A tear test applies a force to a material that already contains a partial fracture or rupture otherwise known as a “tear” in order to bring the material to complete failure. This is done by placing the test sample into a tensile testing or universal test machine so that the section on one side of the fracture is pulled up and the section on the other side is pulled down. A steady load is then applied until the tear has propagated through the entirety of the sample and the two sections have been completely separated from one another.
Purpose of a tear test:
The overall goal of a tear test is to determine a materials ability to resist the growth of the initial tear or its “tear resistance”. The value of a material’s tear resistance is generally accepted to be the force required to complete the tear so that it has gone completely though the material so that it has been separated into two sections. This characteristic is particularly important to applications that use flexible materials that may suffer a partial rupture intentionally or otherwise.
Types of tear tests:
Most tear resistance tests are performed statically and commonly take one of three forms. These forms differ from one another in the configuration and preparation of the test sample, the grip placement, and the rate of testing.
The first form is referred to as the trouser shape tear test and involves a rectangular sample that has a cut down the middle of one half of the sample so that two legs are formed and the sample resembles a pair of trousers. Each leg is then placed into one of the test grips and is then pulled apart so that the tear continues down the middle of the sample.
The second form is the tongue tear test and requires that the sample have a tab cut in the middle. This tab is cut so that so that it is separated the rest of the sample on the bottom and sides but not the top. The tab is then placed in the top grip while the rest of the sample is placed in the bottom grip. As the top grip is moved upward the tab is pulled so that the cuts on both sides are extended upward to the top of the sample and the tab is eventually pulled free leaving a rectangular piece in the top grip and a “U” shaped piece in the bottom.
Finally in the case of the trapezoid tear test the test sample is cut into the form of an isosceles trapezoid with a small cut made in one side. The sample is then loaded into the test fixture so that the tear made by the cut will propagate as the grips move apart. Generally the sample is orientated so the tear extends horizontally.
Types of materials for tear testing:
A tear test is generally only performed upon flexible materials as they are more susceptible to accidental fractures and punctures as well as partial ruptures (tears and holes). The more common materials that benefit from tear testing are textiles, fabrics, elastomers, rubbers, polymers, packaging and paper products. Each of these materials can have serve in a wide range of industries such as clothing, medical, biomedical, and dental.