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1.1      Facets

XJTUDIC observes the deformation of the specimen through the images by means of various square or rectangular facets. The following figure shows 15x15 pixel facets with a facet step of 13 pixels (corresponds to a 2 pixel overlapping area, default setting). The facet size and step can be adjusted according to the requirements.

(a)     Facets                         (b) Settings

Figure 4‑1  Facets and settings

From each valid facet, a measuring point results after computation. Therefore, the adjustment parameters of the facets are important for strain computation and visualization.

1.2      Rectangular Facets

For strain measurements where the specimen is subject to high levels of strain, use rectangular facets according to the figure below in order to get evaluable facet fields.

Figure 4‑2  Facet deformation

1.3      Computation Masks

Computation masks allow the software to carry out facet computations in defined areas of the 2D camera images only. Defining computation masks can improve the computation efficiency and reduce the probability of mismatch. To define computation masks, you can use the dialog window that shown in Figure 4-3.

Figure 4‑3  Computation masks during definition

Figure 4‑4 Computation masks (green area)

1.4      Define Start Point

In order to improve the computation efficiency and reduce the probability of mismatch, the definition of a start point is required before the facets computation. The start point refers to the same facet in all stages, and it is the foundation of the facets computation. However, it is possible to work with different start points in one measuring project, for example, if after computation it turns out that for some stages XJTUDIC could not calculate any facets or the specimen breaks apart.

There are two different methods to create start points:

Manual Start Point Creation:

Open start point definition dialog window, Click in the middle of the area to be calculated in the 2D images with Ctrl and left mouse button to define a start point, and then click the button ¡°Create¡±, XJTUDIC will automatically search and create a start point in each state.

 Define start points only in those areas of a specimen that are subject to the least relative movement within the XJTUDIC measuring volume. Thus, you ensure that the start point creation works through all stages.

Fully Automatic Start Point Creation:

Click the button Automatic in the dialog can accomplish the start point definition automatically.

Figure 4‑5 Add start point

Fully automatic start point creation only works correctly if the pattern of the specimen is good and if the stages were recorded without too large deformation steps.

1.5      Start Point Creation for Torn Specimens

If a specimen tears apart during the test, strain will only be computed on that part of the specimen where the start point is located. As shown in Figure 4-6.

Figure 4‑6  Torn specimen with one-sided strain computation

In order to compute the strain in the torn apart area of the specimen as well, define a second start point as of the stage preceding the crack. This procedure optimizes the computation time (Figure 4-7).

Figure 4‑7  Definition of the second start point

After calculating the project with the second start point, now strain data are also available in the torn apart area. As shown in Figure 4-8:

Figure 4‑8  Torn specimen now with complete strain computation

1.6      Facets Matching 

With Computation->Speckle Computation, you can start the speckle image matching, that is the matching between the facets in the left and right images. The matching effect is shown as Figure 4-10.

Figure 4‑9 Speckle computation

Figure 4‑10 Facets matching of the left and right images

1.7      Strain Computation

After defining computation masks and start point, you can do the speckle computation, 3D reconstruction, strain computation, and then we can get field data of different deformation states.

Figure 4‑11 3D reconstruction

Figure 4‑12 Strain field