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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Nomenclature NDT

Non-destructive testing SHM Structural health monitoring FCG Fatigue crack growth AE Acoustic emission PZTs Piezoelectric transducer f Loading frequency ∆ σ t Stress range FFT Fast Fourier Transform RMS Root mean square β ′ Second harmonic parameter γ ′ Third harmonic parameter DPL Dynamic Piecewise Linear

2. The fatigue testing procedure

A 250 kN Instron hydraulic fatigue testing machine (Fig. 1a) was employed to apply cyclic loading to the specimens. For specimens T1–T3, the maximum and minimum applied stresses were 90 MPa (corre sponding to 62% of the material yield strength) and 9 MPa, respectively, yielding a stress ratio ( R ) of 0.1. Owing to stress concentration around the central hole, fatigue cracks initiated and propagated symmetrically from both sides. The initiation of the crack and subsequent propagation were continuously performed using a Canon EOS 5D Mark II camera equipped with a 21-megapixel full-frame CMOS sensor. Crack lengths were measured using a measurement tape attached directly to the specimen surface, as shown in Fig.1d, and Fig. 1e highlights three representative stages of fatigue damage progression: (I) no visible crack, (II) a crack length of 5.8 mm, and (III) final fracture. Two P-876 DuraAct PZT sensors were mounted on each specimen using thermoplastic adhesive film (Yue et al. (2021)). Their exact positions and geometries, referenced to the lower-left corner of the specimen as the origin, are showed in Fig.1b. Sensors P1 and P2 were placed symmetrically with respect to the central hole and vertically aligned, thus targeting the region most suscep tible to initiation and propogation of fatigue cracks. Following the findings of Pittarresi et al.(Pitarresi et al. (2019)), which demonstrated that loading frequencies above 5 Hz result in a quasi-steady material response, a loading frequency ( f ) of 6 Hz was adopted in this study. The dynamic responses under fatigue loading were acquired using a TiePie Handyscope HS5 oscilloscope and subsequently stored with Multi-Channel Oscilloscope software, as shown in Fig.1c. The proposed method for crack monitoring is summarized in Fig. 2. As illustrated in Fig. 2, the proposed crack monitoring approach integrates experimental loading, signal acquisition, and feature extraction. First, cyclic loading is applied to the specimen using a servo-hydraulic fatigue testing machine, while responses are continuously acquired by surface-bonded PZT sensors. Subsequently, the frequency domain is obtained using FFT, enabling the identification of the fundamental, second, and third harmonic frequency components. Three features are calculated: the RMS of the signal in the time domain, the second harmonic parameter β ′ , and the third harmonic parameter γ ′ , which are used to monitor the crack. Finally, these parameters are tracked over the entire fatigue process and correlated with periodic visual crack length measurements. 3. Method

4. Results and discussion

This section presents the feature extraction process, the crack growth and detection results obtained using the di ff erent features and specimens.