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The development of new adhesives has allowed to expand the application of bonding into the most diverse industrial fields. This review article presents the commonly used experimental methods for the investigation of mechanical performance of adhesively bonded joints in the aerospace, wind energy, automotive and civil engineering sectors. In these sectors, due to their excellent intrinsic properties, composite materials are often used along with conventional materials such as steel, concrete and aluminium. In this context, and due to the limitations that the traditional joining techniques present, adhesive joints are an excellent alternative. However, standardized experimental procedures are not always applicable for testing representative adhesive joints in these industries. Lack of relevant regulations across the different fields is often overcome by the academia and companies' own regulations and standards. Additional costs are thus mitigated to the industrial sectors in relation with the certification process which effectively can deprive even the biggest companies from promoting adhesive bonding. To ensure continuous growth of the adhesive bonding field the new international standards, focusing on actual adhesive joints' performance rather than on specific application of adhesive joints are necessary. [ABSTRACT FROM AUTHOR]

Highlights: • Onset of interface crack is studied using cantilever beam geometry. • Geometrical reconfiguration of initially straight crack front is investigated experimentally and theoretically. • Change from initially straight to curved crack is observed in-situ. • The geometrical process is associated to branching of otherwise linear load response path. • The full reconfiguration process quantitatively relates to transition from the plane strain to the plane stress conditions experienced by the adherend. Abstract The load response during onset and reformation of an interface crack is investigated experimentally and theoretically using linear-elastic analysis. A number of double cantilever beam experiments is carried out under the prescribed displacement and the prescribed displacement rate loading conditions. During the onset of the interface crack with the initially straight front, the crack grows from the centre of the specimen before adopting a semi-elliptic shape. The shape of the crack front eventually becomes self-similar (sizes of the crack front process zone and the ratio between the crack front depth and its width become invariant) when reaching the free edges of the specimen. As a consequence, during the crack reformation the incremental crack area cannot be attributed to the growth along a single direction. Finite element calculations are implemented in which the recorded crack front morphologies are imposed as the crack front boundary conditions. It is concluded that the load response curve can become non-linear upon the geometrical reconfiguration of the crack front and quantitatively is associated with gradual transition from the plane strain to the plane stress conditions experienced by the adherend during the crack front reformation. [ABSTRACT FROM AUTHOR]

Interface and bondline heterogeneities are inherent parts of laminated materials. Recently, these otherwise unwanted features are gaining importance enhanced by the surface patterning techniques. In the present contribution, a double cantilever beam adhesion experiment is performed on joints with patterned interfaces. Weak/strong adhesion bands perpendicular to the crack growth direction are produced along the bonded surfaces. A vast range of specimens with different but systematic weak/strong zone ratios is tested. A novel analytical model, including a rule of mixture scaling, is developed to gain phenomenological insights into the data obtained. Two specific cases are addressed: a single strong/weak transition and multiple transitions. The new model proves a very good agreement with the experimental data obtained for any configuration tested. The role of three length scale parameters, weak, strong and process zone sizes, and their interactions, are emphasized. For instance, once small (in comparison to the process zone size) weak/strong zones are present, a substantial decrease in the fracture energy is recorded. This observation may be of importance in understanding both natural (e.g. geckos feet) and industrial (laminated and bonded materials) bondline discontinuities including the role of fillers, voids or kissing bonds. [ABSTRACT FROM AUTHOR]

The demand for composite materials is higher than ever before. Effective and reliable modelling techniques allowing for better understanding of their failure are critical for designing future optimised light-weight structures. However, failure of composite at the microscopic level is a complex phenomenon and it has often been analysed using empirical approaches. A promising alternative, that instead takes a physics-based approach, is the phase-field model (PFM). This approach has been applied comprehensively to homogeneous materials and has briefly been extended to composite laminae, as modelled by Kanninen's local heterogeneous region (LHR). Despite this progress, a systematic investigation of the implications and requirements of using PFM for studying fracture in composite materials seems incomplete and missing links between the physical and the numerical aspects. This work aims to fill that gap and discusses in detail the role of the critical parameters in a PFM analysis of fracture, including length-scale interactions and concerns on convergence. The present analysis is framed around the problem of toughening of composites by weakening the matrix/fibre interface. Here, the PFM proves very efficient tool allowing recognition of multiple failure modes. [Display omitted] [ABSTRACT FROM AUTHOR]

We investigate fracture toughness of architected interfaces and their ability to maintain structural integrity and provide stable damage propagation conditions beyond the failure load. We propose theoretical and numerical frameworks to evaluate the fracture properties of architected interfaces sandwiched between two (face) materials. The microscopic geometries of these interfaces are chosen as 2D cells – pillar, tetrahedron, and hexagon – as well as their 3D counterparts – namely, pillar array, octet truss, and Kelvin cell. Our model, both numerical and analytical, exhibits a high level of accuracy in predicting the compliance before failure and failure loads. Novel results are obtained during the damage propagation regime, indicating fulfilment of the so-called fail-safe design. Some of the cell geometries unfold during fracture, thus increasing the failure load and ensuring stable and controlled damage propagation conditions. [ABSTRACT FROM AUTHOR]