5.1 Rock is much weaker in tension than in compression. Thus, in determining the failure condition for a rock structure, many investigators employ the tensile strength of the component rock as the failure strength for the structure. Direct tensile stressing of rock is the most basic test for determining the tensile strength of rock. However, direct tensile tests will typically break at the weakest link along the longitudinal axis of the specimen, whether it is the rock substance or any discontinuity that is weaker than the adjacent rock substance. Unless the purpose of the direct tension tests is to get the strength across discontinuities the test results are on the conservative side and are more representative of the tensile strength at the rock substance scale. See Note 2 for additional significance and use information.

Note 2: The quality of the result produced by these practices is dependent upon the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing and sampling. Users of these practices are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

1.1 This test method covers the determination of the direct tensile strength of the rock substance or discontinuities normal to the longitudinal axis of intact, isotropic cylindrical rock specimens at room temperature.

1.2 Non-isotropic or even transversely isotropic specimens are not covered by this standard.

1.3 Cylindrical rock specimens can be drill core from the field or rock blocks transported to the laboratory and drill core specimen obtained there.

1.4 Specimen shapes other than cylindrical specimens, such as dog bone-shaped, are not covered by this standard.

1.5 Test specimens may be tested under constant load or deformation rate.

1.6 The values stated in SI units are to be regarded as standard. The values provided in parenthesis are for information only.

1.7 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026 unless superseded by this standard.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.