360 Slater and Ennos: Assessment of the Remodeling of Bifurcations in Hazel (Corylus avellana L.) head of the testing machine was then made to rise at a rate of 35 mm min-1 until each bifurca- tion was broken, while an interfacing computer recorded the displacement (mm) and force applied (N) at a rate of 10 measurements per second. Aſter this testing, careful observation was made, by eye, of the fracture surfaces of all bifurcations, in relation to their morphology and appearance. To assist with comparing the relative strength of the bifurcations, three-point bending tests of the smaller diameter branch of the bifurcation were carried out, testing the yield strength of the middle of each branch whose structure had not been compromised by the rupture testing. The span for these branches was set at 215 mm for branches up to 20 mm in diameter and 275 mm for branches up to 23 mm in diameter, and the crosshead of the UTM, fitted with a semicircu- lar plastic probe, pressed down on the branch at a rate of 30 mm min-1 until the branch yielded sub- stantially, with the interfacing computer record- ing force, displacement, and calculating the yield strength of each branch tested. This procedure was used successfully in previous testing (Slater and Ennos 2013; Slater and Ennos 2015). Due to limitations of the testing machine in terms of the span length that could be used and the maximum load (900 kN) that could be applied, branches with a mid-diameter of over 23 mm could not be tested to their yield point. Careful observations of the yielding of each branch was undertaken, as these shorter spans could have resulted in shear failures (Vincent 2012) that could have invalidated some of the test specimens; however, no shear failures were observed to occur in these test specimens. In February 2015, after four growing sea- Figure 4. Diagram of the means of attachment of the bifur- cations to the universal testing machine during the rupture tests. In order to estimate their breaking stress, the fol- lowing measurements were taken for each bifurca- tion: the diameter of both branches adjacent to the apex of the bifurcation perpendicular and in-line with the plane of the bifurcation (A1 B2 branch bark ridge, perpendicular and in-line with the plane of the bifurcation (PS1 distances between the two drill holes in the two branches and between both drill holes and the apex of the bifurcation (a, b, and c) (Figure 3). This method of rupture testing of hazel bifurca- ); the diameter of the parent stem just below the and PS2 , A2 , B1 tions was used by Slater and Ennos (2015), when they assessed the strength of hazel bifurcations con- taining bark-inclusions, and the same equations were used to estimate the breaking stress of the bifurcations as are reported in this previous paper. ©2016 International Society of Arboriculture ); and the , and sons for the original set of drilled bifurcations and three growing seasons for the braced and split bifurcations, all the remaining bifurcations were cut from the hazel trees and subjected to the same method of bagging, storage, and rup- ture testing. A different testing machine (Model 3344, Instron, Norwood, Massachusetts, U.S.) had to be used for this second set of mechanical tests, as the original UTM suffered a breakdown in the two-year period between these two tests. The parameters of the rupture tests were the same in nearly all respects; however, the rate of displacement was increased to 50 mm min-1 , due to the large number of bifurcations that had to be processed. This higher rate of displacement for this second set of tests did not make any dis- cernible difference to the kinematics of failure. The bifurcations with bark included within them were classified aſter testing in terms of the relative occlusion of the bark into the bifurcation, giving rise to three types of bark inclusion: embedded,
September 2016
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