Experimental Study On Damage Evolution Of Rock-like Quasi-Brittle Materials Under Tensile Loading

The damage and fracture of rock-like quasi-brittle materials under tensile stress plays an important role in the field of crushing engineering and slope engineering.A number of significant theories about rock damage mechanics have been developed,however,experimental research on the damage properties of rock-like quasi-brittle materials under direct tensile loading is very limited.In particular,the direct tensile experiment which can be able to obtain the full-field strain information of rock specimen has not been reported.In this paper,the damage failure of rock-like quasi-brittle materials under tensile loading is studied.The digital image correlation technique(DIC)was used to achieve a full-field measurement of the specimen under the tensile loading.Based on the full-field strain information,the key factors influencing the damage of the material were determined.Moreover,the mechanism of tensile damage,as well as the fracture evolution in rock-like quasi-brittle materials,was analyzed.Finally,a kind of damage model,which can reflect the main features of tensile damage,was proposed.And also,the methods to determine the parameters in the damage model were given.A uniaxial direct tensile scheme for rock-like quasi-brittle material was designed in this paper.In the cutting of rock-like material specimens,a type of advanced ceramic processing technology was used to ensure the smoothness of the specimen surface.A pneumatic fixture with a universal joint was selected to solve the problem of eccentric loading.A computer program was written to ensure that the CCD image synchronize with the tensile loading.Five types(15 specimens)of rock-like quasi-brittle materials were investigated under uniaxial tension.Combined with the global tensile stress-strain curve,the variation trend of elastic modulus of rock specimen during tensile loading was analyzed.A parameter relating to the tensile damage evolution of specimens is proposed,called the gray correlation coefficient,which describes changes in the value of gray scale on the specimen surface during tensile loading and falls within the range of [0,1].During the early stage of loading,damage is at the initial stage such that the gray correlation coefficient is close to zero.At the end stage of the loading,the gray correlation coefficient near the fracture area is close to 1.This suggests that the parameter can characterize the spatial distribution and the situation of damage in rock-like quasi-brittle materials.Based on the full-field measurements of the DIC and statistical analysis of the experimental data,the mechanical parameters such as displacement field and strain field were given in the time domain and spatial domain.To overcome the "distortion point" phenomenon exists in the damage zone for ordinary DIC system,a formula was proposed to calculate the average gradient of the tensile strain.Before the tensile stress approaches its ultimate value,the experimental results show that one or more strain localized bands emerging on the surface of the specimen.There were significant differences in the strain localized band between artificial rock-like material and natural rock.In time,while the strain localized band in artificial rock-material surface appears at the early stage of loading,the band of the natural rock only becomes visible at the late stage or just prior to failure.In the space domain,many bands appear in the strain field of the artificial specimen,while only one band or some smaller ‘high strain points' can be seen on the surface of the natural rock.While the strain distribution has a single peak in the region where the fracture is finally formed,the distribution of strain gradient tends to symmetrical distribution.Through the analysis of the experimental data,it can be considered that the two mechanical parameters of strain and strain gradient can reflect the damage evolution and damage localization process of rock-like materials.Based on the full-field tensile strain data,high strain bands were extracted to describe three modes of crack propagation,coalescence of the rock-like materials under uniaxial tensile loading.It revealed a mechanism for the fracture failure process of macroscopic cracks caused by coalescences of micro-defects in the interior of the rock specimen without the influence of artificial defects.FE-SEM images showed that micro-defects,such as micro-cracks,micro-holes,and so on,only appear in the damage area where a high strain band exists.The scanning results verify the correlation between the two mechanical parameters(the apparent strain and the strain gradient)and the tensile damage.Based on the experimental results,a type of elastic damage model for rock-like quasi-brittle materials is proposed.Two mechanical parameters such as strain and strain gradient are included in the damage evolution equation.Then,the corresponding analytic or asymptotic analytical solutions for one-and two-dimensional damage evolution problems under tensile load are analyzed.And combined with the direct tensile tests of lath specimens,the methods determining the relevant parameters in the damage evolution equation were given.The comparative results between analyses and experimental tests show that the analytical strain solution in the damage zone is basically consistent with the measured strain data.It proved that the model can describe the damage zone parameters including width,ultimate strain and distribution of strain.