Studies on the Mechanisms of Fracture, Evolution of Microcracks and Size Effect in Concrete through Acoustic Emission Analysis

Abstract

The mechanics of fracture evolution in heterogeneous material like concrete is a distinct and complex phenomenon and still stands as an open problem after decades of research. A cluster of multi-scale cracks ahead of the crack tip is known as a fracture process zone in concrete. The fracture process zone reduces stress singularity ahead of the crack tip by stress redistribution and contains several toughening mechanisms. Therefore, understanding of fracture behaviour of the concrete builds around the fracture process zone evolution. Moreover, fracture process zone also originates the well know size-effect phenomena in concrete. The energy dissipated by cracks in fracture process zone travels throughout the material as a stress wave, which is known as acoustic emission. The acquisition of acoustic emissions by piezoelectric sensors for understanding fracture phenomenon is a recently developed modus operandi for fracture studies in concrete. In this research an attempt is made to develop new paradigms of AE analysis and explore as many as possible aspects of AE information. In this dissertation, the acoustic emission technique is used and a thematic styled scientific inquiry for fracture process evolution in concrete as well as damage inference is presented. The acoustic emission waveforms carry information regarding the source, its mechanism, and the propagation media. Therefore, deciphering the information carried by waveforms is a crucial task. The individual spectra of waveforms as well as the collective population of AE events are explored. The level of information sets the theme of the study in which waveform spectra, magnitude distribution, cumulative AE energy, and non-extensive entropy is investigated consecutively. The wavelet transform for the multiresolution spectral study gives an estimator called wavelet entropy to distinguish signal based on spectral energy distribution. The variation of wavelet entropy reveals the diversification of fracture mechanisms as stress level increases and specimen approaches failure. Therefore, the fracture process zone is not only a result of diversified crack sizes but also diversified fracture mechanism, which eventually contributes to its quasi-brittle behaviour. Further, the traditional b-value analysis using magnitude-frequency distribution of AE events is critically discussed. On the shortfall of Gutenberg-Richter law, a recently developed generalized logistic equation for b-value analysis is applied. The b-value of generalized logistic equation shows damage compliant behaviour which can be used for damage inference. Acoustic emission energy is a parameter determined from AE waveforms and can be used for energy-based damage detection. A global as well as local correlation is established between applied mechanical work for fracture (MWF) and AE energy. A damage parameter, mapping nonlinear AE energy evolution over crack length using piecewise linearization, is proposed for damage assessment. Finally, Tsallis non-extensive statistical mechanical framework is explored to model the probability distribution of AE energy and magnitude distribution. A power-function ansatz for distribution modelling is proposed and compared with already existing formulations based on exponential ansatz. The resulting entropic index is a size-independent parameter of the distribution.