Issue34

J. Saliba et alii, Frattura ed Integrità Strutturale, 34 (2015) 300-308; DOI: 10.3221/IGF-ESIS.34.32 305 zone were also computed using probability and statistics laws by dividing the damage zone into two areas: the zone of confidence with high damage and a weak damage zone based on the distribution either of the number or the amplitude of acoustic emission events [15]. In our study, the length of the FPZ was estimated based on AE events density along the ligament length. Fig. 5(a) shows the density of AE events at each Y location at 10% of the maximal strength in the post-peak region for UN200, UN100, SN200 and LN200 beams. For notched beams, it can be seen that ahead of the notch tip, the number of AE events increases progressively due to the front boundary, attains the maximum value (N max ) and then remains almost constant for some distance along the ligament length and decreases as the crack propagates towards the back boundary. The same behavior can be observed with the AE energy distribution. The curves clearly follow the energy dissipation trend shown by the boundary effect model. The fracture energy through the front and back boundary effect can be taken into account by assuming a trilinear variation of local fracture energy over the ligament length [22, 23, 24, 25, 26]. However for unnotched beams, the AE events density presents an important peak near the front boundary indicating a more important energy dissipation for crack initiation, then decreases along the ligament length. The occurrence of AE events can be considered as a criterion to follow the crack propagation through the beam depth for different loading levels. Therefore, the length of the FPZ was defined as the length of the segment from notch tip to the intersection of the histogram with the horizontal line located at 20% of N max . Fracture examination in concrete through combined techniques as digital image correlation and X-rays can give additional information; however the monitoring of the evolution of the fracture length with AE technique shows similar trends as those later and proved to be quantitatively acceptable [4, 27]. The relative fracture length (Lcrack/D), i.e. the ratio of length of fracture from the crack mouth (Lcrack) to height of specimen (D), obtained with the AE technique at different loading intervals is plotted in Fig. 5 (b) for UN200, UN100, SN200 and LN200 specimens. . For the LN200 beams, fracture initiates earlier at 90% of the maximal strength in the pre-peak regime followed by SN200, while the fracture growth of unnotched beams starts after the peak load. The relative fracture length in LN200 specimen is more important in comparison with SN200, UN100 and UN200 and the relative fracture length of UN100 has values between that of UN200 and SN200 beams. The fracture growth is relatively abrupt at the beginning and decreased at the end. A similar behavior was also observed while studying size effect with a more important crack length for smaller beams [27]. Note here that the AE analysis does not give precise information about the tip of the stress free crack because the criteria used for determining the length of the FPZ may cause a loss of information as it is not possible to know exactly the crack tip position. 0 50 100 150 200 250 300 350 400 0 50 100 150 200 AE events Y (mm) LN200 SN200 UN200 UN100 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0 10 20 30 40 50 60 70 80 90 100 90 80 70 60 50 40 30 20 10 L FPZ /ligament length % Loading UN200 SN200 LN200 UN100 Figure 5: a) Cumulative AE events at each Y location and b) evolution of relative crack length with load steps for UN200, UN100, SN200 and LN200 beams. Based on the localization maps, the width of the FPZ was also estimated based on the horizontal line located at 20% of N max which separate the zone with a high number of events representing the localization of microcracking and the outside zone where the level of damage is lower [15] (Fig. 6). The discrepancy between the widths of the FPZ is relatively important, this could be due to different sources of error: the localization of AE events is realized in 2D on one face of the specimen however the crack path inside the thickness of the beams is different (3D) due to the internal heterogeneity of the material. In addition, the crack path is dominated by the distribution of aggregates and other softening mechanisms and the width of the FPZ cannot be determined correctly when main crack branches into microcracks in multiple directions. However, the results show that the width of FPZ tend to increase with the decrease of the relative notch depth. This behavior was also observed in Otsuka and al. [4]