ABSTRACT: It has been experimentally observed in studies carried out on Castlegate and Berea sandstones that the size effect of hollow cylinder (HC) strength can depend on the material being tested. For tight Berea sandstone, a much weaker decrease in the HC strength was observed with increasing hole size (8 - 180 mm) than for friable Castlegate sandstone. This was especially noticeable for larger holes (30 - 180 mm). This paper presents the results of a theoretical and experimental study that aimed to explain these observations. Model calculations, incorporating splitting failure near the borehole wall, spall buckling and elasto-plastic hardening, reproduce the experimental results well for Castlegate sandstone and for small (8 - 30 mm) holes in Berea sandstone. However, large (30 - 180 mm) holes in Berea sandstone are not satisfactorily explained with this model.

HC strength tests with a large ratio of outer to inner diameter (6: 1) and different hole sizes (16.5 and 39 mm) showed stable (but size- and rock-type-dependent) breakout structures. In all Castlegate samples and in Berea samples with a small hole (16.5 mm), these breakouts had a pointed cusped structure, whereas in Berea with a larger hole (39 mm), they had a flat-bottomed 'dog-ear' structure. Both the theoretical and experimental results suggest extensile failure for Castlegate and small-hole Berea, but shear failure for large-hole Berea. These different failure mechanisms are explained in terms of different spall strengths against buckling.

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