PSI - Issue 39
Riccardo Cappello et al. / Procedia Structural Integrity 39 (2022) 179–193 Author name / Structural Integrity Procedia 00 (2019) 000–000
190 12
4.2. Sample #2 The results from Sample #2, in terms of crack growth and thermoelastic maps, are here presented. Sample #2 is nominally identical to Sample #1, but an overload after 2 mm of crack growth produced a big plastic enclave ahead of the crack tip (Figure 14, left).
Figure 14 – Picture of the overloaded specimen (left) and crack length vs the number of cycles (right). The big plastic enclave almost stops the growth of the crack for the first 100k cycles.
The overload causes a slowdown in the crack growth, until the crack manages to overcome the zone of residual compressive stresses. This effect can be clearly seen looking at Figure 14 (right), where the length of the crack measured through the pictures captured via digital camera is reported vs the number of cycles. After about 230000 cycles and 2 mm of crack growth, there is a remarkable increase in the crack growth rate that regains its normal trend. In this case, only the optical measurement of the crack length is performed, since the analytical solution has not been used due to the large amount of plasticity. 4.2.1. Thermoelastic first harmonic signal The thermoelastic isopachics shape (Figure 15) is influenced by the overload, being different with respect to Sample #1. In this case, the thermoelastic signal shows a more rounded shape near the crack tip area. Only after the crack tip distances itself from the highly plasticized area the isopachics start regaining the expected cardioid shape.
Thermoelastic Signal
T
Thermoelastic Signal
T
Thermoelastic Signal
T
Sample #2: N = 190000
Sample #2: N = 301000
Sample #2: N = 100000
[°C]
[°C]
[°C]
0.8
0.8
0.8
0.76
0.76
0.76
20
20
20
0.72
0.72
0.72
0.68
0.68
0.68
40
40
40
0.64
0.64
0.64
0.6
0.6
0.6
60
60
60
0.56
0.56
0.56
80
80
80
0.52
0.52
0.52
0.48
0.48
0.48
100
100
100
0.44
0.44
0.44
0.4
0.4
0.4
120
120
120
0.36
0.36
0.36
140
0.32
140
0.32
140
0.32
0.28
0.28
0.28
160
160
160
0.24
0.24
0.24
0.2
0.2
0.2
180
180
180
0.16
0.16
0.16
0.12
200
0.12
0.12
200
200
0.08
0.08
0.08
220
220
0.04
220
0.04
0.04
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
mm
Figure 15 – FH thermoelastic amplitude isopachics evolution for the case of the overloaded specimen
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