Results_and_Discussion1

3.2.2.3 Results and Discussion

During the first exposure the interaction of the cutting wedge with the test specimen under gradual loading by forces F = 793 N, 1586 N and 2379 N was holographically displayed. During the second exposure, the cutting wedge was recorded by a force slightly smaller, but still of sufficient value for the interference structure to be well distinguished mainly for the needs of quantitative analysis. Some interferograms of the cutting wedge surface changes are successively displayed in Fig. 3–31, Fig. 3–32 and Fig. 3–34. Even from the sole qualitative analysis of the interference fringe structure we can draw conclusions about the deformation process of the cutting wedge as well as about crack propagation within the test specimen.

In Fig. 3–31 we can see the photographs of holographic interferograms taken from two different viewpoints under different angles in the initial stage of the cutting wedge penetration into the test specimen. In this stage the wedge acts by compression force parallel with the fibres while the interference fringes are perpendicular to the loading force direction. This stage is specific also by the event where the concentration of wood substance around the edge of the cutting wedge appears as it is shown by the shape of the interference fringes in Fig. 3–31 a, Fig. 3–31 b.

The opening of the crack by the sides of the cutting wedge is evident from Fig. 3–32 a, b whereas the wedge is in the depth of approximately 2 mm. The cutting wedge tends to be led by the curve of the fibres from the plane perpendicular to the surface of the test specimen basis, which is manifested by displacements of the interference fringes on the wedge. In the interferograms we can also see the shapes of the interference fringes (loops) corresponding to anatomic directions of the test specimen.

In the third stage, the crack propagation in front of the cutting wedge can be observed (Fig. 3–34). From the photograph of the reconstructed interferogram we can also see the formation of the analogical zone of the tension-deformation state at a certain distance from the cutting wedge.

The individual components of the tensions obtained by the combined method (by using equations 3.51 to 3.53 and the holographic interferograms presented in Fig. 3–32 and Fig. 3–34) are graphically displayed in Fig. 3–33 and Fig. 3–35.

In the previous case the wood deformation was caused by cleavability where the cutting wedge acted parallel to the grain direction. In Fig. 3–36 the interferogram of the cutting wedge acting perpendicular to the fibres of the test specimen (spruce) is shown. In the wood dividing technologies this process of destruction is called shearing. From the figure it is evident that the shapes of the interference fringes depend on anatomic directions of the test specimen. To illustrate this, Fig. 3–37 shows the tension-deformation state of the cutting wedge penetration into the test specimen (beech) with an artificially created crack. This crack was created by cutting to the depth of up to 2 mm whereas its width was given by the thickness of the used saw blade. The cut was led in quasi radial direction perpendicular to the base surface of the test specimen. As seen from the interferogram, in this case the concentration of the wood substance around the cutting wedge does not occur, but the pressure force of the cutting wedge is transferred on the total volume of the body, which is evident from the character of the interference fringes. This can be caused by small loading force, small depth of the artificially created crack as well as by the hardness of the test specimen.

From the experiments realised, we can conclude that by this method it is possible to observe not only the tension-deformation state of the tool and the machined material but also the crack arising in the direction followed by the cutting wedge.