A maximum stress at a distance criterion for the prediction of fracture in adhesively-bonded joints

The present work relates to the numerical prediction of the mode I failure of metal-to-metal adhesive joints under quasi-static, steady-state conditions by means of a criterion based on attaining a critical value of the maximum principal stress at a critical distance ahead of the crack tip. Firstly, the values for the maximum principal stress/critical distance for three different epoxy-based structural adhesives have been identified from the experimental data, and the values of the required maximum principal stress/critical distance have been interpreted, with the help of SEM images, in terms of the physical mechanisms responsible for fracture. Secondly, numerical simulations have been undertaken employing these values of the respective maximum principal stress/critical distance and have revealed that the proposed model predicted very accurately the failure of the three adhesives (i) over a wide range of the thickness of the adhesive layer from 0.1 to 1 mm, and (ii) for two very different test geometries: namely the tapered double-cantilever beam (TDCB) test and the wedge-peel test. Finally, a non-dimensional theoretical study has been undertaken to ascertain the effect of the bulk properties of the adhesive and of the parameters of the failure criterion on the predicted value of the adhesive fracture energy as a function of the bondline thickness. The results have revealed that low values of the yield stress and of the hardening exponent as well as large values of the critical maximum principal stress tended to magnify the value of the adhesive fracture energy