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Currently, adhesive bonding processes are developed and optimised in a time-consuming trial and error procedure, which rarely leads to an optimal solution due to the high complexity of the adhesive flow behaviour during application. The ideal adhesive layer has precise geometric specifications; entrapped air bubbles or overfilling of the bond should be avoided. Numerical methods, such as Computational Fluid Dynamics (CFD), are only capable of calculating squeeze-flow processes to a limited extent. Apart from high computing times, mesh and convergence problems often occur due to the small ratio of adhesive layer thickness to adhesive layer length and width. The Generalised Partially Filled Gap (GPFG) model, published in a companion paper uses fundamental characteristics of every bonding process to derive clever assumptions, and thus provide an efficient simulation tool for adhesive squeeze-flow. The GPFG model simplifies the squeeze-flow to a two-dimensional problem, as the flow in thickness direction can be neglected for most bonding processes – without significant loss of accuracy compared to analytical or CFD solutions. The experimental validation of the model is presented in this study. Both stresses and flow geometry were evaluated, and a very good agreement between experiments and model was proven. [ABSTRACT FROM AUTHOR]

Throughout the past, heating by electromagnetic induction has been frequently used to design speedier curing processes of adhesives. With this method, bonded components are exposed to an alternating electromagnetic field (EMF), which generates heat in EMF-sensitive adherends, like steel and aluminium, or in susceptors that are admixed to the polymers to be cured, e.g. fibres or particles. Recently, specially designed susceptors, so-called Curie particles (CP), have shown their great potential for induction curing. Heat generated in CP is capped by the materials Curie temperature (Tc), preventing the adhesive from overheating. As a result, curing proceeds way faster and independently from ambient temperatures, opening up new application fields for bonded components. Although practical applicability has already been demonstrated, CP-induced heating – and consequently curing – has proven to be very sensitive to the boundary conditions of the considered application. Therefore, induction times needed to achieve full cure must currently be determined by cumbersome and costly experimental investigations. To compensate for this disadvantage, the present study – representing the second part of a series – aimed at offering a first approach of a numerical model in which thermal and kinetic aspects of CP-induced accelerated curing were combined. For that, curing kinetics of two kinetically different 2K epoxy adhesives were linked to a transient heat flow simulation in Ansys based upon experimentally determined heat loads. Since the first part of this series concentrated on presenting all preliminary experimental work as well as analytics applied during modelling, this part focuses on the validation of the developed FEA using the exemplary application of CP-cured Glued-in Rod (GiR) specimens. In the following, various numerical parameter studies were carried out, demonstrating principal functionality of the new FEA technique and highlighting in particular its contribution for the design of more efficient and target-orientated CP-curing processes. [ABSTRACT FROM AUTHOR]

Accelerating the polymerisation of adhesives has been a long-established field of research in context of adhesive bonding technology as long curing times – sometimes up to days – represent a decisive disadvantage in contrast to most mechanical joining techniques like riveting or screwing. In addition to, e.g., curing via UV, microwave or IR radiation, electromagnetic induction represents a promising solution for speeding up the cure of polymers. With this method, adhesively bonded components are subjected to an alternating electromagnetic field (EMF) that induces heat in EMF-sensitive materials, e.g., steel or aluminium. If intrinsic EMF-sensitivity of the materials to be bonded is not given, different types of susceptors like meshes, fibres or particles are added to the adhesive in order to be heated inductively and thus cure the adhesive from the inside. Recent research has focused on a special type of susceptors, so-called Curie particles (CP), which can only be heated inductively up to their specific Curie temperature (Tc) at which CP-induced heating automatically stops. As a result, a curing process is created that eliminates the need for external temperature monitoring while simultaneously preventing overheating of the bond. As underlying curing kinetics – most importantly polymerisation enthalpy and curing time – significantly differ depending on the considered adhesive, induction times needed to achieve full cure must currently be determined through costly preliminary investigations on an experimental level. Thus, to contribute for a more efficient and controllable bonding process, the present study aimed at developing a numerical model based on the Finite Element Method (FEM) and capable of predicting the curing degree α in dependency of curing temperature profiles TCure(t) and the CP content ccp. For this purpose, the curing kinetics of two fundamentally different 2K epoxy adhesives were linked to a transient heat flow simulation based upon experimentally determined heat loads. The validation of the developed FEA technique was successfully carried out using the example application of inductively cured large-scale Glued-in rod (GiR) specimens, whereby experimentally determined temperature profiles showed excellent agreement with the numerical predictions. The present paper focused on presenting preliminary experimental work as well as all analytical methods implemented for the numerical modelling. [ABSTRACT FROM AUTHOR]

In the manufacturing industry, the process of applying adhesive to join two substrates can be challenging. It requires precise placement of adhesive so that it is distributed in the desired pattern, ensuring a strong bond between the substrates. Overfilling is a common practice to ensure that the bond is not underfilled after the substrates are pressed together. However, this approach leads to unnecessary adhesive waste and can result in additional cleaning work. To address these issues, an optimization algorithm has been developed that can compute an initial adhesive pattern to flow into any desired geometry. The algorithm is based on a transient squeeze flow simulation, which iteratively removes fluid material as it flows out of the target bond domain. For example, to obtain a square bond geometry after a parallel squeezing process, the adhesive needs to be initially applied in a star shape. However, the algorithm is highly adaptable and can be used for many desired bond geometries as shown in this paper. The results are independent of the mesh resolution and fluid flow law, thus eliminating the need for complex rheological fluid characterization. The algorithm requires only two input parameters, namely the target bond geometry and the degree of compression. This highly simplifies the adaption of the proposed optimization method for manufacturers, and their design requirements can be considered with ease. The results are reliable and consistent despite the simulation's complexity. Overall, this new optimization methodology represents a significant advance in adhesive application technology, eliminating the need of overfilling and thus making the manufacturing process more efficient, sustainable and cost-effective. [ABSTRACT FROM AUTHOR]

Over the past years, a large number of studies about the implementation of adhesive bonding in timber engineering were published. Current practice is to use two-component (2K) epoxies and polyurethanes in conjunction with threaded steel bars, reinforced concrete bars, and rods made of fibre reinforced polymer. All these material might be considered questionable in terms of sustainable ecology. The current article investigates the possible substitution of aforementioned metallic bars by hardwood dowels, and the polymer-based adhesives by glutine- and casein-based ones. It summarises experimental investigations carried out with different geometric parameters, in which beech (a hardwood) rods were glued into spruce (a softwood) blocks. Joint capacities achieved using glutine- and casein-based adhesives were compared with results using 2K epoxies and 2K polyurethane adhesives under dry conditions, and under the influence of moisture. Results indicate that current glutine and casein adhesives, if used under dry conditions, achieve similar strength; however, the influence of moisture is still a limiting factor that severely restricts their usage under adverse conditions. The presentation of the results is completed by a numerical modelling of the considered joints, including a methodology to predict their capacity with a reasonable degree of accuracy. [ABSTRACT FROM AUTHOR]

Abstract Glued-in-rods (GiR) represent a class of joints being used in timber engineering that are mostly used to transfer axial loads in structural members with Glass Fibre Reinforced Polymers (G-FRP) increasingly being considered as rod material in substitution for the usually considered metallic rods. The primary objective of the research presented herein was to apply a probabilistic capacity prediction method to timber joints with G-FRP GiR. Necessary secondary objectives were to mechanically characterise the involved components (adhesive, rods, and timber) and to numerically investigate the stress state inside the tested joints. The methodology shall be applied to a recently reported in literature Zhu et al. (2017) that was specific in two ways: firstly, G-FRP rods were bonded into both ends of the timber block with both ends being tested to failure—a deviation from common practice; and secondly, as opposed to most previous studies exhibiting stiff adhesives, a ductile Polyurethane with markedly non-linear behaviour was used. All material characterisation was performed with methods that can be reproduced by any standard laboratory equipment, to provide parameters for the subsequent numerical analyses. Based thereupon, a probabilistic method was used and provided reasonably accurate predictions of the joint capacities of 25 different geometrical GiR configurations. The probabilistic method was extended for realistic estimations of the experimental capacity scattering in form of upper and lower percentiles. [ABSTRACT FROM AUTHOR]

Dimensioning adhesively bonded joints remains a challenging task. Their diversity in geometry and material (including combinations thereof), characteristic stress peaks, scatter and brittleness are aspects few practitioners are prepared to handle for dimensioning purposes. As developed in the first part of this two-part paper, direct stress based methods, despite diverse fudging techniques, proved to be unsuitable. This left the field open for fracture mechanics, which, however, departed very much from the engineering reality leading to only lukewarm acceptance from practitioners. Almost unnoticed, probabilistic methods, much closer to classical mechanics, may offer an alternative. Based upon the presentation, in the first part of this series, of the concepts, the authors suggested an implementation in terms of design procedure. The second part of this two-part paper, is dedicated to the presentation of several examples, from available literature, of adhesively bonded joints involving a large variety of typologies (single and double-lap, tubular, and hybrid joints) and materials (fibre-reinforced polymers, steel, wood). The focus therein is set on practitioner oriented aspects, in particular on techniques for implementation. [ABSTRACT FROM AUTHOR]

Recent adhesive research starts to consider more process oriented problems, in addition to the classical static stress and strength related topics, thus shifting towards a more global view on adhesive bonding. This includes the consideration of adhesive flow during application and joining of the substrates with all the information about initial application pattern and the resulting adhesive distribution contained after joining. Since the surface condition plays a major role and various different pretreatments are commonly used to improve the strength and durability of bonded joints while modifying their surface structure, the question of whether this influences the dynamics of the squeeze flow – or not – is addressed in this paper. In particular the materials glass and steel in combination with two designated surface pretreatments (blasting and flame-treatment) as well as their respective untreated reference surface are considered. For validation purposes the surfaces were characterized by means of roughness R a and contact angle θ measurements, performed before and after the pretreatment, which resulted in a range of surface parameters for R a from almost 0 to 3.3 μ m and for θ from approximately 0 to 35°. However based on the measured force–displacement behaviour during the squeeze process no statistically significant difference regarding the dynamics of the adhesive flow could be observed between the untreated and pretreated surfaces for all investigated materials. [ABSTRACT FROM AUTHOR]

This paper presents a simple experimental validation of a method for finding optimal application patterns in adhesive bonding, as previously introduced by Flaig et al. (2023). The study uses two examples to demonstrate that the patterns found in theory effectively fill the target gaps after pressing in practice. The results confirm that the identified application patterns fill the target gaps almost perfectly, ensuring excellent joint integrity after the pressing process. In particular, the applicability of the star pattern to fill square gaps is experimentally validated. In addition, the method of creating a rectangle by pressing a cut star connected by a straight beam is also effective. While the theoretical results in Flaig et al. (2023) were already promising, this research now provides experimental evidence of their practical applicability. [ABSTRACT FROM AUTHOR]

The highly automated world of manufacturing industry offers new possibilities for the application of adhesives which are not fully utilized yet. In addition to the classical static stress and strength related topics, a more global view on adhesive bonding is needed. This includes the consideration of adhesive flow during application and joining of the substrates with all the information about initial application pattern and the resulting adhesive distribution contained after joining, and thus benefit from the modern manufacturing possibilities. To do so, research which addresses this issue is needed in the first place to elaborate innovative approaches on how to design and develop adhesive application pattern which minimize adhesive spillage and waste while achieving a bond without any air enclosures or other imperfections. This paper systematically investigates different phenomena which can occur during the joining process, such as the formation of air pockets and how they behave once formed. Furthermore, optimized application pattern and the merging behaviour of different flow fronts are considered. In total, four different adhesive application patterns were investigated experimentally and analysed using Particle Image Velocimetry. The investigations were complemented by a numerical simulation of the corresponding squeeze flow experiments — which achieved a high level of agreement. It was found that during the joining process, existing air pockets can reform due to the phenomenon of viscous fingering and subsequently cure into flat cavities, which are more critical in terms of bonding strength than spherical ones. [ABSTRACT FROM AUTHOR]

Adhesive bonding is increasingly being considered by engineers for load-bearing joints to complement, or supplement, traditional mechanical fasteners for a large variety of structural materials, including fibre-reinforced polymers, timber, and steel. However, practitioners are still largely applying analysis and design methods, including codes and standards, that rely heavily on fudge factors. This is not due to the relative novelty of the joining technique, and the complexity of the associated mechanics. This paper guides the reader through some of the most important aspects of adhesively bonded joints, and the reasons why classical mechanics can only be applied in conjunction with fudge factors. Subsequently, the principles of fracture mechanics (FM) are presented; it is shown that, despite its conceptual strengths, FM is rather difficult to implement, as it introduces mechanical concepts practitioners are not familiar with. Then probabilistic methods (PM) are introduced; it is shown that PM can be considered as an extension of classical mechanics, as they can be easily implemented as a post-processing routine of the latter. Additionally, it is shown that PM, unlike FM, neither require specific test procedures for characterisation, nor complicated numerical analyses. Lastly, the paper offers a step-by-step guide for the implementation of PM for the dimensioning of adhesively bonded joints. The second part of this paper illustrates the latter analysis and design of adhesively bonded joints on a series of examples involving FRP, timber, and steel. [ABSTRACT FROM AUTHOR]

Current studies show great, but unutilized potential for mechanical optimization of adhesively bonded joints. There are different approaches, e. g. mixed adhesive joints; tapered adherends or joints with spew fillets, all of which try to redistribute the stress peaks, occurring at the ends of adhesive layer overlap, in favor to the load capacity. However, these investigations fail to take a general approach to find optimal geometric solutions to the problem of joining two materials by means of adhesive bonding. Instead the geometry is changed by predefined parameters such as tapering angles, spew fillet radii or the number of adhesives in mixed adhesive joints. To allow a non-parametric optimization, this paper proposes and investigates an extended bi-directional evolutionary structural optimization methodology based on a failure criterion to account for othotropic materials, which are often used in the context of bonding. The optimized joint geometry, generated by the proposed algorithm, resulted in a fairly constant distribution of elemental failure probability compared to the non-optimized state, thus eliminating underutilized spots of material and increasing the efficiency of the connection. Furthermore the influence of different parameters such as the overlap-length to adherend thickness ratio; the stiffness between adherend and adhesive as well as the adherend degree of orthotropy were investigated for optimal joint geometry. [ABSTRACT FROM AUTHOR]

This study examines grouted joints for offshore wind turbine systems, where gaps between steel pipes are filled with grout, relying on adhesion, friction and compression struts formed by shear keys. Concerns about the limited load carrying capacity of large diameter grouted joints in offshore structures have prompted investigation. Whilst adhesive bonds are recognized for load transfer in structural steel, the complete replacement of grouting with bonding alone is proving impractical for large gaps in offshore structures, posing challenges in terms of cost and handling. To address these challenges, the study presents an innovative hybrid grouted joint for steel structures that combines grout and adhesive layers. This novel approach replaces traditional large shear keys with distributed micro-shear keys — small granules embedded in the grout material. Under axial loading, the hybrid joint demonstrates robust performance, with a maximum nominal shear stress on the inner pipe of 30.1 MPa and consistent load capacity across tests. Average shear strength is in line with, and occasionally exceeds, expectations. Notably, the hybrid joint shows resilience in different configurations and maintains a low coefficient of variation of 5.5%, indicating consistent performance. When the influence of the adhesive material on the hybrid joint is examined, the effect on the joint stiffness is minimal, despite variations in adhesive stiffness. Sikadur 370, which has a higher modulus of elasticity than DuploTEC, shows similar deformation curves in the elastic region, emphasizing the robustness of the joint. Differences in the maximum loads are attributed to the thickness of the adhesive layer and the lower modulus of DuploTEC, resulting in different shear strengths. The proposed hybrid joint offers a promising solution to improve the performance of conventional grouts and adhesives in offshore structures. The study also formulates a failure criterion, performs numerical stress analysis, and models the effect of geometric dimensions, eccentricity, and misalignment on load capacity. The methodology's accuracy in predicting load capacity, supported by numerical analysis and large-scale joint simulations, contributes to a comprehensive understanding of hybrid joint performance in engineering applications. [ABSTRACT FROM AUTHOR]

The choice of a suitable joining method is of major importance for the performance and durability of structural connections in any type of construction and in other branches of industry, such as the manufacturing of sliding vehicles. In addition to mechanical joints using high-strength bolts as well as welding, adhesive bonding is more and more being considered as a serious alternative. Hybrid joints are the focus of the research, especially to increase load-bearing capacity and reduce deformation. This project specifically aims at the combination of elementary bonded joints and High Strength Friction Grip Bolts to form the so-called pre-loaded hybrid joint. The pre-loaded hybrid joint optimally combines the specific advantages of both elementary joining methods and thus creates an efficient structural joint. This research takes up the findings of a previous project and focuses on the characterisation of the pre-loaded hybrid joint with regard to its application in the field. For this purpose, a comprehensive experimental test programme is carried out, which served for the validation of a numerical model generated for the computational load prediction in a companion paper. [ABSTRACT FROM AUTHOR]

Strength prediction of bonded joints remains a challenging task made even further complicated for hybrid joints in which adhesive is supplemented by mechanical fasteners. The present paper complements a series of experimental investigations on hybrid joints combining adhesive bonding with pre-tensioned bolts on galvanised and coated steel substrates, and considering two adhesives. The paper starts with modelling the strength of the bonded connection under the simultaneous action of tensile and compressive normal stresses, and shear. It then presents how these data are moulded into appropriate failure criteria and used the latter as input data for a subsequent full probabilistic finite element analysis. Following the paths of Weibull and subsequent researchers, joint capacity is predicted with reasonable accuracy for a dozen different combinations of substrates, adhesive, and pre-tension level – for which, on average, an accuracy slightly below 10% has been achieved. The results deliver meaningful insights into the subtle relationship between adhesive bonding, mechanical fastening, failure criteria, and resulting predictions methods. [ABSTRACT FROM AUTHOR]

This last paper of a series of three concludes the analysis of a series of adhesively bonded tube-in-tube (TiT) joints composed of glass-fibre reinforced polymers (G-FRP) for which induction heating with Curie-particles (CP) was used to accelerate curing. It focuses on the prediction of their load capacity using a probabilistic method (PM). First, the G-FRP material was characterised with regard to transverse (tensile) and shear stresses, which allowed to determine a valid failure criterion, including the related statistical description subsequently required for the PM. Then, finite element analysis (FEA) was used to determine the stresses within the TiTs, for not only a perfect geometry, but also considering a series of possible geometrical imperfections (offset and misalignment of the centrelines, and ovalisation) with three degrees of severity. The influence of aforesaid imperfections on stresses was determined, and discussed. Based thereupon, load capacity was numerically determined, compared to the experimental values, and the influence of imperfections highlighted. Predicted joint capacities agreed well with the experimental ones. The influence of centreline offset and ovalisation was found to be of minor importance, if compared to that of the misalignment. By an extension of the procedure, scattering of the experimental data was also predicted in form of upper and lower quantiles, which also agreed well. [ABSTRACT FROM AUTHOR]

The installation of offshore wind turbines has achieved rapid and substantial progress worldwide and further increase is predicted by enhanced technologies that will reduce costs and increase service time. Secondary structures applied to the primary structure, as the transition piece of a monopole, can be e.g. cable support, boat landing or anode systems. These structures are often welded, which leads to problematic notch effects and hydrogen embrittlement, especially for underwater applications. Also the handling of technical equipment as power current or artificial housings for scuba divers for underwater welding is challenging. Adhesive bonding will lead to cost reduction as the mentioned negative aspects can be avoided. The corrosion protection coating and the primary structure will no longer be damaged and therefore do not need a subsequent coating. This article focuses on the area which is permanently exposed to water. A critical point is how the capability to form adhesion and cohesion, will be influenced by the application process under water. Therefore, stud bonding fasteners are designed that enable the injection of adhesive to the bonding area under water. The load capacities for different adhesives, surface pre-treatments and the degradation by exposure to artificial sea water were investigated. Adhesion was achieved with two different adhesives, which were able to cure and realize reasonable strength under water. Furthermore, two selected coating systems were able to improve the performance of the adhesive bond. [ABSTRACT FROM AUTHOR]

Previous research indicated that glued-in rods (GiR), an adhesively bonded joint in which metallic rods are glued into wood blocks, are an efficient method to connect or reinforce structures in timber engineering, and represent a success story for adhesive bonding in civil engineering. In literature, GiR are mostly manufactured under perfect conditions, regarding both surface states and geometry, with outermost care taken to avoid any deviation thereof. However, in reality GiR are typically manufactured by staff that received little to no training, in particular with regard to the peculiarities of adhesive bonding. Thus the very high standards that apply in other industries, as for example aerospace, are hard to achieve. Additional constraints result from the relatively narrow gap in which the adhesive is injected, and the difficulty to monitor its complete and correct filling. For the manufacturing of GiR-connections several different techniques are used, which were mostly validated empirically. The influence of different manufacturing methods and potential defects on joint performance was investigated. Additionally, this paper sheds additional light on the influence of different typical defects on the load capacity of GiR with steel rods. Several defects, such as corrosion, contamination by oil, dust, wood chips, sand and moisture were considered, and performance of joints manufactured thereof compared to joints without such defects. The results of the research demonstrate that defects on GiR result in a non-negligible reduction of the load capacity, but that this effect is somewhat smaller than expected. Accordingly, GiR connections might be considered as relatively robust with regard to manufacturing. [ABSTRACT FROM AUTHOR]

• With regard to the high quality standards for adhesive bonding, GiR have proven to be very robust. • Only voids had a great influence on the performance; dirt , oil , and rust had little to no. • The influence of the defect is relatively independent from that of the adhesive of wood. • GiR in beech LVL achieve higher load capacities compared to joints made of spruce GLT. • An Analysis of Variance (ANOVA) showed that the effect of most defects is not significant. Glued-in rods (GiR), which consist of rods glued into structural elements of wood, or engineered wood products, share the manufacturing quality concerns associated to adhesively bonded joints in general. Because of the difficulties associated with the bonding process, it is often argued within the timber engineering community that under the specific conditions prevailing in construction sites, it is not possible to meet the high-quality requirements for bonded joints typically encountered in other industries. The present study presents experimental evidence gathered on 240 individual probes in which the effect of several different types of defects (excessive wood moisture content, wood chips not removed from the borehole, simulated air inclusions by mixing polystyrene spheres, oil contaminated rods, and corroded rod) was investigated with threaded rods glued into beech spruce glued laminated timber (GLT) and beech laminated veneer lumber (LVL) using four adhesives (including 2K-epoxies and a 2K-polyurethane). Results were analysed by means of an analysis of variance (ANOVA). The findings showed that glued-in rods represent a relatively robust class of adhesively bonded joints, as the majority of defects types could not be distinguished on statistical ground from the reference sets, with the notable exception of air voids leading to a significant decrease of joint capacity. [ABSTRACT FROM AUTHOR]

• A joining concept combining adhesive bonding, welding and bolting is presented. • Climate data from platforms in the North and Baltic Seas are obtained and evaluated. • A real offshore jacket is numerically modelled in order to determine mechanical stresses. • Adhesives meeting the requirements are analyzed mechanically and thermo-mechanically. • The relationship between stress, temperature and adhesive properties is highlighted. Due to the advantageous wind conditions, many new structures are planned offshore. The construction of offshore structures with ever increasing sizes requires new concepts to cope with the particular requirements thereof. One often overlooked possibility is adhesive bonding as a structural joining method, which upholds significant optimisation potential. This paper, the first in a series of two, illustrates the potential of adhesive bonding on the widely used jacket structure. For that, the structural concept is adapted towards ease of manufacturing, hybrid joints are defined, adhesives are selected according to requirements resulting from structural and climatic constraints, structural verifications performed, and the validation therefor presented in preliminary small-scale tests—which will be upscaled in the second part of this series. This paper thus serves as a roadmap for practitioners, and outlines the various steps to evaluate the potential of adhesive bonding in complex structures. [ABSTRACT FROM AUTHOR]