LAUSR

Accurate knowledge of the spatiotemporal evolution of damage within mechanically loaded structural elements is of great importance for a wide range of engineering fields. In this direction, the acoustic emission (AE) technique has been long ago recognized as an extremely valuable and flexible tool, which permits monitoring and even quantifying dynamic processes within loaded structural elements. In specific cases, the data provided by the AE technique allow gaining some insight into the deformation mechanisms (Ohtsu 2010; Aggelis et al. 2013), and also they provide timely warning signals concerning forthcoming failure (Miller et al. 2005; Kourkoulis et al. 2018a, b). In laboratory experiments, with specimens prepared according to the respective international standards and submitted to monotonically increasing load until the final fracture, the AE technique allows recording of a number of AE hits ranging from several hundreds to several tenths or even hundreds of thousands, depending on the material, the exact size of the specimen and the loading mode. The AE hits rate (hits per second) and the energy release rate are some of the parameters that characterize the acoustic emission activity and can provide direct information concerning the time rate of generation of micro-cracks (Vidya et al. 2013; Moradian et al. 2016). Moreover, proper elaboration of data concerning the average frequency and the rise time per amplitude (or simply the rise time) of the acoustic signals permits proper classification of the internal damage processes according to whether they are due to tensile micro-cracking (Mode-I) or due to shear (and friction) phenomena (Mode-II) or even due to a combination of the above, i.e., mixed-mode phenomena (Ohno and Ohtsu 2010; Aggelis 2011). Usually the data recorded by the AE sensors during any type of experiment are represented and analyzed with the aid of plots using a logarithmic scale for the hits rate and the energy release given that the relation between these quantities and the respective mechanical ones (like for example load or stress) is strongly non-linear, at least after a specific load (or stress) threshold. The aim of this study is to introduce an alternative way for the representation of the data concerning the acoustic activity, in an attempt to enlighten what happens during the very last loading steps just before the fracture of the specimens. This alternative representation is based on a function of the inter-event times (denoted from here on as F function), which is plotted in terms of an “inverse” time arrow [sometimes denoted as “time-to-failure” (Li and Ma 2014)] using logarithmic scales. In fact, the term “time-to-failure” denotes the time parameter (tf − t), where tf is the time instant of fracture. The pros and cons of this representation are here analyzed taking advantage of data from three-point bending tests with specimens made of either concrete or marble.