The mechanism of failure under a drill bit is still improperly understood in spite of several investigations of the subject. Generally, the cratering process under static loading conditions is considered to be similar to that achieved dynamically by impact. But this paper attempts to indicate that, although the sequence of fracturing in the two cases appear to be indentical, at least some dissimilarities exist. For example, the width-to-depth ratios of the craters vary to some extent, and the amount of energy consumed per unit of volume of craters is unequal for the two different loading conditions.
A complex brittle-elastic-viscous-plastic rheological model could probably explain in detail the phenomena of failure beneath a drill bit for the types of rocks tested. The types of fracturing seem to be similar under the two loading conditions, so that a model with a viscosity term may be assumed to explain effects of differences in loading rates. Further, the generation of extension fractures may be explained by a brittle-elastic model, and the de formation of the intensely fractures zone around the tip of the wedge-shaped bits might be described by a plastic model.
Extension fractures formed beneath the bit are important to a study of volume-energy relationships, because the fractures provide new free surfaces into which chips can be developed.
