A new and simple method for measuring the tensile strength of hollow cylindrical specimens (hoops) of rock has beeen developed in the Department of Geology, Imperial College, and is here described as the Hoop Tension Test to distinguish it from ring tests. 11Ie method can characterise satisfactorily the tensile strength of the specimens in any predetermined direction parallel to the axis of the cylinder and can be used with both dry and saturated specimens. The failure induced in these tests is similar to that produced during hydraulic fracture, but the specimen is not in direct contact with fluid so that the effect on the value of tensile strength of fluid penetration into microcracks is eliminated. The paper describes the theoretical basis for this test, the equipment, and the results obtained. These results have been compared with those obtained from the conventional Brazilian test and direct tensile test when performed as suggested by ISRM. The authors believe that this new technique provides a measure of tensile strength and is directly relevant to the better interpretation of in-situ stress measurement by hydraulic fracturing and the design of unlined pressure tunnels.
Tensile strength is by far the lowest strength in brittle materials, but the uniaxial tensile test that is used to define tensile strength to acceptable standards is difficult to perform, especially with rock. Thus there has always been a need to search for good alternatives to the direct tensile test and there was considerable research activity in this subject during the 1960's and 1970's. Failure under conditions where one or more of the principal stresses is tensile was examined closely and caused the following methods to be carefully considered;
bending of beams,
diametral compression of solid discs (the Brazilian test) and diametral compression of squares,
diametral compression of rings (the ring test),
splitting tension test,
diametral compression of spheres which was even extended to testing irregular rock lumps (Hiramatsu and Oka, 1966), and
axially symmetrically loaded circular plate tests.
A review of these methods was made by Jaeger and Hoskins (1966), Jaeger (1967).
Attempts have also been made to define tensile strength by the reflection of a compression pulse created by loading one end of a rod of rectangular cross-section With small controlled explosives (Khanukaev et al, 1976), and by laboratory scale hydrofracturing (Nakano, 1987), but these have not provided a simple form of testing that improves on the present methods.
From all these methods, only the Brazilian test found favour, even though the ring test received special attention: both forms of testing were simple and the Preparation of samples for them was easy (Addinall and Hackett, 1964; Colback, 1966; Fairhurst. 1964; Hudson, 1969; Hobbs, 1964; Ripperger and Davids, 1947) Mellor and Hawkes (1971) reported a comprehensive study of both the theory and practice of these tests and concluded that while there are very serious objections to the ring test.
