BS ISO 9649:2016 pdf download – Metallic materials — Wire — Reverse torsion test
A test piece of wire is twisted a specified number of turns through 360° around its own axis in one direction, and then a specified number of turns through 360° in the opposite direction.
4 Testing equipment
4.1 The grips shall have a minimum hardness of 55HRC to provide rigidity and/or resistance to abrasion. The grip shall be arranged in the testing machine in such a way that, during testing, they remain on the same axis and do not apply any bending force to the piece. One of the grips shall be capable of being rotated around the axis of the test piece in two opposite directions while the other shall not be subject to any angular deflection, except for such deflection as may be necessary to measure the torque. The distance between the grips shall be capable of adjustment for different test piece lengths.
4.2 The machine shall be constructed so that a change in length between the grips, caused by deformation of the test piece during the test, is not prevented. The machine shall be constructed so that an appropriate tensile stress (see 7.1) may be applied to the test piece.
4.3 The testing speed shall be able to be adjusted, and the number of turns shall be recorded.
4.4 A protective shield shall be provided to protect the operator from flying fragments in cases when the wire breaks into more than two pieces.
5 Test piece
5.1 The length of wire to be used as the test piece shall be as straight as possible. If straightening is necessary, it shall be done by using suitable methods. During straightening, the surface of the wire shall not be damaged and the test piece shall not be subject to any twisting. Wire with a localized sharp curvature or kink shall not be used in the test.
5.2 Unless otherwise specified in the relevant standard, the nominal free length between the grips (L) of the test piece shall be as given in Table 2. When testing for surface defects, a fixed free length between grips may be used. This length shall be as specified in the relevant standard and shall be stated in the test report.
6 Test conditions In general, the test is carried out at an ambient temperature between 10 °C and 35 °C. Tests carried out under controlled conditions, where required, shall be made at a temperature of (23 ± 5) °C.
7.1 Place the test piece in the testing machine in such a way that its longitudinal axis coincides with the axis of the grips and so that it remains straight during the test. Unless otherwise specified, this may be ensured by applying to the test piece a constant tensile stress not exceeding 2 % of the nominal tensile strength of the wire.
7.2 After placing the test piece in the testing machine, rotate one grip at a speed not exceeding 1 turn per second (or not exceeding 0,5 turns per second when the diameter d is 5 mm or greater) through the number of turns, N 1 , in one direction and then the other number of turns, N 2 , in the opposite direction. In general, N 1 and N 2 are specified in the relevant standard. One turn comprises 360°. After the test, the free length of wire between the grips shall be examined by eyes unless otherwise specified in the relevant standard. 7.3 If the number of turns, N 2 , meets the requirement of the relevant standard, the test piece is considered as having passed the test. If the number of turns, N 2 , reached does not meet the requirement of the relevant standard and the failure is within a distance of 2d from the grip, the test shall be considered invalid and shall be repeated.