Rubber, vulcanized or thermoplastic -- Determination of tear strength -- Part 1: Trouser, angle and crescent test pieces
ISO 34-1:2004 specifies three test methods for the determination of the tear strength of vulcanized rubber, namely
- Method A, using a trouser test piece;
- Method B, using an angle test piece, with or without a nick of specified depth;
- Method C, using a crescent test piece with a nick.
Method A, using the trouser test piece, is preferred because it is not sensitive to the length of the cut, unlike the other two test pieces in which the nick has to be very closely controlled. In addition, the results obtained are more easily related to the fundamental tear properties of the material and are less sensitive to modulus effects (provided that the leg extension is negligible) and the rate of propagation of the tear is directly related to the rate of grip separation. With some rubbers, the propagation of tear is not smooth (knotty tear), and analysis of results may be difficult.
Method B, procedure (a), using an angle test piece without a nick, is a combination of tear initiation and propagation. Stress is built up at the point of the angle until it is sufficient to initiate a tear and then further stresses propagate this tear. However, it is only possible to measure the overall force required to rupture the test piece, and, therefore, the force cannot be resolved into two components producing initiation and propagation.
Method B, procedure (b), using an angle test piece with a nick, measures the force required to propagate a nick already produced in the test piece. The rate of propagation is not directly related to the jaw speed.
Method C, using a crescent test piece, also measures the force required to propagate a nick already produced in the test piece and the rate of propagation is not related to the jaw speed.
A separate method for the determination of the tear strength of small test pieces of vulcanized rubber (Delft test pieces) is specified in ISO 34-2.