Combining of results from visual inspection, non-destructive testing and semi-destructive testing to predict the mechanical properties of Western Hemlock

Current methods of assessing the mechanical properties of in-situ timber are limited, particularly with regard to estimating bending strength and stiffness. The aim of this research is to combine non-destructive testing (NDT), visual inspection and the taking and testing of very small (micro) samples to make a better predictive model. The modulus of elasticity of 150 structural sized joists of Tsuga heterophylla was measured using acoustic resonance NDT, and knots were plotted, before testing to destruction in four point bending. The moisture content and density were then obtained and the ring width and the slope of grain of the broken joists recorded. Micro clear (6.5 mm diameter 91 mm long) specimens were taken from undamaged regions of tested joists and small clear (20 × 20 × 300 mm) specimens were taken from material adjacent in the tree. Both were tested to destruction in three point bending. The results are analysed statistically and it was found that in predicting properties of the structural sized joists: (i) the single measurement of dynamic stiffness was the best predictor of static stiffness; (ii) the averaged density from a pair of micro clear specimens was the best predictor of density and (iii) the combination of dynamic stiffness and visual inspection of knots was the best predictor of bending strength