The project addressed the characterisation of the fracture resistance in layered structures, with special focus on debonding of adhesive joints or delamination of composite laminates, which are very important issues particularly for the aerospace and automotive industry. For these problems, several industrial standards have been released. For a mode-I (opening) case a ’double-cantilever-beam’ (DCB) is mostly used (Fig. 1). For a mode mode-II (sliding) case the ‘end-notched flexure’ (ENF) specimen is often used (Fig. 2). For a mixed-mode case, a simple test uses the ‘mixed-mode-fracture’ (MMF) specimen (Fig. 3). Peel tests are also used in some cases (Fig. 4).
Current standards are mostly based on linear elastic fracture mechanics (LEFM). They also need the measurement of the crack length, which is a main obstacle to their adoption by the industry. Moreover, many authors and indeed even textbooks question the validity of LEFM for the case of ductile adhesives and suggest that alternative approaches, including J-integral and cohesive-zone models (CZMs), better account for ductile behaviour of adhesive before fracture.
Indeed, CMZs are widely used to simulate delamination because they are easy to implement in a finite-element (FE) model and are able to accurately model the deformability and damage of the adhesive before failure. In many cases, the rate-dependence of the process (i.e. dependence on the loading speed) cannot be ignored and has been recently incorporated within CZMs by a number of authors, with important contributions given by the host researcher before this project.
The main aim of the project was to revisit the current standards to overcome the above-mentioned issues, taking advantage of the fact that, because of their simple geometry, the specimens used in these tests can be modelled using relatively simple FE models (beam models). For the same level of complexity and accuracy, these models can simulate delamination tests faster than any available models (e.g. 2D and 3D models).
The specific objectives, which can be found in the detailed technical report and in the DoA, can be summarised in less technical terms as follows:
(a) Develop a new software code for the computer-aided simulation of the above mentioned tests by using computationally efficient FE models accounting for mode I, mode II and mixed-mode crack growth, as well as for rate dependence.
(b) Conduct experimental tests using DCB, ENF, MMF and peel-test adhesive-joint specimens, at a sufficient number of different speeds to characterise the rate dependence of the adhesive.
(c) Validate the numerical models against the experimental results.
(d) Develop a friendly graphic user interface (GUI) for the code, with a view to promoting the developed models as new tools for the characterisation of fracture resistance of layered structures in the industry.
