Selected mechanical properties and the structural grading of young Pinus patula sawn timber

[摘要] ENGLISH ABSTRACT: In this study 1345 pieces of 38x114 mm timber sawn from 16-20 year-old Pinus patula trees were obtained from a wide variety of sites along the Mpumalanga escarpment in South Africa. The samples were tested for various mechanical and physical properties. The objectives of this study were (1) to determine the variation in the bending strength, tension strength, and stiffness of the sawn timber, (2) to assess the efficiency of the current visual and mechanical grading rules on this sample, and (3) to evaluate the potential of some indicator properties to be used as structural grading parameters on this resource.A large number of non-destructive measurements were performed on all the samples, including scanning of the boards with a commercial X-ray density scanner, moisture content measurement, growth ring measurements, warp measurement, acoustic frequency measurement and measurement of the stiffness using a mechanical grader. The data from the X-ray density scanner was used to calculate a number of knot-related variables for each board. Visual grading according to the SANS 1783-2 (2005) specifications for structural timber was performed by a certified grader on all the boards. The timber was divided into two groups for destructive testing, one group each for bending and tension tests. Tests were conducted according to the SANS 6122 (1994) method for in-grade testing. Due to the sampling method followed, the destructive tests effectively used a random placement of defects with regard to the load application position. From the destructive tests the modulus of elasticity (MOEedge), bending strength (MOR) and tensile strength were determined.The study showed that the MOEedge of the sawn timber was far below the requirements of the current national standard (SANS 10163-1) for all of the visual and mechanical grades. The mean MOEedge of the visually graded S5 timber was about 30% lower than required. The 5th percentile values for bending strength of all the visual and mechanical grades were above the required values of SANS 10163-1. The 5th percentile values for tensile strength of all the visual and mechanical grades were similar to that required by the SANS 10163-1 standard.Correlations between flatwise measured MOE (MOEflat) and edgewise measured MOE (MOEedge) were smaller than expected, as well as the correlations between both MOEedge and MOEflat with MOR. Dynamic MOE (MOEdyn), calculated from acoustic frequency tests on the timber, was found to be the best single predictor of MOEedge, MOR and tension strength. Multiple regression analysis showed that a combination of MOEdyn, density and knot parameters can be used to improve the predictability of some of the strength and stiffness characteristics of the timber.It is recommended that a comprehensive study on the structural grading of SA Pine be performed which includes (1) an analysis of market requirements in terms of strength and stiffness properties of timber, (2) in-grade testing of a representative sample of structural timber in South Africa, and (3) a review of the standards used in South Africa to regulate the structural grading of timber.

[发布机构] Stellenbosch University