In addition to the mechanical properties, the selection of adhesives is ruled by a lot of other factors, which, according to Ref , can be categorized in terms of product requirements, loads and actions, resistances related to aging, durability, and other factors.
Product requirements involve aspects regarding the type of substrate (steel, glass), thicknesses, tolerances, surfaces (contamination, grease, oil), coatings (screen print, zinc), pre-treatments (primer, activator), connection type (structural, dis-/continuous, sealing, assembling), production conditions (on-site, workshop, laboratory), application behavior (viscosity, pot life), and processing properties (curing mechanism, mixing).
Loads and actions can be static, strain-rate, or temperature dependent; creeping; and relaxation or dynamic. Another significant load is summarized by exposure classes, which include effects from climate change, weathering, corrosion, and ultraviolet (UV) light.
The resistance to aging and temperature is strongly influenced by durability aspects such as the projected life cycle, compatibility (polyvinyl butyral (PVB) foils), inspection, and reparability.
Other issues are economical (costs), architectural (color, visual attraction), or constructional ones that determine the adhesive selection.
Therefore, mechanical values are not always the only decisive selection criterion.
Concerning bonded hybrid beams, the adhesive selection firstly depends on the bonding geometry (Table 1, II), which has to ensure the bubble-free filling of joints and complete curing and guarantee protection from UV radiation and weathering, which would lead to a considerable strength decrease or loss of adhesion. Butt joints and connections with U-profiles seem to be the most promising in this regard. Additionally, the cured bonded joint has to meet static and constructional requirements such as the following:
• Load transfer of shear forces.
• Compensation of fabrication tolerances.
• Reduction of stress peaks.
• Compensation of constraint forces due to possible thermal expansion.
In particular, the adhesive thickness is one of the decisive factors for the
load bearing behavior and stiffness of the adhesive joint. In the past, numerous research projects revealed that every adhesive has an optimal thickness (for most adhesives it is between 0.05 and 0.2 mm) at which the load bearing behavior and carrying capacity are best . For the large-scale hybrid steel-glass beams developed within the INNOGLAST project, it was not possible to achieve adhesive thicknesses smaller than 3 mm without avoiding steel-glass contact due to the tolerances of the thin steel flanges and glass fins of at least 4 m in
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length. Smaller adhesive thicknesses of about 1 mm would increase the stiffness significantly and lead to higher ultimate loads and beam stiffnesses, but from a manufacturing point of view they are not achievable in practice. However, the main advantage of these high thicknesses is the high temperature compensation capability for temperature differences between steel and glass, even for very stiff adhesives. Therefore, in all cases an adhesive thickness of 3 mm was used.
Based on these considerations, seven cold-hardening (room temperature) adhesives were selected, most of them two-component structural adhesives: one two-component high-strength epoxy resin with high temperature resistance, four two-component polyurethanes with different strengths, one UV-curing acrylate, and, as a reference, a two-component silicone generally applied for structural sealant glazing. These adhesives were selected in such a way as to include both very stiff adhesive systems with high Young’s moduli and strength values and flexible adhesive systems with low strengths and stiffnesses. Adhesives with minor temperature resistance were generally avoided, as were adhesives with limited pot life or viscosity and those with a curing mechanism unsuitable for linear steel-glass connections, such as warm – or humidity-curing systems.
Beyond that, the choice of adhesives was made with regard to potential “inside" and “outside" application, which means the adhesives should withstand weathering, UV radiation, cleaning agents, and temperature changes without any relevant change of their mechanical properties or any loss of bearing capacity. Not all of the four joining geometries (Table 1, II) are well suited for each adhesive; thus the advantages and disadvantages must be checked for each application. In addition to the required bonding length, high workmanship and optical criteria are of particular importance.