The market share of thermally modified wood (TMW) has increased in Europe during the past few years as an environmentally friendly and durable building product. However, TMW products of today are not permitted for use in structural applications, because the reduction in strength that is caused by thermal treatment cannot be accounted for. The purpose of this paper was to investigate the bending properties of thermally modified timber (TMT) of Norway spruce, and to explore possibilities to predict the bending properties of TMT. A sample of 100 boards from a 2X-log sawing pattern of 100 logs was thermally modified according to the ThermoWood® process, while the mirror 100 boards served as an unmodified control sample. Two non-destructive methods were employed: (1) a novel method based on scanning of fibre directions to obtain the lowest edgewise bending modulus of elasticity (MOE) along a board, and (2) a conventional excitation method to determine the first axial resonance frequency used to calculate the axial dynamic MOE. Finally, the boards were bent to failure according to European standard EN 408. Despite the fact that bending strength was reduced by 42% due to thermal treatment, the type and location of failure in TMT remained related to the presence of knots. Prediction of bending strength based on local fibre direction and axial dynamic MOE, gave coefficients of determination of 0.51 for the thermally modified boards and 0.69 for the control boards, whereas axial dynamic MOE alone gave 0.46 and 0.57, respectively. These results indicate that although Norway spruce TMT has lower bending strength compared to unmodified timber, predictions of the bending strength can be made with good accuracy.

Thermally modified wood is available through a number of manufacturers in Europe on today’s market for interior and exterior building products. Thermal modification of wood allows for improvement of dimensional stability and durability, but a considerable decrease in strength properties occurs. Despite this loss in strength, thermally modified wood shows potential to be further exploited in structures exposed to loading. For such applications, accurate prediction of its static bending behaviour is essential. This paper studies the applicability of two different non-destructive test (NDT) techniques in estimating the bending properties of thermally modified timber (TMT). The study was done on 100 Norway spruce logs. One hundred (100) boards (i.e. one from each log) were thermally modified and the mirrored 100 boards were used as controls. After modification, resonance-based and time-of-flight measurements of axial wave velocity were carried out. Subsequently, all 200 boards were bent to failure following European standard EN408. This study shows that although TMT has a lower bending strength than unmodified timber, predictions of bending strength and stiffness using the NDT techniques are possible and with sufficient accuracy. The resonance-based method gave better predictions of the bending properties of TMT in respect to time-of-flight method.

The effect of thermal modification (TM) on the chemistry, anatomy and mechanical properties of wood is often investigated using small clear samples. Little is known on the effect of growth-related and processing defects, such as knots and checks, on the bending strength and stiffness of thermally modified timber (TMT). Nine boards of Norway spruce with different combinations of knot types were used to study the combined effects of checks and knots on the bending behaviour of TMT. Digital image correlation (DIC) measurements on board surfaces at sites of knots subjected to bending allowed to study strain distribution and localise cracks prior to and after TM, and to monitor development of fracture (around knots) in TMT to failure. DIC confirmed that checking in knots was increased after TM compared to kiln-dried timber, specifically for intergrown knots and intergrown parts of encased knots. Effects appear local and do not affect board bending stiffness at these sites. Bending failure in TMT initiated mainly at knot interfaces or besides knots and fractures often propagated from checks. Scanning electron microscopy analyses of fracture surfaces confirmed this, and fractures were typically initiated around knots and at knot interfaces due to crack propagation along the grain in the longitudinal–radial plane (TL fracture) under mixed mode I and II loading, such that boards failed in simple tension like unmodified timber. Images of fracture surfaces at the ultrastructural level revealed details of the brittle behaviour of TM wood. This was especially apparent from the smooth appearance of transwall failure under mode I loading across the grain.

Previous studies have shown that thermally modified wood (TMW) performs well in outdoor, above‐ground conditions in terms of resistance to wood‐decaying fungi. Yet, little is known about the development of defects such as checks and the corresponding mechanical properties of TMW in this condition. This experiment focused on the effect of 30 months outdoor above‐ground exposure (weathering) on the degree of checking, dynamic stiffness and static bending properties of thermally modified timber (TMT) of Norway spruce. Two board pairs per log were cut from 190 logs; one board of each pair was thermally modified and the other used as control. Then, 90 board pairs were exposed to the weather in south Sweden. Surface checking and axial stiffness were monitored at six‐month intervals by using digital photography and non‐destructive tests (time‐of‐flight and resonance method) to monitor changes in the material upon weathering. Finally, all boards were tested destructively in a 4‐point static bending test following EN 408 standard. Results showed that weathering had no significance influence on static bending properties of TMT even though the degree of checking was considerably higher in TMT than unmodified timber after weathering. In particular, checks along growth rings were deeper, longer and more common in TMT after weathering, especially on the pith side of boards. The maximum depth of these checks did not depend on board orientation (i.e., which side was exposed) and exceeded limits given in strength grading standards for 7% of the modified boards included. Axial dynamic stiffness determined at 6‐month intervals was less influenced by fluctuations in moisture content for TMT compared to unmodified timber, but did not confirm the increase in the degree of checking of TMT. The presence of checks from weathering did influence failure modes in TMT; horizontal shear failure became more frequent and some boards failed in compression.