## 1 Introduction  
Wood has always been a versatile material throughout human evolution, initially used as a solid fuel, tools, or weapons. One of the oldest wood samples was discovered in a brown coal mine in Germany in 1995 and dated to be 400,000 years old [1]. Wood has also been frequently used as construction material for ships and buildings [2, 3]. Despite its versatility, wood has a major limitation: it is predominantly an organic material that can undergo deterioration under certain conditions, such as exposure to humidity, ultraviolet radiation, and mechanical stress [4; 5]. These conditions may induce a disintegration of wood's structure, significantly reducing its mechanical properties, particularly after an attack by fungi, insects (beetles and termites), or bacteria. As a result, frequent inspections of wooden structures in ancient buildings are essential not only to evaluate the safety of the structures but also to preserve and restore cultural heritage buildings [6; 7; 8]. In heritage buildings, the used timber species is often unknown, which further complicates the assessment of the mechanical properties and stiffness of structural wood members, considering the high variability of wood properties, even in the same wood species [4]. The mechanical properties of wood can be evaluated at the laboratory scale using small, defect-free (free of knots, cracks, reaction wood, etc. ), and conditioned wood samples by following relevant norms such as DIN EN 52182 [9]., ISO 13061 [10], or EN 384 [11]. Alternatively, this evaluation may be performed using auxiliary non-destructive (NDT) visual grading and advanced methods, semi-destructive or low-destructive techniques (SDT), or a combination of these approaches [12; 13; 14; 15; 16; 17]. The SDT techniques are particularly relevant in the analysis of ancient woods, standing trees, and logs [7; 18; 19; 20; 21]. Starting from 1950s, several advanced non-destructive techniques (NDTs) were introduced to study wood, including X-ray radiation diffraction, X-ray computer tomography (CT), near infrared spectroscopy (NIR), infrared spectroscopy (FTIR), ultrasound techniques, neutron imaging (NI), active thermography, etc. [18; 22; 23; 24; 25; 26]. These techniques have proven highly effective for the detailed characterization of wood structures, but they are often costly, require surface preparation, and are usually unsuitable for an in situ analysis. On the other hand, mechanical wood inspection techniques such as drilling resistance (DR), needle penetration resistance (NPR), screw withdrawal resistance (SWR), and core drilling (CD), which are SDTs, offer advantages over the advanced NDTs, such as a lower cost, rapid application, and in situ application [14]. Moreover, it is easier to evaluate the results of mechanical wood inspection techniques in wood constructions compared with the often complex interpretation required for advanced NDTs [27]. Density is one of the key indicators of wood strength (the other important indicators being moisture content, anatomical features, and chemical composition) [4]. Generally, the density of wood is directly proportional to its mechanical strength [4]. Conversely, moisture content (\(u\)) is inversely related to wood strength below the fiber saturation point (\(u=30\%\)), which represents the maximum \(u\) in which wood is usually used [4]. Anatomical features of wood, such as the ring width and the proportions of latewood and juvenile wood, influence the density profile [28]. Chemical composition also plays a critical role in wood strength because the density as well as the linking and spatial arrangement of cellulose, hemicelluloses, and lignin differ significantly; thus, their relative content directly affects mechanical performance [4; 5]. Previous studies demonstrated that density and the holocellulose (cellulose + hemicelluloses) content can serve as indicators of the residual strength in wood structures [29].