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A novel approach for describing the acoustic activity in brittle structural materials while they are loaded mechanically at levels close to those causing macroscopic fracture is proposed. It is based on the analysis of the rate of acoustic emissions in terms of the Natural Time concept. Experimental data from protocols with either intact or notched beams, made of cementitious materials, subjected to three-point bending are analyzed. It is concluded that in case the acoustic activity is described with the aid of the F-function in the Natural Time domain, its evolution is governed by a power law, independently of geometrical details and the type of the loading scheme. It appears that the onset of validity of this law provides an interesting pre-failure indicator. [ABSTRACT FROM AUTHOR]

The acoustic activity developed in marble specimens under various loading schemes is explored in terms of the recently introduced F-function. The novelty of the study is that instead of describing the temporal evolution of the F-function in terms of conventional time, the Natural Time concept is employed. Although completely different geometries and loading schemes were considered, the evolution of the F-function in the Natural Time domain exhibits a self-consistent motive: its values increase progressively with fluctuations of varying intensity, however, while the fracture is approaching, a power law appears to systematically govern the response of the specimen/structure loaded. The exponent of this law, somehow corresponding to the intensity of the acoustic activity within the loaded complex, varies within broad limits. The onset of validity of the power law designates that the system has entered into its critical stage, namely that of impending fracture, providing a useful pre-failure signal. [ABSTRACT FROM AUTHOR]

Here, we study the scaled distributions for the variance κ 1 of natural time χ when analysing the energy time-series of acoustic emission recorded when marble and cement mortar specimens were subjected to mechanical loading until fracture. It is shown that the κ 1 scaled distributions exhibit – for at least three orders of magnitude – a characteristic exponential tail like that previously found for the order parameter in equilibrium (e.g., two-dimensional Ising model), and in non-equilibrium (e.g., turbulent flow) critical systems as well as in seismicity. The presented findings, which are also compared to those coming from the non-conservative Olami–Feder–Christensen earthquake model and the Burridge–Knopoff train model, advocate for the use of κ 1 as an order parameter for the acoustic events preceding failure in a fashion similar to that in the case of seismicity. • The scaled PDF of the variance κ 1 of Acoustic Emission(AE) in natural time is studied • We use AE energy recorded from mechanically loaded brittle materials until fracture • An exponential tail earlier found for the order parameter in critical systems arises • We compare with the Olami–Feder–Christensen and the Burridge–Knopoff models results • The results advocate for using κ 1 as an order parameter for the AE preceding rupture • The similarities arising from the comparison may be useful for practical applications [ABSTRACT FROM AUTHOR]

The acoustic emissions observed before the failure of typical construction materials like marble and cement mortar are analysed in natural time. The study of the variance κ 1 of natural time, the entropy S , as well as the entropy S − under time reversal reveals the presence of two behaviours which can be understood either by the Burridge–Knopoff (BK) train model or the Olami–Feder–Christensen (OFC) earthquake model. The present results may be of practical importance for the identification of the time of failure of materials subjected to increasing stress. • Acoustic Emissions before failure of the materials are studied in natural time. • The quantities κ 1 , S , S − defined in natural time are calculated using the AE energy. • The results reveal that criticality has been reached well before failure. • Comparison with the Olami–Feder–Christensen and the Burridge–Knopoff models is made. • Similarities arising from this comparison may be useful for practical applications. [ABSTRACT FROM AUTHOR]

• The acoustic and electric activities in brittle materials are studied comparatively. • Specimens made of either plain or fiber-reinforced concrete under 3 PB are used. • The study is implemented in terms of the Natural rather than the conventional time. • Clearly distinguishable criticality indices are detected. • The acoustic and electric criticality indices detected are in very good agreement. The target of this study is to detect the existence of criticality indices, namely, indices which may be considered as in-time warning signals about the entrance of a mechanically loaded system into its critical stage, i.e., the stage of impending fracture. To achieve this target advantage is taken of the experimental data provided by two sensing techniques, namely the Acoustic Emissions and the Pressure Stimulated Currents ones. The innovation of the present study is that the temporal evolution of the acoustic and electrical activities is implemented in terms of the Natural Time rather than in terms of the conventional time. The experimental protocol included three-point bending tests with centrally notched beam-shaped specimens made of either plain concrete or concrete reinforced with either plastic or metallic short fibers. The comparative analysis of the acoustic and electric activities in the Natural Time Domain revealed that both techniques provide clearly distinguishable criticality indices. Moreover, it was proven that the overall evolution of the acoustic activity is quite compatible with the respective one of the electric activity, concerning both the instant at which the pre-failure indices are detected, and, also, the duration of the respective regions of criticality. [ABSTRACT FROM AUTHOR]