Elastic constants of green Pinus radiata wood
 Nicholas T. Davies^{1}Email author,
 Clemens M. Altaner^{1} and
 Luis A. Apiolaza^{1}
DOI: 10.1186/s404900160075x
© The Author(s). 2016
Received: 21 January 2016
Accepted: 27 September 2016
Published: 16 October 2016
Abstract
Background
Mathematical modelling is often used to investigate phenomena difficult or impossible to measure experimentally.
Findings
This paper presents the constants needed to mathematically model green Pinus radiata D.Don core and outerwood. The constants include all three elastic and shear moduli along with the six Poisson ratios needed for describing orthotropic materials in the elastic domain. Further proportional limit surfaces are presented.
Conclusions
The constants provided allow for an increase in realism of mathematical models examining the mechanical performance of standing trees.
Keywords
Corewood Elastic modulus Neverdried Poisson ratio Shear modulus StrengthFindings
Introduction
Mathematical modelling is increasingly being used to investigate tree biomechanics (Ciftci et al. 2013; Coutand et al. 2011; Fourcaud et al. 2002; Fourcaud and Lac 2002; Guillon et al. 2012; Moore and Maguire 2008; Ormarsson et al. 2010; Sellier et al. 2006; Wagner et al. 2012). Such models need to be parameterised with material constants expressing the elastic behaviour as well as failure characteristics. Wood is often described as an orthotropic material, implying independent material properties for the radial, tangential and longitudinal direction (Bodig and Jayne 1982). Nine independent material constants are required to describe the elastic behaviour of orthotropic materials, including elastic moduli, shear moduli and Poisson ratios.
The failure characteristics of wood are complex. A simple way to express the failure characteristics is proportional limits. These are defined as the point at which stress and strain stop being linearly related. Proportional limit surfaces can be used to describe the proportional limit when a material is deformed in multiple directions at once. This is different to ultimate tensile failure which is the point when catastrophic failure occurs within a sample resulting in two separate pieces. Wood is also a hygroscopic material, and its mechanical behaviour is influenced by changes in moisture content (Skaar 1988). As wood is usually used in a dry state, most published material constants for wood are for the dry state (e.g. Ross 2010) which are a poor representation when modelling live trees.
A further complication is that wood as a biomaterial varies greatly in its properties even within a tree. Radial changes in wood properties within trees are well documented (Lachenbruch et al. 2011). Also, most research on these radial patterns within stems focused on longitudinal material properties, neglecting material properties in transverse direction. These radial gradients are particularly important when considering younger trees, for example from commercial fastgrowing, shortrotation plantations (Zobel and Sprague 1998).
Full sets of material constants for green wood are hard to find. A number of full or near full sets of elastic constants for wood above fibre saturation, along with other texts of interest, are provided in Additional file 1. To the best of our knowledge, no full sets of material constants have been published for green corewood.
The constants used to parameterise mathematical tree models are generally estimated from generic relationships to known properties, typically in longitudinal direction and below fibre saturation point due to a lack of empirical values. The purpose of this paper is to provide the material constants necessary for describing green core and outerwood as an orthotropic material. Tree stems can be modelled more realistically with the provided data. The study used Pinus radiata D.Don selected from plantations in Canterbury, New Zealand.
Materials and method
Green P. radiata boards cut from trees approximately 28 years old were sourced from a sawmill in Canterbury, New Zealand. Four boards (50 × 100 × 2400 mm), two outerwood and two corewood, were chosen from approximately 30 boards to represent high and low green density and dynamic modulus. Corewood (found adjacent to the pith) was defined as wood with widely spaced rings and high ring curvature, and outerwood (found adjacent to the bark) as the opposite (Burdon et al. 2004). No distinction could be made regarding the tree(s) the boards came from nor heights of the samples in the trees.
The samples were stored in sealed bags with excess water at 5 °C for a maximum period of 2 weeks. Subsequently, six defectfree, straightgrained test specimens of the required shape and orientation were machined from each of the four boards, totalling in 216 (6 replicates × 4 boards × 3 orientations × 3 tests) test specimens. All testing was conducted at room temperature and humidity over the 2week period. Prior to mechanical testing, each specimen was weighed and its volume measured by water displacement. Acoustic velocity was measured using TREETAP, a prototype nondestructive direct transmission timeofflight acoustic tool developed at the University of Canterbury.
The specimens were placed into a universal testing machine (UTM) (Instron 5566). Displacement was tracked on the two faces perpendicular to the axis of loading with two digital cameras fitted with 60mm macro lenses taking images every 3 s.
The UTM fitted with a load cell (0.1N accuracy) was run at a constant velocity of 1.5 mm min^{−1} for 300 s. For compression testing, the samples with dimensions of 30 × 15 × 15 mm were used, with the 30mm direction under load. Tension testing used boneshaped samples with dimensions 50 × 25 × 6.5 mm with the breakage plane having an area of 6.5 × 6.5 mm. With the exception of the longitudinal direction, where a straight stick test of dimensions 100 × 6.5 × 6.5 mm was used, with the 100mm direction under load. For the shear plane, Lshaped samples with a shear plane of 15 × 15 mm were used. Once the specimens had been mechanically tested, they were ovendried at 104 °C, weighed and volumemeasured using the displacement method. The microfibril angle (MFA) and standard deviation of the microfibrils for one specimen of each sample and test (i.e. 12 in total) were obtained using Xray diffraction (Cave and Robinson 1998a, 1998b).
The Tsai and Wu (1971) failure criterion provides a general theory of strength for anisotropic materials. Wood has been reported to behave differently under compression and tension (Bodig and Jayne 1982; Ozyhar et al. 2013), so separation of the two load types is needed for the strength criteria. This theory has been applied to wood and wood products previously (MackenzieHelnwein et al. 2005; MackenzieHelnwein et al. 2005).
Proportional limit surfaces were calculated using Eqs. 3–10 in two dimensions.
Ultimate failure is difficult to define in the compression case, so this criterion is reported only for the tensile tests. In the tensile direction, ultimate failure results in a sudden drop in stress when the sample breaks.
Results and discussion
Descriptive wood properties of the samples tested
Property  Stiff outerwood  Nonstiff outerwood  Stiff corewood  Nonstiff corewood 

Green density (kg m^{−3})  1143 (3)  1099 (9)  933 (21)  818 (22) 
Dry density (kg m^{−3})  531 (5)  458 (9)  438 (8)  393 (5) 
Acoustic velocity (m s^{−1})  4651 (8)  4221 (26)  4191 (34)  3413 (14) 
Number of rings  >20  >20  <10  <10 
MFA (degrees)  7 (1)  8 (1)  9 (2)  21 (1) 
Standard deviation of MFA (degrees)  9 (1)  12 (1)  11 (1)  12 (0) 
Variation in the measurements represents not only experimental error but also the underlying inhomogeneity of wood. The confounding effect between board and wood type should be noted as only a single board was chosen to represent each wood type. It is not possible to separate these two effects without a larger study. Additional file 2 provides the mean and standard error of elastic (in both tension and compression tests) and shear moduli. Additional file 3 provides the mean and standard error of the Poisson ratios, from both tensile and compression testing.
Elastic moduli and Poisson ratios for green core and outerwood Pinus radiata
Direction  Stiff outerwood  Nonstiff outerwood  Stiff corewood  Nonstiff corewood 

E _{ r } (GPa)  0.49  0.3  0.26  0.31 
E _{ t } (GPa)  0.25  0.19  0.24  0.17 
E _{ l } (GPa)  4.36  2.81  3.5  2.38 
TL (GPa)  0.11  0.21  0.11  0.13 
LR (GPa)  0.06  0.03  0.04  0.03 
RT (GPa)  0.05  0.02  0.02  0.04 
v _{rt}  0.64  0.54  0.60  0.77 
v _{rl}  0.03  0.05  0.03  0.06 
v _{tr}  0.33  0.33  0.55  0.42 
v _{tl}  0.03  0.01  0.03  0.04 
v _{lr}  0.29  0.47  0.36  0.44 
v _{lt}  0.60  0.16  0.37  0.50 
Mean proportional limits for green core and outerwood of Pinus radiata
Direction  Stiff outerwood (MPa)  Nonstiff outerwood (MPa)  Stiff corewood (MPa)  Nonstiff corewood (MPa) 

rt_{ t }  0.8 (0.2)  1.3 (0.5)  1.2 (0.3)  0.9 (0.1) 
rt_{ c }  −3.2 (0.1)  −2.5 (0.2)  −2.1 (0.1)  −3.2 (0.3) 
rl_{ t }  1.0 (0.1)  0.8 (0.2)  1.7 (0.4)  0.5 (0.1) 
rl_{ c }  −3.3 (0.2)  −2.6 (0.1)  −2.3 (0.1)  −3.3 (0.2) 
tr_{ t }  0.8 (0.2)  0.7 (0.2)  0.9 (0.1)  0.6 (0.1) 
tr_{ c }  −0.27 (0.1)  −2.4 (0.3)  −1.8 (0.2)  −2.0 (0.2) 
tl_{ t }  1.0 (0.3)  0.5 (0.1)  0.8 (0.2)  0.8 (0.2) 
tl_{ c }  −2.6 (0.2)  −2.8 (0.2)  −2.0 (0.1)  −2.3 (0.3) 
lr_{ t }  46.0 (7.5)  26.0 (1.3)  7.6 (1.8)  21.8 (4.1) 
lr_{ c }  −17.4 (1.0)  −6.6 (2.9)  −14.0 (1.1)  −17.6 (1.9) 
lt_{ t }  35.3 (3.8)  27.0 (4.5)  9.6 (2.4)  20.6 (2.4) 
lt_{ c }  −15.5 (1.5)  −8.5 (3.5)  −13.6 (1.6)  −21.4 (2.8) 
TL  2.2 (0.4)  4.5 (0.3)  2.7 (0.2)  3.2 (0.3) 
LR  1.8 (0.1)  0.9 (0.2)  1.6 (0.3)  1.3 (0.1) 
RT  1.1 (0.2)  0.6 (0.2)  0.7 (0.1)  0.9 (0.1) 
Mean ultimate tensile strength of green core and outerwood of Pinus radiata
Direction  Stiff outerwood (MPa)  Nonstiff outerwood (MPa)  Stiff corewood (MPa)  Nonstiff corewood (GPa) 

rt  3.1 (0.3)  3.1 (0.3)  2.9 (0.2)  2.2 (0.2) 
rl  3.1 (0.3)  3.0 (0.3)  2.8 (0.3)  2.2 (0.2) 
tr  2.1 (0.4)  1.6 (0.3)  1.9 (0.1)  2.1 (0.2) 
tl  2.1 (0.5)  1.4 (0.3)  1.9 (0.1)  2.1 (0.2) 
lr  64 (6)  45 (5)  21 (2)  33 (3) 
lt  65 (6)  44 (6)  20 (2)  33 (3) 
The results showed that it can be important to consider the differences between corewood and outerwood when modelling the mechanical behaviour of trees. The presented data will allow researchers to build more realistic biomechanical models of trees.
Conclusion
Different material properties should be considered when mathematically treating tree stems as engineered structures because of the mechanical differences between core and outerwood. Wood properties should be derived from samples in the green state that most closely represents the living tissue. Here, the nine elastic constants needed to represent an orthotropic material are presented for green core and outerwood P. radiata. Green core and outerwood reach limits of proportionality at different strains in different directions. Proportionality limit surfaces are also presented.
Abbreviations
 MFA:

Microfibril angle
 UTM:

Universal testing machine
Declarations
Acknowledgements
The authors wish to thank New Zealand Foundation for Research, Science and Technology Compromised Wood (P2080) Programme for financial support.
Authors’ contributions
ND designed and carried out the experimental work, data analysis and paper preparation. CA contributed to experimental design and paper preparation. LA contributed to data analysis. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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