Tear strength describes how much resistance a film offers to a tear that has already started from spreading. In bags, most damage occurs when a tear that starts from a small nick or hole grows; this is why tear strength is a property noticed in everyday use.
Film usually shows different tearing behaviour in two directions: the machine direction and the transverse direction. This difference comes from the balance between haul-off speed and blow-up ratio during production. The raw material is also decisive; adding LLDPE generally increases resistance to tearing. Tear strength is measured in the laboratory using standard methods on samples.