PSI - Issue 39
Riccardo Cappello et al. / Procedia Structural Integrity 39 (2022) 179–193 Author name / Structural Integrity Procedia 00 (2019) 000–000
184
6
Thermoelastic Signal
T
Thermoelastic Signal
T
Thermoelastic Signal
T
[°C]
[°C]
[°C]
Sample #1: N = 368250
Sample #1: N = 137280
Sample #1: N = 503445
0.8
0.8
0.8
0.76
0.76
0.76
0.72
0.72
0.72
50
50
50
0.68
0.68
0.68
0.64
0.64
0.64
100
100
100
0.6
0.6
0.6
0.56
0.56
0.56
0.52
0.52
0.52
150
150
150
0.48
0.48
0.48
0.44
0.44
0.44
200
200
200
0.4
0.4
0.4
0.36
0.36
0.36
0.32
0.32
0.32
250
250
250
0.28
0.28
0.28
0.24
0.24
0.24
300
300
300
0.2
0.2
0.2
0.16
0.16
0.16
350
350
350
0.12
0.12
0.12
0.08
0.08
0.08
0.04
0.04
0.04
400
400
400
0
0
0
50
100
150
200
50
100
150
200
50
100
150
200
mm
Figure 3 – Thermoelastic signal maps of FH amplitude at three successive instants
Williams LSF
Williams LSF
Williams LSF
2
2
2
I [MPa]
I [MPa]
= 0.976
= 6 mm; R
= 0.8mm ; r
r
= 1.6 mm ; r
= 9 mm; R
= 0.98
r
= 2.1 mm ; r
= 13 mm; R
= 0.97
r
I [MPa]
max
min
max
min
max
min
500
500
500
Fitted data
Fitted data
Fitted data
Experimental data
Experimental data
Experimental data
450
450
450
400
400
400
350
350
350
300
300
300
250
250
250
200
200
200
150
150
150
100
100
100
50
50
50
50
100
150
200
50
100
150
200
Figure 4 – Contour plots of the fitted and Experimental data. The number of terms used in the Williams’ series is N w =7
4.1.2. Thermoelastic second harmonic signal The SH thermoelastic maps (in terms of both amplitude and phase) of the tests shown in Figure 3 and Figure 4are here discussed. This time the maps are further zoomed near the crack tip area to better highlight the relevant features. Also, the crack tips as obtained from the optical images and from the numerical fitting are pinpointed in the amplitude maps. The SH amplitude signal in the crack tip area, shown in Figure 5, present a peculiar shape resembling a turtle shape . Whenever this shape is detected, it is a clear signal of the presence of crack closure, as also reported in previous works [3], [5], [6], [8] . The origin of this “signature” is, in-fact, well explained by analyzing the influence of rising compression stresses, localized in the wake of the crack, during a specific portion of the loading cycle. It is in particular possible to distinguish two areas:
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