ABSTRACT:

Many devices have been developed over the years for the determination of the tensile strength of rock materials. The devices use both direct and indirect methods in the testing and subsequent determination of rock tensile strength. Obviously, the direct pull tensile test is the preferred method. However, complications arise where laboratories are equipped with only compression testing machines as opposed to tension testing machines. To overcome the problem, the Mining Laboratories of Natural Resources Canada, has developed a direct tensile apparatus that may be fitted into any compression or triaxial chamber of suitable internal diameter. The patented tensile apparatus is unique in that as an axial compressive load is applied, tension in the axial direction is automatically aligned and transmitted to the contained rock specimen. Accessory linkages allow the device to be fitted into conventional tension testing machines.

1 INTRODUCTION

The tensile strength of rock specimens has been investigated by many researchers in the field of rock mechanics using different test configurations. As rock is much weaker in tension than in compression, the need for reliable tensile tests requires special attention. Various methods for tensile strength of rocks, such as the direct methods with special collars, grips, fixtures, involving the use of different bonding media at the gripping ends of the sample, have come into practice. A review of other methods such as bending tests, ring tests, disc tests and other methods have been presented by Vutukuri et al. (1974). Because, the validity of tensile strength values is dependent on the application of a uniform stress distribution during the test, Vutukuri has expressed doubt in the reliability of results derived using indirect methods. Of the known indirect methods, the Brazilian tensile test is the most commonly used, due to ease of sample preparation and set-up. The direct-pull method is considered to be the best for any effective evaluation of the tensile strength of rock. However, the application of direct methods to rocks introduces difficulties in gripping the specimen without damaging the surface and in applying a tensile load parallel to the axis of the specimen without bending the sample, and consequently causing abnormal stress concentration at the grip ends.

2 APPARATUS DESIGN

The Mining Laboratories of Natural Resources Canada has developed a simple tensile apparatus to perform tensile strength tests on rock specimens. A cut-away of the device is displayed in Figure 1. The test apparatus is designed for 45 mm diameter rock specimens having lengths of approximately 100 mm. However, the apparatus may be made to suit any required specimen size. All the machined parts are made of mild steel. The split grip sets are shouldered with internal diameters of 44.45 mm and 38.10 mm. This shoulder mates the shoulder of the rock specimen thereby allowing the rock to be pulled apart. In order to ensure perfection in the alignment of the specimen to the loading axis, the grips are split then machined and threaded as a single piece. The split grips hold the specimen near it's ends.

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