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Cao, Y., Street, J., Mitchell, B., To, F., DuBien, J., Seale, R., and Shmulsky, R. (2018). "Effect of knots on horizontal shear strength in southern yellow pine," BioRes. 13(2), 4509-4520.

Abstract

Knots are inevitable components found in wood that can adversely affect the mechanical properties of the lumber. The objective of this study was to investigate the effect of knots on the horizontal shear strength of southern yellow pine. Knot condition (sound/unsound) and shear plane (radial/tangential face) were studied as the factors of shear strength. The standard ASTM D143-94 (2014) was used to compare 120 pairs of clear shear blocks and shear blocks containing knots. Paired t-test results showed that regardless of the direction of the grain compared with the shear plane (perpendicular or parallel), sound knots increased the shear strength and the unsound knots decreased shear strength. Based on this study, the unsound knot volume was found to be a significant factor in decreasing the shear strength in the radial or tangential face direction. Furthermore, no significant relationship between the knot angle and shear strength was found. Shear failure occurred in the wood when an encased knot sample was tested and shear failure occurred in the knot when an intergrown knot sample was tested.


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Effect of Knots on Horizontal Shear Strength in Southern Yellow Pine

Yawei Cao,a Jason Street,a,* Brian Mitchell,a Filip To,b Janice DuBien,Roy Daniel Seale,a and Rubin Shmulsky a

Knots are inevitable components found in wood that can adversely affect the mechanical properties of the lumber. The objective of this study was to investigate the effect of knots on the horizontal shear strength of southern yellow pine. Knot condition (sound/unsound) and shear plane (radial/tangential face) were studied as the factors of shear strength. The standard ASTM D143-94 (2014) was used to compare 120 pairs of clear shear blocks and shear blocks containing knots. Paired t-test results showed that regardless of the direction of the grain compared with the shear plane (perpendicular or parallel), sound knots increased the shear strength and the unsound knots decreased shear strength. Based on this study, the unsound knot volume was found to be a significant factor in decreasing the shear strength in the radial or tangential face direction. Furthermore, no significant relationship between the knot angle and shear strength was found. Shear failure occurred in the wood when an encased knot sample was tested and shear failure occurred in the knot when an intergrown knot sample was tested.

Keywords: Shear strength; Knots; Angle; Volume; Shear failure; Southern yellow pine

Contact information: a: Department of Sustainable Bioproducts, Mississippi State University, P. O. Box 9820, Mississippi State, MS 39762-9820 USA; b: Department of Agriculture & Biological Engineering, Mississippi State University, Box 9632, Mississippi State, MS 39762 USA; c: Department of Mathematics and Statistics, Mississippi State University, P. O. Box MA, Mississippi State, MS 39762 USA;

* Corresponding author: jason.street@msstate.edu

INTRODUCTION

Branches are essential for tree growth so that leaves can propagate on a tree for photosynthesis. Knots found in lumber are the remnants of those branches. These knots can appear as encased knots or intergrown knots. An encased knot can form when a tree grows around a dead branch, and these type of knots are usually surrounded by a dark ring with a decaying center. Encased knots are also referred to as “loose” knots because the bark inhibits the knot from tightly binding to its surrounding wood. Encased knots are those whose rings of annual growth are not intergrown with those of the surrounding wood. An intergrown knot typically refers to the base of a living branch on a tree. Intergrown knots are usually surrounded by a halo of circular growth rings. These intergrown knots are also referred to as “tight” knots because these knots are securely bound to the wood surrounding them. Encased and intergrown knots can further be divided into sound and unsound knots. Unsound knots tend to have decay, while sound knots are solid across the face and show no symptoms of decay. Examples of these classifications can be seen in Table 2,

In general, the branch tissue of softwoods is characterized as material that has a higher density than the stem wood, small or incomplete annual rings, a high proportion of compression wood, an increased microfibril angle, increased lignin content, and a decreased fiber length (Shigo 1985). Knots have been widely considered as defects in regard to wood quality, which can adversely affect the strength properties (USDA 1999). Guindos and Polocoser (2015) investigated the influence of the slope of the grain on the strength-reducing effect of face knots. They claimed that the modulus of rupture (MOR) of beams containing knots could be reduced by up to 50%. In addition, knots were shown to significantly affect the modulus of elasticity (MOE) (Hossein et al. 2011). Dávalos-Sotelo and Ordóñez Candelaria (2011) stated that knots negatively affected the bending strength of pine wood and their presence significantly decreased the value of the wood.

Douglas fir shear blocks have been previously tested to determine their shear strength (Gupta et al. 2004). Specimens tested included those with knots parallel and perpendicular to the shear plane, as well as those with no knots. The results showed that there was no significant difference in the mean shear strength of clear and knotted specimens, regardless of their orientation. Baño et al. (2013) used the finite element analysis method to investigate the effect that knots have on the bending strength of beams using the knot condition, size, and position as variables. Their research considered knot sizes with diameters of 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm. It was discovered that the bending strength of the beams decreased as the knot size increased, and this bending strength decrease was enhanced when the distance from the neutral axis was increased. A theoretical model (Ping 2000) revealed that knots negatively affected the stiffness (MOE) of pine lumber. To date, the influence of knot angles and the influence of sound and unsound knots on the shear strength of southern yellow pine have not been widely researched.

The objective of this study is to investigate the effects of knots on horizontal shear strength when the shear plane is parallel to the tangential and radial face of southern yellow pine. The knot size, knot condition, and the knot angle are investigated using shear blocks to determine how these factors affect shear strength. This study involves the consideration of various knot conditions (Fig. 2, Table 2) using shear block analysis of southern yellow pine for the first time.

EXPERIMENTAL

Materials

Sample preparation

Southern yellow pine (Pinus spp.) lumber was obtained from the Shuqualak Lumber Company located in Shuqualak, MS, USA. A table saw, an arm saw, and a band saw were used to cut the Southern yellow pine lumber into blocks with the specifications shown in Fig. 1. The samples were produced with dimensions of 1.5 in × 1.5 in × 2.5 in (modified ASTM D143-94 (2014)). Literature suggests that although these dimensions reduced shear area when compared to the ASTM D143-94 (2014) standard, this modification has an insignificant effect on the ultimate shear strength (Bendtsen and Porter 1978; Lang and Kovacs 2001). A total of 120 paired shear blocks were prepared. Each pair included one clear sample and one sample containing a knot. These samples were cut adjacent to one another to perform a comparison. They were assembled into four groups based on the knot condition and the shear plane direction. Group A contained 30 pairs of clear samples and sound knot samples in which the shear plane was parallel to the tangential face.

The knot condition was visually classified. Sound knots were solid from face to face with no decay symptoms. Unsound knots had noticeable decayed characteristics according to the Southern Pine Inspection Bureau. Group B contained 30 pairs of clear samples and unsound knot samples in which the shear plane was parallel to the tangential face. Group C contained 30 pairs of clear samples and sound knot samples in which the shear plane was parallel to the radial face. Group D contained 30 pairs of clear samples and unsound knot samples in which the shear plane was parallel to the radial face. Groups A, B, C, and D are shown in Figs. 2a, 2b, 2c, and 2d, respectively. Figure 2 shows that Groups A and B had growth rings positioned parallel to the shear tool (tangential), and Groups C and D had the growth rings positioned perpendicular to the shear tool (radial). The southern yellow pine lumber, which was used for this study, was conditioned at 20 °C ± 2 °C and 65% ± 5% air relative humidity for one month. All knots were classified as Type 3 knots as stated by the ASTM D4761-13 (2013) standard.

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Fig. 1. Shear block sample dimensions