I perform the conventional fatigue tests on salt according to different upper and lower stress limit,temperature and loading rate;(3)Develop the fatigue life model to describe the ability for salt to endure the number of loading-unloading cycle.During the cyclic loading-unloading,salt behave rather differently,showing new strong hardening/strengthening.The constitutive relationship taking account the move of yield surface is established to describe the cyclic behavior of salt.(4)Based on the practical data from one running storage,series of interval(discontinuous)fatigue tests were carried out,to reveal the effect of intervals(which was ignored in most cases before)on the fatigue properties for salt.The coupled effect between ratcheting effect and intervals was studied.(5)Explain the behavior(including plastic flow,dilatancy change)of salt from the perspective of dislocation theory and fracture mechanics;by introducing the weakening parameter,which describe the yield surface periodic change occurring during the intervals,a new constitutive relationship for salt rock under discontinuous cyclic loading was developed.