Search

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]

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]

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]

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]

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]

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]