Evolution of deformation property and strength component mobilization for thermally treated Beishan granite under compression

The cohesion weakening and friction strengthening(CWFS)model for rock reveals the strength components mobilization process during progressive brittle failure process of rock,which is very helpful in understanding mechanical properties of rock.However,the used incremental cyclic loading−unloading compression test for the determination of strength components is very complicated,which limits the application of CWFS model.In this paper,incremental cyclic loading−unloading compression test was firstly carried out to study the evolution of deformation and the strength properties of Beishan granite after various temperatures treated under different confining pressures.We found the axial and lateral unloading modulus are closely related to the applied stress and damage state of rock.Based on these findings,we can accurately determine the plastic strain during the entire failure process using conventional tri-axial compression test data.Furthermore,a strength component(cohesive and frictional strength)determination method was developed using conventional triaxial compression test.Using this method,we analyzed the variation of strength mobilization and deformation properties of Beishan granite after various temperatures treated.At last,a non-simultaneous strength mobilization model for thermally treated granite was obtained and verified by numerical simulation,which demonstrated the effectiveness of the proposed strength determination method.

Physical and mechanical properties of Beishan granite after different high temperature treatments

In China,Beishan granite is chosen as a potential host surrounding rock of a high-level radioactive waste(HLW)repository.For this research,Beishan granite specimens were heated up to 300℃,400℃ and 500℃,respectively.And conventional triaxial compression tests were conducted after cooling down the samples.The results show that after 300℃,400℃ and 500℃ heating treatment,the diameter of samples increases by 0.066%,0.143%and 0.409%,respectively,which is a little larger than the axial length changes.Mechanical tests show that peak strength increases slightly with increasing temperature.However,the dilatancy threshold is lower than that observed for samples which have not experienced heating treatment.Peak strain and dilatancy threshold strain show a strong temperature dependence.The higher the temperature,the greater the strain.Furthermore,increasing temperature has negative influence on threshold elastic modulus E_(c) and tangent elastic modulus E_(t).Poisson ratio decreases when temperature increases from 300℃ to 500℃,but it is still larger than that observed for samples which have not experienced heating treatment.In addition,AE monitoring shows a quiet period in the initial loading stage,which proves that the micro cracks are closed during heating and contribute to the increase of peak strength.

Mechanical properties and energy evolution of Beishan shallow-layer granite under different unloading paths

Rock has mechanical characteristics and a fracture damage mechanism that are closely related to its loading history and loading path. The mechanical properties, fracture damage features, acoustic emission(AE) characteristics, and strain energy evolution of the Beishan shallow-layer granite used in triaxial unloading tests were investigated in this study. Three groups of triaxial tests, namely, conventional triaxial compression test(Group Ⅰ), maintaining deviatoric stress synchronously unloading confining pressure test(Group Ⅱ), and loading axial pressure synchronously unloading confining pressure test(Group Ⅲ), were carried out for the cylindrical granite specimens. AE monitoring device was utilized in these tests to determine the degree to which the AE waves and AE events reflected the degree of rock damage. In addition, the crack stress thresholds of the specimens were determined by volumetric strain method and AE parameter method, and strain energy evolution of the rock was explored in different damage stages. The results show that the shallow-layer granite experiences brittle failure during the triaxial loading test and unloading test, and the rock has a greater damage degree during the unloading test. The crack stress thresholds of these samples vary greatly between tests, but the threshold ratios of all samples are similar in the same crack damage stage. The Mogi-Coulomb strength criterion can better describe the unloading failure strength of the rock. The evolution of the AE parameter characteristics and strain energy differs between the specimens used in different stress path tests. The dissipative strain energy is the largest in Group Ⅱ and the smallest in Group Ⅰ.

Creep constitutive model considering nonlinear creep degradation of fractured rock

Stability analysis of underground constructions requires a model study of rock masses’ long-term performance. Creep tests under different stress conditions was conducted on intact granite and granite samples fractured at 30° and 45° angles. The experimental results indicate that the steady creep strain rates of intact and fractured rock present an exponential increase trend with the increase of stress level. A nonlinear creep model is developed based on the experimental results, in which the initial damage caused by fracture together with the damage caused by constant load have been taken into consideration. The fitting analysis results indicated that the model proposed is more accurate at identifying the full creep regions in fractured granite, especially the accelerated stage of creep deformation. The least-square fit error of the proposed creep model is significantly lower than that of Nishihara model by almost an order of magnitude. An analysis of the effects of elastic modulus, viscosity coefficient, and damage factors on fractured rock strain rate and creep strain is conducted. If no consideration is given to the effects of the damage, the proposed nonlinear creep model can degenerate into to the classical Nishihara model.

Physico-mechanical properties of granite after cyclic thermal shock

Understanding rock mechanical behaviors after thermal shock is critically important for practical engineering application.In this context,physico-mechanical properties of Beishan granite,Gansu Province,China after cyclic thermal shock were studied using digital image correlation(DIC),acoustic emission(AE)monitoring,and microscopic observation.The results show that the peak strength and elastic modulus decreased gradually with increase in thermal shock cycle.However,the above two parameters showed no further changes after 10 thermal shock cycles.The loading stress ratio(i.e.the ratio of the current loading stress level to the peak stress in this state)corresponding to the occurrence of the uneven principal strain field and the local strain concentration zone on the surface of the granite specimen decreased with increase in thermal shock cycle.Three transformation forms of the standard deviation curves of the surface principal strain were found.For granite with fewer thermal shock cycles(e.g.no more than 2 cycles),the standard deviation curves exhibited approximately exponential growth in exponential form.With increase in thermal shock cycle,the S-shaped curve was dominant.After 10 thermal shock cycles,an approximate ladder-shaped curve was observed.It is displayed that AE activity was mainly concentrated around the peak strength zone of the granite specimen when the rock samples underwent fewer thermal shock cycles.With increase in thermal shock cycle,AE activity could occur at low loading stress levels.Microscopic observation further confirmed these scenarios,which showed that more microcracks were induced with increase in thermal shock cycle.The number of induced microcracks at the edge location of the granite specimen was significantly larger than that at the interior location.Finally,a continuum damage model was proposed to describe the damage evolution of the granite specimen after cyclic thermal shock during loading.