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Güntekin, E., Aydin, T. Y., and Niemz, P. (2015). "Prediction of compression properties in three orthotropic directions for some important Turkish wood species using ultrasound," BioRes. 10(4), 7252-7262

Abstract

Compression properties in three orthotropic directions for some important Turkish wood species, including Calabrian pine (Pinus brutia Ten.), Taurus cedar (Cedrus libani), Oriental beech (Fagus orientalis), and sessile oak (Quercus petraea), were studied using non-destructive and destructive techniques. The materials used in the study consisted of 720 small clear specimens of nominal dimensions of 20 x 20 x 60 mm. The influence of equilibrium moisture content (EMC) was studied over four batches of 15 specimens each, conditioned for six to eight weeks before testing at a temperature of 20 ± 2 °C and at four different relative humidity conditions (50%, 65%, 85%, and 95%). Time of flight values were measured with a commercial ultrasonic tester. Using the time results from the ultrasound device, the wave velocities (length/time) and Edyn values were calculated. Samples were also tested in uniaxial compression to determine the Young’s modulus and compression strength values in three orthotropic directions. The Edyn correlated well with the Young’s modulus and compression strength of the specimens; coefficients of determination ranged between 0.75 and 0.96. Moisture content seems to have more influence than density on sound velocities. Results showed that there is a weak and mostly negative correlation between the density of the specimens and the sound velocity values.


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Prediction of Compression Properties in Three Orthotropic Directions for Some Important Turkish Wood Species Using Ultrasound

Ergün Güntekin,a,* Tuğba Yılmaz Aydın,a and Peter Niemz b

Compression properties in three orthotropic directions for some important Turkish wood species, including Calabrian pine (Pinus brutia Ten.), Taurus cedar (Cedrus libani), Oriental beech (Fagus orientalis), and sessile oak (Quercus petraea), were studied using non-destructive and destructive techniques. The materials used in the study consisted of 720 small clear specimens of nominal dimensions of 20 x 20 x 60 mm. The influence of equilibrium moisture content (EMC) was studied over four batches of 15 specimens each, conditioned for six to eight weeks before testing at a temperature of 20 ± 2 °C and at four different relative humidity conditions (50%, 65%, 85%, and 95%). Time of flight values were measured with a commercial ultrasonic tester. Using the time results from the ultrasound device, the wave velocities (length/time) and Edyn values were calculated. Samples were also tested in uniaxial compression to determine the Young’s modulus and compression strength values in three orthotropic directions. The Edyn correlated well with the Young’s modulus and compression strength of the specimens; coefficients of determination ranged between 0.75 and 0.96. Moisture content seems to have more influence than density on sound velocities. Results showed that there is a weak and mostly negative correlation between the density of the specimens and the sound velocity values.

Keywords: Compression; Prediction; Ultrasound

Contact information: a: Department of Forest Products Engineering, Suleyman Demirel University, 32260, Isparta, Turkey; b: Institute for Building Materials, Wood Physics Group, ETH Zürich, Switzerland; *Corresponding author: ergunguntekin@sdu.edu.tr

INTRODUCTION

Compression properties, particularly the Young’s modulus, in the three principal directions are important in the design of wood members in structures. Young’s modulus, also known as the elastic modulus, is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. In general, there are many physical parameters that may affect the Young’s modulus, such as the moisture content (MC), specific gravity, temperature, creep, knots, number of annual growth rings, and grain angle. Investigations regarding the influence of MC on the Young’s modulus have shown that if MC increases, the Young’ modulus will decrease. While the influence of MC on the mechanical behavior of wood in the longitudinal (L) direction is relatively well known (Gerhards 1982a), investigations of the behavior in the perpendicular directions (radial, R and tangential, T) are limited. Interest in the moisture-dependent orthotropic behavior is not new. So far, only a few studies have investigated the moisture-dependent elastic properties of wood in the R and T directions (McBurney and Drow 1962; Hering et al. 2012a,b; Ozyhar et al. 2013a,b). Furthermore, moisture-dependent wood strength in the R and T directions remains unrevealed for most wood species. The usable data is limited to a few references (Kretschmann and Green 1996; Ozyhar et al. 2013a,b), while selected moisture-dependent elastic properties for some wood species have also been reported (Kretschmann and Green 1996; Ross 2010).

The Young’s modulus can be determined using both destructive and non-destructive methods. Use of non-destructive testing (NDT) and non-destructive evaluation (NDE) in the field of wood and wood-based materials is advancing every day. There are widespread NDT techniques, equipment, and evaluation procedures available today that resulted from early NDT research (Brashaw et al. 2009; Dündar and Divos 2014).

Ultrasonic wave velocity has advantages over other techniques in practical terms (Esteban et al. 2009). The ultrasonic technique has been utilized in many applications including tree quality evaluations in forests (Wang et al. 2004) and condition assessments of wood structures in service (Ross and Pellerin 1994). Determination of the ultrasonic modulus of elasticity in a solid depends on its elastic properties and its density (Oliveira and Sales 2006). The velocity of sound in wood is influenced by factors such as MC, grain orientation, density, decay, temperature, and geometry (Beall 2002; Oliveira et al. 2005).

Information on the Young’s modulus of wood in the orthotropic directions is not available for the majority of Turkish species. Most studies deal with bending modulus of elasticity (MOE) and bending, tensile, and compression strength at constant MC. Although data are needed for three-dimensional modeling of mechanical behavior depending on the MC change, no information is available for this purpose. In this study, the Young’s modulus in compression for some important Turkish wood species is determined by non-destructive and destructive testing under various moisture conditions.

EXPERIMENTAL

Materials

For this study, two softwood and two hardwood species were chosen. The sample trees of sessile oak (Quercus petraea) and Oriental beech (Fagus orientalis) were harvested from a beech-oak mixed stand in the Devrek Forest Region of the Western Black Sea region of Turkey. The sample trees of Calabrian pine (Pinus brutia Ten.) and Taurus cedar (Cedrus libani) were selected from a pine-cedar mixed stand in the Bucak Forest Region of the Southwest region of Turkey. Calabrian pine covers the largest area (3096 064 ha) among conifers grown in Turkey, which corresponds to about 15.3 percent of the total forest area in Turkey. The woods of other selected species are important raw material for various fields of forest industry and have high importance in trade.

The ages of the pine, cedar, beech, and oak trees considered in this work were 60, 80, 140 and 170, respectively. The pine, cedar, beech and oak logs were 37 to 50 cm in diameter at breast height. All the samples came from the sapwood planks cut from the trunk section 1 to 3 meters from the ground level, except for oak, which has very narrow sapwood. 60 samples with nominal dimensions of 20 x 20 x 60 mm for each direction (L,R,T) from radial or tangential planks were prepared.

Prior to testing, specimens were divided into four matched groups conditioned for six to eight weeks at a temperature of 20 ± 2 °C, and at four different relative humidity conditions (50%, 65%, 85%, and 95%). A total of 180 specimens used in testing for each species.

Methods

Apparent densities (ρ) of the samples were calculated according to TS 2472 (2005) using the stereometric method which is based on measurements of the sample volume and mass.

Time of flight values were measured with an ultrasonic commercial device (Steinkamp BP-V, Germany) using conical sensors with a frequency of 22 kHz. Measures were made in end to end directions (L, R, T) on each specimen with a constant sensor coupling pressure, as shown in Fig. 1. According to the time results of the ultrasound devices, the sound velocities (SV, length/time) and Edynwere calculated using the following equation,

Edyn = ρ V2 10(1)

where Edyn is the dynamic modulus of elasticity, in N/mm2ρ is the density, in kg/m3; and V is the velocity of the ultrasound wave, in m/s.