Testing the strength of glue involves gluing various materials together using different kinds of adhesives. Common methods for testing the strength of glue include peel, cleavage and shear tests.
Shear tests are used to test the adhesive strength of tape, labels and other flexible bonded materials. People can test the glue strength on flexible objects at 90 degrees, 180 degrees or using a loop-style test. In this case, the two items are glued together. Once the glue has dried, the glued objects are bent to an 180 degree angle, or a 90-degree angle if they are only semi-flexible. If the two objects are especially flexible, such as paper on soft plastic, they can be bent into a full loop. People can then observe which glue holds the objects together best.
A shear test is for items that are glued one on top of the other. The test determines the shear strength of the adhesive, which is its ability to resist forces that cause the internal structure of the material to slide against itself. After the two items are glued together, they are forcefully pushed in an attempt to cause sliding. Cleavage strength is an adhesive's ability to resist being pulled apart. After materials are glued together, they are pulled in an attempt to force them apart. Any of these tests can be used with different glues to discover which is the strongest.