Issue34

J. Saliba et alii, Frattura ed Integrità Strutturale, 34 (2015) 300-308; DOI: 10.3221/IGF-ESIS.34.32 303 for SN200 and at 20% for LN200. The position of the peak of occurring AE hits changed also with the relative notch depth and shifted to the latter part of the descending branch as the relative notch depth decreased. For unnotched beams, the peak appeared earlier with a more important peak magnitude however the width of the occurring AE hits distribution peak is less important and the decrease of AE hits distribution becomes steeper in the descending branch following the same trend of the load-CMOD curves. This behavior indicate a more brittle behavior for unnotched concrete beams. For UN100 beams, the AE distribution presents a peak near the maximal strength with a larger width and shows an intermediate behavior between UN200 and notched beams. This indicates a more ductile behavior for UN100 beams in comparison with UN200 beams and thus a similar influence of the notch depth and specimen’s depth on fracture process after the initiation of the crack. 0 50 100 150 200 250 300 350 400 450 500 0 2 4 6 8 10 12 14 16 18 0 0,1 0,2 0,3 0,4 0,5 AE hits number Load (kN) Cumulative AE hits x 1500 CMOD (mm) LN200 Relative notch depth =0.5 0 50 100 150 200 250 300 350 400 450 500 0 2 4 6 8 10 12 14 16 18 0 0,1 0,2 0,3 0,4 0,5 AE hits number Load (kN) Cumulaive AE hits x 1500 CMOD (mm) SN200 Relative notch depth = 0.2 0 50 100 150 200 250 300 350 400 450 500 0 2 4 6 8 10 12 14 16 18 0 0,1 0,2 0,3 0,4 0,5 AE hits distribution Load (kN) Cumulative AE hits x 1500 CMOD (mm) UN200 Relative notch depth = 0 AE hits distribution Load cumulative AE hits 0 50 100 150 200 250 300 350 400 450 500 0 2 4 6 8 10 12 14 16 18 0 0,1 0,2 0,3 0,4 0,5 AE hits distribution Load (kN) Cumulative AE hits x 1500 CMOD (mm) UN100 Relative notch depth = 0 AE hits distribution Load cumulative AE hits Figure 3: Correlation of load vs. CMOD curves with the AE characteristic in UN200, UN100, SN200 and LN200 beams. D AMAGE LOCALIZATION uring the formation of a crack, energy is emitted as an elastic wave and propagates from the crack location to the AE transducers at specimen surface. The localization map of AE events is based on arrival times of the first wave at each transducer and their respective velocity in concrete specimen. Once the arrival time is picked, least- square method is used to estimate the event location. The cumulative acoustic events are placed in 2D. The detected AE events of notched and unnotched beams are presented over a window covering the specimen height and a width of 300 mm centered at the notch (Fig. 4). The localization maps of AE sources are drawn at the peak, at 50% of the maximal strength in the post-peak region and at the end of the rupture test. Each plotted point indicates a detected AE source. In addition to number of AE events, it is important to investigate AE signal parameters. The initiation and propagation of cracks in concrete are generally correlated to AE signals amplitude. Extensive studies have shown that the absolute acoustic energy is also an important parameter to characterize an event [19]. Thus, the 2D localization maps of AE events are classified in terms of their levels of energy. Six energy levels are defined. It can be seen that AE events of higher energy levels are located in the core of the FPZ [4] indicating the crack path with a rough and complex fracture surface inside the specimen. D