Experimental investigation of the mode I fracture toughness behaviour of timber adhesive joints: The synergistic effect of the adhesive type and the bondline thickness

Abstract

Timber is a construction material widely used for structural applications. In the last decades, the improvements of both the bonding techniques and the quality of the adhesives enabled to overcome the intrinsic limitations of sawn timber, resulting in a wide spread of engineered wood products (EWPs). Understanding and assessing the bonding quality of structural EWPs has become nowadays a crucial point to assure their load-bearing capacity and even to increase their performance. One of the most revealing properties to assess the bonding quality and behavior is the fracture energy (or toughness), which could be estimated by performing suitable tests. In the present work, the results of an experimental campaign aimed to define the mode I fracture toughness of timber-to-timber adhesive bonding are presented and discussed. The fracture toughness behavior of three different categories of adhesives for structural applications was investigated, namely melamine-urea-formaldehyde (MUF), polyurethane (PU) and phenol-resorcinol-formaldehyde (PRF). In addition, the effect of the adhesive bondline and precrack thickness (0.15 and 0.30 mm) was also investigated. The results revealed different fracture toughness behaviour between the various adhesives and bond line thickness, in terms of both crack initiation and propagation. In fact, the mode-I fracture toughness appears to be highly dependent on the adhesive type, as also seen by the related analysis of variance. The fractured surfaces are also characterized by means of optical microscopy, giving insights about the local failure modes.