Nondestructive characterization of wood by monitoring of local elastic anisotropy and dynamic nonlinearity

Abstract

Abstract The results of non-contact measurements of local in-plane elastic anisotropy in wood by monitoring the flexural plate wave velocity as a function of azimuth angle of propagation are reported. The Focused Slanted Transmission of air-coupled ultrasound is used to generate and detect locally the flexural waves in wood as well as to measure their velocities. The analysis presented shows that for thin plates the flexural wave velocity can be readily used to evaluate the anisotropy of Young’s modulus. This conclusion is confirmed by measurements of the plate wave anisotropy factors for veneer laminae and cross-ply laminates of spruce and beech. The technique is sensitive enough to detect the anisotropy variation of earlywood caused by the incorporation of latewood areas in growth rings. A new approach based on dynamic nonlinearity of wood is developed and applied to acoustic imaging of wood structure and detecting of defects in wood. The hysteresis mechanism of the dynamic nonlinearity is shown to dominate in clear wood and to manifest in a primary generation of odd acoustic harmonics. Local maxima of the odd higher harmonic amplitudes in the LR- and LT-planes of softwood are observed in the earlywood area close to latewood/earlywood transition interface that indicates the lower stiffness and strength of wood in those areas. The higher-order even harmonics and subharmonics in the nonlinear vibration spectra of wood are mostly produced by “clapping” in defect areas. Measurements of local amplitudes of these modes are applied to nonlinear acoustic imaging of cracks, delaminations and knots in wood and wood composites.