Failure, adhesive peel loading

F-BDAF Tg for various blend compositions, see Fig. 14. The microphase-separated morphology further manifests itself in the self-adhesion behavior of polyimide films derived from such mixtures. For mixture containing at least 25 wt% of the flexible component, peel tests of polyimide bilayer samples prepared by solution casting, bulk failure of the test specimens was observed. Since the flexible component contained fluorine, the samples could be examined by X-ray photoelectron spectroscopy to determine the surface composition. At only 10% loading, the flexible component comprised 100% of the top 75 A of the sample. The surface segregation of the flexible component is believed to be responsible for the adhesion improvements. 

The thickness of the TDCB specimens (S = 10 mm) is sufficient to ensure plain strain conditions. It should be noted that during the test the arms remain within their elastic limit. Therefore, from simple beam theory [7], and by the use of linear elastic fracture mechanics, the strain energy release rate of the adhesive can be obtained using Eqn. 2, where P is the load at failure and E, is the substrate modulus. The calculated adhesive fracture energy was employed in the simulation of the TDCB and impact wedge-peel (IWP) tests. 

The failure of adhesive joints depends on the combination of peel and shear loading. In fracture mechanics, three basic modes of loading (Fig. 6.10) are the following ... 

In many RPs, the most convenient joint will be a bonded type (possibly produced during the lay-up process), and the geometry is therefore very influential. The weakest joints are those where failure is limited by inter-laminar failure of the adherend, or peel of the adhesive. Next strongest are those where the load is limited by the shear strength of the adhesive. The strongest designs will fail outside the joint area, at a load equivalent to the strength of the adherend. 

Bond strength— The unit load (force) supplied in tension, compression, flexure, peel, impact, cleavage, or shear, required to break an adhesive assembly, with failure occurring in or near the plane of the bond (the interface). The term adherence (q.v.) is frequently used in place of bond sttraigth. 

If peeling forces are counteracted by constraining the joint with lubricated clamps, this type of failure is not observed. Indeed, when high-performance adhesives are used, the shear loading of restrained joints usually tears the adherends - even metal ones - and the joint itself remains intact. 

Designs deliberately incorporating flexible rubbery inserts between two stiff adherends are well known. For example, cyanoacrylate adhesives are successfully used to bond spectacle lenses to frames through an intermediate rubber layer. Without the rubber to dissipate peel and cleavage loads, the joint between the lens and the metal frame would be readily over-stressed, resulting in premature failure. 

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