A tear growth test determines the tear growth of foam. When subjected to tensile load, a stress peak forms at the end of the crack in which material failure occurs. Tear resistance is specified in N/cm in relation to the width.
Trouser and angle specimens are defined by the standard, whereby the trouser specimen is addressed only in the ISO standard. As opposed to rubber, it is loaded to crack opening (mode I).
For trouser specimens, crack prorogation takes place in the longitudinal axis of the specimen over a distance of approximately 25 mm, whereby a large surface is involved. This increases the probability of encountering an area with a closer cellular structure, which is then responsible for a higher maximum force. Since the crack is introduced by an incision, this method evaluates the force required to propagate a crack.
The angle specimen is loaded without an incision. As a result, a stress peak forms, however not before a sharp incision. In this method, the crack is propagated transverse to the longitudinal axis of the specimen.
The results of the trouser specimen are significantly lower and scatter much more as compared to those of the angle specimen. In a round robin test performed to ISO 8067, the degree of reproducibility for trouser specimens are, at 16 to 36%, three times that of angle specimens.
The test method can be performed with an electro mechanical testing machine. Since the forces are relatively low, a load cell that achieves Class 1 measurement accuracy should be used in the force range used. Displacement measurement is not required, since tear resistance is calculated from the maximum force that occurs during the test. In practice, the tear growth is often depicted as a graph that follows the travel of the tightening clamp.
The testXpert II software controls the test sequence and evaluates the tear growth to the standard.