A reassessment of the compressive strength properties of southern yellow pine bark

Samples of southern yellow pine outer bark and wood were tested in compression to determine values for modulus of elasticity, stress at proportional limit, and maximum crushing strength. Results reported here resolve inconsistencies in the compressive strength data previously reported by others for pine bark. Testing of solvent-treated bark blocks suggests that although extractives are present in significant amounts, their contribution to the mechanical properties is minimal. Bark comprises roughly 10 to 20 percent of a tree stem with higher amounts for the branches in the crown (Fengel and Wegener 1983). The inner bark (phloem), as the conductive tissue necessary for transporting the products of photosynthesis, is protected by the outer bark (rhytidome) which includes the mostly nonliving tissues outside the innermost periderm. Differences in the basic anatomy between bark and wood are manifested in differences in their mechanical properties. In a recent study, the stiffness of the bark was one half that of the respective wood; however, it was found that the bark contributed significantly to the resistance of stem segments to bending forces (Niklas 1999). The mechanical properties of bark are also important for practical reasons since bark has been suggested as a resource for composite manufacture (Maloney 1973, Chow 1975, Blanchet et al. 2000). Nevertheless, only a few studies have focused on the mechanical properties of bark. Whereas the radial compressive strength of Douglas-fir (Pseudotsuga menziesii Franco) bark was significantly lower relative to that in the longitudinal direction (Lin 1973), a study including several southern pine species (e.g., P. taeda L., P. echinata Mill., P. palustris Mill., P. elliottii Engelm.) suggested that the compressive strength in the radial direction was equal or higher than that in the other directions (tangential, longitudinal), especially at higher bark SGs (Martin and Crist 1968). To address this inconsistency, a study was undertaken to determine the compressive strength of southern yellow pine (SYP) bark. Since the extractives in wood may influence wood mechanical properties to a limited extent (Panshin and de Zeeuw 1980, Wood Handbook 1999), solvent-treated bark specimens were tested in an effort to determine if the high extractives content of the bark influences the compressive strength properties. Materials and methods