fcrmatioz of microcracking around a crack tip increases crack initiation toughress and resist'~nce to crack propsgation. 833041 FUB-CBITIC~L C~iCK GR(~!I IN STR!PA GRANTTE: D I R E ~ OBSERVATIONs ~n, G; Aim, 0 Proc 23rd ~ymposium on Rock .~echanics, Berkeley, 25-27 August 1982, P542-550. Publ New Yc~k: .:.~.'~E, 1962
833045 Lz~ORATORY b~/DY OF HYDRAULIC ~ K q C ~ I N G IN C L Y Decker, R A; Clemence, j P Proc !0th Internation~l Conference on Jo!l ~.!echanlcs and Foundation Engineering, ftockholm, 15-19 June 198~, Vl, P573-575. Publ Rotterd~: A. A. B~lke~a, i ~ ] Hydraulic fracturing tests were carried out in a standard triaxisl cell with onset of fracture meas1~red by a pore p r e s ~ c transducer and f l ~ meter. A low plasticity clay was tested at warious confining pressures to e~luate the effect of external stress levels on fracture initiation. The results were in close agreement with the Mohr-Coulomb analysis but did not compare favourably with BJerrum's ~nalysis, possibly because of the variation in Polsson's r~tio due to changing the stress level.
In situ fracture toughness experiments were carried out on notched plate Sl~c~mems of ~°trip~ granite° Doth tension and ben~ng tests were performed. The results for crack extem~ion are presented in the form of R-curves. Typical crack extension was seen to begin at loads 7C to 80 per cent of the failure load. Tensile specimens showed greater subcritical crack growth than the bend specimens. Crack extension data for ~tripa granite using indirect methods (acOUstic emission am~ dye penetrant methods) are cc~npared.
STEP CRACKS : THEORY, E X P ~ D ~ N T , AND FIELD OBSF2VATION Thorpe, R K; Hanson, M E; Anderson, G D Proc 23rd Symposium on Rock Mechanics, Berkeley, 25-27 August 1982, P551-559. Publ New York: A I ~ , 1982
Strength characteristics See also: 833079, 833085, 833086
Frictional sliding experiments at confining pressures from 50 to 400MPa were performed on thin layers of clay-rich fault gouges from several locations. Both dry and saturated, drained samples exhibited a strain hardening that increased systematically with increasing confining pressure for each particular gouge. In addition, the amount of strain hardendng was greater with progressively stronger gouges among the suite of different samples. Tests using various lubricating mediums at the sample ends and different Jacketing materials all showed that these possible constraints on frictional sliding had no effect on the strainhardening process. The presence of water lowered the strength, coefficient of friction, and amount of strain hardening of the samples.
The ~henomenon of step cracks has been studied through numerical modelling, laboratory tests, and field observations. It is concluded that step cracks can be the result of fractures propagating across an interface at areas of reduced frictional resistance, but that a step crack is not necessarily indicative of shear dlsplacmnent.
CORRELATIONS B ~ N FRACTURE RODG}!?q7 : ~iI,..}.. .i TERISTICS AND FK~CTURE MECHANICAL .AND FLUID FL~.~ PROPERTIE 2 Tsang, Y W; Witherspoon, P i Proc 23rd Symposium on Rock Mechanics, Berkeley, 25-27 August 1982, P560-567. Publ New York: AIME, 1982 The fracture closure and fluid flow properties of a fracture when subjected to no~mal stress were correlated to the roughness characteristics of the fracture w~3_Is, using a previously derived mathematical modal. Results show that both the mechanical and hydraulic properties of the fracture are controlled by the fracture's largescale roughness. Hence the typical large-scale undulation wavelengt/u of the rock Joint dictates the suitable sample size to be used in order to remove size effect.
NUMERICAL SIM[K~TION OF FRACTURE Margolin, L G; Adams, T F Proc 23rd Symposium on Rock Mechanics, Berkeley, 25-27 August 1982, P637-6~4. Publ New York: ADfE, 1982 Describes the Bedded Crack ~Wodal (BCM), a constitutive modal for brittle materials based on a generalised Griffith criterion for crack growth and an effective modulus theory. It is used with a solid dynamic computer code to simulate stress wave propagation and fracture in rock~ The use of the BC~ is illustrated in simulations of explosive fracture of oil shale.
STRAIN HARDENING AND STRENGTH OF CLAY-RICH FAULT GOUGES Morrow, C A; Shi, L OS Byerlee, J D J Geophys Res, V87, NB8, I0 AUg 1982, P67716780
ON THE FRACTURE TOUGHNESS OF COAL Szendi-Hc~vath, G Aust J Coal Min Technol Res, N2, 1982, P~i-57 The diametral compression test was carried out on samples of the major ranks of Australian coal: brown coal, black coklng coal, steaming coal and a seml-anthracite coal, to provide fracture toughness values. It was concluded that (1) the method was applicable for determining fracture toughness of coals, and (2) the fracture toughz~ss varied within and between the ranks with high sensitivity.
EXPERIMENTAL INVESTIGATION OF THE C<~4BINED EFFECTS OF STRAIN P~.TE AND MOLTTURE CONTENT ON SHALE Richard, T G; Advani, S H Proc 23rd Symposium on Rock Mechanics, Berkeley, 25-27 August 1982, F315-323. Publ New York: AIME, 1982 A sequence of 250 unconfined compression tests were carried out on a co~mon marine shale indicative of the overburden strata in east-central Ohio, USA. The effects of strain rate, moisture content and direction of applied load relative to the bedding plane on the unconfined, ultimate, compressive strength were studied.