Annual Layers Affect Resistance of Wood to Impact

Using test material from a relatively large log of Douglas-fir, an attempt was made to observe whether differences in the resistance of wood to impact loading may be related to certain anatomical characteristics of the test specimens. Wood with moderately wide annual layers from the central region of the log was carefully machined to produce test specimens having single bands of earlywood or latewood exposed on the tangential surfaces. Under impact normal to the annual layers, these specimens did not appear to show any consistent relationship between toughness and the position of the bands of earlywood or latewood on the surfaces subject to maximum tension or compression stress. The average toughness of green specimens loaded on the radial face was less than half the average toughness of specimens loaded on the tangential faces. In addition to the observed difference between loading specimens on the radial as compared to the tangential face, significantly higher toughness was recorded for tangential specimens loaded on the inner (pith) side than those loaded on the outer (bark) side. In order to observe these differences further, new test specimens were prepared from wood near the outside of the log, in matched sets, for loading on the radial and both tangential faces. Since the initial tests were made on green wood, part of the new material was tested airdry. The difference between loading tangential specimens on the pith side and loading them on the bark side remained highly significant in the green tests but lacked significance in the airdry material. The difference between radial and tangential loading remained highly significant throughout.

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