PSI - Issue 3

S.K. Kourkoulis et al. / Procedia Structural Integrity 3 (2017) 326–333 6 S.K. Kourkoulis, D. Triantis, I. Stavrakas, E.D. Pasiou and I. Dakanali / Structural Integrity Procedia 00 (2017) 000–000

331

2.0

2.0

PSC cum AE hits

1.5

1.5

cum AE hits (x1000)

1.0

1.0

PSC [pA]

0.5

0.5

0.0

0.0

0

120

240

360

480

Time [s]

Fig. 5. The PSC and the cumulative number of hits near the right notch for the specimen mentioned in Fig.4a.

which corresponds to about 92% of the maximum load attained. From this point on, the PSC values increase abruptly indicating, according to the literature (Stavrakas et al. 2003, Triantis et al. 2006, 2007, Kyriazopoulos et al. 2011), that the specimen entered a critical state and failure is impending. The PSC results are in good agreement with the acoustic activity near the same notch as it is seen, also, in Fig.5. The curve describing the time variation of the cumulative AE hits (green line) can be divided in three almost linear segments. For the first one (0 s - 330 s) the acoustic activity is very low while in the third one (440 s - 470 s) the acoustic activity is strongly amplified. The threshold at which the acoustic activity becomes very intense corresponds to about 95% of the maximum load attained. The number and positions of the acoustic sensors used, permitted (as it was already mentioned) the detection of the source of the acoustic events recorded. Taking advantage of this feature of the AE technique, an attempt to predict the tip from which fracture would start was undertaken. In this context, data from another typical specimen (with a=4 cm) are here analyzed. The events recorded are grouped according to their amplitude. The results of this classi fication are plotted in Fig.6a. It is obvious that the source of the vast majority of events with high amplitude are located in the close vicinity of the left notch, i.e. the one from which the crack initiated, as it is seen in the photo of Fig.6c.

(a)

(b)

(c)

0.12 0.08 0.04 0.00 -0.04 -0.08 -0.12

sensor 7

sensor 2

sensor 3

sensor 5

sensor 8

sensor 4

sensor 1

sensor 6

Fig. 6. (a) The location of the acoustic events colored according to their amplitude. Sensor 5 is attached at the rear side of the specimen; (b) The specimen (a=4 cm) before the test; (c) The same specimen after the test. -0.08 -0.04 0.00 0.04 0.08