Analysis of Fracture Processes in Steel Fibre Reinforced Cementitious Composites using Acoustic Emission Testing

Abstract

Plain concrete has low tensile strength. Steel fibre reinforced concrete (SFRC) is an alternative to plain concrete. One of the major reasons of using the SFRC is due to its high tensile strength. The fracture process in SFRC has been studied by researchers over the years. It is known that microcracking or any other change in a solid releases strain energy. A part of this released strain energy is transformed into stress waves that propagates in the medium. These stress waves are referred to as acoustic emission (AE). From literature review it was observed that the study of the fracture process in SFRC using AE testing is a relatively unexplored area. Most of the studies have been limited to monitoring of SFRC using conventional AE waveform parameters such as AE energy, AE peak amplitude, and AE counts. These waveform parameters can be dependent on various external factors such as the acquisition system, detection threshold and attenuation characteristics of the material. Because of these reasons the AE parametric data may not be reproducible in certain cases and can be qualitative in nature. In this thesis the aim is to study the fracture processes in SFRC using AE testing. An attempt has been made to apply different data analysis techniques such as (i) wavelet packet decomposition (WPD) (ii) AE information entropy (iii) power law relations to the AE waveform data and extract useful information related to the fracture mechanisms. AE waveform contains information regarding its source of fracture mechanism. Fracture process in SFRC can be classified broadly into two types namely (i) cementitious matrix cracking (ii) steel fibre pullout. AE released during the fracture process in SFRC is a combination of these two mechanisms. Using WPD and based on the frequency domain features it was possible to separate the AE waveforms due to cementitious matrix cracking and steel fibre pullout. To make the damage monitoring independent of the AE system and the detection threshold, AE information entropy was used. AE information entropy is based on the probability distribution of the AE waveform amplitude data. Therefore, AE information entropy is only dependent on the unfiltered waveform characteristics. Therefore, AE information entropy is independent of the AE acquisition system. The AE information entropy showed a direct relationship with damage when it was compared to an available AE based damage index. Also, its behaviour was observed to change with the SFRC. The cumulative information entropy exhibited an increase in slope with the increase in the steel fibre content. Criticality which is a concept in the statistical mechanics was studied for the AE avalanches generated during the compressive fracture process and Mode I fracture process. The experimental analysis focuses on the AE absolute energies of the individual AE events as well as on the time correlations between successive events. The AE absolute energy distributions exhibited critical behaviour truncated by an exponential damping factor in both plain concrete and SFRC specimens. The test (plain concrete and SFRC) samples showed similar waiting time distributions. However, some differences were observed in the Omori’s power law. The exponent of the absolute energy distribution (Gutenberg-Richter exponent) showed a relation with development of fracture process in the material. The exponent decreased as the fracture in the specimens progressed. Finally, the fracture process zone (FPZ) in SFRC was studied using AE testing. FPZ consists of several toughening mechanisms such as aggregate bridging, aggregate interlocking, and steel fibre bridging. The FPZ has been divided into three zones namely (i) major damage zone (ii) moderate damage zone (iii) low damage zones. These zones have been identified based on the AE parameters such as AE peak amplitude, AE peak frequency, AE information entropy, AE duration and Gutenberg-Richter exponent. It was observed that the zone of major damage consisted of AE events with high peak amplitude and low information entropy. The above studies may be useful to understand the fracture process in SFRC in a different perspective, using AE testing.