C. Mattheck, K. Bethge KIT Karlsruhe Institute of Technology, Institute for Applied Mechanics, P.O. Box 3640, D- 76021 Karlsruhe, Germany
Alexander Bauhuis IML Instrumenta Mechanik Labor, Parkstrasse 33, 69168 Wiesloch, Germany
The IML drilling resistance measurement device is powered by two independent electric motors: one provides the axial feed energy in direction of the needle, while the other generates the torsional energy for needle rotation. The feed energy is represented by the blue curve in the measurement results shown in the following figures, while the torsional energy is depicted by the green curve.
The image shows the complete penetration of a pear tree trunk. After the needle exits, a residual drilling resistance is still measured, which results from the friction of the needle shaft.
Drilling into ice or a cork reveals that the feed energy tends to increase with stiff, hard materials, while strong, fracture-tough materials lead to a rise in torsional energy.
Cracks and bark inclusions are neither stiff nor strong. Consequently, they cause both curves to drop, as demonstrated by the examples of a crack in oak and a bark inclusion near root strangulation in beech.
Drilling diametrically through a branch produces different curves when drilling from top to bottom or drilling from bottom to top. This is explained by growth stresses and the lateral pressure from the branch’s weight-induced bending, which influences friction on the needle shaft. When the upper side of a branch is infected by sac fungi (e.g. Massaria disease), we always attempt to drill from top to bottom.
The annual rings, clearly contoured near the bark, become increasingly blurred inwards until reaching the actual decayed core. Here, only remnants of decomposed latewood tips are visible. The wood can be crumbled by hand into fragments and, in the final stage, resembles cocoa powder.
The wind load at the base of this beech tree is distributed to the root buttresses. As a result, more load-bearing wood is deposited there, but not between the buttresses, where the wood bears less load. This is reflected in the measurement curves, which show higher values in the root buttresses.
The wet core is known to slightly soften the wood of poplar tree without significantly reducing its strength. Due to the increased fracture path, it also raises the fracture energy. Therefore, it was expected that the torsional energy would not decrease upon entering the wet core but rather slightly increase. However, one would have expected the feed energy to decrease due to the softness of the wet core. The rise of the blue curve is initially surprising. Other effects must be causing the curve’s increase, compensating for the softening of the wood. Hypothetically, these could include the higher density of wet wood or the work required to displace water from the drilling channel. The exact reason is not yet known.
Drilling from dry wood into wet wood of the same tree part does not result in such a sharp increase in axial feed force as observed with a true wet core. The most noticeable increase occurs in the Douglas fir sample. The wet core apparently alters the wood beyond just its water content.
The annual rings of this Douglas fir can be identified in a radial drilling by the late-wood tips, which represent locally high drilling resistance when penetrating denser, load-bearing wood. Latewood forms before the end of the growing season and con-stitutes the actual load-bearing wood. During axial drilling along the trunk’s axis, no annual ring boundaries are crossed. The needle tends to find its path in the earlywood between two adjacent latewood walls, occasionally scraping along them. A periodic drilling pattern, as seen in radial drilling, is therefore not expected.
Residual Drilling Resistance and Needle Shaft Friction
When a trunk is completely penetrated, the drilling resistance does not drop to zero, as would be desirable, but rather to a residual drilling resistance. Moreover, the two curves decline to different extents. This will be explained below through mechanical estimates applicable only to the residual drilling resistance after the needle exit.
The influence of the n/v ratio on the residual drilling resistance in Black locust:
As the drilling power ratio increases, the shear resistance ratio decreases, which ben-eficially limits the difference in curve amplitudes.
Conclusions:
With the IML-RESI PowerDrill®, axial feed energy and torsional energy are measured. Defects in the tree, whether softening or becoming brittle, were unequivocally detected in this study.
Upon entering cavities or completely penetrating tree parts, residual drilling resistance remains. This is significantly higher for torsion due to the considerably larger rotational path compared to the smaller axial path that the feed force encounters over the time interval.
An inherently fortunate circumstance is the difference in shear resistance, which limits the differences in curve amplitudes, thus keeping the axial feed still clearly readable.
With an increasing “n/v” ratio, e.g., rotational speed to feed speed, the shear resistance ratio decreases. Hypothetically, this is understood based on the tangential versus axial deformation behaviour and the “roll resistance” of the drill chips.
These correlations also explain why the two measurement curves show different per-centage drops in drilling resistance upon needle exit. The authors thank Norbert Kuhn, head of the Bretten Forestry Office, for supporting our tree research.
