Determination of modulus of elasticity and poisson's ratio using strain gauges

SCOPE


This method of test is intended to determine stress-strain curves and Young‟s modulus and Poisson‟s ratio in uniaxial compression of a rock specimen of regular geometry. The test is mainly intended for the classification and characterization of intact rock.


APPARATUS


(a) A suitable machine shall be used for applying and measuring axial load to the specimen. It shall be of sufficient capacity and capable of applying load at a rate conforming to the requirements.


(b) Electrical resistance strain gauges for measuring strain of the rock sample. Their design shall be such that the average of two circumferential and two axial strain measurements, equally spaced, can be determined for each increment of load. The length of the gauges over which axial and circumferential strains are determined shall be at least ten grain diameters in magnitude and the gauges should not encroach within D/2 of the specimen ends, where d is the diameter of the specimen.


(c) An apparatus for recording the loads and deformations; preferably an X-Y recorder capable of direct plotting of load-deformation curves.


PROCEDURE


(a) Test specimens shall be right circular cylinders having a height to diameter ratio of 2.5-3.0 and a diameter preferably of not less than NX core size, approximately 54 mm. The diameter of the specimen should be related to the size of the largest grain in the rock by the ratio of at least 10:1.


(b) The ends of the specimen shall be fiat to 0.02 mm and shall not depart from perpendicularity to the axis of the specimen by more than 0.001 radians (about 3.5 min) or 0.05 mm in 50 mm.


(c) The sides of the specimen shall be smooth and free of abrupt irregularities and straight to within 0.3 mm over the full length of the specimen.


(d) The use of capping materials or end surface treatments other than machining is not permitted.


(e) The diameter of the test specimen shall be measured to the nearest 0.1 mm by averaging three diameters measured at right angles to each other at about the upper-height, the mid-height, and the lower height of the specimen. The average diameter shall be used for calculating the cross-sectional area. The height of the specimen shall be determined to the nearest 1.0 mm.


(f) Moisture can have a significant effect on the deformability of the test specimen. When possible, in situ moisture conditions should be preserved until the time of the test. When the characteristic of the rock material under conditions varying from saturation to dry is required, a proper note shall be made of moisture conditions so that correlation between deformability and moisture content can be made. Excess moisture can create a problem of adhesion of strain gauges which may require making a change in the moisture content of the sample.


(g) Load on the specimen shall be applied continuously at a constant stress rate such that failure will occur within 5-10 min of loading, alternatively, the stress rate shall be within the limits of 0.5-1.0 MPa/sec.


(h) Load and axial and circumferential strains or deformations shall be recorded at evenly spaced load intervals during the test. At least ten readings should be taken over the load range to define the axial and diametric stress-strain curves.


(j) The number of specimens tested should be determined from practical considerations but at least five are preferred.


Instructor: Dr. Radhakanta Koner, Department of Mining Engineering, IIT(ISM) Dhanbad

Students: MTech students of Mining Engineering, IIT(ISM) Dhanbad

Course Name: Geomechanics Practical (MNC505)

Session: 2020-21, Monsoon

Acknowledgment
Prabhat Kumar Mandal
Krishna Murari
Birju Gope
Vijay Karanga