The generation mechanisms of acoustic emission in metal cutting
The objective of the present thesis is the investigation of the generation mechanism of the ultrasonic vibrations, commonly called acoustic emissions (AE), detected during the course of metal cutting, since, although quite a lot of research effort has been put into the use of AE to monitor metal cutting condition, the mechanism by which AE is generated is still not fully understood. If chip generation is continuous, without built-up edge, and a sharp tool is used, continuous type AE is normally assumed. Most published models relate the energy of AE to the total cutting power, but this can be shown to be rather incorrect. Consequently, as continuous-type AE is mostly generated due to plastic deformation, and as dislocation motion is the main mechanism of plastic deformation of metals, a relationship between AE and dislocation motion is developed for the typical plastic deformation regimes encountered in metal cutting (due to the high temperatures, flow stress decreases with temperature in the so-called diffusion controlled regime, and due to the high strain rates, opposing viscous damping becomes the dominant mechanism governing dislocation movement). Although viscous damping governs the mechanics of deformation in metal cutting, it is proposed that AE is generated due to the interaction between dislocations and obstacles, since as a dislocation approaches an obstacle, strain energy is stored, which is rapidly released as soon as the dislocation surmounts the obstacle, resulting in the emission of an AE event. The detected AE is a result of many consequential likewise events. Consequently, a qualitative original model of AE generation is developed, in which the energetic level of AE is predicted to increase with strain and strain rate, but decrease with temperature, and the frequency content of AE is predicted to increase with strain rate, decrease with temperature, and remain unchanged with strain. In order to access the validity of the above-mentioned model, two sets of metal cutting experiments were accomplished for four different work materials, in which the cutting conditions were varied over a wide range, and the workpiece temperature was artificially modified. Both energy and frequency information were computed from the experimental data using the most appropriate data processing technics, i.e. AE mode and mean frequency, respectively. In addition, a semi-empirical metal cutting theory was utilized to predict basic metal cutting parameters. As the experimental results are in close agreement with the predictions provided by the qualitative model, it is concluded that the main source of AE in metal cutting comes from the interaction of moving dislocations with obstacles, whose dynamics is, however, dictated by viscous damping.
Networked Digital Library of Theses and Dissertations