Radial variation in the dynamic modulus of elasticity of Scots pine wood assessed by ultrasonic time-of-flight and drilling resistance measurements

Acoustic and drilling resistance (DR) techniques are widely used for the nondestructive evaluation of wood quality; however, their combined application for characterising radial variation in wood elastic properties remains insufficiently explored. This study investigated the radial variation of ultrasonic velocity (UV), wood density, and dynamic modulus of elasticity (MOE) in Scots pine ( Pinus sylvestris L.) wood using ultrasonic time-of-flight (ToF) and DR measurements. Twenty-three pith-centred Scots pine specimens, each obtained from a different tree, were analysed. UV parallel to the grain was measured using two devices operating at nominal frequencies of 60 and 54 kHz, while radial density profiles were estimated from DR measurements smoothed using locally weighted polynomial regression. Dynamic MOE profiles were calculated from predicted density profiles and UV data. Both ultrasonic devices produced comparable mean velocity values (4997 and 4945 m·s − 1 ), although statistically significant differences between instruments were detected. Radial UV profiles exhibited a pronounced increase from the pith towards the outer wood zones, reaching a plateau approximately 50 mm from the pith. Meanwhile, predicted density increased continuously from pith to bark. As a result, dynamic MOE showed the strongest increase within the corewood zone, rising by approximately 340%–370% during the first 65 mm from the pith, followed by a more gradual increase towards the cambium. Generalised regression models describing the averaged radial variation in UV, predicted density, and dynamic MOE exhibited excellent goodness of fit ( R 2 = 0.99). The results demonstrate that the combined DR–ToF approach provides an efficient means of characterising radial variation in wood elasticity without the need for extensive laboratory-based density profiling. The proposed methodology has potential applications in wood quality assessment, structural timber grading, and the nondestructive evaluation of standing trees and roundwood by combining DR radial profiles, a single UV measurement, and the generalised model developed in this study.