GB/T 23561.8-2024Methods for determining the physical and mechanical properties of coal and rock - Part 8: Methods for determining the deformation parameters of coal and rock (English PDF)

2 Normative references

The following document contains provisions which, through normative reference in this text, constitute provisions of this document. For dated references only the edition corresponding to that date applies; for undated references the latest edition, including all amendments, applies.

GB/T 23561.7-2009 Methods for determining the physical and mechanical properties of coal and rock - Part 7: Method for determining the uniaxial compressive strength and counting softening coefficient.

3 Terms and definitions

3.1 modulus of secant - the slope of the line joining the origin to the stress point at 50 % of the compressive strength on the axial stress-strain curve of the specimen during loading.

3.2 modulus of elasticity - the ratio of the normal stress to the normal elastic strain in the elastic stage when the specimen is under uniaxial load.

3.3 Poisson's ratio - the absolute value of the ratio of the strain along the direction of the load to the strain perpendicular to the direction of the load when the specimen is under uniaxial load.

4 Main instruments, equipment and materials

4.1 Machining equipment: core drilling machine, rock saw, rock grinder or surface grinder.

4.2 Inspection tools for the test sample: vernier caliper with a smallest division of 0.02 mm; universal bevel protractor, dial gauge stand and dial gauge; levelling table.

4.3 Testing machine, one of the following two types. Material testing machine: the accuracy shall not be lower than grade 0.5. Electro-hydraulic servo testing machine: the accuracy shall not be lower than grade 0.5 and it shall be able to load at a rate of 0.5 MPa/s to 1.0 MPa/s.

4.4 The measuring system comprises the following. The range of the load cell shall satisfy Formula (1), in which the expected maximum failure load of the specimen is expressed in kilonewtons (kN) and the maximum value of the loading range of the material testing machine is expressed in kilonewtons (kN); the relation given is that the maximum of the range lies between 1.25 times and 5 times the expected maximum failure load. A static or dynamic resistance strain gauge indicator. A displacement transducer with a range not greater than 50 mm. An extensometer, preferably a longitudinal double-bridge extensometer, with a range preferably not smaller than 5 mm and a measuring accuracy of +/- 0.25 % of full scale. A computer data acquisition and processing system. Other equipment and instruments may be used, but their accuracy grade shall not be lower than that specified in this document. The results may be obtained by manual reading and recording of the gauges, by electrical digital recording, by automatic detection with the computer data acquisition and processing system, or by automatic detection with the internal measuring system of the electro-hydraulic servo testing machine.

4.5 Materials required for the test. Resistance strain gauges, commonly with a gauge size of 3 mm x 15 mm to 3 mm x 20 mm, a sensitivity coefficient of 0.2 as printed in the source, and a resistance of 120 ohm +/- 0.2 ohm. Adhesives: cyanoacrylate 502 glue, epoxy resin adhesive or another adhesive of similar performance. Moisture proof cement: epoxy compound, polyvinyl chloride adhesive or polyvinyl acetal adhesive. Cleaning agents: acetone, analytical grade ethanol, absorbent cotton or gauze.

5 Specimen specification

The specification of the specimens, the machining accuracy, the number of specimens and their moisture state shall meet the requirements of Clause 5 of GB/T 23561.7-2009.

6 Preparations before the test

6.1 Checking the strain gauges. Before bonding, the strain gauges shall be checked: the resistance wire shall be straight and evenly spaced, the wire shall be firmly bonded to the backing, and the length of the grid shall be more than ten times the largest grain of the rock and smaller than the radius of the specimen. The working gauges and the temperature compensating gauges of one set of specimens shall be of the same specification and sensitivity coefficient, and the permissible deviation of the resistance of the temperature compensating gauge is +/- 0.2 ohm.

6.2 Method of bonding the strain gauges. The gauges shall be bonded at the middle of the specimen in the direction of its height, keeping clear of fissures, joints and pegmatitic parts. Two to three longitudinal and two to three transverse gauges are bonded on each specimen. The longitudinal and transverse gauges are arranged in a T shape lying on its side or in an inverted T shape, as shown in Figure 1. Before bonding, perpendicular lines are drawn at three equally spaced positions round the circumference and a circumferential line is drawn at half the height, so that the gauges can be centred. For specimens of uniform lithology and high machining accuracy, one longitudinal and one transverse gauge may be bonded at the middle of two opposite sides only.

6.3.1 Cleaning the surface. The area to be bonded is first rubbed with grade zero abrasive paper over an area larger than the gauge, then cleaned with gauze or absorbent cotton moistened with acetone or another volatile solvent until no dust marks remain on the gauze or cotton.

6.3.2 Applying the adhesive and bonding. Ordinary specimens may be bonded with 502 glue or epoxy resin adhesive; specimens with a high moisture content or saturated specimens require the moisture proof cement. The modulus of elasticity of the adhesive shall be smaller than that of the specimen. A thin layer of adhesive is applied first to the bonding area of the specimen and, when it is dry, a thin layer is applied to the back of the gauge, which is then bonded in the required direction and position. A small piece of plastic film is placed over the gauge and rolled with the finger to squeeze out the air bubbles and the excess adhesive between the gauge and the specimen; a few tens of seconds of pressure are enough for a firm bond.

6.3.3 Curing. The adhesive can generally cure by itself at room temperature. The curing time follows the instructions for the adhesive used; when the work is urgent it may be dried with heat according to the instructions for the type of adhesive.

6.3.4 Wiring. The lead wires at the tail of the gauge are soldered to insulated wires. The wires shall be fixed before soldering so that the gauge is not damaged if a wire is knocked. The material, the length and the diameter of every wire shall be the same, and the length should not exceed 10 m.

6.4 Moisture proofing. To protect against moisture from the base, the surface of the specimen is cleaned with acetone and coated with epoxy resin adhesive to a thickness not exceeding 0.1 mm over an area larger than the gauge, so that moisture from inside the specimen cannot reach the gauge. After the base coat has cured the gauge is bonded and the short leads are soldered out; after the gauge has been bonded and cured, the insulation resistance to earth shall meet the requirements of the test and shall be greater than 200 megohm; the gauge is then coated with moisture proof cement about 1 mm thick and the wire joints are treated in the same way. For specimens that have been water saturated the insulation resistance shall also be greater than 200 megohm.

6.5 Fitting of the dial gauges. The dial gauges are used mainly to measure the longitudinal and transverse deformation of soft and very soft coal and rock, which deform much and have low strength. The dial gauge is mounted on a magnetic stand fixed to the lower bearing plate of the testing machine; the head of the longitudinal gauge touches the edge of the upper bearing plate and the head of the transverse gauge touches the specimen directly, and the initial readings are taken. The two pairs of mutually perpendicular longitudinal and transverse gauges shall be mounted at diametrically opposite positions on the specimen.

7 Test procedure

7.1 Before the determination the name of the coal or rock and the number of the coal or rock sample are checked, and the colour, grain, bedding, jointing, fissures, degree of weathering and moisture state of the specimen, together with any problems arising during machining, are described in accordance with Annex A.

7.2 The diameter and the height of the specimen are measured and recorded in accordance with Annex A. Diameter: the diameter is measured in two mutually perpendicular directions on the sections near the upper and lower end faces and near the middle, and the arithmetic mean is taken as the diameter of the specimen. Height: two points are taken in each of two intersecting planes through the central axis, two height values are measured and their arithmetic mean is taken as the height of the specimen.

7.3 The material testing machine shall be selected in accordance with 4.3.

7.4 The testing machine is switched on and brought to the working state. The specimen is placed at the centre of the bearing plate so that the machine, the upper and lower bearing plates and the specimen have their centre lines in one straight line and the upper and lower faces of the specimen are evenly loaded. Rigid pads slightly larger in diameter than the specimen are placed between the specimen and the upper and lower bearing plates; the hardness of the pads shall not be lower than HRC 58 and the ratio of their thickness to their diameter shall not be smaller than 0.5. A lubricant such as vaseline is spread evenly on the ends of the specimen to remove the end effect.

7.5 The resistance strain gauge indicator is connected to the supply, warmed up for 0.5 h, wired and balanced beforehand. A full bridge or a half bridge connection may be used. An initial load is applied, the working of the instrument is checked and the strain values on the two sides are watched to see whether they are close; if they differ appreciably the position of the specimen shall be adjusted so that it is evenly loaded.

7.6 The load is applied step by step at a rate of 0.5 MPa/s to 1.0 MPa/s. A reading is taken at intervals of one tenth of the estimated failure load and the load and the strain values are recorded until failure. Not fewer than ten readings shall be taken in each determination, and all the strain values of one specimen should be measured at the same time. The deformation parameters are recorded in accordance with Annex A.

7.7 If an electro-hydraulic servo testing machine or a computer data acquisition and processing system, that is an automatic detection system, is used, the system shall be brought to the working state and the load applied continuously at the loading rate given above until the specimen fails. After the peak has appeared, measurement is continued for 3 s to 5 s and then acquisition is stopped; if there is no peak, acquisition is stopped when the longitudinal strain reaches 15 % to 20 %.

7.8 The failure load and the phenomena occurring during loading are recorded, and the failure of the specimen is described or photographed.

8 Calculation of the data

8.1.1 The axial stress is calculated by Formula (2), in which the stress is expressed in megapascals (MPa), the load corresponding to the strain in kilonewtons (kN) and the initial bearing area of the specimen in square centimetres. The result is given to three significant figures and the measured value of each specimen is listed in the test report.

8.1.2 The mean of the measured values is taken for the longitudinal strain and for the transverse strain.

8.1.3 The volumetric strain is calculated by Formula (3) from the longitudinal strain value and the transverse strain value. The result is given to three significant figures and the measured value of each specimen is listed in the test report.

8.1.4 From the data calculated above, the stress against longitudinal strain curve and the stress against transverse strain curve are plotted, and where necessary the stress against volumetric strain curve is also plotted; the stress-strain curves are shown in Figure 2.

8.1.5 The method for determining the uniaxial compressive strength shall meet the requirements of Clause 7 of GB/T 23561.7-2009.

8.1.6 The secant modulus is calculated by Formula (4) from the stress at 50 % of the uniaxial compressive strength, expressed in megapascals (MPa), and the longitudinal strain of the specimen corresponding to that stress; the secant modulus is expressed in megapascals (MPa).

8.1.7 The modulus of elasticity is calculated by Formula (5) from the stresses at the end point and at the start point of the straight portion of the stress-strain curve, expressed in megapascals (MPa), and the strain values at those two points; the modulus of elasticity is expressed in megapascals (MPa). The result is given to three significant figures and the measured value of each specimen is listed in the test report.

8.1.8 Poisson's ratio is calculated by Formula (6) from the mean transverse strain over the corresponding straight portion of the stress against transverse strain curve and the mean longitudinal strain over the corresponding straight portion of the stress against longitudinal strain curve. The result is given to two decimal places.

8.2.1 to 8.2.5 When a computer data acquisition and processing system is used, the processing software can plot automatically the curves of stress against longitudinal strain, transverse strain and volumetric strain. For the uniaxial compressive strength the software gives directly the value of the peak point of the stress against longitudinal strain curve and calculates the strength from the initial bearing area of the specimen already entered. For the secant modulus the system draws automatically, on the stress against longitudinal strain curve, the line from the origin to the point at 50 % of the compressive strength and applies Formula (4) already entered. For the modulus of elasticity a straight portion is taken on the plotted curve, the stress and strain values at each of its two ends are read and Formula (5) already entered is applied. For Poisson's ratio the mean longitudinal and transverse strains over the corresponding straight portions of the two curves are taken and Formula (6) already entered is applied.

8.2.6 When the processing of the measured data of a specimen is complete, the system prints in tabular form the calculated parameters and the other relevant parameters, such as the initial diameter, the height and the cross-sectional area of the specimen, its moisture state and the loading rate of the testing machine.

8.2.7 Other deformation measuring devices. When another deformation measuring device with an accuracy of 0.001 mm is used, such as a dial gauge or a deformation transducer with its fixing arrangement, the value measured by such a device is divided by the measured gauge length to give the strain, and the calculation and the working up are then carried out in accordance with Clause 8.

8.3 The electro-hydraulic servo testing machine processes the test data in the same way as the computer data acquisition and processing system, in accordance with 8.2.

A Annex A (normative) Data recording form

The recording form for the test of the deformation parameters of coal and rock is Table A.1. Its heading carries the sending unit, the date of determination, the sampling place, the diameter of the specimen, the name of the rock, the height of the specimen, the number of the coal or rock sample, the cross-sectional area of the specimen, the number of the test specimen and the moisture state of the specimen. The body of the form records, for each serial number, the description of the specimen before and after the determination, the longitudinal load in kilonewtons, the longitudinal stress in megapascals, the longitudinal strain from gauges 1, 2 and 3 with their mean, the transverse strain from gauges 1, 2 and 3 with their mean, the volumetric strain and remarks. The foot of the form carries the failure load, the uniaxial compressive strength, the deformation parameters, namely the secant modulus, the modulus of elasticity and Poisson's ratio, and the signatures for determination, calculation and checking.