Tear strength expresses how well a rubber resists tearing and is one of its important physical properties. It is the force divided by the thickness (F/t), usually in kN/m. For the same material, the product's geometry and the direction and angle of the pull all change the measured value.

The standard and its three test pieces

DIN ISO 34-1 covers the tear strength testing of vulcanized and thermoplastic rubbers. It provides three test pieces:

• A: the trouser test piece.
• B: an angle test piece, with or without a nick.
• C: a crescent test piece with a nick.

The trouser method

The trouser method takes a thin strip with a slit at one end and clamps the two "legs" in the upper and lower grips of a tensile tester. The maximum force needed to tear the strip completely apart is recorded and divided by the thickness to give tear strength in kN/m.

The standard treats the trouser piece as the preferred method for several reasons. A strip cut slightly longer or shorter has little effect on the result; the other two pieces need the nick to be controlled carefully, which the trouser piece does not; the result relates more readily to the basic tearing behavior of the material; and the measured tear propagation rate is directly tied to the grip separation rate. Some rubbers, natural rubber in particular, tear unevenly, which produces scattered data that are hard to interpret.

What affects the result

Specimen shape and thickness, calendering (grain) direction, cut depth, test temperature and tearing speed all influence tear strength. When comparing batches or suppliers, keep these conditions the same.