Modeling Moisture Content-Mechanical Property Relationships For Clear Southern Pine

The objective of this study was to determine the effect of moisture content on a wide range of clear wood properties. Specimens were cut from commercially dried 38- by 140-mm (nominal 2- by 6-in.), southern pine lumber and sorted, based on specific gravity, into five matched moisture groups of approximately 40 specimens each. Empirical models are presented for predicting the effect of moisture content on strength and stiffness for the various test properties. Results indicate that tensile stress parallel- and perpendicular-to-grain and both Mode I and Mode II stress intensity factors for fracture toughness increase with decreasing moisture content from green to a peak between 7% and 13% moisture content. Upon additional drying, these properties decrease. Maximum fiber stress in bending, compression parallel- and perpendicular-to-grain, shear parallel-to-grain, and all elastic modulii increase with decreasing moisture content from green to 4% moisture content. For some of these properties, the increase is not linear at lower moisture content levels. Because specific gravity is known to affect clear wood properties, models were also developed to account for using moisture content and specific gravity. Theoretical approaches to moisture absorption that may explain experimental results are discussed