Simultaneous measurement of elastic constants of engineered wood-based panels by modal testing

Abstract

With the advent of mass timber panels and the development of mid- to high-rise wood constructions, the renaissance of wood construction is underway from Europe to North America and throughout the world. Engineered wood-based panel products, especially mass timber panels, play an important role in the evolution of wood construction. Elastic properties are not only fundamental mechanical properties for structural design but also important indicators for quality control purposes. Accurate measurement of the global elastic properties of full-size panels is critical for their applications as load-bearing building components. An efficient and reliable non-destructive technique is required for the purposes both of characterizing elastic properties and of grading engineered wood-based panel products in the production line before processing for all kinds of structural applications. In this study, two vibrational non-destructive techniques employing modal testing for natural frequencies and other modal parameters were developed for simultaneous measurement of elastic constants of engineered wood-based panels. Both vibrational methods adopted modal testing of a rectangular plate with the boundary condition of a pair of opposite edges in the width direction simply supported and the other pair free. Compared with the elastic constant values by conventional static tests, both vibrational methods generally showed close agreement. The first method was developed for measuring the moduli of elasticity in both major and minor strength directions and the in-plane shear modulus of a panel based on free transverse vibration of rectangular thin orthotropic plates. A simplified modal testing procedure together with frequency identification methodology based on sensitivity analysis and an iterative algorithm were proposed as the means of achieving an efficient and reliable measurement with three and/ or four sensitive natural frequencies from only three impacts. The method was first verified with standard static test values in laboratory for full-size cross laminated timber, oriented strand board and medium density fibreboard. Then, 55 full-size cross laminated timber panels with different characteristics and from three manufacturers were tested in factory environments. The results showed that non-edge bonding and gap size had a negative effect on both Ey and Gxy and led to a large variation compared with edge bonded panels as well as with their corresponding prediction models (i.e., k-method, gamma method and shear analogy method). The second vibrational method was developed for determination of effective bending and shear stiffness values based on Mindlin plate theory with an exact frequency solution and a genetic algorithm for the inverse problem. The results showed that the transverse shear moduli of cross laminated timber panels can be accurately determined with proper shear correction factors and were verified by planar shear test values. According to an in-depth comparative study, the first vibrational method shows great potential for future development of a standard testing method and on-line quality control over other existing vibrational methods in terms of setup implementation, frequency identification, accuracy and the calculation efforts required. The second vibrational method is suggested for engineered wood-based panels with small transverse shear moduli and/ or small length/ width to thickness ratio. Both methods are deemed to be applicable to all kinds of composite plates.