Keywords: adhesive failure, energy released, failure criterion, peel stress, shear stress, finite elements.

This paper concerns the analysis and comparison of the suitability and precision of some interface failure criteria obtained from the literature on structural adhesive joints. The comparison is based on the predictions of the criteria for the failure for the failure onset in mechanical tests that assure an adhesive failure. The tests are torsion-tension tests on butt joints. Three types of adhesives were tested with steel substrates with different diameters. For the prediction, stresses calculations are made using the finite element software, COMSOL Multiphysics 3.3. The calculations were linear elastic with small strains. It is well known that stresses at the interfaces exhibit singularities at the edges and, therefore, a simple point-wise criterion of stresses cannot be applied to predict the onset of adhesive failure. The application of a criterion for the interface failure onset in structural adhesive joints giving correct predictions for different loads and geometries has not been published yet.

Three criteria were analyzed. The first one is a quadratic expression of average interfacial stresses over a characteristic distance from the edge. This criterion has been widely applied for predicting delamination in composite laminates. The second criterion applies a mixed criterion introduced by Leguillon which involves simultaneously stress and energy conditions [1]. The stress condition is a point-wise quadratic expression of interfacial stresses and the energy condition makes use of the calculation of an incremental strain energy release rate arising from a spontaneous crack. The third criterion is also a mixed criterion but it includes an original condition of the stresses.

For the same butt-joint subjected to any combination of tension and torsion loads, the three criteria correctly predict interface failure initiation. The third criterion provides the best fit of experimental data and is suitable for predicting failure for any load condition. When different diameters of butt joints are considered, the average stress criterion is not suitable for predicting failure onset whereas the mixed criteria are suitable for this task.

The results in this paper may constitute an important advance in the development of new tools and the selection of better criteria to predict the initiation of adhesive failure in adhesively bonded joints.

1

D. Leguillon, "Strength or toughness? A criterion for crack onset at a notch", European Journal of Mechanics - A/Solids, 21(1), 61-72, 2002. doi:10.1016/S0997-7538(01)01184-6

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