Mechanics Characteristic And Constitutive Model Studies Of Gabbro Under Triaxial Cyclic Loading Condition

In order to systematically understand mechanical property of gabbro and provide theoretical basis and mechanical parameter for the study of stability of the high slope and mechanical property of gabbro in the Panzhihua open-pit mine, this paper conducted experimental study under different stress level conditions on gabbro samples from Panzhihua mine by using MIS815 in the state key laboratory of Chengdu University of Technology. As this mine involves blasting operation of ore body, the stress-strain whole procedure test under uniaxial compression condition, conventional triaxial compression test and triaxial cyclic loading vibration test were conducted to better simulate force condition of gabbro. According to results of the above three tests, the mechanical parameter that could reflect strength and deformation characteristics of gabbro was obtained by studying these characteristics under different loading conditions. The macroscopical morphology of rupture surface and failure mechanism of gabbro under these three loading conditions. Constitutive model under triaxial cyclic loading condition was built and verified according to results of triaxial cyclic loading vibration test. The major results of this study are as follows:(1) The uniaxial compression test has yielded the stress-strain whole procedure curve of gabbro. According to the curve shape after the peak, the stress-strain whole procedure curves of 25 samples can be divided into the following three types: type?, multistage drop of the stress; type?, drop-rebound of the stress; type ?, rapid drop of the stress. In the stress-strain curves of the gabbro samples under uniaxial compression condition, the stress decreases rapidly after the peak, indicating a kind of brittle fracture of the samples. The stress-strain whole procedure curve of gabbro could be divided into 4 stages as the above three curves have high similarity and reproducibility. According to three times the standard deviation method, the modulus of elasticity and Poisson's ratio values of gabbro are 12.62 GPa and 0.16, and the uniaxial compressive strength is 55.29 MPa.(2) The conventional triaxial compression test has yielded the stress contrast-axial strain curve of gabbro under different confining pressures. The stress contrast-axial strain curve is concave before rock destruction. 40 analyses on the samples indicate that the average Angle of internal friction and cohesion are 56.29° and 9.628 MPa under confining pressures of 1-4MPa and are 49.462° and 9.372 MPa under 2-8MPa.(3) The triaxial cyclic loading tests on 27 gabbro samples have resulted in the following two results:(1) Under the same confining pressure condition, dynamic modulus of elasticity gabbro increases with increase of cyclic times; Under the same cyclic times, dynamic modulus of elasticity gabbro decreases with increase of the confining pressure; Dynamic modulus of elasticity gabbro decreases with increase of its residual axial strain.(2) Under the same loading amplitude and confining pressure conditions, damping ratio of gabbro decreases rapidly with increase of cyclic times at first, and then becomes stable; Under the same cyclic times, damping ratio of gabbro increases with increase of confining pressure; Under the same confining pressure conditions, the average damping ratio value decreases with increase of loading amplitude; Under the same loading amplitude, average damping ratio value increases with increase of confining pressure.(4) Study of failure characteristic of rocks under different loading path indicates that the destructional forms under triaxial compression condition include tension and shear failure; The destructional forms under conventional triaxial compression and triaxial cyclic compression conditions are dominated by shear failure; SEM mesoscopic morphology analysis of rupture surface produced by the above three destructional forms further confirmed the destructional forms determined by macroscopic observation.(5) This paper has provided empirical constitutive model and fatigue mechanical model based on internal variable theory. Fitted values of the theoretical model and measured values have a very high fitting precision, indicating that the proposed models could well reflect fatigue mechanical behaviors of rocks under cyclic loading condition.