Random acts of elasticity: MoE, G and EN408

Later this month I'm travelling to Bled in Slovenia to attend the Cost E53 working group meetings and wood drying seminar. I'll be presenting a poster entitled "Random acts of elasticity: MoE, G and EN408" which looks at the variation in modulus of elasticity (MoE) and shear modulus (G) within sawn timber and the implications for testing timber in accordance with EN408:2003 ...which ties in nicely with the draft revision of EN408 (09/30159969 DC), in which a new torsion testing method to obtain G replaces the old flexural method.

The main reason for the difference between global and local MoE is not shear, but the variation of MoE within a specimen. This raises serious doubts as to whether the current methods provided in EN408 for estimating G from bending tests are valid.

Furthermore, the practice of correcting global MoE based on an assumed value of G that is proportional to MoE would not work for individual specimens because MoE and G are not correlated.

The exact location of low stiffness defects within the central span has a large influence on the value of local MoE obtained. Global MoE is less sensitive and, perhaps counter intuitively, may be a better measure of wood stiffness. The scale of such a defect relative to the span explains how species and size influence MoE comparisons

You can download the full paper here, the poster here, and play with an interactive simulation of a four point bending test here.