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Glued-in Rods (GiR) are nowadays widely established as an adhesively bonded connection type in timber engineering. The assemblies consist of rods, e.g. threaded rods or rebars, made of steel, or Fibre-Reinforced Polymers (FRP), which are bonded into wooden construction elements. In contrast to mechanical fastening, adhesive bonding often results in a better load transfer between the adherends, and consequently a reduction of local stress peaks in comparison to mechanical fasteners. Bonding of GiR is typically performed with 2 K polyurethanes (2 K-PUR) or 2 K epoxies (2 K-EPX), for which full curing can last up to several days. Consequently, before the connection can be stressed, or moved for further processing, components have to be maintained in fixed position. This process can be very time, and cost, consuming for contractors. In order to create a speedier, thus more efficient, bonding process, the present study investigated accelerated curing of large-scale GiR specimens with the help of inductively heated Curie particles (CP). The CP are added to the adhesives, and exposed to a high frequency (HF) electromagnetic field (EMF). The EMF heats the mix up until the Curie temperature (TC) is reached, at which point the heating automatically stops. Due to the generated heat, polymerisation is significantly accelerated in a process independent of external monitoring techniques that prevents adhesive overheating. In order to design this practitioner-friendly and controllable manufacturing process, kinetic models were developed, allowing the prediction of the degree of cure, α, in dependency of CP content. The kinetic models were applied to experimentally determined temperature profiles during inductive heating of large-scale GiR specimens, considering three commercially available 2 K-EPX, and one 2 K-PUR, all of which are widely used. The kinetic modelling allowed for a much better interpretation of the heating behaviour, highlighting, in particular, the contribution of enthalpy for adhesive curing, and by extension of the whole induction process. [ABSTRACT FROM AUTHOR]

Induction heating of timber/glass-joints at the level of lap-shear specimens is used to accelerate curing of a silane-terminated epoxy. It was shown that handling strength could be achieved within only 10 minutes of heating, instead of hours or days. This finding identifies inductive heating as a promising method for accelerated curing in the construction industry. The correlation of heat development with mixing ratio of particles was shown. This allows controlled post-curing until final strength is achieved. Fracture patterns were investigated using various surface analytical methods. The results showed almost no differences between induction cured and room temperature curing of the adhesives, regardless of the systems having different chemical composition due to the filler material. However, the results of the XPS have shown that in both cases, inductively cured samples and samples cured at room temperature, the added particles have good adhesion to the adhesive itself and therefore do not weaken the bond when it comes to mechanical testing. [ABSTRACT FROM AUTHOR]

Glued-in Rods (GiR) connections are widely used in timber engineering as a connector between individual load-bearing members. Typically, GiR consist of a rod, e.g. rebars or threaded rods, glued into timber-based elements. However, rods made of new materials, as Glass-Fibre Reinforced Polymers (G-FRP) are increasingly being used due to their advantageous material properties like high tensile strength and good resistance against corrosion or chemical media. The cure of usually used and commercially available adhesives for GiR applications, mostly cold-curing 2 K-epoxies (2 K-EPX) and 2 K-polyurethanes (2 K-PUR), is time consuming. After adhesive application, the components must be fixed and maintained in position from several hours to days before the connection can be stressed. This process, depending on the connection type, can be significantly time, and cost, consuming for contractors, which makes the process difficult on-site. In order to overcome these restrictions, the present study investigated accelerated curing with the help of Curie-Particles (CP) exposed to a high-frequency (HF) alternating electromagnetic field (EMF). The CP are added to the 2 K adhesives, the resulting mix is then exposed to the EMF, generating thermal energy, which in turn accelerates the chemical reaction and associated adhesive curing. Due to the very nature of the CP, heating proceeds until their Curie temperature is reached, at which point the heating automatically stops. As a result, accelerated curing is achieved independently of external monitoring. In the present study, G-FRP rods were glued into blocks of spruce Glued Laminated Timber (GLT), and inductively cured through the addition of 33.3 w/w-% of CP. In total, four commercially available adhesives (three 2 K-EPX, and one 2 K-PUR) were investigated. The authors demonstrated on a total of 67 large-scale GiR specimens that inductively heated specimens achieved almost identical failure loads, and fracture patterns (wood failure), compared to cold-cured references accompanied by a reduction of curing time from as long as 1–10 days down to 10 min. [ABSTRACT FROM AUTHOR]

The constantly growing demand for cost-intensive lightweight structures is accompanied by an increased demand for reliable non-destructive testing (NDT) methods. Previous investigations have shown that Lamb Waves (LW) are very well suited for the use in Structural Health Monitoring (SHM) due to their relatively undamped propagation over long distances. Therefore the numerical modelling of a non-contact and automated system for non-destructive testing of C-FRP laminates was investigated in the present study. The method is based on the propagation of LW, which are excited by an actuator-driven piezo transducer and interact with defects in the structure. In contrast to classical Structural Health Monitoring, a scanning inspection system is used instead of a sensor permanently integrated into the structure to detect the measurement signals. For the representation of the wave propagation a three-dimensional simulation model with the commercial FEM software PZFlex was modelled. The wave interactions in defective C-FRP coupons are investigated and visualized through different output formats like A-, B- and C-Scans. Additionally the model allows the determination of the dispersion characteristics of the LW in the examined laminate. The validation of the numerical results was successfully carried out at different levels with the help of experimental investigations. [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]