Crack formation, strain distribution and fracture surfaces around knots in thermally modified timber loaded in static bending

Abstract

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.

Performance of Thermally Modified Spruce Timber in Outdoor Above-Ground Conditions: Checking, Dynamic Stiffness and Static Bending Properties

“…Checking was found to increase by thermal modification compared to kiln-dried timber [23,49,50]. The pattern of checks is similar to kiln-dried timber; however, checks are wider, deeper and more abundant in TMT.…”

“…Today's use of TMW is, therefore, limited to low-key and volume structures such as exterior cladding and decking or internal wall and ceiling panels and flooring, as a sustainable alternative to toxic preservative-treated timber or tropical hardwood. Thus, there is a lot of unexplored potential as has recently been seen in a preceding investigation on thermally modified timber (TMT) [21][22][23]. It was shown with sufficient accuracy in prediction that reasonable levels of strength remain after thermal modification of spruce timber.…”

Review of Wood Modification and Wood Functionalization Technologies

“…In contrast to small and clear test samples, full-sized boards also suffer from growth-related and processing defects. Thermal modification increases the checking in knots compared to kiln-dried wood and fractures during bending often propagate from such checks [193]. Overall, these effects typically limit the use of thermally modified wood to non-load bearing applications.…”

Prediction of Mechanical Properties of Artificially Weathered Wood by Color Change and Machine Learning

“…The propagated wave velocity, and consequently the wood dynamic MOE, are typically calculated in wave propagation techniques using the time-of-flight method. The prediction of the mechanical properties [ 13 , 14 ] and the detection of internal check formation [ 15 , 16 ] in weathered thermally modified timber have been performed using the ultrasonic wave method. The physical and mechanical properties of thermally modified wood have also been predicted using the stress wave method [ 17 ].…”