Issue 50

I. Dakanali et alii, Frattura ed Integrità Strutturale, 50 (2019) 370-382; DOI: 10.3221/IGF-ESIS.50.31 377 Moreover, the characteristic slope changes of the load-time curve are accompanied by increased AEs duration. After the maximum load, when the connection has lost its integrity, there are many acoustic signals of high duration that are attributed to the prevailing co-existing friction as the bar is sliding through the hole. The variation of the bar’s displacement and that of the cumulative energy of the AE versus the load induced are plotted in Fig.9 for two typical experiments (1 st class). It is very interesting to observe that almost simultaneously with every slope change of the bar’s displacement curve, a significant increase of the energy of the AEs is recorded. Despite the fact that the 1 st class pull-out tests give very encouraging AE results, it is obvious that the acoustic signals produced by the experimental set-up dominate the final results and the pure pull-out events are ‘covered’ by those parasitic events. Considering that the alternative design of the pull-out tests (2 nd class) focuses on the tested area, the results are expected to be clearer. The time variation of the duration and energy of the AEs, the load and the data gathered from the LVDT in contact with the bar’s lowest end for a typical 2 nd class test are plotted in Fig.10a. It is obvious that during the first two periods regarding the changes of inclination for the LVDT curve, there is not a significant number of acoustic signals. The accumulation of the AEs started slightly before the third slope change of the LVDT curve. The location of the Acoustic Events and specific- ally their “y” position along the bar-paste area versus time is plotted in Fig.10b. The system started to record AE along the anchoring length when the LVDT’s indications curve changes inclination for the third time before the maximum load. Considering that the above results are clearer and most of the parasitic signals have been removed, it could be useful to categorize the recorded signals produced due to the pull-out phenomenon. Based on the average frequency and RA para- meter analysis [16, 17] the respective diagrams for two characteristic 2 nd class tests are drawn (Fig.11). There are four time ranges, three according to the LVDT curve inclination’s change and one after the maximum load. The first and second area’s signals (A&B) presented a significant decrease of the RA parameter. The third area (C) presents a worth mentioning Figure 10: ‘2 nd Class’ test: (a) Time variation of the load and the LVDT indications with the duration and energy (contour plot) of the AEs; (b) Time variation of the LVDT indications, the load and the y position of the AEs (along the anchoring length). (a) (b)