In this paper, the nuclear magnetic resonance (NMR) measurements are carried out to evaluate the micro‐cracking characteristics of sandstones during the creep stage under the different levels of creep stresses. The variations in the parameters, including transverse relaxation time (T2) spectra distribution, percentage of the pore distribution, incremental value of porosity and ultrasonic P wave velocity are analysed. The results show that, during the creep stage, small pores in the rocks gradually evolve into large pores, resulting in an increase in damage of rocks. With the increase of the loading ratio, the increasing rate of porosity increases sharply, implying that the damage degree becomes more serious under the high loading ratio. When the loading ratio is larger than 0.7, the increasing rate of porosity and the decreasing rate of the P wave velocity both increases rapidly. During the creep stage, the relationship between micro‐crack and macro‐creep characteristics of rocks is established based on the analysis of the deformation and the porosity evolution during creep. It is found that, as the loading ratio increases, the increasing rate of porosity is linear with the creep strain. The NMR method offers a feasible option to identify micro‐cracking process of rocks during the creep stage, which can directly reflect the damage mechanism of rock caused by creep. The porosity of rocks is an effective parameter to evaluate the damage of rocks. The damage evolution of the specimen is highly anisotropic and is heavily related to the loading ratio during the creep stage. The relationship between the damage parameter and the loading ratio is exponential.

In many major rock projects, services have spanned decades or even hundreds of years. For engineering design, it is necessary not only to ensure the safety of personnel during construction but also to ensure the safety of long‐term use and operation in the future. Therefore, it is indispensable to study the creep damage mechanical properties of rocks before the construction of the project. For a long time, rock creep has been an important reason for large deformation and even instability in building foundations, underground tunnels, and slopes. Moreover, the surrounding rock generally exhibits obvious creep damage characteristics under an environment of high stress, high temperature, and high water‐pressure. Under certain geological conditions, this creep damage behavior is extremely prominent. Therefore, the study of rock creep damage mechanics is very important. This paper reviews the advancement of rock creep damage mechanics from the aspects of mechanisms, research methods, constitutive models, and so forth. Rock creep is a process of interaction between structural damage and the hardening effect. The structural damage and hardening effect are opposite to each other, but they exist in the same physical process. The creep research method introduces mainly the research objects, influencing factors, monitoring methods, and experimental methods. Creep constitutive models introduce mainly empirical models, linear element combination models, nonlinear combination models, and other nonlinear models based on new theory. Finally, based on the current research progress, several important research directions in rock creep damage mechanics are pointed out.

In this paper, the effects of materials and confining pressures on the mechanical properties of flawed coarse and fine sandstone is first analyzed. Then, the evolution processes of spatiotemporal acoustic emission (AE) events of coarse and fine sandstone specimens containing two flaws subjected to biaxial compression are investigated to character the damage characteristics and the fracture mechanism. Third, the ultimate failure modes of coarse sandstone specimens are compared with those of the fine sandstone specimens. Finally, the Electron Probe Microscope Analysis (EPEI) technique is conducted to observe the main macrofracture surfaces of coarse and fine sandstone specimens. Besides, the distribution characteristics of two AE indexes are also employed to qualitatively reveal the failure mechanism of tested specimens. The experimental results can provide new insight into the mechanical responses and the cracking behaviors of flawed rocks in complex stress loading conditions.

In the previous works conducted by Zhou and Yang, the deformation induced by wing crack coalescence was not investigated. In this paper, a novel micromechanics‐based rate‐dependent constitutive model is proposed to investigate the effect of wing crack coalescence on the mechanical properties of rocks at intermediate strain rate. The rate‐dependent damage mechanism is revealed using the frictional sliding crack model. Wing crack interaction is defined using pseudo‐traction method as well as the dynamic stress intensity factors. Strain energy density factor approach is applied to determine the critical condition of wing crack initiation, propagation, and coalescence. The strain softening behaviors of flawed rocks is studied by using thermodynamic theory.