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The bondline thickness is a crucial parameter affecting the strength and toughness of adhesive joints. This parameter is responsible for the stress distribution, and thus, the efficiency of the stress transfer between the adherends, the failure modes, and the correspondent failure loads. However, the available literature reports different trends and relationships during testing between the thickness and strength, or thickness and toughness. In this chapter, the effect of adhesive thickness on the mechanical properties of adhesive joints is explored using the fracture mechanics framework. Two testing cases are discussed in detail: (i) soft bondlines joining rigid adherends, that is, butt joint and single end notch testing specimens, and (ii) rigid bondlines joining flexible adherends, that is, double cantilever beam geometry. These cases represent two extremes in terms of where the energy is stored while testing: in the first one, the energy is stored in the adhesive while in the second, the energy is stored in the adherends. From this discussion, the bondline thickness emerges as a critical geometrical parameter dictating transition between the two extreme cases. A single theoretical framework is provided that merges the two cases and is used to disclose the recently obtained experimental results., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Structural Integrity & Composites

Thicker bondlines along with manufacturing-tolerant and fracture-resistant adhesives are trends visible across different industries, especially maritime. In this work, two contrasting adhesives: an elastic-brittle epoxy-based, and a nonlinear-ductile methyl methacrylate (MMA) are characterized and compared via tensile, compact tension (CT) and double cantilever beam (DCB) testing, for steel-to-steel adherends. Significant differences are captured between the two bonding materials in terms of the energy required for crack growth: in the MMA “ductile” adhesive is ∼4 and 10 times more than for the epoxy “relatively brittle” adhesive for CT and DCB testing, respectively. While epoxy bondlines fail due to a symmetric in-plane crack, the MMA bondlines experience multiple cracking originating from high deformations and Poisson's effects. Moreover, in the case of MMA adhesive, the DCB testing led to plastic deformation of steel adherends. The existing evaluation protocols are adopted for data reduction and the effect of plastic dissipation is theoretically addressed. Despite adherend plasticity, it is concluded that the crack growth is driven by the elastic energy release, and thus, after small correction taking into account initial adherend plasticity, the existing simple models can still be used. This study highlights the potential use of ductile adhesive instead of the commonly used brittle ones to significantly improve the adhesively bonded joints in maritime applications in which thick bondlines, manufacturing-tolerant and fracture-resistance characteristics are required., Structural Integrity & Composites, Aerospace Structures & Computational Mechanics

The intensive use of Carbon Fibre Reinforced Polymer (CFRP) Composites in aerospace is motivated by their high strength to weight ratio. However, the manufacturing of complex aeronautic structures combining metallic and composite parts requires an efficient bonding process [1]. Joints obtained through bonding with adhesives suffer from low to moderate toughness [2] and the process requires multiple steps separating the manufacturing of the composite parts and the bonding. Several strategies have been proposed to enhance the joint toughness, such as introducing a mesh or cavities in the bondline [3]. One strategy to improve the efficiency of the process is to bond the metal part to the composite panels during the curing process, without using any adhesive. On the road to the integrated bonding of hybrid metallic composite structures, the present works investigates potential toughening strategies by bonding composites via co-curing of two composites parts while relying on joint architecturing principles.

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.

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines (≈10 mm) under global mode I loading. The role of adherend-adhesive modulus-mismatch and pre-crack length are scrutinized. The parameters controlling the crack path directional stability are also discussed. Single-material (i.e. steel-steel and GFRP-GFRP) and bi-material (i.e. steel-GFRP) double-cantilever beam joints bonded with a structural epoxy adhesive are tested. The joints are modelled analytically, considering a beam on elastic-plastic foundation, to include characteristic length scales of the problem (e.g. adhesive thickness, plastic zone) and numerically using Finite Element Model. An empirical relation, in terms of geometrical and material properties of the joints, that defines the transition between non-cohesive and cohesive fracture onset is found. Above a specific pre-crack length the stress singularity at pre-crack tip rules over the stress singularity near bi-material corners, resulting in mid-adhesive thickness cohesive fracture onset. However, the cracking direction rapidly deflects out from the adhesive layer centre-line. Positive T-stress along the crack tip is found to be one of the factors for the unstable crack path., Structural Integrity & Composites