Abstract
An approach combining maps of wood morphology and digital image correlation was developed to investigate the drying of Eucalyptus nitens wood. Maps of morphological features (vessel and ray distribution) and cell cross-section dimensions were acquired by confocal laser scanning microscopy. Shrinkage maps were generated using digital image correlation. There were statistically significant correlations between shrinkage/collapse and wood morphology at two levels. Firstly, there were positional relationships, with for example, both radial and tangential shrinkage increasing with increasing distance from vessel elements. Secondly, there were dimensional relationships, such as, cells with large perimeters (relative to their wall thickness) on average showing greater shrinkage. Generally, the positional relationships dominated the dimensional relationships. Detailed analysis over large areas allows for a fuller analysis of the interrelationship between wood morphology and drying shrinkage and collapse.
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Using Cell Cross-section Dimensions and Digital Image Correlation to Evaluate Drying Shrinkage and Collapse in Eucalyptus Nitens Wood
Alan Dickson * and Bernard Dawson
An approach combining maps of wood morphology and digital image correlation was developed to investigate the drying of Eucalyptus nitens wood. Maps of morphological features (vessel and ray distribution) and cell cross-section dimensions were acquired by confocal laser scanning microscopy. Shrinkage maps were generated using digital image correlation. There were statistically significant correlations between shrinkage/collapse and wood morphology at two levels. Firstly, there were positional relationships, with for example, both radial and tangential shrinkage increasing with increasing distance from vessel elements. Secondly, there were dimensional relationships, such as, cells with large perimeters (relative to their wall thickness) on average showing greater shrinkage. Generally, the positional relationships dominated the dimensional relationships. Detailed analysis over large areas allows for a fuller analysis of the interrelationship between wood morphology and drying shrinkage and collapse.
Keywords: Wood drying; Wood shrinkage and collapse; Digital image correlation; Wood morphology; Confocal microscopy; Eucalyptus nitens
Contact information: Scion, Te Papa Tipu Innovation Park, 49 Sala Street, Rotorua 3010, Private Bag 3020, Rotorua 3046, New Zealand; *Corresponding author: alan.dickson@scionresearch.com
INTRODUCTION
Upon drying, Eucalyptus nitens (Maiden) wood is prone to cracking and internal checking, limiting its utilisation for timber applications. This is due to high levels of differential cell collapse (deformation) and shrinkage (Blackburn et al. 2010). The role of wood structure in shrinkage and collapse has received extensive interest over the years, but the causality remains unclear. Several studies on Eucalytus point to a relationship between high levels of checking and low wood density (McKenzie et al. 2003; Rebolledo et al. 2013), but the correlations are generally low and may be related to a higher frequency of vessels (Rebolledo et al. 2013). Checking (Lausberg et al. 1995; Shelbourne et al. 2002) and collapse (Ananías et al. 2014) in E. nitens can be more prevalent in the middle growth rings. However, McKenzie et al. (2003) saw no significant relationship between checking and ring number or cell microfibril angle or cell dimensions. Likewise, they saw no relationship between density (between earlywood and latewood) and collapse (McKenzie et al. 2003). According to Kube and Raymond (2005) tree diameter and basic density were the best predictors of collapse but only explained 31% of it.
Wood rays are often linked with collapse. Wu et al. (2006) showed a positive correlation between residual collapse (collapse above the fibre saturation point) and the proportion of ray material. They hypothesized that increased proportions of ray parenchyma, in species susceptible to collapse, may make a greater contribution to the generation of collapse than other anatomical features and would be regarded as one of the main indicators of cell collapse intensity. According to Bariska (1992), collapsed fibres were generally found in contact with rays (or in rays) and not with vessels.
Most work investigating drying/shrinkage/swelling in relationship to wood morphology has been on softwoods with a focus on earlywood and latewood. Earlywood and latewood respond differently during drying (and swelling) with latewood tending to show the greatest overall response. Earlywood shrinkage is generally anisotropic, with tangential much greater than radial shrinkage. Latewood shrinkage is generally more isotropic (Boutelje 1962; Perré and Hubber 2007; Derome et al. 2011, 2013; Almeida et al. 2014; Lanvermann et al. 2014; Perré et al. 2016). There is also an interaction between earlywood and latewood affecting total shrinkage behaviour (Murata et al. 2001; Patera et al. 2018). The two tissues have a restraining influence on each other, making the combined shrinkage more isotropic (Patera et al. 2018). This appears to be a more complex relationship than where tangential shrinkage remains approximately constant from earlywood to latewood and radial shrinkage increases from earlywood to latewood (Perré et al. 2007; Almeida et al. 2014).
Variation in collapse between latewood and earlywood is often cited as a cause of internal checking and/or washboarding in sawn eucalypt timber (Hamilton et al. 2009). In E. nitens the checks are primarily located in the earlywood, have a lenticular shape, and are oriented in the radial direction (Rebolledo et al. 2013). In Eucalyptus regnans, earlywood is generally more collapse-susceptible than latewood. This is because earlywood fibres often have similar external diameters to latewood fibres but are generally thinner-walled. Thus, latewood fibres are more resistant to internal water tension than earlywood fibres and cell collapse occurs when cell walls are not strong enough to withstand the greater internal tension (Innes 1996). Han et al. (2016) used digital image correlation (DIC) to investigate moisture related shrinkage. They found slightly higher shrinkage values for latewood than earlywood but both had the expected trend of tangential shrinkage being greater than radial.
Numerous studies have applied DIC to wood drying and swelling, whether at the tissue level (earlywood and latewood) (Derome et al. 2013; Lanvermann et al. 2014; Han et al. 2016) or over larger areas (Bigorgne et al. 2011; Kang et al. 2016; Lee and Jeong 2018). With the exception of the work in oak by Badel et al. (2001, 2006), little of the DIC related literature deals with the anatomy of hardwoods in relationship to wood drying. In a study looking at shrinkage in oak wood, both the properties of individual cells, and the gross wood morphology, were needed to predict shrinkage (Badel et al. 2007). It was noted that some of the deviations from the predicted model behaviour were due to the presence of cell collapse and reaction tissue.
Most previous studies have either dealt with gross variations in wood morphology over large regions, or detailed analysis over very small regions. This study presents the first steps to bridge the gap between these two scales by combining cell and tissue level observations. It is hypothesised that the exposed cell cross-section dimensions will show some relationship with the drying deformation of the bulk wood underneath and that local drying responses (small regions) will be influenced by more global responses (larger regions). These relationships were investigated to determine their relative influences and to inform analyses over larger size scales. Ultimately, this approach can be used to understand the relationship between drying deformation at the cell level and wood anatomy/morphology at macro scales up to several centimetres or greater.
EXPERIMENTAL
Overview
Confocal laser microscopy was used to image part of the transverse surface of E. nitens wood (12 × 12 × 12 mm cubes) before and after drying. Images were stitched together and covered an area > 3 mm2. DIC was applied to the resultant images to obtain maps of shrinkage. Image analysis was used to obtain maps of the cell dimensions. Statistical correlation analysis was used to analyse the relationship between shrinkage and cell dimensions (Fig. 1).
Preparation of Samples
Two cubes of 12 × 12 × 12 mm were cut from the same piece of green (wet), approximately 14 year old E. nitens heartwood. They came from adjacent positions in the height dimension of the tree (the top surface of one was close to the bottom surface of the other) (Fig. 2). The transverse surfaces were prepared with a Leica SM2010R sliding microtome (Leica Biosystems, Nussloch, Germany). A central region of 4 × 4 mm was marked-out on one transverse surface and by following the grain on the radial and tangential surfaces. The equivalent region was marked out on the other transverse surface. These central regions were imaged and did not contain growth rings.
Drying
The air-dried sample was kept in a controlled temperature (25 °C) and humidity (65% relative humidity) for 3 weeks. The oven-dried sample was in an oven at 105 °C for 2 days.
Fig. 1. Overview of approach taken
Imaging
To avoid drying during imaging, the wet cubes were immersed in water (in a small container) to just below the surface to be imaged. A few drops of 0.0002% safranin stain was added to aid imaging. A no. 1.5 cover glass was on the surface of the cube. A Leica SP5 II confocal laser scanning microscope (CLSM) (Leica Microsystems, Mannheim, Germany), a 20× objective lens and 2× zoom (2048 × 2048 pixels, giving a pixel size of 0.189 µm) were used. Type F immersion oil (Leica Biosystem, Wetzler, Germany) was used (between cover glass and lens). The excitation wavelength was 488 nm, and emission was detected in the 500 to 700 nm range.