Issue34

J. Saliba et alii, Frattura ed Integrità Strutturale, 34 (2015) 300-308; DOI: 10.3221/IGF-ESIS.34.32 306 Those differences in the length and the width of the FPZ show a non-uniform stress distribution in the cross section of the beams with different sizes and different relative notch depth which could be responsible, in combination with the softening behavior of concrete, of the size effect in concrete structures. 0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 Width of FPZ (mm) UN 00                 UN200  SN200  LN200 Figure 6: The width of the FPZ for UN200, UN100, SN200 and LN200 beams T HE AE BASED IB - VALUE he AE peak amplitude is related to the magnitude of cracks developed during fracture. Micro-cracks generate an important number of small AE hits amplitude, while macro-cracks generate few hits with higher amplitude. To determine the overall fracture quantitatively on basis of peak amplitude, the b-value originated from seismology is performed. It is defined as the absolute value of the slope of the cumulative distribution of AE hits amplitude. If small- scale fractures are superior to large-scale fractures, the Ib-value tend to increase, in the contrary if large-scale fractures are superior to small-scale fractures, the Ib-value tends to decrease. The Ib-value was calculated using the peak amplitudes of 50 successive hits as:                     10 1 10 2 1 2 log log N N Ib where  is the standard deviation of the amplitude distribution,  is the mean value of the amplitude distribution,  1 is the coefficient related to the smaller amplitude and  2 is the coefficient related to the fracture level.  1 and  2 are constants and are equal to 0 and 1 respectively. Fig. 7 shows the evolution of the improved Ib-value. The results show that Ib-value decreases near the peak and then fluctuates strongly in the post peak region. Thus small-scale fracturing with higher frequencies were dominantly generated up to the peak then large scale fractures with lower frequency started near the peak and continue to occur. C ONCLUSION n experimental study is presented in order to monitor fracture growth during three point bending tests in notched and unnotched concrete beams. The AE technique was also employed to follow the damage evolution and to characterize the FPZ. The global mechanical response of the beams can be then translated from the local fracture process in the material. The total number of the released AE hits and AE energy increases with the decrease of the relative notch depth. In addition, the correlation between the load–CMOD curves and AE analysis shows that as the notch depth increases the peak of AE records becomes lower and wider, and moves along the horizontal axis indicating a more ductile behavior. The fracture process zone length was also monitored with AE technique. The evolution of the fracture length was highly dependent on the relative notch depth. The initiation of the fracture is detected earlier in notched beams compared to unnotched beams with a higher relative crack length respectively. An additional energy dissipation for crack initiation was also observed in unnotched beams. T A