Spatio-Temporal Distribution of the Sources of Acoustic Events in Notched Fiber-Reinforced Concrete Beams under Three-Point Bending

Abstract

The acoustic activity, generated in notched, beam-shaped concrete specimens, loaded under three-point bending, is studied in terms of the position of the sources of acoustic events, and the frequency of their generation. Both plain specimens (without any internal reinforcement) and specimens reinforced with various types of short fibers were tested. The target of the study is to investigate the existence of indices that could be considered as pre-failure indicators of the upcoming fracture. In addition, an attempt is undertaken to classify the damage mechanisms activated to tensile or shear nature. Considering comparatively the spatio-temporal evolution of the position of the acoustic sources and the respective temporal evolution of the frequency of generation of acoustic events, it was concluded that for relatively low load levels the acoustic sources are rather randomly distributed all over the volume of the specimens. As the load increases toward its maximum value, the acoustic sources tend to accumulate in the immediate vicinity of the crown of the notch and the average distance between them approaches a minimum value. When this minimum value is attained, the load is maximized and the generation frequency of the acoustic events increases rapidly. The simultaneous fulfillment of these three conditions is observed a few seconds before the onset of propagation of the catastrophic macrocrack for all classes of specimens tested, providing a kind of warning signal about the upcoming fracture. Moreover, the classification of the damage mechanisms to tensile and shear ones revealed a crucial difference between the plain and the reinforced specimens after the maximization of the load applied. Indeed, while for the plain specimens, the prevailing damage mechanism is tensile microcracking, for the reinforced specimens a balance between tensile and shear damage mechanisms is observed after the load applied has attained its peak and starts decreasing.

Revealing the Proximity of Concrete Specimens to Their Critical Damage Level by Exploring the Cumulative Counts of the Acoustic Emissions in the Natural Time Domain

“…This combination signals the transition from micro- to macro-cracking, with a fatal macro-crack front propagating from the crown of the pre-machined notch to the interior of the specimens’ volume in the direction of the load application point. The specific statement is well validated in a very recently published paper (with specimens of identical composition to the ones studied here), the aim of which was to study the spatiotemporal evolution of the position of the sources of the AEs [ 59 ].…”

“…A detailed description of the specimens’ geometry, their composition and the loading scheme adopted can be found in ref. [ 59 ].…”

“…Details about the characteristics of the fibers are found in refs. [ 59 , 60 , 61 , 62 ]. It is emphasized here that, since the present study focuses on temporal critical quantities and observations, the issue of data synchronization becomes a crucial one.…”

Combined Effect of In Situ Stress Level and Bedding Anisotropy on Hydraulic Fracture Vertical Growth in Deep Marine Shale Revealed via CT Scans and Acoustic Emission

“…Although hydraulic fracturing technology has been successfully used in reservoir stimulations, this approach might lead to undesirable outcomes in deep-buried formations with high structural complexity, such as those in the Sichuan Basin in South China [5]. The shale strata on the margin of the Sichuan Basin are frequently folded and Energies 2023, 16, 7270 2 of 14 faulted [3,4], and the varied bedding inclinations significantly influence the hydraulic fracture propagation [6,7]. Moreover, the fracturing operations of deep shale reservoirs face challenges of insufficient fracture growth and a rapid decline in productivity due to the increasing in situ stress level [1,2].…”

“…The acoustic emission method is also used in analyzing shale hydraulic fracturing since millimeter-scale fractures and meter-scale faults hold highly similar physical processes of seismic activities [13]. Also, the acoustic emission energy derived from the integral of the voltage signal over the duration of acoustic emission waveform is widely used to reflect the fracture initiation and propagation [14][15][16][17]. Gehne et al [7] analyzed the seismic response of hydraulic fracture propagation in anisotropic shale, and they demonstrated that a hydraulic fracture crossing the bedding required a higher fluid pressure and generated more acoustic emission activities than splitting the bedding.…”