Experimental and numerical investigation of the effect of micro-cracks on the mechanical behavior of rock specimens

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Khodayar amin; NEJATI HAMID REZA

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Journal: Modares Civil Engineering journal Year:2018 | Volume:18 | Issue:6 Page(s): 95-105

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Information Journal Paper

Title

Experimental and numerical investigation of the effect of micro-cracks on the mechanical behavior of rock specimens

Author(s)

Khodayar amin | NEJATI HAMID REZA | Issue Writer Certificate

Keywords

failure modeQ1

micro-crackQ1

mechanical behaviorQ2

GraniteQ1

Abstract

It is seldom possible that rock engineering structures found without joints, cracks, or discontinuities. On the other hand, the application range of these structures are steadily increasing in recent years and includes bridges, tunnels, slopes, underground gas storage and etcetera. Thereby, their impact is to be considered in the rock structure design. In the present study, it is intended to evaluate the effect of induced micro-cracks on the mechanical behavior of rock specimens. For this purpose, 24 cylindrical specimens of Granite were prepared and some of them heated up to 800 degrees Celsius to induce micro-crack in the specimens. In the next, uniaxial compression test for determination of stress-strain curve of heated and unheated specimens were performed based on International Society for Rock Mechanics (ISRM) suggested method on a cylindrical specimen with 110 mm and 54 mm in length and diameter, respectively. The tests were conducted using a load controlled testing machine and the loading rate was kept at 0. 5 MPa/Sec. Results of experimental tests indicate that mechanical properties of heated specimens change with increasing the temperature. For example elastic modulus, uniaxial compression strength, tensile strength of the fractured specimens is smaller than the intact ones and the strain corresponding to the failure increases with inducing the micro-cracks in the specimens. Also, it should be noted that tensile strength compared to the compression strength in more sensitive to thermal stress and induced micro-cracks. Therefore, brittleness index of the specimens (ratio of compression strength to tensile strength) decreases with increasing the fracture density. Further, induced micro-cracks influence on the failure pattern of specimens. The failure pattern of unheated specimen is axial splitting mode, while the failure pattern of heated specimen up to 800 degree Celsius changes to shear mode failure. In the next step, numerical modeling of the uniaxial compression test was provided to evaluate the reasons for changing of the failure pattern of heated specimens compared to the unheated ones. Numerical simulation was conducted using rock failure process analysis (RFPA) tool as a finite element code which has been developed on the basis of elastic damage model. The heterogeneity of rocks at a mesoscopic level is considered by assuming that the material properties follow the Weibull distribution. Elastic damage mechanics is used for describing the constitutive law of the meso-level element. The numerical models were adopted using mechanical properties of heated and unheated specimens extracted from laboratory tests. On the basis of numerical modeling results, it is found that in the specimen with induced micro-cracks, ratio of the number of shear cracks to tensile ones increases, under loading sequences. With increasing the fracture density in the specimens, the index of brittleness reduces and consequently the probability of nucleation and propagation of shear cracks, under uniaxial compression, increases. This is the main cause for changing of the type of specimen failure mode from axial splitting to shearing.

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APA: Copy

Khodayar, amin, & NEJATI, HAMID REZA. (2018). Experimental and numerical investigation of the effect of micro-cracks on the mechanical behavior of rock specimens. MODARES CIVIL ENGINEERING JOURNAL, 18(6 ), 95-105. SID. https://sid.ir/paper/256714/en

Vancouver: Copy

Khodayar amin, NEJATI HAMID REZA. Experimental and numerical investigation of the effect of micro-cracks on the mechanical behavior of rock specimens. MODARES CIVIL ENGINEERING JOURNAL[Internet]. 2018;18(6 ):95-105. Available from: https://sid.ir/paper/256714/en

IEEE: Copy

amin Khodayar, and HAMID REZA NEJATI, “Experimental and numerical investigation of the effect of micro-cracks on the mechanical behavior of rock specimens,” MODARES CIVIL ENGINEERING JOURNAL, vol. 18, no. 6 , pp. 95–105, 2018, [Online]. Available: https://sid.ir/paper/256714/en

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