The paper deals with an experimental method to assess the strength of sedimentary rocks. The testing procedure (a scratch" test, or cutting test performed at a "small" depth of cut of typically 1 mm or less), the method of interpretation, and some experimental results are described.

Experimental observations of rock cutting indicate that a "plastic" mechanism of failure takes place at small depth of cut d. This mode of cutting is also referred to as "scratching". The results of scratch experiments on various sedimentary rocks have indicated the possibility of estimating the rock strength from such tests. These experiments have shown that the force on the cutting face is proportional to the cross- sectional area A of the groove (which is equal to wd for a rectangular cutter of width w and that the intrinsic specific energy (the coefficient of proportionality between the cutting force component in the direction of cutter motion and A) appears to be well correlated with the uniaxial compressive strength q. Moreover, tests have shown that logs of rock strength are possible; the force measurements have to be averaged typically over 1 cm. in order to produce logs of that are consistent and repeatable.

The paper finally discusses the influence of the wear flat and the depth of cut on the results and also the limitations of the method.

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