## 1 Introduction  
Thermal modification would prolong the service life of wood in outdoor applications, because its biological durability and dimensional stability are improved by a decrease in hygroscopicity after modification [1, 2]. During the thermal modification process, wood is heated up to a target temperature of 160-240 \({}^{\circ}\)C that is maintained for a few hours while oxygen levels are kept low. Compared to other wood modification technologies, thermally modified wood (TMW) is available in largest volumes and at the lowest cost [3, 4]. The total production capacity of TMW was estimated at 300,000 m\({}^{3}\) in 2015 of which roughly half was modified according to the ThermoWood(r) process [3, 5]. In 2019, the total production of ThermoWood(r) was slightly over 220,000 m\({}^{3}\)[5]. The softwood species spruce (_Picea abies_ [L] Karst.) and pine (_Pinus sylvestris_ L) are primarily used for thermal modification, and TMWcurrently retails for about 1500 euro/m\({}^{3}\)[5; 6]. TMW is not suitable for ground contact situations since decay can already be discovered after a few years of exposure [7]. Previous studies also pointed out that TMW performs much better in outdoor, above-ground conditions [8; 9], i.e., use class 3.2, according to EN 335-2 [10] standard. In this use class, Thermo-D can be applied, which is the most durable and dimensionally stable ThermoWood(r) product on the market [11; 12]. The decrease in hygroscopicity after thermal modification is mainly explained by the thermal degradation of hygroscopic hemicelluloses in the wood cell wall, and measured by a reduction in equilibrium moisture content (EMC) of 50-60% and a reduction in swelling of 40-80% [1; 13; 14]. This loss in hemicelluloses is also considered in large part responsible for the decrease in some of the mechanical properties, particularly bending and tensile strength, and toughness [1; 15; 16]. These strength properties can be reduced by as much as 50% due to thermal modification depending on wood species and treatment conditions [15; 17; 18; 19; 20]. Mechanical properties of TMW have been investigated mainly using small specimens of clear wood, which makes it difficult to apply in structures. Today's use of TMW is, therefore, limited to low-key and volume structures such as exterior cladding and docking 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. It was also found that although checking in knots was increased after thermal modification compared to kiln-dried timber, effects appear locally and did not affect bending stiffness of TMT at these sites. Yet, little is known about the development of checks and the corresponding mechanical properties of TMT in outdoor above-ground conditions. Longer service-life of timber members relies on reliable estimation of material mechanical properties. Dynamic test methods used to obtain acoustic velocity in wood (based on natural resonance frequency or time-of-flight) have been proven useful in this respect [24; 25]. The dynamic elastic modulus calculated from acoustic velocity and density has shown moderate to strong association with static bending strength and modulus of elasticity (MOE) for both timber and TMT [21; 22; 26; 27; 28]. Weather conditions, however, may have a considerable influence on in situ evaluation of exposed wooden members with dynamic tests. This is because acoustic velocity and density depend on wood moisture content (MC) and temperature. For example, acoustic velocity determined with both resonance and time-of-flight tools has been found to decrease rapidly with increasing MC until fibre saturation point, whereas above this point the rate of change in velocity was much slower [29; 30].