PSI - Issue 80

Yuhang Pan et al. / Procedia Structural Integrity 80 (2026) 43–52

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YuhangPan / Structural Integrity Procedia 00 (2023) 000–000

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(a) (b) Fig. 3: Response of time and frequency domain obtained from the intact specimen (1000 cycles, blue solid line) and specimen with a 11.8 mm fatigue crack (134,000 cycles, red dash line): (a) time-domain signals and (b) frequency-domain spectra

4.2. Crack growth result

Fig.4 illustrates the crack growth behavior of specimens T1–T3, where cracks initiated from the edges of the central hole and were monitored using the camera system. The first detectable crack in specimen T1 was observed at approximately 90,000 cycles with a crack length of 0.5 mm, whereas in specimen T2, a detectable crack appeared at around 86,000 cycles with a crack length of 0.7 mm (Fig.4a). Specimen T3 exhibited the latest crack initiation, with detection occurring at approximately 110,000 cycles and a corresponding crack length of 0.4 mm. The variability observed across specimens is attributed primarily to di ff erences in the machining quality of the central hole. As shown in Fig.4b, although all specimens were manufactured from the same aluminum sheet, minor variations in geometric dimensions, thickness, and mass, together with subtle di ff erences in edge finishing, contributed to discrepancies in crack initiation and propagation behaviour.

(a)

(b)

Fig. 4: (a) Crack growth result on coupons T1-T3 and (b) The variability of length, width, thickness and mass of coupons T1-T3

4.3. Crack detection result

Our previous studies Pan et al. (2024, 2025) have systematically demonstrated the e ff ectiveness of higher order harmonic features for fatigue crack monitoring. In particular, the second harmonic parameter ( β ′ ) and the third harmonic parameter ( γ ′ ) were shown to enable online, baseline-free crack detection and crack