PSI - Issue 28

Dimos Triantis et al. / Procedia Structural Integrity 28 (2020) 502–510 D. Triantis, I. Stavrakas, A. Kyriazopoulos, E. D. Pasiou, S. K. Kourkoulis / Structural Integrity Procedia 00 (2019) 000–000 3

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phenomena, i.e., lattice separation, bond breaking, acceleration of free electrons etc. (Varotsos and Alexopoulos 1986; Enomoto and Hashimoto 1990; Slifkin 1993). It is experimentally verified that these electrical signals are strongly related to the level of the mechanical stress induced, while the crucial instant for their appearance is somewhere within the transient time interval between linear and non-linear mechanical response, related, also, to the onset of irreversible deformations (Stavrakas et al. 2003). From the physical point of view the above phenomena can be explained in terms of the Moving Charged Dislocations (MCD) theory. According to this approach, dislocations and other types of mechanical imperfections appear as a result of mechanical loading. The electrical neutrality around a physical defect is not maintained in case the material is loaded further since movement in the net of the structure of the material would generate dipoles of opposite signs and therefore the net polarization would not be zero anymore (Vallianatos and Tzanis 1999). Clearly, any net electrical polarization of a given sign, must be the result of a net excess of charged dislocations. Given that the motion of charge carriers is an electric current, detection of electric current emissions is possible when mechanical loading is applied on a sample. According to the Moving Charged Dislocations model an analogy between the PSC and the strain rate, dε/dt, exists due to the propagation of arrays of charged edge dislocations through the material during micro-fracturing (Vallianatos et al. 2004).   d PSC t dt   (1) In case the material obeys Hooke’s law (i.e., the stress does not exceed the material’s yield limit, σ y ) and assuming a constant rate dσ/dt, PSCs are not expected to be emitted. However, stress changes even in this range seems to cause some material damage and PSC emissions are detected. On the other hand, in case the applied stress exceeds the yield limit σ y , the modulus of elasticity starts decreasing and internal damage appears. Then a damage variable, D, is introduced, the value of which ranges in the (0, 1) interval, somehow quantifiying damage as (Turcotte and Shcherbakov 2006):     y 0 y y E 1 D ,            (2) For σ>σ y the material enters its non-elastic region of the stress-strain relation (ignoring the nonlinear elastic portion of its response) and therefore permanent changes take place in the material. As a result, the strain rate dε/dt will not be constant, thus inducing electrification, measurable in terms of weak PSC emission. In general, the electric emission phenomenon is of quite complex nature and origin, strongly depending, among others, on the kind of the loading scheme. As a result, the PSC recorded cannot be uniquely standardized. It is quite often observed that during the loading procedure the damage/fracture mechanisms activated result to PSC recordings of non-stationary nature with strong fluctuations (Vallianatos and Triantis 2008; Stergiopoulos et al. 2013; Kourkoulis et al. 2018; Pasiou et al. 2019). From the laboratory point of view, the electric signals are detected in the form of a very weak electric current using ultra sensitive electrometers and are noted as Pressure Stimulated Currents (Anastasiadis et al. 2004). Usually the sensors are pairs of gold plated electrodes properly attached at strategic point of the loaded specimen. The techique has been already applied for various types of brittle materials, like marble (Triantis et al. 2006) and amphibolite (Triantis et al. 2007) as well as for various types of cement-based materials (Kyriazopoulos et al. 2011). Moreover, it has been proven that the time evolution of the PSC provides valuable indices indicating proximity of the system (structure or specimen) to its critical stage, or equivalently to fracture. 3. The experimental protocol 3.1 Material and specimens The specimens were prepared by the Greek cement manufacturer TITAN using ordinary Portland cement (OPC). The constituents were fine aggregates (sand with 3 mm maximum and 0.6 mm minimum grain size, fineness 2.8, specific gravity 2.6), OPC and water at a mixing rate 3:1:0.5, respectively. Low speed was chosen for mixing in order to achieve optimum moisturizing conditions. Then, quick spinning of the mixture followed for 1 min, and finally the