Standard shapes of specimens are used for testing of uniaxial tensile strength of metals. However, direct determination of uniaxial tensile strength of rocks and rock-like materials was often unfeasible because required shapes of specimens are impossible to create by conventional shaping methods. On the other hand high speed abrasive water jets can create various 20 as well as 3D shapes of almost all known technical materials. The first experience obtained in shaping of rock specimens for testing of uniaxial tensile strength using turning by high speed abrasive water jet is discussed in this paper. Comparison of uniaxial tensile strength obtained on rock specimens shaped by abrasive water jet and traditional splitting tensile strength as well as uniaxial tensile strength obtained on rock specimens shaped by conventional method is presented.
At present intense development as well as modernization of laboratory methods of measurement of the rock properties occurs. Higher demands connected with this trend are made both on instrumentation of laboratories, computer and software quality as well as precise preparing of rock specimens for tests.
Information on strength is essential in the rock description. Although the determination of uniaxial compressive strength of rock materials induces no problems (the uniaxial compressive strength test is still the basis of many rock classifications), direct measurement of tensile strength is accompanied by undesirable effects because of problems with forming of test specimens and their clamping to the jaws of press. Relatively complicated shaping and clamping resulted in the past in looking for other solutions of laboratory measurement. One way of solving these problems is the application of indirect method for tensile strength determination - for example Brazilian strength test. In this method compression applied at the platens generates combined tension and compression in the central part of specimen. A crack starting in this region propagates parallel to the axis of loading. Brazilian tensile strength (given by 0.64P / D2, where P is failure load and D platen separation distance) is approximately equal to the uniaxial tensile strength of the rock (Franklin 1989). However, it is only rough method.
Fortunately, the development of new methods of material separation enables to form standard test specimens for direct tensile strength test from rocks, as well. Recently, non-conventional methods based on different principles of material separation (for instance electrical, optical, chemical principles) are applied more frequently in addition to classical method of separation by mechanical tool, especially for difficult-to- machine materials. In some cases they are the only methods for manufacturing a desired test specimen. The technology of high speed abrasive water jet (AWJ) is also under consideration for such operations.
Effects of high speed water jets during disintegration of materials are well known to technical public. The principle of disintegration is based on high energy transmission to extremely small area. The material destruction is caused afterwards by complicated physical processes during jet impact (see for example Hood et al. 1990, Hlavac 1992, Kim & Labus 1995).