A wide range of testing devices have been devised to assess the crack resistance of glued structures in pure mode I, pure mode II or in mixed mode. Most of these devices are beam type specimens. We will very soon review the most common:

 

•           Double cantilever beam tests (DCB) determine the mode I crack resistance of adhesives in a fracture mechanics framework. These tests consist in opening an assembly of 2 beams by applying a force at the ends of the two beams. The trial is unstable and a adapted version of this test characterised by a non constant inertia was planned called the tapered double cantilever beam (TDCB) specimen.

 

•           Peel tests measure the fracture resistance of a thin layer bonded on a thick substrate or of two layers glued together. They consist in measuring the energy needed for tearing an adherent layer from a substrate or for tearing two adherent layers one from another. Whereas the configuration is not symmetrical, various mode mixities can be introduced in these tests. This is one of the more common methods of evaluating paper strength in library and archival preservation.

 

•           Wedge tests calculate the mode I dominated fracture resistance of glues used to bond thin plates. These tests consist in inserting a wedge in between two glued plates. A critical energy release rate can be derived from the crack length during testing. This experiment is a mode I test but some mode II component can be introduced by bonding plates of different thicknesses.

 

•           Mixed-mode delaminating beam tests (MMDB) consist in a glued bilayer by two starting cracks loaded on four points. The test presents about the similar amount of mode I and mode II with a small reliance on the ratio of the two layer thicknesses.

 

•           End notch flexure tests consist in two glued beams built-in on one side and laden by a force on the other. As no normal opening is allowed, this device allows testing in basically mode II condition.

•           Crack lap shear tests (CLS) are usage-oriented crack resistance tests. They consist in two plates glued on a limited length and loaded in tension on both ends. The test can be either symmetrical or dis-symmetrical. In the primary case two cracks can be initiated and in the second only one crack can propagate.