PSI - Issue 39

9

Riccardo Cappello et al. / Procedia Structural Integrity 39 (2022) 179–193 Author name / Structural Integrity Procedia 00 (2019) 000–000

187

Figure 8 – Theoretical phase values predicted from the analysis of the harmonic content of the loading signals.

The analysis of the experimental data can confirm this theoretical prediction. To do so, one acquisition is chosen (i.e., the last test) and the mean temperature signals from the crack tip (white area of Figure 9, left, and solid blue line in Figure 9, right) and from the wake of the crack (black area of Figure 9, left, and red dotted line in Figure 9, right) are extracted. The signals behavior clearly follows the one predicted in Figure 7 (a) (obviously, since this is the temperature variation linked to a load like the one reported in Figure 7 (a), a 180° phase shift has to be considered). Then, the DFT of these two signals is performed and the harmonic content analyzed (Figure 10 (a)), confirming its complexity. The first and second harmonics are also filtered using the LSF approach described in section 2.1.

160

temperature on crack tip (

head

)

temperature on crack closure area (

body

)

24.4

180

24.3

24.2

200

24.1

24

220

23.9

23.8

Temperature [°C]

240

23.7

23.6

23.5

260

0

10

20

30

40

50

60

70

80

90

100

frames

130

140

150

160

170

180

Figure 9 – Crack closure and selection areas for the extraction of signals: the mean temperature of the points inside the white area (left) is the blue curve (right), while the points in the black area (left) are represented by the red-dotted line (right).

In Figure 10 (b) the filtered temperature signals allow to highlight that the crack closure also affects the temperature signal at the crack tip. In the unloading phase (temperature increment) the blue curve appears to be flatter than the pure sinusoidal signal that one would expect. This might be due to the remaining of a tensile stress at the crack tip to counterbalance the compression induced by the presence of the crack closure.