PSI - Issue 17

Moritz Hartweg et al. / Procedia Structural Integrity 17 (2019) 254–261 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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deviation as a function of the cycle number is shown in figure 5b. The mean value of the raw data approaches very well the angle of 0° as it was expected from the fracture surface (figure 2). The good approach is also shown by the filtered data. After 30,000 cycles, the scatter has nearly disappeared, indicated by a standard deviation below 5° and the curve shows very well the expected angle of 0°. At this point, the crack covers 5.92 % of the cross section. A distinct reduction of the scatter can already be seen after 10,000 cycles, where the crack spreads only over 1.48 % of the cross section area. At this point the standard deviation shows a drop down to about 20°. Beside the absolute value of the standard deviation the scatter of this value is also decreasing with the cycle number (figure 5b). This indicates that the scatter of the calculated angle seems to be a promising parameter for crack detection.

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Fig. 5. Calculated angle (a) and standard deviation of 50 points (b) for the specimen shown in figure 2. The colored lines correspond to the overload marks.

Figure 6 shows the run of the calculated angle of a specimen without a secondary notch. As shown in figure 6b, the crack was initiated near potential probe 3 at an angle of 103° referring to potential probe 1. A first crack was visible at the overload mark after 115,000 cycles (green line). The following lines in figure 6b show the crack size after 130,000 (blue), 145,000 (red) and 160,000 cycles (orange). The angle to the center of the cracked area was measured and the corresponding data points are shown as colored points in figure 6a. After about 110,000 cycles the angle starts to change and the scatter decreases. After 115,000 cycles a crack with a size of 0.45 mm 2 representing 0.4 % of the cross section has formed. After 130,000 cycles a distinct reduce in the scatter is found. At this point the crack has reached a size of 1.44 mm 2 (1.27 % of the cross section). After about 145,000 cycles the crack spreads over 4.8 % of the cross section and the scatter in the calculated angle has nearly disappeared. The angle calculated from the potential drop differs from the values determined on the crack surface, but for all measured values the deviation is below 10°.