Experimental Study on Mechanical Characterization of Spruce Wood

Spruce wood (Picea abies) is one of the most widely used softwood in the global timber industry, valued for its straight grain, workability, and favorable strength-to-weight ratio [1]. In agreement with Kollmann et al. [2], the physical and mechanical properties of spruce wood account for its widespread use across a broad range of applications. Owing to its relatively high mechanical performance combined with a comparatively low density, spruce wood is particularly well suited for lightweight structural construction. These desirable properties stem from the wood's microstructure, which is primarily composed of cellulose, hemicellulose, and lignin [2, 3]. In particular, cellulose molecules form long, slender crystalline chains (4–5 µm in length) which contribute significantly to the material's high strength and low density [4]. Owing to these properties, spruce wood is extensively used in structural applications ranging from traditional sawn timber to glued-laminated timber (glulam) and cross-laminated timber (CLT). The increasing use of such materials requires an accurate understanding of their mechanical properties. In this context, Brandner et al. [5] provided a comprehensive state-of-the-art review of CLT, addressing its production, material properties, structural design, and connections. The mechanical behaviour of spruce wood, as with all wood, is inherently orthotropic and strongly governed by two fundamental physical properties: density and moisture content.

The relationship between wood density and mechanical properties is one of the most robust and well-quantified in materials science. Numerous studies have demonstrated that density, which reflects cell wall thickness and the amount of solid wood substance per unit volume, exhibits a strong positive correlation with strength and stiffness. Simic et al. [6] reported statistically significant effects of different planting densities on both timber density and modulus of elasticity. Jaakkola et al. [7] showed that growth rate affects the density of Norway spruce, with slower growth leading to higher density. For Norway spruce, air-dry density typically ranges between 350 and 470 kg/m³, although substantial variations arise from genetics and silvicultural conditions. These variations translate directly into mechanical performance. For instance, the correlation between density and compressive strength in spruce is exceptionally strong, with reported coefficients (R²) often exceeding 0.7-0.8. Rais et al. [8] demonstrated that high plant densities result in improved mechanical quality of sawn timber. Commercial grading machines extensively use this relationship, employing X-ray or microwave scanning to non-destructively estimate density and thereby predict mechanical performance. Brougui et al. [9] evaluated the mechanical properties of timber structures, through non-destructive testing (NDT) methods, particularly ultrasonic waves. Eder et al. [10] and Huang et al. [11] determined cell wall structure and wood properties.

Similarly, the detrimental effect of moisture content (MC) on the mechanical properties of wood is a fundamental principle of wood mechanics. Indeed, Ike et al. [12] conducted an experimental investigation on the influence of moisture content on the structural behavior of Norway spruce glued-laminated (glulam) timber. Samples were conditioned to six moisture content levels ranging from 5% to 30%. The results demonstrated that increasing moisture content led to a significant reduction in strength in all evaluated mechanical properties. As moisture content increased from dry conditions to the fibre saturation point (approximately 30%), compressive and tensile strengths decreased by more than 50% and 30%, respectively, in both grain directions. Similarly, flexural and shear strengths were reduced by up to 31%, while flexural stiffness exhibited a more pronounced decline of approximately 58%. As a hygroscopic material, wood absorbs water into its cell walls, weakening the hydrogen bonding within the cellulose microfibrils and the lignin-hemicellulose matrix. It is well-established that for most mechanical properties, both strength and stiffness decrease approximately linearly with increasing moisture content (MC) from oven-dry to the fibre saturation point (FSP) [13, 14]. Fajdiga et al. [15] focused exclusively on the three-point bending behavior of clear Norway spruce specimens in order to develop numerical models based on the finite element method.

While the individual effects of density and moisture are well documented [16, 17], their combined and interacting effects on the full range of mechanical properties (compression, tension, and bending) for Norway spruce under rigorously controlled conditioning states are less comprehensively documented within a single study. Existing data are often dispersed across decades of literature, derived from different species or obtained under varying testing conditions. Furthermore, advanced numerical modelling, particularly for simulating complex multi-physical phenomena such as the fire performance of wood-metal composite structures requires a complete and self-consistent dataset obtained from the same sample batch under defined conditions. Therefore, the primary objective of this study is not merely to reaffirm known relationships, but to generate a rigorous, empirical dataset that quantifies the mechanical properties of Norway spruce across its natural density range and under three precisely controlled moisture states: oven-dried (0% MC), air-dried (~12% MC), and fully moisture-conditioned (>FSP). This work addresses two key and novel objectives:

(1) To systematically isolate and analyze the interactive effects of density and moisture content on the orthotropic mechanical properties of spruce, leading to predictive relationships that can be directly applied in material selection and structural design.

(2) To establish a robust experimental benchmark that underpins the development and validation of advanced thermo-mechanical models within ongoing research on the fire performance of wood-aluminum hybrid structures.

By achieving these objectives, the present study delivers critical, high-quality data for engineers and designers aiming to optimize the structural use of spruce in demanding applications, while supporting the advancement of next-generation numerical simulation tools for performance-based timber design.

3.1 Physical properties

Spruce wood exhibits a moderate density with relatively limited variation among the tested specimens. The average density of 507.25 kg/m³ falls within the typical range reported for spruce [3, 5, 6] which is widely recognized as a lightweight softwood material. According to Huber et al. [22], Norway spruce and other conifers generally display lower variability in density and mechanical properties than deciduous woods, while achieving a more favorable balance between density and bending strength. The results in Table 1 show that the moisture content of the specimens was relatively low averaging around 15.35%, which is consistent with values expected for air-dried wood conditioned under ambient environments [23]. Wood with a moisture content below 20% is considered suitable for structural and construction applications [6, 24], as it minimizes the risks of shrinkage, warping, and decay. Variations in density observed within the same wood type can be attributed to natural growth and material imperfections, such as knots, and moisture condition. As expected, oven-dried specimens displayed the lowest densities due to moisture removal, whereas moisture-conditioned specimens showed increased density due to water absorption.

Table 1. Density and moisture content of used wood

The data reveal a clear positive correlation between moisture content and density, with increasing moisture content leading to higher wood density, consistent with findings reported in the literature [25]. Although relatively high standard deviations were observed (approximately 92.7 for density and 11.1 for moisture content as shown in Table 1), these values primarily reflect natural variability among individual specimens rather than experimental uncertainty. Even for specimens extracted from the same trunk, substantial differences can arise due to variations in annual ring width, the proportion of earlywood to latewood, and radial position relative to the pith. Local anatomical features including grain deviation and small knots, further contribute to variability in both density and moisture, as reported by previous studies [26, 27]. For moisture-conditioned specimens, this variability is further amplified by non-uniform water penetration and differential swelling of cell walls above the fiber saturation point.

3.2 Mechanical properties

3.2.1 Compression test (parallel to grain)

Compression tests conducted on spruce wood specimens (S1-S9) under three different conditioning states (Figure 3), oven dried (S1-S3), air-dried (S4-S6) and moisture-conditioned (S7-S9), revealed distinct mechanical behaviors influenced by moisture content and inherent material variability. As shown in Figure 3(a), the oven-dried specimens (S1-S3) exhibited predominantly brittle fracture, characterized by limited deformation, as evidenced by sharp fractures or sudden buckling, attributed to increased stiffness at low moisture content. In contrast, air-dried specimens (S5-S6) displayed moderate deformation prior to failure, reflecting a balance between strength and flexibility. The moisture-conditioned specimens (S7-S9) showed pronounced fiber crushing and buckling, indicative of reduced strength and increased plasticity associated with higher moisture levels. The corresponding tress-strain curves reveal an initial non-linear response for all specimens, likely arising from the viscoelastic behavior of wood and progressive cell-wall buckling. Following the peak stress, a short plateau region was observed, resembling a ductile-like phase, before a gradual reduction in stress. This failure pattern indicates that spruce wood fails in a non-fragile manner, typical of softwoods under axial compression [27].

Figue 3. Spruce wood samples after compression testing parallel to the grain, (a) Oven-dried, (b) Air-dried, and (c) Moisture-conditioned

Failure of the specimens generally occurred through a combination of buckling and shear fracture along the grain, with minimal fiber tearing. Among the oven-dried specimens, S2 which contained a small knot, failed at a lower stress than S1 and S3, highlighting the weakening effect of defects. Overall, the oven-dried specimens exhibited the highest peak stresses, as the absence of moisture increased stiffness and load-bearing capacity. Their stress-strain curves displayed a steep linear elastic region (Figure 4) followed by abrupt post-peak stress softening, which is characteristic of dry wood where microfibril collapse occurs suddenly with limited plastic deformation, in agreement with Báder et al. [24].

Figure 4. Stress-strain curves of oven-dried spruce wood samples subjected to compression parallel to the grain

For the air-dried specimens, peak stresses were approximately 10-15% lower than those of the oven-dried specimens due to moisture-induced plasticization of the cell wall constituents. A longer post-yield plateau was observed (Figure 5), indicating progressive cell-wall collapse rather than sudden failure and reflecting the enhanced energy-absorbing capacity of air-dried wood. Failure was governed by a combination of shear and crushing, with visible fiber buckling but no explosive splitting (Figure 3(b)). Specimen S5, which exhibited straight grain, sustained higher strains prior to failure than specimen S4, which showed slight grain deviation, emphasizing the influence of grain orientation on deformability.

Figure 5. Stress-strain curves of air-dried spruce wood samples subjected to compression parallel to the grain

Moisture conditioned specimens exhibited the lowest strength and stiffness among all groups, with peak stresses reduced by approximately 25–30% relative to oven-dried samples. The elastic region of the stress-strain response was notably shorter, with early microfibril yielding leading to a more gradual curve. The post-peak plateau was the most pronounced of all conditioning states, indicating high deformability. These specimens showed clear barreling deformation (lateral expansion) and fiber kinking (Figure 3(c)). Across all conditioning states, variations in density, moisture content and natural imperfections (knots, grain deviations) contributed to the observed dispersion of stress and strain responses, consistent with observations reported in the literature.

As shown in Figure 6, the moisture-conditioned specimens exhibited a predominantly ductile mode of failure, characterized by crushed and deformed surfaces with poorly defined cracking. This reflects the increased deformability and energy absorption capacity of spruce wood under compressive loading. Overall, increasing water content shifts fracture behavior from brittle to ductile, reducing crack severity and favoring material yielding over fracturing.

Figure 6. Stress–strain curves of moisture-conditioned spruce wood samples subjected to compression parallel to the grain

Compression test results for spruce wood specimens (S1-S9), shown in Table 2, under three conditioning states reveal clear moisture-dependent trends. Oven-dried specimens exhibited the highest mechanical performance, characterized by consistent Young's Modulus and Stiffness, indicating superior strength and reliability for structural applications. Air-dried specimens showed intermediate performance but with greater variability, suggesting less predictable behavior under compressive loading. In contrast, moisture-conditioned specimens displayed the lowest strength and stiffness with particularly poor results for specimen S9, underscoring the detrimental effect of elevated moisture content on load-bearing capacity. These observations highlight the strong relationship between moisture content and mechanical properties. As reported in the study [26], the stiffness modulus of spruce wood is affected by the structure of the annual rings, while the study [27] showed that crystallinity increases from fourth to the tenth ring measured from the pith and remains approximately constant beyond this region.

Table 2. Mechanical properties of spruce wood specimens subjected to compression testing

3.2.2 Profilometry and optical microscopy of samples after compression tests

Profilometry images of the spruce wood specimens (S1-S9) obtained after compression testing reveal distinct surface morphologies and fracture features that vary with the conditioning state.

The profilometry images presented in Figure 7 illustrate the surface behavior of oven-dried spruce wood specimens subjected to compression testing. The upper row of 3D surface maps reveals the surface topography and highlights dominant deformation features. The left image shows relatively smooth areas with minor surface markings, indicating either elastic or early plastic deformation. In contrast, the central and right images display distinct longitudinal cracking and raised fiber structures, suggesting the onset of tensile splitting and fiber pull-out along the grain direction, a typical failure mechanism in wood subjected to axial compressive loading.

Figure 7. 3D profilometry (top) and optical microscopy (bottom) images of oven-dried spruce wood samples (S1–S3) after compression tests parallel to the grain highlighting cracks and surface imperfections

The lower row, consisting of high-resolution optical images, provides a closer look at the microstructural changes. The left image clearly shows a deep, sharp crack, characteristic of brittle failure and localized shear or tensile stress. The central and right images further confirm the presence of cracks oriented parallel to the grain, indicating longitudinal splitting. This crack pattern reflects the anisotropic nature of spruce wood, which possesses high strength along the grain but reduced resistance to stresses acting in transverse directions [28]. The raised, torn fibers suggest micro-fiber pull-out and local crushing around the cracked zones. Overall, the observed surface features are consistent with the brittle behavior of oven-dried spruce wood, where low moisture content reduces ductility and promotes crack propagation along the grain.

The profilometry images of air-dried specimens (Figure 8) confirm the presence of a large, open cavity with delaminated internal surfaces, indicating tearing and separation between fiber layers. such features are commonly observed when the internal cohesion of the wood matrix is compromised under compressive loading. The second image reveals a pronounced longitudinal crack with rough edges, characteristic of brittle fracture initiated along weak zones within the grain structure. The third image shows more subtle surface damage with partially separated fibers and micro-cracking, indicating areas that may have experienced progressive failure under compressive stress.

Figure 8. 3D profilometry (top) and optical microscopy (bottom) images of air-dried spruce wood samples (S4–S6) after compression tests emphasize cracks and deformation features

Overall, the air-dried spruce specimens exhibited a mixed mode failure behavior under compression, dominated by longitudinal splitting, fiber separation, and delamination. Compared with the oven-dried samples, the air-dried wood retained slightly more ductility, allowing some localized deformation before final failure. Nevertheless, the prevailing failure mechanism still reflects the inherently brittle compressive response of spruce wood, particularly along the grain direction.

Consistent with the findings of Kollmann et al. [2], the mechanical properties of wood are strongly affected by its growth ring structure, the presence of defects such as knots and spiral grain, and moisture content. The profilometry and optical microscopy images of moisture-conditioned spruce wood specimens (Figure 9) reveal surface features indicative of a more ductile failure behavior under compressive loading. The 3D surface profiles (top row) exhibit less aggressive surface cracking compared to oven and air-dried samples. Observed cracks are narrower, smoother, and more continuous, suggesting that plastic deformation and energy dissipation through fiber buckling and internal shear rather than abrupt fiber rupture. The fiber structure appears more compliant, exhibiting localized compression and surface folding rather than widespread delamination or tearing. These characteristics are typical of wood at elevated moisture contents, where water absorbed within the cell walls acts as a plasticizer, softening the lignin and hemicellulose matrix and thereby reducing the material’s brittleness. Although an increased lignin content may contribute to compressive strength, its effect appears to be minimal.