EXPERIMENTAL STUDY ON GRANITE AND THE DETERMINATION OF ITS TRUE STRAIN-RATE EFFECT

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YUBIN LU

Abstract

SPLITTING, UNIAXIAL AND TRIAXIAL COMPRESSION TESTS WERE USED TO STUDY THE MECHANICAL RESPONSE OF GRANITE UNDER DIFFERENT STATIC STRESS CONDITIONS WITH THE MTS ROCK MECHANICS TEST SYSTEM, AND THE IMPACT COMPRESSION TESTS WERE PERFORMED AT DIFFERENT STRAIN-RATES BY THE SPLIT HOPKINSON PRESSURE BAR (SHPB). THE TEST RESULTS SHOW THAT THE COMPRESSIVE STRENGTH INCREASES WITH THE INCREASE OF CONFINEMENT, BUT THE INCREASE RATE DECREASES AS THE CONFINEMENT GETS LARGER. THE AXIAL FAILURE STRAIN ALSO INCREASES WITH THE INCREASE OF CONFINEMENT. FAILURE IS RELATED TO THE COMPOSITION AND STRUCTURE OF GRANITE, AS WELL AS THE STRESS STATE. WITH INCREASING CONFINEMENT, THE SAMPLE IS MORE CONSTRAINED, THE ELASTIC LIMIT STRAIN BECOMES SMALLER, AND THE ELASTIC MODULUS BECOMES LARGER ACCORDINGLY. IN ADDITION, SHEAR SLIP FAILURE TAKES PLACE UNDER TRIAXIAL COMPRESSION. IN THE DYNAMIC COMPRESSION TESTS, THE STRAIN-RATE AFFECTS NOT ONLY THE STRENGTH OF GRANITE, BUT ALSO THE DEGREE OF FRAGMENTATION AND THE BREAKING PATTERN. ALSO, IT HAS BEEN FOUND THAT THE DYNAMIC COMPRESSIVE STRENGTH ENHANCEMENT OF ROCKS UNDER IMPACT LOADING IS DUE TO THE COMBINED EFFECTS OF THE MATERIAL STRAIN-RATE, LATERAL INERTIA AND END FRICTION; HOWEVER, IN SHPB TESTS THEY ARE COUPLED TOGETHER AND COULD NOT BE SEPARATED FROM EACH OTHER. TO DETERMINE THE MATERIAL STRAIN-RATE EFFECT OF ROCKS IN THE SHPB TESTS, THE DYNAMIC COMPRESSIVE STRENGTH ENHANCEMENT CAUSED BY THE LATERAL INERTIAL EFFECT AND END FRICTION EFFECT NEEDS TO BE REMOVED. ASSUMING THAT THE EFFECT OF THE MATERIAL STRAIN-RATE, LATERAL INERTIA AND END FRICTION IS UNCOUPLED, THE NUMERICAL SIMULATION METHOD HAS BEEN EMPLOYED TO SIMULATE THE SHPB TESTS ON GRANITE. THE TRUE STRAIN-RATE EFFECT OF GRANITE IN SHPB TESTS IS THUS DETERMINED.

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