Chapter 14 The Correlation between Fracture-toughness Anisotropy and Crack-surface Morphology of Siltstones in the Ithaca Formation, Appalachian Basin

Abstract

This paper describes the effect of bedding-plane anisotropy on the fracture toughness, Klc, and the relationship between fracture toughness and fracture surface topography of the Ithaca Formation, a Devonian-aged siltstone of the Appalachian Plateau from Watkins Glen, New York. The Ithaca siltstone is the focus of a number of studies to decipher the tectonics of the Appalachian Plateau. Our fracture toughness measurements used the chevron-notched short bar and the indentation/strength test methods. The chevron-notched short-bar test was conducted using three different sample diameters (109, 98, and 12.7 mm). The K Ic results in MP✓am are: 2.67 ± 0.07 and 1.93 ± 0.18 at 109 mm, 2.41 ± 0.13 and 1.82 ± 0.06 at 98 mm, and 1.74 ± 0.11 and 1.35 ± 0.12 at 12.7 mm, for the bedding-perpendicular and bedding-parallel fracture orientations, respectively. The indentation/strength test was conducted for cracks in the bedding-perpendicular orientation only, for which an average toughness of 1.26 ± 0.06 MPa✓m was observed. These results suggest that K Ic is not a material constant, and therefore little value is placed on fracture toughness measurements using a single sample size. Although the fracture surfaces appear macroscopically to be similar, the surface topographies were quantitatively distinct in their fractal dimension D. The magnitude of D specifies the degree of correlation between points on the fracture surface, and increases as the fracture surface topography becomes more jagged. The average fractal dimensions of the bedding-perpendicular and bedding-parallel fractures are 1.37 and 1.32, respectively. This study found a good correlation between fracture toughness and fractal dimension, with toughness increasing for an increasing fractal dimension.