PSI - Issue 33

Dimos Triantis et al. / Procedia Structural Integrity 33 (2021) 330–336 Dimos Triantis et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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proper signals that could warn about upcoming catastrophic failures of structures in-service. The proper description of such signals is sine-qua-non for successful Structural Health Monitoring (SHM) of any kind of engineering structures. The issue seriously concerns the engineering community long ago and it is currently studied intensively worldwide by means of sensing techniques which provide data mainly from the interior of the loaded specimens or structures. Among these techniques, the most widely used one is based on the detection/recording/exploitation of Acoustic Emissions (AE). The AE technique is the most mature and firmly founded one concerning the soundness of its theoretical basis. Addi tional techniques, based on quantification of electric resistance changes of embedded fibers with carbon nanotubes, applications of optical fibers, Global Positioning Systems (GPS) etc., are also used, although at a rather limited extent. Besides numerous advantages the sensing techniques used nowadays for SHM purposes suffer from some limita tions as it is, for example, the fact that they provide qualitative rather than quantitative data, the fact that direct cor relation of their outcomes with quantities measured using traditional sensing tools is not as yet available, and, last but not least, the fact that their in-situ application is difficult, experienced personnel is necessary, installing them for long term sensing/monitoring of many structural members is expensive, while for some of them their application demands interventions not always permitted (as it is, for example, the SHM of Cultural Heritage monuments). In an attempt to cure some of the above-mentioned drawbacks and limitations, advantage is recently taken of well known experimental observations indicating that when specific classes of brittle building materials are subjected to mechanical loads, at levels leading to the formation and propagation of networks of micro-cracks, various types of emissions are recorded, including among others electromagnetic ones. A comprehensive review of the sensing tech niques based on electromagnetic emissions was recently published by Sharma et al. (2021). Special attention has been paid to the extremely weak electric signals, usually called Pressure Stimulated Currents (PSCs), which are produced when specific classes of materials, both piezoelectric and nonpiezoelectric, are stressed mechanically (Stavrakas et al., 2004). It is considered that the phenomenon is associated with the development of microcracks producing electric charges due to the formation of electric dipoles, which constitute charged systems (Varotsos et al. 2002), responsible for an electric potential across the crack, leading to the flow of electric current. Given that the detection/recording of these weak electric currents is relatively easily achieved by means of small and cheap sensors and the respective set-up is quite simple, it seems that this technique could be potentially considered as an alternative SHM technique, assuming that it is properly validated against the most mature and widely accepted sensing technique, i.e., that of the AEs. In this context, the present study, besides describing the acoustic activity during the stage of impending fracture, aims, also, to comparatively consider this activity against the respective electric one, quantified in terms of the PSC technique. It is indicated that, at the stage of impending fracture, the acoustic activity is governed by a power law, independently of whether it is described in terms of the cumulative counts or the F-function. The specific law has been recently found to, also, govern the respective electric activity (Triantis et al. 2021) during the same time interval.

Nomenclature AE(s) Acoustic Emission(s)

PSC(s) Pressure Stimulated Current(s) SHM Structural Health Monitoring F(t)

F-function, representing the time rate of appearance of AE hits

DENT Double Edge Notched Tensile (specimens)

2. The experimental protocol and the raw experimental data Dionysos marble plates of thickness equal to 1 cm, with two symmetric (with respect to the loading axis) edge notches were loaded under direct tension. Two acoustic sensors (S1, S2) were attached on the front face of the specimens very closely to the crowns of the two notches (Fig. 1a) while two electric sensors (ES1, ES2) were attached on the rear face of the plates again very closely to the crowns of the notches. Four classes of specimens were tested with notches’ depths equal to a 0 =20, 40, 60 and 80 mm. Displacement-control conditions were adopted at a rate ensuring quasi-static loading conditions. A stiff INSTRON servo-hydraulic loading frame was used for all the experiments of the protocol. The tensile strength is plotted in Fig. 1b versus the normalized (over the semi-width of the plates) length of the notch. It can be seen that the differences between the four classes tested lie well within the unavoidable experimental scattering,