Test machine effect in the determination of modulus of elasticity for strength-graded timber

This paper investigates if the relationship between the global and local moduli of elasticity (MoE), measured in bending tests, is affected by the test machines employed, and how that may influence strength grading. The ultimate aim is to study the potential influence of those factors on the yields of graded timber in Europe, and improve the standards used. More than 2000 structural timber pieces of Douglas fir (Pseudotsuga menziesii), larch (Larix spp.) and spruce (Picea sitchensis & P. abies) grown in Ireland and the United Kingdom, were tested using two different test machines. The results are examined, and compared to the dynamic MoE measured using longitudinal vibration resonance. Results found that there can be a sufficiently large systematic effect of the test machine on the measurement of MoE, to potentially be transferred to the grading process with important consequences in the yields of structural grade timber. Furthermore, adjustment factors for MoE, as used in the standards, may not be transferable between laboratories, even when the species and timber source are the same. Caution is therefore recommended when deriving or using these MoE adjustment equations, with due care to ensuring the timber is of similar characteristics and tested under as similar as possible setups. The extrapolation of models derived from other sources is best avoided, unless confirmed by crosschecking measurements.