PSI - Issue 5

Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 416–421 Andrzej Katunin/ Structural Integrity Procedia 00 (2017) 000 – 000

418

3

resolution in all three axes of 0.025 mm, which allows observing resulting defects with a necessary magnification and sharpness.

3. Results and discussion

The acquired CT scans were exported as sets of planar slices and subjected to image processing in order to emphasize different types of damage occurred in the specimens. The processing was performed using image filtering, local adaptive thresholding and morphological operations. The processed slices were finally reconstructed to 3D matrices in order to visualize damage for further analysis. The main purpose of this analysis was an identification of influence of the self-heating effect on character of damage initiation and its evolution during the self-heating temperature growth. For the reference purposes a typical maximal self-heating temperature history curve is presented in Fig. 1. One can observe three phases of structural degradation during fatigue accompanied with the self-heating effect characteristic for the non-stationary self-heating. The first phase corresponds to exponential temperature growth. Then, the temperature increases linearly in the second phase, and during this phase damage initiation takes place. The cumulating microcracks transform into a macrocrack at the end of the second phase, and in the third phase intensive crack propagation with rapid temperature growth is observed, which finally leads to a structural failure. The exemplary plot presented in Fig. 1 allows for direct comparison of the results of fatigue tests with those obtained from XCT studies with further image processing.

Fig. 1. Exemplary self-heating temperature history plot obtained during fatigue tests with the self-heating effect.

The reconstructed images were subjected to further processing in order to classify flaw types occurring during fatigue. Using advanced filtering and morphological operations on the acquired scans it was possible to distinguish delaminations (marked in blue) and matrix cracks (marked in red). In order to show resulting damage after fatigue loading at certain self-heating temperature values visualization for selected specimens were prepared and presented in Fig. 2. In each case, a visualization was presented in a form of isometric 3D view (left side image) as well as in the form of a planar XY-projection (upper right) and a lateral YZ-projection (lower right). Note that values on axes denote millimeters. The selection of representative cases was driven by observations as well as characteristic self-heating temperature values corresponded to a fatigue degradation degree.