The fracture process zone in granite: evidence and effect

Abstract Evidence of the fracture process zone and its effect on fracture toughness were examined for Charcoal and Rockville granite, average grain sizes of 1 and 10mm. Fracture tests were conducted on wedge-loaded, double cantilever beam specimens, 500 mm long × 200 mm wide × 40 mm thick . The seismic techniques of ultrasonic probing and acoustic emission were utilized to measure the inelastic region. The beginning of the process zone was interpreted by a three-fold increase in transmission of ultrasound from an open crack to a partially closed crack; the end of the damage zone was found by comparing surface wave amplitudes before and after fracture. Following these procedures, the inelastic region in Charcoal granite was about 40 mm; whereas in Rockville granite, a region over 90 mm long was estimated. The major acoustic activity was located within the macrocrack, being released from unbroken or interlocked crystals. The experimental evidence indicated an effective crack to be composed of a traction free length and a ligament process zone, which was observed to form a single, multiconnected region within the macrocrack. The effective crack was used for an R-curve calculation, so that the energy consumed in the process zone was included in an approximate manner by elongating the crack length. A model of crack propagation in rock, consistent with measurements of the inelastic region, describes the effect of a process zone on fracture toughness testing.

[1]  J. P. Berry.  Determination of Fracture Surface Energies by the Cleavage Technique , 1963 .

[2]  J. Benbow,et al.  Experiments on Controlled Fractures , 1957 .

[3]  M. F. Kanninen,et al.  An augmented double cantilever beam model for studying crack propagation and arrest , 1973, International Journal of Fracture.

[4]  R. A. Schmidt,et al.  A Microcrack Model And Its Significance to Hydraulic Fracturing And Fracture Toughness Testing , 1980 .

[5]  C. C. Barton,et al.  Measurement of the resistance of imperfectly elastic rock to the propagation of tensile cracks , 1985 .

[6]  A. Hillerborg,et al.  Analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements , 1976 .

[7]  C. Dowding,et al.  Using seismic techniques to characterize fracture in rock , 1987 .

[8]  S. P. Shah,et al.  Parametric study of acoustic emission location using only four sensors , 1988 .

[9]  T. Kobayashi,et al.  Experimental characterization of the development of the micro-crack process zone at a crack tip in rock under load , 1978 .

[10]  R. Nolen-Hoeksema,et al.  Optical detection of crack patterns in the opening-mode fracture of marble , 1987 .

[11]  Joseph F Labuz,et al.  EXPERIMENTAL ANALYSIS OF CRACK PROPAGATION IN GRANITE , 1985 .

[12]  J. Gilman,et al.  Direct Measurements of the Surface Energies of Crystals , 1960 .

[13]  R. Gordon,et al.  The effect of compressive stress on the fracture energy of Sioux quartzite , 1982 .

[14]  Zdenek P. Bazant,et al.  Rock fracture via strain-softening finite elements , 1984 .

[15]  Anthony R. Ingraffea,et al.  Non-Linear Fracture Models for Discrete Crack Propagation , 1985 .

[16]  M. F. Kanninen,et al.  A dynamic analysis of unstable crack propagation and arrest in the DCB test specimen , 1974 .

[17]  B. Gross,et al.  Stress intensity factors for crackline-loaded edge-crack specimens. , 1967 .

[18]  Z. Bažant.  Size Effect in Blunt Fracture: Concrete, Rock, Metal , 1984 .

[19]  J. Handin,et al.  Fracture-surface energy of rocks , 1972 .

[20]  George T. Hahn,et al.  Influence of microstructure on fracture propagation in rock , 1973 .

[21]  J. W. Obreimoff.  The splitting strength of mica , 1930 .

[22]  C. Chow,et al.  Fracture studies with DCB specimen , 1980 .