PSI - Issue 5

K.S.C. Kuang et al. / Procedia Structural Integrity 5 (2017) 1168–1175 Dong Yang et al. / StructuralIntegrity Procedia 00 (2017) 000 – 000

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3. Experimental methodology

To simulate the initiation and propagation of crack, a three-point-bending fatigue test was carried out on a steel specimen using a universal testing machine (INSTRON MODEL 1334) as shown in Figure 3. The dimensions of the specimen(200 mm × 40 mm × 20 mm ， length × width × thickness respectively). A notch of 6 mm was machined at the bottom mid-span to guide the crack. A sinusoidal cyclic loading was applied on the specimen. The proposed IPOFs sensors were attached to the bottom of the specimen shown in Figure 3 in order to collect the information of the crack initiation and propagation as much as possible. The POFs sensor readings were sampled at 50 z H . As shown in the Figure 3, two AE sensors were attached to the top and side surfaces of the specimen, the sampling rate and gain of preamplifier were 5 MKz and 40 dB respectively, and the threshold for AE hits was set at 60 dB .

Fig. 3. The fatigue crack propagation monitoring system

3.1. Signal processing algorithm

According to the structural mechanics, crack initiation and propagation under a sinusoidal fatigue load are apparently non-stationary, containing envelop modulation. The retrieval results show that the methods used in this field are usually for stationary signal, and there is no commercial solution for monitoring crack initiation and propagation. So a suitable algorithm for the crack monitoring has remained elusive. A crack monitoring algorithm is proposed in this section using the root mean square (RMS)-based envelope detector and a Hilbert-based filter. With this algorithm, the crack initiation and propagation can be properly obtained.