ONLINE ISSN 2069-7430 ISSN-L 1841-4737 PRO LIGNO Vol. 13 N° 2 2017 www.proligno.ro pp. 21-26 21 INFLUENCE OF MOISTURE CONTENT ON SOME ELASTIC CONSTANTS OF BLACK PINE SUBJECTED TO COMPRESSION Ergün GÜNTEKIN * Assoc. Prof. Dr. - Department of Forest Products Engineering, Faculty of Forestry Address: Suleyman Demirel University, Isparta, Turkey E-mail: rgngntkn@gmail.com Seda DEMIRATLI Graduate Student - Suleyman Demirel University Address: 32260 Isparta, Turkey Abstract: Moisture content is an important factor influencing physical and mechanical behaviors of wood materials. This research aimed at quantifying effect of moisture content on some orthotropic mechanical properties of Black pine wood. The elastic properties investigated include E L , E R , E T , ʋ LR , ʋ LT , ʋ RL, ʋ RT, ʋ TL and ʋ TR under compression. Compression strength in all orthotropic directions was also studied. Specimens were cut from sapwood of pine logs and sorted into four matched MC groups. Clear wood samples were conditioned at 20°C and 45%, 65%, 85%, 95% RH and subjected to compression tests. A bi axial extensometer was used to measure active and passive strain during loading. Young modulus, Poisson ratios, and compression strength were calculated and compared for all orthotropic directions. Results indicated that elastic and strength properties are significantly different in principal directions. Both Young’s modulus and compression strength of the samples tested were strongly affected by moisture content. Both properties decrease seemingly linear with increasing moisture content. Poisson ratios seem to be insensitive to the MC changes. Key words: moisture content; elastic constants; Poisson’s ratio; black pine. INTRODUCTION Wood is a popular construction material which its mechanical properties strongly influenced by surrounding relative humidity. Thus mechanical properties of wood are adjusted by its moisture content (MC) in practical use, particularly in structural applications. MC is known to be one of the major strength and stiffness reducing factors (Ross 2010). Most of the strength and elastic properties of wood vary inversely with the MC of the wood below fiber saturation point (Panshin and de Zeeuw 1980). Above fiber saturation point the mechanical properties are constant with changes in MC. At very low MC (0-10%), some strength properties may decrease again after reaching a maximum value (Ross 2010). The various mechanical properties have a different sensitivity to changes in MC, with strength properties more sensitive than stiffness properties and static properties more sensitive than dynamic properties (Dinwoodie 2000). A detailed discussion on the influence of MC on mechanical properties of wood can be found in earlier studies conducted by Gerhards (1982), Green and Kretschmann (1994), Kretschmann and Green (1996), and recent investigations of Hering et al. (2012a and 2012b), and Ozyhar et al. (2013). While the influence of MC on the mechanical behavior of wood in the L direction is relatively well understood (Gerhards 1982), investigations on the behavior in the perpendicular directions (R and T) are less investigated. The understanding of moisture dependent orthotropic behavior of wood species is necessary for advanced computational models such as finite elements used in engineering analysis. The mechanical investigation regarding Turkish wood species generally concerned with behavior at constant MC of 12%. Although data needed for three dimensional modeling of mechanical behavior depending on the MC change, a few references is available for this purpose (Güntekin et al. 2015, Güntekin et al. 2016a, Güntekin et al. 2016b). OBJECTIVE The purpose of this study was to determine a set of elastic and strength parameters in compression tests at different moisture conditions. The parameters evaluated and reported here comprise the Young’s modulus, Poisson‘s ratios, and compression strengths in principal directions. MATERIALS AND METHODS Small clear wood specimens were prepared from black pine (Pinus nigra Arn. subsp. Pallasiana (Lamb.) Holmboe) logs harvested from Bucak Forest District in Turkey. They were approximately 70cm in * Corresponding author