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.

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.

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.

The aim of this study is to investigate the effect of the adherend material on the mode I fracture behaviour of bi-material composite bonded joints. Both single-material (steel-steel and composite-composite) and bi-material (steel-composite) joints bonded with a structural epoxy adhesive are studied. Additionally, two adhesive bondline thicknesses are considered: 0.4 mm (thin bondline) and 10.1 mm (thick bondline). The Penado-Kanninen reduction scheme is applied to evaluate the mode I strain energy release rate. The results show that the mode I fracture energy, G_Ic, is independent of the adherend type and joint configuration (single or bi-material). G_Ic shows average values between 0.60 and 0.72 N/mm for thin bondlines and 0.90–1.10 N/mm for thick bondlines. For thin bondlines, the failure is cohesive and the similar degree of constraint that is imposed to the adhesive by the high-modulus (i.e., steel) and/or relatively thick (i.e., composite) adherends results in similar values of G_Ic for both single- and bi-material joint types. For thick bondlines, the crack grows closer to one of the adhesive-adherend interfaces, but still within the adhesive. The results show that the adhesive could deform similarly, although the crack has been constrained on one side by different types of adherends, either a steel or composite.

The fracture behaviour of joints bonded with a structural epoxy adhesive and bond line thicknesses of 0.1–4.5 mm has been studied. However, limited research is found on similar joints with thicker bond lines, which are relevant for maritime applications. Therefore, the effect of the adhesive bond line thickness, varying from 0.4 to 10.1 mm, on the mode I fracture behaviour of steel to steel joints bonded with a structural epoxy adhesive was investigated in this study. An experimental test campaign of double-cantilever beam (DCB) specimens was carried out in laboratory conditions. Five bond line thicknesses were studied: 0.4, 1.1, 2.6, 4.1 and 10.1 mm. Analytical predictions of the experimental load-displacement curves were performed based on the Simple Beam Theory (SBT), the Compliance Calibration Method (CCM) and the Penado-Kanninen (P-K) model. The P-K model was used to determine the mode I strain energy release rate (SERR). The average mode I SERR, G _Iav., presented similar values for the specimens with adhesive bond line thicknesses of 0.4, 1.1 and 2.6 mm (G _{I av.}=0.71, 0.61, 0.63 N/mm, respectively). However, it increased by approximately 63% for 4.1 mm (G _{I av.}=1.16 N/mm) and decreased by about 10% (in comparison with 4.1 mm) for the 10.1 mm (G _{I av.}=1.04 N/mm). The trend of the G _Iav. in relation to the bond line thickness is explained by the combination of three factors: the crack path location, the failure surfaces features and the stress field ahead of the crack tip.

This paper addresses the fracture behaviour of bonded composite plates featuring a kissing bond along the crack growth path. Double cantilever beam (DCB) experiments are carried out under a displacement controlled loading condition to acquire the load response. The experimental data are collected and analysed analytically for specimens with and without kissing bond. The following aspects are observed and discussed: effect of the adhesive carrier film, non-smooth crack growth and rising R curve. An analytical model taking into account the aforementioned effects is proposed. The kissing bond leads to unstable crack growth resulting in a loss of the load carrying capacity. The presence of the knit carrier in the adhesive film results in the crack growth process characteristic for the stick-slip phenomena and a significant increase of the resistance to fracture of the bondline by triggering a bridging phenomenon. The model shows a very good agreement with the experimental data. A sound understanding of the fracture process is gained enabling analysis and prediction of the effects of kissing bonds and supporting carrier.