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Glued-in rods (GiR) are a successful class of structura bonded joints for timber engineering applications. However, they suffer from two limitations: relatively slow curing processes, and requirements related to minimum temperatures for the bonding process. Both, to some extent, limit their wider application, in particular on site. Inductive heating was investigated as a method to overcome both drawbacks by speeding up the process, and enabling it under low temperatures. To verify that, a series of four commercially available adhesives was used to manufacture GiR with beech LVL and Ø16 threaded rods embedded by 100 mm, which were then cured under a variety of conditions. Firstly, cold-curing according to the prescriptions of the technical data sheets and inductively, both at +23°C; secondly, inductively at +5°C; and thirdly, at – 10°C. Numerical modelling complementing these studies allowed for insights related to temperature distributions and curing progress otherwise inaccessible through purely experimental means. The results of this study clearly show that, if selecting appropriate adhesives, curing of glued-in rods is not only accelerated from several days down to a few minutes, but that the process can also be carried at much lower temperatures than those prescribed by the technical datasheets. Based thereupon, a completely new range of practical applications, in which gluing of rods can potentially be made independent of ambient temperatures on-site, has been unleashed. [ABSTRACT FROM AUTHOR]

Accelerated curing of glued-in rods (GiR) has been demonstrated in recent papers to represent an interesting option to shorten dramatically the process time. It has been shown that once heated up within minutes, GiR require meaningfully longer to cool down; it is posited that the cooling phase significantly contributes to the curing of the adhesive. This hypothesis has been experimentally tested in the study presented herein on GiR consisting of M16-8.8 threaded steel rods bonded into blocks of laminated veneer lumber (LVL) with a cross section of 120 × 120 mm2 considering four different two-component adhesives (one polyurethane, and three epoxies). Accelerated curing was achieved by transient heating regimes, in which temperatures were raised to different peak temperatures, followed by their cooling. Results indicate that, depending upon the adhesive, transient heating has, for some adhesives, the potential to achieve joint capacities comparable to cold-cured reference GiR, but that is not true for, in particular, 2K polyurethanes. Comparisons between experimental data and results obtained through the characterisation of the curing kinetics pointed out the complex relationship between the development of joint strength and that of chemical conversion degrees. [ABSTRACT FROM AUTHOR]

Adhesive bonding is nowadays an integral part of timber engineering and sets the basis for efficient joint typologies, as well as for most Engineered Wood Products (EWP). Regarding the manufacturing of joints, in particular under on-site conditions, two-component (2K) adhesives, in particular epoxies (EPX) and polyurethanes (PUR), are preferred over one component (1K) adhesives, because of their ability to cure under ambient conditions. If compared to mechanical fastening, however, the relatively slow curing of most 2K-adhesives used in practice remains a serious disadvantage. This paper summarises investigations on induction heating as a way to speed up significantly the curing process of Glued-in Rods (GiR), a joint type used in timber engineering. Firstly, four commercially available adhesives were investigated with regard to their potential for accelerated curing, resulting in the determination of maximum curing temperatures, and heating rates. Based thereupon, a series of large-scale GiR, involving threaded rods and beech Laminated Veneer Lumber (LVL), were manufactured, and inductively cured. The curing time was dramatically shortened from days to 5 min. Resulting joint capacities depended upon the time elapsed since induction. The resulting findings served as a basis for subsequent investigations to be presented in the second part of this series Figure 16. [ABSTRACT FROM AUTHOR]

The curing behaviour of inductively heated glued-in rods (GiR) experimentally tested in Part I of this work was further investigated with regard to the kinetics of adhesive curing, and evolution of strength with time. For that, dynamic DSC measurements were performed, and appropriate kinetic models were derived. These investigations were complemented with rheological experiments in which the adhesive were subjected to the same thermal regime as in the inductive heating of the GiR. Curing kinetics of the GiR were then compared to the increase of strength with time. The comparison showed that curing progress, defined by conversion degree, does not result in proportional strength increase. It was then attempted to estimate the influence of potential residual stresses from the induction curing on joint capacity. A relatively simple FE model was thus derived, which considered two different loading cases: a tensile force acting on the rod, and a shrinkage resulting from the induction process. Their superposition allowed for a general description of the fact that joint capacities increased with time elapsed since induction heating. Depending upon the adhesive considered, fair to very good quality of prediction were achieved. [ABSTRACT FROM AUTHOR]

Glued-in rods (GiR) represent a very successful type of adhesively bonded joints in timber engineering. Despite their apparent geometrical simplicity, their dimensioning still challenges practitioners. A major source of mechanical complexity resides in the orthotropic nature of the wood, or engineered wood products, as laminated veneer lumber (LVL). This paper presents a relatively simple design approach based upon fracture mechanics (FM) and associated double-cantilever beam (DCB) tests that complemented tensile tests for material characterisation. In comparison with the state-of-the-art related to FM in timber engineering, the paper presents a practitioner oriented approach of a yet complex set of GiR geometries involving beech LVL (M16-8.8 threaded rods embedded in cross sections of 120 × 120 mm2 and embedment lengths of 96mm, 128mm and 160mm). The developed numerical model resulted in a good description of the load-displacement of a series of full scale, including very good estimates of their load capacities. Additionally, it allowed for significant insights regarding the complicated relationship between geometry, orthotropy, strength of the component and failure of the GiR, as for examples the complex fracture process, and the importance of transverse tensile stresses, which play a preponderant role, equal in importance to shear stresses. [ABSTRACT FROM AUTHOR]

Fatigue behaviour of materials is traditionally performed on probes that ensure that stresses (or strains) are very uniform. This is a situation seldom encountered in most adhesively bonded joints, where stresses usually peak at the end of the overlaps. This paper presents a relatively simple model to predict the fatigue behaviour of glued-in rods (GiR) involving hardwood in previously investigated in an extensive experimental campaign. The model is based on strength and stiffness degradation of the components of the GiR, which were experimentally estimated in small scale tests. Based thereupon, Finite Element Analysis (FEA) was used to estimate the effect of material degradation on the residual strength of the GiR, which was then interpreted as S-N-curves. The influence of several parameters, e.g. strength and stiffness degradation rates and the magnitude of the residual strength threshold were numerically investigated. The result showed that it is possible, using a practitioner adapted numerical model, to predict the fatigue behaviour of GiR, based upon comparatively simple fatigue characterisation on their components. [ABSTRACT FROM AUTHOR]

Glued-in Rods (GiR) are among the high-performance joining technologies used in timber engineering. Engineering design procedures for GiR almost exclusively regulate softwood and softwood engineered wood products (EWP) under quasi-static loads. Since the use of hardwood is expected to significantly increase due to climate change, and potentially establish itself as predominant in Europe's forests, GiR in combination with hardwood and corresponding EWP need to be investigated. Timber constructions subjected to cyclic loads are increasingly being used, as for example in timber bridges, high rise buildings, and wind turbine towers. No normative regulation is yet available for fatigue of GiR, which is for most part due to incomplete or completely missing experimental data. This paper is the first of a two part series; it presents fatigue investigations on GiR in combination with hardwood EWP with different types of rods, wood species and adhesives. Further, the embedment length and stress ratio were varried. In total, more than 70 cyclic tests were performed resulting in a high number of SN-curves showing the fatigue characteristics of GiR. The results show that two different damage mechanisms exist: in low cycle fatigue range (LCF), timber and adhesive fracture become dominant, while rod failure is the limiting factor in high cycle fatigue range (HCF). Based upon the documented experimental findings, existing standards for cyclic load on softwood connection technologies were discussed. The compagnion paper will present a design methodology based on a wood damage accumulation, giving a complete picture combined with existing metal fatigue models. [ABSTRACT FROM AUTHOR]

To manufacture multi-material hybrid parts for the automotive industry, mechanical joining processes like riveting are well established. However, adhesive bonding is increasingly being used in the car body to realize lightweight structures with high crash performance using a multi-material design. The combination of the specific advantages of both joining techniques in form of hybrid joints yields in synergies of manufacturing, crash, and durability performance. Critical issues when combining riveting with bonding are the formation of adhesive pockets in the region of the mechanical junctions, and global deformations of the metal blanks between the mechanical joints. Pockets in the region of the mechanical junctions decrease the strength of the mechanical joints. Global deformations caused by the formation of adhesives pockets while the multi-point mechanical fixation process takes place, influence the smoothness of the bonded part’s surface. This project focuses the numerical simulation of the time dependent formation process of both types of pockets with the aim to better understand the parameters affecting them, and to develop better processes. [ABSTRACT FROM PUBLISHER]

Timber engineering sets high expectations on adhesive bonding as a joining method to overcome a series of limitations related to partly obsolete traditional techniques. Research on adhesively bonded timber joints has proved their superiority over mechanical fasteners in terms of strength and stiffness, but this research was often limited to softwood. Despite its abundant availability in Central Europe and its high mechanical resistance, beech is only rarely considered as a structural material. Furthermore, research on adhesively bonded timber joints almost exclusively focused on tests at room temperature. Elevated temperatures, however, are critical in conjunction with adhesives, making it paramount to shed more light on that particular aspect. Based on experimental and numerical investigations, it was found that the capacity of adhesively bonded hardwood joints increased asymptotically with overlap length to a ceiling value; furthermore, it was concluded that temperature negatively impacts capacity. Glass transition temperature,Tg, marked a clear transition, but joints still sustained relatively high loads beyondTg. A probabilistic approach was validated and successfully applied to predict the joint capacity. The research contributes to fill knowledge gaps by offering the basis for subsequent dimensioning methods that at term will enable practitioners to design their structures accordingly. [ABSTRACT FROM PUBLISHER]