Acoustic Emission Analysis of Nanoindentation-Induced Fracture Events
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Acoustic Emission Analysis of Nanoindentation-Induced Fracture Events Pawel Dyjak and Raman P. Singh Mechanics of Advanced Materials Laboratory Department of Mechanical Engineering, Stony Brook University Stony Brook, NY 11794-2300 ABSTRACT Monitoring of acoustic emission (AE) activity was employed to characterize the initiation and progression of local failure processes during nanoindentation-induced fracture. Specimens of various brittle materials were loaded with a cube-corner indenter and AE activity was monitored during the entire loading and unloading event using an AE transducer mounted inside the specimen holder. As observed from the nanoindentation and AE response, there were fundamental differences in the fracture behavior of the various materials. Post-failure observations were used to identify particular features in the AE signal (amplitude, frequency, rise-time) that correspond to specific types of fracture events. Furthermore, analysis of the parametric and transient AE data was used to establish the crack-initiation threshold, crack-arrest threshold, and energy dissipation during failure. It was demonstrated that the monitoring of AE signals yields both qualitative and quantitative information regarding highly local failure events in brittle materials.
INTRODUCTION Nanoindentation, also known as ultra-low load indentation, is a powerful technique for characterizing the mechanical properties of materials at very small length scales [1-3]. In this technique, a well-defined diamond tip is indented into the material of interest while the applied load and displacement are continuously monitored during one complete loading and unloading cycle. Current nanoindentation systems can typically provide resolutions of 1 nN and 0.1 nm for force and displacement measurements, respectively, which enable the mechanical sampling of very small material volumes. Typically, nanoindentation has been employed to determine hardness and elastic modulus [2], but it can also be used to determine the fracture toughness of brittle materials [4]. When a brittle material is loaded by a sharp indenter, radial cracks propagate out of the indenter corners if a sufficiently large load is applied. In such a case the fracture toughness can be estimated by measuring the length of radial cracks produced at a given indentation load [5]. This procedure has proved to be adequate for providing an averaged measure of fracture toughness in homogeneous materials. However, it provides no information regarding the progression of crack initiation and growth. Furthermore, this technique cannot delineate energy dissipation by various fracture processes that occur for a locally heterogeneous material. Additionally, the need for measuring the crack length after each test introduces complications in the calculations of the fracture toughness. Appropriate identification of the events during crack initiation and growth is an essential step towards characterizing the overall failure behavior of locally heterogeneous materials using nanoindentation
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