9 TEAR TESTS

In a normal tensile test, taken to break, the force to produce failure in a nominally flawless test piece is measured. In a tear test the force is not applied evenly but concentrated on a deliberate flaw or sharp discontinuity and the force to continuously produce a new surface is measured. This force to start or maintain tearing will depend in rather a complex manner on the geometry of the test piece and on the nature of the discontinuity. Hence, it would be expected that different tear methods, using different geometries, will yield different tear strengths. However, there is evidence that, for at least a number of rubbers, the ranking of compounds is the same regardless of which tear method is used, ^[125], ^[126], ^[127] and is the same ranking as found from a tensile test ^[125]. Dozortsev ^[128] goes so far as to say that tear measurements are unnecessary for the assessment of the quality of rubbers.

What is certain is that the initiation and propagation of a tear is a real and very important factor in the failure of rubber products, being involved in fatigue and abrasion processes as well as the catastrophic growth of a cut on the application of a stress. There is, therefore, considerable interest in the tearing resistance of rubbers. What is uncertain is how tear resistance should be measured and the results interpreted.

It is not surprising that, given the importance of tearing and the...