Reaction kinetics investigation in relation to the influence of humidity on fatigue behavior of wood lap joints

Abstract

It is generally assumed that the properties of wood against fatigue are good, but little is known about the properties of adhesively bonded wood, which represents today most of the wood-based products. Lap-shear samples glued with three common wood adhesives [two ductile one-component polyurethane (1C-PUR) systems and one brittle phenol resorcinol formaldehyde (PRF) adhesive] were tested under cyclical loads at three different climates [20°C, 35% − 50% − 85% relative humidity (RH)]. For the analysis of data, an empirical model based on reaction kinetics was developed. In addition, a probabilistic model was used to estimate the endurance limit and the expected run-out lifetime. Both models were combined to accurately model fatigue at high and low relative stress intensity. It was shown that ductile 1C-PUR adhesives perform better than the brittle adhesive system under dry climates (35%–50% RH). However, for higher RH, the brittle PRF adhesive showed better performance, most probably due to a better wood-adhesive adhesion in wet climate. An average endurance limit for tensile shear stresses between 20% and 48% of the mean tensile shear strength (TSS) was estimated for the tested adhesives. It was shown that the model parameters could be linked to fundamental physical constants through the reaction kinetics approach; however, further research is needed to correlate these parameters to specimen-specific quantities.

Quasi-static and fatigue performance of bonded acetylated rubberwood (Hevea brasiliensis, Müll. Arg.)

“…For the lowermost tested relative strength level of 0.3 (300 N), all samples exceeded the run-out limit, which had been set to 10 6 cycles. This large scattering for one relative strength value has already been observed in the few previous studies on dynamic loading of wood and bonded wood (Bachtiar et al 2017b;Clerc et al 2020).…”

“…More specifically, the fatigue behavior at low stress level should be studied in more detail with a larger number of samples to estimate more accurately the endurance limit and differentiate the effect of the acetylation for long loading time. In this matter, a probabilistic model such as described by Castillo and Fernandez-Canteli (2009) can be successfully used to estimate the long-term behavior, as shown by Clerc et al (2020) on glued shear samples.…”

“…Due to the low number of samples, it is difficult to describe the influence of the treatment at low force. Other models exist such as presented by Clerc et al (2020) but were not applied here due to the low number of samples and the large scatter in the data. However, comparing the two extremes (considering the run-outs as fractured samples and neglecting them for the analysis) shows the same ranking in the order of the simplified S-N curves.…”

Selected previous findings on the factors influencing the gluing quality of solid wood products in timber construction and possible developments: A review

“…The thickness of the glue line influences the strength of the joint (Kläusler 2014). Stiffer adhesives lead to a slightly increased wood failure rate (Kollmann 1955, Clauss 2011, Hass 2012, Kläusler et al 2013, Knorz et al 2016, Clerc et al 2020b. The adhesion behavior of the glue towards the wood surface depends on the wood species and its chemical-physical compatibility with the adhesive.…”

Wood Derived Cellulose Scaffolds—Processing and Mechanics

“…[ 59 ] Damping is characterized by the frequency‐dependent ratio of storage and loss modulus, while fatigue is determined by the number of cycles until failure at a number of different load levels relative to the (static) strength of wood. [ 60 ] By performing in situ DMA at different environmental conditions such as temperature, humidity, or pH, phase changes (glassy‐to rubbery‐transitions) can be made visible by significant changes of the ratio of storage to loss modulus. [ 61 ] In particular, the thermo‐mechanical behavior of wood is of practical relevance.…”