Fracture Initiation and Progress in Wood Specimens Stressed in Tension. Part III. Clear Wood Specimens with Various Slopes of Grain

Abstract

Clear spruce wood specimens with 30, 20, 10, and 5° slope of grain were subjected to in-situ tensile failure tests in order to study fracture initiation and propagation. Fracturing of the specimens was carried out inside a SEM with a bending and a tension device, respectively. The bending and the tension specimens consisted of two to three growth rings. Fracture process in 30°-, 20°-, and 10°-specimens was quite uniform but differed from that of 5°-specimens. There was no difference between bending and tension specimens. With one exception, the specimens fractured in a stepwise manner. Cracks initiated at wood rays in the latewood and were arrested in the adjoining earlywood. This is attributed to the greater stiffness of the latewood tracheids and the capacity of the thin-walled earlywood tracheids to deform thereby arresting cracks. In 30°-, 20°-, and 10°-specimens, the main fracture mode was cleavage along the grain due to tensile stress perpendicular to the grain. Latewood tracheids showed intrawall fracture while earlywood tracheids showed longitudinal transwall failure. In the 5°-specimens, tensile stress parallel to the grain prevailed.

Mechanical Properties of Low-Stiffness Out-of-Grade Hybrid Pine—Effects of Knots, Resin and Pith

“…Typical failures can be seen in Figure 28. Bodner, et al [71] and Galicki and Czech [80] experienced the same types of failure in their testing including separation at the earlywood-latewood boundaries. Bodner, et al [71] found that wood rays were structurally weak and were frequently the location of either initiation or discontinuity of fractures under tensile load.…”

“…Bodner, et al [71] and Galicki and Czech [80] experienced the same types of failure in their testing including separation at the earlywood-latewood boundaries. Bodner, et al [71] found that wood rays were structurally weak and were frequently the location of either initiation or discontinuity of fractures under tensile load. The knot samples failed in the deviated grain around the knots and frequently through the pith of the branch wood.…”

“…Pith was exposed in the shear failure plane of all samples that contained pith showing its poor strength in longitudinal shear. This may also be associated with the wood rays that radiate out from the pith and are known to be structural weak points [71].…”

Growth Ring Orientation Effects in Transverse Softwood Fracture

“…Given that cell separation is governed primarily by lignin fracture, our results support this. Additionally, Bodner et al [ 22 , 23 ] showed that rays play an important role in wood fracture characteristics. Specifically, they showed that rays can act as stoppers for cracks propagating in the longitudinal direction (RL or TL).…”

Verbundverhalten von Vollgewindeschrauben in Brettschichtholzbauteilen

“…Weiterhin konnte das Risswachstum durch die Zuwachslänge des Risses innerhalb eines feststehenden Intervalls zwischen zwei Aufnahmen beschrieben und dadurch einen Längenzuwachs pro Zeit ermittelt werden. Frühere Untersuchungen von B odner et al [8] hatten gezeigt, dass kurze Risse im Spätholz entstehen und im Frühholz enden, was auf die höhere Steifigkeit des Spätholzes gegenüber dem Frühholz zurückzuführen ist [9]. Diese Annahme konnte durch den Einsatz der fotooptischen Messtechnik überprüft und bestätigt werden.…”