For example, with
rubber, tear resistance measures how the test specimen resists the growth of any cuts when under
tension, it is usually expressed in
kN/
m. Tear resistance can be gauged via the same ASTM D 412 apparatus used to measure
tensile strength,
modulus and
elongation. ASTM D 624 can be applied to measure the resistance to the formation of a tear (tear initiation) and the resistance to the expansion of a tear (tear propagation). Regardless of which of these two is being measured, the sample is held between two holders and a
uniform pulling
force applied until the aforementioned deformation occurs. Tear resistance is then calculated by dividing the force applied by the thickness of the material. Materials with low tear resistance sometimes have poor resistance to
abrasion and when damaged will quickly fail (this includes
hard materials, since hardness is not related to tear resistance). ==Tear resistance of different materials==