Fasteners and Sealing
4 Structural adhesives are used to join a variety of similar and dissimilar materials effectively while achieving an optimal distribution of mechanical loads, stresses and vibrations. Hermann Handwerker reports.
4 Les adhésifs structuraux sont utilisés pour assembler efficacement une variété de matériaux similaires et différents tout en parvenant à une répartition optimale des charges, tensions, et vibrations mécaniques, déclare Hermann Handwerker.
4 Strukturkleber werden zur wirksamen Verbindung von gleichen und ungleichen Materialien verwendet und verteilen dabei die mechanischen Lasten, Spannungen und Schwingungen optimal. Hermann Handwerker berichtet.
Optimal bonding for industrial constructions
O
ver the past decades, structural adhesives have evolved into effective and dependable alternatives to conventional joining techniques such as bolting, riveting
and welding. They can be used to bond almost all materials commonly employed in industrial manufacturing. One essential aspect is selecting the right bonding solution for each particular construction. But which adhesive solutions are available and which factors must be considered in order to get the best results? The range of applications stretches from
bonding of single-use medical articles through to the assembly of heavy equipment. In order to ensure the structural strength of the bondlines, the structural design of the materials to be joined and the choice of adhesive technology must be very carefully evaluated.
Structural design considerations
The strength and long-term durability of a structural adhesive bond depend especially on the adhesive used, the materials involved, and the loading to which the construction will be exposed. The loads, in other words the external forces acting on a construction, can be classified according to five types of stresses: tensile, compressive, shear, peel and cleavage forces. For each type of loading, there is an adhesive technology which optimises the ability to withstand the forces acting on the component in question. Tensile force is the force acting on a bondline
when the component is pulled or stretched in a direction perpendicular to the bondline and the
substrates it joins. With a pure tensile load, the stress distribution within the bondline is relatively uniform. The same load acts on each part of the bond area. This is also true of compressive loads. A compressive force is an external force acting on a joint when the bonded materials are subjected to pressure. In practice, pure tensile or compressive loads are generally rare and usually also involve a degree of peel loading. In industrial constructions, the most common
loads are shear, peel and cleavage forces. A shear force is the force acting on a bondline when the two joined parts are pulled in parallel and opposite directions. Peel and cleavage forces are very similar and are the least desirable forces acting on a structural bondline. These two forces occur when a load is applied to the end of a bondline, being of a peel nature when at least one of the parts can be easily deformed.
Although shear loads are by far more desirable
than peel or cleavage loads, in all these three types of loading, the resulting stresses are distributed unevenly across the bondline. Adhesive bonds subjected to shear loading exhibit stress peaks at both ends of the bondline, while the stresses in the middle region are less intense. This load mode is probably the most frequent, especially in overlapped joints, which are the most common adhesively joined configurations. However, peel loads result in high stress concentration at just one end of the joint. If the adhesive starts to come away from the substrate at the edge of the bondline, small cracks will then propagate throughout the entire joint. An effective adhesive bond is therefore characterised by uniform stress distribution, which is achieved by
Fig. 1. Figs. 1 and 2. Tensile and compressive forces.
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Fig. 2.
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