PSI - Issue 39
Riccardo Cappello et al. / Procedia Structural Integrity 39 (2022) 179–193 Author name / Structural Integrity Procedia 00 (2019) 000–000
191 13
4.2.2. Thermoelastic second harmonic signal Figure 16 shows the evolution of the SH amplitude signal at different loading cycles. This time, the body of the turtle has an elongated shape, since the considerable amount of plastic deformation and compressive stress state induced by the overload result in a higher extent of crack closure in the wake of the crack (remote crack closure).
Second Harmonic
T
Second Harmonic
T
Second Harmonic
T
Sample #2: N = 100000
Sample #2: N = 190000
Sample #2: N = 301000
0.05
0.05
0.05
20
20
20
0.045
0.045
0.045
40
40
40
0.04
0.04
0.04
60
60
60
0.035
0.035
0.035
80
80
80
0.03
0.03
0.03
00
100
100
0.025
0.025
0.025
20
120
120
40
140
0.02
0.02
140
0.02
60
160
160
0.015
0.015
0.015
80
180
180
0.01
0.01
0.01
200
200
200
0.005
0.005
0.005
220
220
220
0
0
0
20
40
60
80
100
120
140
160
180
20
40
60
80
100
120
140
160
180
20
40
60
80
100
120
140
160
180
Figure 16 – SH amplitude maps of the overloaded specimen: the higher amount of plastic deformation induces a remote crack closure in the wake of the crack. The SH phase maps are reported in Figure 17. Again, the same 180° phase shift found in Sample #1 is here reported. This time, being the contact area between the flanks of the cracks higher, a bigger zone behind the crack tip is subject to the 180° phase shift.
Second Harmonic Phase
Second Harmonic Phase
Second Harmonic Phase
[°deg]
Sample #2: N = 190000
Sample #2: N = 301000
Sample #2: N = 100000
[°deg]
[°deg]
180
180
180
40
150
150
20
150
20
60
120
120
120
40
40
80
90
90
90
60
60
100
60
60
60
80
80
120
30
30
30
100
100
140
0
0
0
120
120
160
-30
-30
-30
140
140
180
-60
-60
-60
160
160
200
-90
-90
-90
180
180
-120
-120
-120
220
200
200
-150
-150
-150
240
220
220
-180
-180
-180
20
40
60
80
100
120
140
160
180
20
40
60
80
100
120
140
160
180
20
40
60
80
100
120
140
160
180
mm
mm
Figure 17 – SH phase for the overloaded specimen: the presence of an overload does not affect the expected 180° phase shift.
5. Conclusions In this work, cracks propagating from Single Edge Notched Tension specimens made of AISI 304L steel, under fatigue loading testing, have been monitored by means of a cooled sensor infrared camera. The acquired signal is then processed, to evaluate the harmonic content of the temperature. The analysis of the first harmonic is used for the determination of the thermoelastic signal and the derivation of a Stress Intensity Factor and crack-growth monitoring, enabling the determination of a Paris Law. The analysis of the second harmonic allowed an in-depth study of plasticity induced crack closure. In addition, the analysis has considered also a previously overloaded specimen of the same material, allowing to evaluate the impact that an extended plastic enclave has on the crack growth and on crack closure. The main results of the work can be summarized as follows:
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