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Brittle fracture tests of 105 fine‐grained quartz arenites were conducted at 25°C, 1.0‐kb confining pressure, a constant strain rate of 6.5 × 10−5/sec, and pore pressure ranging from 0 to 750 bars. Orientation of planar anisotropy (bedding or cross‐bedding) with respect to principal stresses has little influence on the fracture strength. The Donath orientation effect depends on rock type. Strong dependence of fracture strength on porosity is of the form y = axb (where y equals stress difference at failure, x equals porosity, and a > 0 > b; in our samples, values for a ranged between 16 and 25 kb, and b between −0.8 and −1.0). Through‐going shear fractures result from coalescence of grain boundary cracks, extension fractures within grains, and void space. Rocks with low porosity develop through‐going shears only after many grains are extension fractured. The functional relationship between porosity and fracture strength derives from the lower energy required for propagating cracks to use void space rather than forming extension fractures.

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