Fracture mechanisms of quasi-brittle materials based on acoustic emission
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C. Ouyang Research Associate at NSF Science and Technology Center for Advanced Cement-Based Materials at Northwestern University, Evanston, Illinois 60208 S. P. Shah Professor and Director of NSF Science and Technology Center for Advanced Cement-Based Materials at Northwestern University, Evanston, Illinois 60208 (Received 10 March 1989; accepted 15 August 1989)
Recently acoustic emission (AE) techniques have been used to study crack propagation in materials. The application of these techniques to heterogeneous, quasi-brittle materials such as concrete requires a better understanding of how the signal generated from a microfracture is transformed due to wave propagation and due to the transducer response. In this study, piezoelectric transducers were calibrated using displacement transducers. The validity of an elastodynamic Green's function approach was examined for cement-based materials. The acoustic emission source was characterized using moment tensor analysis. Acoustic emission measurements were analyzed for center-cracked-plate specimens of mortar and concrete. It was observed that, as expected, the dominant mode of cracking was mode I (tensile). However, mode II (shear) and mixed mode cracks also occurred, perhaps due to grain boundary sliding and interface debonding. Microfractures appear to localize prior to critical crack propagation. Mode I cracks generally required more energy release than mode II and a smaller inclusion provided a stronger interface bond than the larger ones. I. INTRODUCTION
Acoustic emissions are microseismic waves generated from microcracking, dislocation movement, phase transformation, and other irreversible changes in a material. These waves could be detected on the surface of the material by transducers which convert the mechanical acoustic vibrations to electric signals which are digitized, stored, and analyzed to obtain useful information about the AE events. There are various irreversible processes responsible for AE in different materials. Some examples are phase transformation in ceramics and dislocation movement in metals. In cementitious materials microcracking is generally accepted as the primary cause for irreversible deformation. Acoustic emission measurements have been widely used to study the crack propagation in materials.1 AE signals generated during material damage have been analyzed in several ways: (1) The rate and sum of occurrence of AE activity have been used to predict the extent of internal damage of material.2 Since the AE events are caused by irreversible energy dissipation associated with the damage of materials, this method can be used to assess damage accumulation. It may be possible to identify impending structural failure from monitoring of AE activity. 206
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J. Mater. Res., Vol. 5, No. 1, Jan 1990
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(2) AE energy has been used to determine the critical energy release rate.3 In addition, it could be possible to distinguish the energy associated in different failure mechanisms such as interface de
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