Analysis of Spruce wood Deformation During Compression Testing

As testing progressed, localized splitting along the wood fibres was observed across all fibre orientations. In some instances, slight sliding occurred along these fracture lines. Despite these failures, the material retained partial internal cohesion and did not completely separate. This gradual failure was accompanied by a slow decline in deformation resistance, indicating that the structural integrity had been compromised. Beyond this point, the material’s behavior became increasingly unpredictable.

In the 0° direction, the wood sample initially showed a uniform distribution of compressive stress, increasing steadily within the elastic range up to a subtle yield point. Beyond this, the material entered a plastic phase, where resistance to deformation remained relatively stable. The end of the test was determined at the point where a definite fracture can be observed. Following image shows this moment for the specimen no.1.

At an angle of 45°, fibre compression begins from the start of the test along one of the planes parallel to the grain. It can be assumed that this represents the compression of the softer (springwood) layer of the growth ring between two harder (summerwood) layers. A strong concentration of deformation soon appears along another plane, also parallel to the grain.

The specimen deforms through gradual sliding along this plane, resulting in the development of localized shear stress. The remaining surface of the sample is subjected to a uniform load.

A brittle fracture occurred along the shear plane.
The sample split into two separate bodies, which slid freely along the fracture surface. The graph has a similar shape to that in the 0° direction, but with significantly lower values of maximum deformation resistance. Failure of the sample also occurred at seemingly random points within the plastic region.

In the 90° direction, i.e., perpendicular to the grain, deformation concentration was again observed from the beginning of the test. In the sample shown in the image, fibres were significantly compressed along at least three planes across the specimen. As deformation progressed, it became evident that on one side, the wood compressed faster in these regions, leading to an inclined deformation of the entire sample. This tilting may have been caused by geometric imperfections in the specimen or by internal structural flaws. In the later stages of the test, waviness appeared on the side surfaces of the samples. Complete failure of the specimen did not occur under this loading direction, but cracks formed at the bulging areas of the wavy surface. In these locations, local stresses can be observed in the 3D visualization. In the 90° direction, the differences in behaviour between individual samples were smaller than in other directions. A linear relationship was observed in the elastic region up to a clearly defined yield point. Beyond that, the material deformed with slight, seemingly linear strain hardening. At the end of this region, the hardening began to increase sharply.