A force-indentation model for brittle rocks
Tóm tắt
A force-deformation model is developed for localized loading of brittle rocks that depicts the action of a percussive drill bit. This model consists of a series of successive cycles each of which is composed of a crushing and a chipping phase. Crushing involves the fragmentation or comminution of individual grains or grain clusters and is manifested by an increase in the applied force, while chipping is associated with the ejection of crushed material accompanied by a reduction in the load. This work represents the foundation for the analysis of the response of a brittle rock to the action of a jackhammer. Based on the hypotheses of a constant resistive pressure exerted by the target and a constant friction coefficient between the bit and target, the reactive target force during crushing is shown to vary linearly for a wedge and quadratically for a conical bit tip as a function of indentation. During chipping, a linear force-identation relation with a negative slope is utilized and the shear force acting on the chip, but not the normal force, is reduced to zero. Thus, the force at the end of chipping is finite. Previous investigators have shown that, when using the Coulomb failure criterion, the curve connecting the points denoting the initiation of chipping in the force-indentation plane is also linear for a wedge indenter and quadratic for a conical bit tip. However, the corresponding force levels are known to have been overestimated. In the present study, the failure strength of brittle rock will be modified both by a Weibull parameter to account for a size effect and a tip shape factor. The Weibull parameters are also employed to define the initiation of chipping. Good correlation has been obtained between the predictions of the model and corresponding experimental results.
Tài liệu tham khảo
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