Growth strain is a recent addition to wood properties of interest to breeders. The splitting tests are the fastest methods for testing for growth strain, which is commonly believed to be the driver for deformation during green sawing. Unfortunately they are destructive tests, making directly testing their precision impossible . Here a method using multiple testing scenarios and a minimization scheme was used to estimate the precision of a single splitting test measurement to be \(\pm 1006\) micro-strain. It is suggested that the resulting interval makes the splitting tests only suitable for removing very poorly performing individuals to make future breeding trials more efficient.
Introduction
In recent years wood quality parameters have started to be included
in tree breeding programs \cite{Davies2017,Chauhan_2013,Apiolaza_2011}. Particular wood properties provide
advantages for different applications of timber. For example, high
stiffness timber is sought after for structural uses, and a premium is
paid. Growth-strain has previously been identified as a
wood quality parameter which reduces the value of some Eucalypt species
\cite{Yamamoto2007,Chauhan2010}. Some timber species, particularly hardwood such as Eucalypts have
a tendency to produce high internal strains during growth, when these
strains are released during processing, the cut boards deform resulting
in an excessive value recovery loss as they must be re-sawn to gain
straight boards. For more details on growth-strain refer to \citet{Alm_ras_2016} and for an older but comprehensive review see \citet{Kubler1987}.
Until recently the quickest growth strain test
required approximately half an hour and substantially sized trees rendering growth-strain too time consuming and expensive to incorporate as part of a breeding trial particularly within an early selection trial with small trees. \citet{jacobs1945} developed the paring test (splitting stems down the pith and measuring the movement of each side) and reported results from Eucalyptus gigantea along with a number of other species. However he did not take the final steps of calculus required to convert his deformation measurements to strains. \citet{Chauhan2010}
developed the splitting test which (although the paper makes no mention of Jacobs) uses the same method with a number of assumptions to complete the calculus required to estimate surface strain from the paring test. The updated pairing test (now refereed to as the splitting test) substantially reduced the time and
cost involved in measuring growth-strain, this method was further
refined for production breeding trial assessment and a pilot study conducted by \citet{Davies2017}, who showed it had potential for suitably large scale
trials on Eucalypts. The splitting test essentially involves cutting a stem longitudinally along the pith and measuring the opening, along with diameter and cut length a numerical value related to the strain in the sample can be obtained.
\citet{Entwistle_2014} calculated the theoretical values of strain lost due to kerf, these become negligible with stems of significant size. \cite{Chauhan_2010} took strain gauge measurements at the surface of the stem and used them to predict the test opening, which showed a reasonable correlation on large stems. The results of this analysis were misleading for two reasons; one, the strain gauges were placed on on the surface perpendicular to the cut, and therefore only indicate the reliability of the splitting test to produce results when measured in such a way as to maximize the correlation and two, were conducted on large stems which was not the intended use for the test (very early selection). Larger diameter stems (anecdotally) exhibit similar growth strain variation by angle as smaller stems, as a result higher correlations can likely be created when tested in this particular way on larger stems, as there is a higher margin for error with gauge placement. \citet{Cramer2018} analysed Eucalyptus nitens stems with a diameter ranging from 58 to 120mm under the same method as \citet{Chauhan_2010} and found correlation between strain gauge surface strain and splitting test predicted surface strain of approximately 0.27 compared to \citet{Chauhan_2010} reporting of 0.92 on Eucalyptus nitens stems with diamters between 111 and 192mm. It is suspected that one of the reasons for this marked difference in correlation between \citet{Cramer2018} and \citet{Chauhan2010} is due to the substantially smaller openings observed by \citet{Cramer2018} due to the smaller tree size, which are similar to the tree size used in this study. It has been well reported that surface growth strain can vary markedly over small areas of stem \cite{Okuyama_1994,Saurat_1976}. The de-facto standard procedure for measuring surface strain on logs with CIRAD or strain gauge methods is to take 8 measurements around the perimeter \cite{cirad} this is to account for the radial variation which can occur over small distances on the log surface \cite{Fournier_1994}. The major trade-off with the speed of the splitting test is the resolution which can be achieved, surface strains near the cut are suppressed due to the geometry of the test, while the strains perpendicular to the cut are primarily responsible for the opening. Hence placing one strain gauge on each side, perpendicular to the plane of cut of the splitting test produces a misleading assessment of how accurately the splitting test predicts the average growth strain of the sample.
It should be noted that the opening in the test is not solely caused by the longitudinal strain, tangential and radial strains also likely play a role. The way longitudinal, tangential and radial strains develop and interact is still unknown, with no work (to our knowledge) having been conducted to understand the underlying mechanical relationship, past work such as \cite{Chauhan2010} have reported correlations obtained from strain gauges. Grain angle with respect to the longitudinal cut direction likely influences the results of both the splitting and strain gauge tests, however no known research has investigated this.
The splitting tests are used within breeding programs \cite{Davies2017} in order to gain a quantitative measure of how much the stem would deform during green sawing. The primary objective of this paper is to present a method (and results from) for estimating how reliable splitting test measurements are when predicting this trait. A cut directly through the pith is a procedure which while not a typically used cutting strategy for eucalypt timber production, allows for (in normal circumstances) the largest manifestation of movement during cutting. Having knowledge of how much trust to place in splitting test measurements of individuals is important for tree breeders when they are deciding on selection weights for various traits. 174 samples were measured using the original splitting test \cite{Chauhan2010}, the rapid splitting test \cite{Davies2017} and the newly proposed "quartering test". The results from each were compared, and used to estimate the reliability of the testing procedures when considered at an arbitrary angle.
Method
The trees used in this study were thinnings of five year old Eucalytus argophloia grown on a moderately steep east facing slope with a rainfall of approximately 700 mm per year in Marlborough New Zealand. The 176 samples from 115 individual trees were labeled and cut when they provided at least 400 mm of suitably straight
stem with an estimated under bark diameter of greater than 20 mm. Under bark diameters ranged from 21 mm to 71 mm with a mean of 40 mm. The samples were cut from the stems in autumn using a
chainsaw and were packed into air tight containers with excess water and transported to a cool store where they were stored at 5C until processing, which took place over four weeks, there where no signs
(visual or statistical) of sample quality degradation.
Testing procedures: Samples were removed individually from their
containers. Bark was removed using knives
to hand peel the samples, with care taken not to damage the underlying
wood. The longest sufficiently straight section of sample
which could be obtained from the big end was marked (and the length from
the big end recorded), with a maximum length being 50mm short of the
small end. The diameter of the big end was recorded as was the small end diameter
at the marked point.
Rapid splitting test: The Rapid Splitting test outlined in \citet{Davies_2017} involves splitting with a band saw through the pith from the big end to
the marked point. The resulting opening is measured and recorded; From
the diameter d, slit length L and opening o, the strain within the sample can
be estimated using Equation \ref{eq:split_eq}.
Splitting Test: The original splitting test, presented by \citet{Chauhan_2010} involves splitting the sample the whole way down the pith (in this case
it was achieved by docking the remaining intact end from the above
procedure to gain two half rounds), clamping the two halves in the
centre and measuring the opening at each end. Here the method is
slightly modified due to the (compared to the original paper) low
openings to reduce measurement error. In this case, the small end was
clamped and the opening was measured at the big end. Note that when the curvature is sufficiently low, these two methods are approximately equivalent. The opening, big end diameter, small end diameter and split
length are measured and processed through Equation — to obtain strain. —
inc Figure \ref{171010}