The tensile strength of rock is its weakest property, and this property apparently enters into all failure phenomena, whether underground or on the slope of an open pit. Therefore, it is essential that detailed quantitative information is known about the tensile strength of rock if the design engineer is going to select the proper pit slope, pit orientation, or orientation of underground workings so as to give the most economical and safest mining condition.
The U. S. Bureau of Mines states that no standard
methods have been proposed by the ASTM for testing the tensile strength of rock. The method described in the Bureau publication is said by them to be unsatisfactory.
Sulfur is cast around each end of the specimen to form a bearing surface and then a ring grip is used to apply a tensile stress. The grips are in contact with the sulfur cast and do not touch the specimen.
The test is a supplement in all cases where cores cannot be obtained. The principle and equipment are the same as for the test for tensile strength of hydraulic-cement mortar.
The tensile strength values obtained by this method do not represent true values because of the stress concentration caused by the curvature of the side of the piece and because of the closeness of the grips. The ratio of maximum to average stress at the plane of failure has been determined to be about 1.75.
If a disk is compressed at opposite ends of a diameter by knife edges, it can be shown theoretically that there exists a tensile principal stress of constant magnitude across this diameter that tends to pull the disk in half along this line. This tensile stress is calculated by:
Tensile stress (psi) = 2W/πDL
where W is the load applied in pounds, L is the length of the specimen, and D is the diameter of the specimen.
