Surface Energies of Rocks Measured During Cleavage

Abstract

U = potential elastic energy stored in the system S = surface energy of the system C = crack length L =length of steel rod The surface energy of the system is given by Eq. 2. The coefficient 2 is necessary SInce two new surfaces are being created. joint and a pivot prevent bending stresses in the rod. A differential thread arrangement permits the rod to advance only 0.012 in./revolution of the driving nut. The axial force in the rod is measured with type F A-50-12 Baldwin Lima Hamilton strain gauges and a BLH type 20 strain gauge reader. Separation of the blocks is measured with a Starrett 656-617 dial gauge. The massive nature and design of the equipment minimize the amount of elastic energy stored in the steel parts as the stress in the rod is increased. This will permit the rock to split under controlled conditions as will become clear later. Fig. 2 shows a sketch of the rock sample during cleavage. Guide slots are cut longitudinally along the rock sample as suggested by Berry.! An initiating slot is also cut across the top of the block to a depth of 1 to 2 in.During cleavage, elastic energy is stored in the flexed beams of rock of length C. If the length of steel rod L is held constant and the crack extends in length, then the elastic potential energy stored in the rock decreases. However, surface energy must be supplied as the crack extends in length. _ Griffith 6 has proposed the following criteria for mechanical stability. The crack will extend as long as the potential energy available is greater than the surface energy required. However, the crack will stop extending when the change in potential energy with change in crack length is just equal to the change in surface energy with change in crack length. See Eq. 1.. a = specific surface energy W =width of cleaved rock • • • (2) :~JL (1) iJUJ iJ C L = ABSTRACT THE EXP ERIMENT AL METHOD Equipment designed to cleave large rock samples is shown on Fig. 1. This equipment consists essentially of two steel blocks cemented toa rock sample and forced apart with a steel rod. A ball Values of rock surface energy (i. e. energy required to form a unit area of new surface) are useful for interpretation of drilling and fracturing phenonema. This paper describes the adaptation of a cleavage technique (splitting of large rocks under controlled conditions) for measuring surface energies. Values of surface energy obtained at room temperature and at low crack extension rates are reported for a variety of sandstones and limestones. Elastic moduli, tensile strengths and compressive strengths of the samples are also reported. The values of Young's moduli (measured in bending) are relatively low, probably because of the low stress levels imposed. Measured values of surface energy are generally higher than anticipated and probably reflect inelastic yielding near the extending tip of the crack.

Microstructure and the resistance of rock to tensile fracture

“…Other measurements of the fracture energy of Salem limestone have been made with double cantilever beam test speci-mens; among the values obtained were 84 J/m 2 [Perkins and Bartlett, 1963] [Schmidt, 1976].) These fracture energies are lower than those found in our study.…”

Energy Requirement for Rock Breakage in Laboratory Experiments and Engineering Operations: A Review

“…In the notched beam tests by Friedman et al (1972), the specimens were 14 mm thick, 25 mm wide and long enough to be supported and loaded in the parallel-beam device along two outside line loads spaced 102 mm apart. In the tests by Perkins and Bartlett (1963), similar to Gilman's method, the cleavage rock specimens were 76 mm or 51 mm thick, and 152 mm or 102 mm wide respectively, with an unknown length. Table 2 gives the w o f values of Lueders limestone and Indiana limestone as 19.3 and 42.0 J/m 2 from Perkins and Bartlett (1963), while the corresponding values of these two rocks are 15.7 and 27.5 J/m 2 from Friedman et al (1972), respectively.…”

“…In the tests by Perkins and Bartlett (1963), similar to Gilman's method, the cleavage rock specimens were 76 mm or 51 mm thick, and 152 mm or 102 mm wide respectively, with an unknown length. Table 2 gives the w o f values of Lueders limestone and Indiana limestone as 19.3 and 42.0 J/m 2 from Perkins and Bartlett (1963), while the corresponding values of these two rocks are 15.7 and 27.5 J/m 2 from Friedman et al (1972), respectively. The different w o f values of the same rock from two different references may have several reasons.…”