Issue 40

E. D. Pasiou et alii, Frattura ed Integrità Strutturale, 40 (2017) 41-51; DOI: 10.3221/IGF-ESIS.40.04 50 clear the point where cracking becomes more severe, it seems to be more conservative. The knee points are observed when strain equals about 50-60% of the fracture strain for all three materials studied here (while the stress level equals about 70-80% for marble specimens and about 55-65% for mortar and soda glass specimens). AE technique has already been mentioned as a valuable tool for detecting the impending failure paying attention either to the general acoustic activity or to specific acoustic characteristics in accordance mainly to the stress induced on the specimens [18-19, 35-36]. PSC technique is more recently developed compared to the AE technique and it is not yet fully standardized. It should be mentioned that the specific technique cannot be applied in heavily distorted electromagnetic environments since very low electrical currents (in the order of pA) are measured and the external electrical noise acts as additive electrical noise on the actual signal. In order to minimize such effects special care must be given to the shielding of the experimental apparatus, electrical ground and low noise cabling. Despite the aforementioned limitations, the electric current recorded using the PSC technique has already been related to the mechanical behaviour of the tested material [4, 7]. Concluding, taking into account that strains are much easier to be measured, compared to the stress field developed, the fact that the critical state is detected by both techniques and it is determined at almost the same strain level for all three materials is an interesting finding which could be very useful for monitoring and assessing the integrity of structures. In addition, in the direction of quantifying the correlation of the AE and the PSC techniques, a successful attempt is made for the very first time in the present paper. Indeed a very good correlation exists between the energies measured by both techniques, which obeys a power law with exponent equal to 0.7-0.8 for the materials studied here. R EFERENCES [1] Rao, M.V.M.S., Lakschmi, P.K.J., Analysis of b-value and improved b-value of acoustic emissions accompanying rock fracture, Current Science, 89 (2005) 1577-1582. [2] Colombo, S., Main, I.G., Forde, M.C., Assessing damage of reinforced concrete beam using ‘‘b-value’’ analysis of Acoustic Emission signals, Journal of Materials in Civil Engineering (ASCE), 15 (2003) 280-286. [3] Shiotani, T., Yuyama S., Li, Z.W., Ohtsu, M., Application of the AE improved b-value to qualitative evaluation of fracture process in concrete materials, Journal of Acoustic Emission, 19 (2001) 118-132. 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