Measurement of moisture-related strain in bonded ash depending on adhesive type and glueline thickness

Journal Article (2016)
Author(s)

Markus Knorz (Technische Universität München)

Peter Niemz (ETH Zürich)

JWG Van de Kuilen (TU Delft - Steel & Composite Structures, Technische Universität München)

Research Group
Steel & Composite Structures
DOI related publication
https://doi.org/10.1515/hf-2014-0324
More Info
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Publication Year
2016
Language
English
Research Group
Steel & Composite Structures
Issue number
2
Volume number
70
Pages (from-to)
145-155

Abstract

Structural wood-adhesive bonds (WAB) have to be durable while subjected to considerable stresses caused by mechanical loads and moisture content changes. To better understand the moisture-related durability of WABs, knowledge is important of how moisture changes generate strain in the bond. In this paper, strain on end-grain surfaces of bonded ash specimens was analyzed by means of digital image correlation. Strains were generated by wood shrinkage, and the evaluation was focused on shear strain (SStr). The bond lines were studied depending on the adhesive type - phenol resorcinol formaldehyde (PRF), melamine urea formaldehyde (MUF), polyurethane (PUR), and emulsion polymer isocyanates (EPI). Moreover, three different glueline (GL) thicknesses of MUF were taken into consideration. Comparing the adhesive types, SStr distributions (SStrD) were strongly influenced by adhesive elasticity. MUF and PRF bonds were quite rigid and were associated with pronounced strain amplitudes in and close to the GL together with strain dissipation reaching deep in the wood. PUR and EPI adhesives were more elastic and therefore allowed for smoother strain transition showing less distinct strain peaks. GL thickness had significant impact on SStrD. A high strain level and direct strain transition between adherends was found for the 0.01 mm GL, whereas a pronounced strain decrease was observed in the 0.1 and 0.2 mm GLs. This indicates different stress levels in the wood-adhesive interface dependent on GL thickness.

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