Characterisation of cubic oak specimens from the Vasa ship and recent wood by means of quasi-static loading and resonance ultrasound spectroscopy (RUS)

Abstract

The cylindrical orthotropy, inherent timedependency response, and variation between and within samples make the stiffness characterisation of wood more challenging than most other structural materials. The purpose of the present study is to compare static loading with resonant ultrasound spectroscopy (RUS) and to investigate how to combine the advantages of each of these two methods to improve the estimation of the full set of elastic parameters of a unique sample. The behavior of wood as an orthotropic mechanical material was quantified by elastic engineering parameters, i.e. Poisson's ratios and Young's and shear moduli. Recent and waterlogged archaeological oak impregnated with polyethylene glycol (PEG) from the Vasa warship built in 1628 was in focus. The experimental results were compared, and the difference between RUS and static loading was studied. This study contributes additional information on the influence of PEG and degradation on the elastic engineering parameters of wood. Finally, the shear moduli and Poisson's ratios were experimentally determined for Vasa archaeological oak for the first time.

Elastic characterization of wood by Resonant Ultrasound Spectroscopy (RUS): a comprehensive study

“…In particular, this will allow the definition of an error function to be minimized during the inversion process. Scanning Laser Vibrometer (SLDV), and/or holographic techniques, can be used to experimentally determine the sample modes shapes (Vorobyev et al 2016). In this paper, it is proposed to use the following steps for the identification:…”

“…Ideally, using only one sample is preferable and allows studying the natural variability. In this context, quasi-static measurements on a single sample of wood have been taken (Bruno and Poggialini 2005;Bruno et al 2008;Majano-Majano et al 2012;Seichepine 1980;Vorobyev et al 2016). This method can be accurate for determining the Young's moduli and some of the Poisson's ratios, but is not so suitable for the determination of the shear moduli on a heterogenous material like wood, due mainly to a non-homogeneous stress field, and it is time-consuming (Hassel et al 2009).…”

“…In this cited work, the elastic tensor of a cubic sample of beech (Fagus sylvatica L.) was estimated by minimizing the difference between measured and simulated resonant frequencies. This method has been extended and compared to quasi-static measurements in Vorobyev et al (2016) on cubic samples of oak.…”

The influence of chemical degradation and polyethylene glycol on moisture-dependent cell wall properties of archeological wooden objects: a case study of the Vasa shipwreck

“…Decreased macroscopic stiffness upon impregnation with PEG has been reported previously by Bjurhager et al (2010). A recent study by Vorobyev et al (2016) investigated the combined effect of aging and impregnation of PEG in Vasa oak on the macroscopic elastic properties as compared with the case of recent oak and found that all Young's moduli and shear moduli along the principal material axes were lower for the Vasa material. For wood with comparable densities and microfibril angles, cell wall softening correlates with an associated softening of the wood on the macroscopic scale (e.g., Gibson et al 2010).…”

“…This procedure aimed at replacing the water in the wood cell walls by PEG and thus preventing shrinkage and formation of cracks during drying (Håfors 2001). Since salt precipitation on numerous surfaces on the ship had been reported (Sandström et al 2002), a series of investigations of the chemistry (Glastrup et al 2006;Almkvist and Persson 2008a, b;Lindfors et al 2008;Almkvist et al 2016) and also of the mechanical properties of Vasa oak wood (Ljungdahl 2006;Bjurhager et al 2012;Vorobyev et al 2016) have been performed over the last years. These research activities showed that the oak wood is affected by degradation, i.e., by depolymerization of polysaccharides.…”