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The first four sources are microscopic duplication of

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The first four sources are microscopic duplication of other typical machining processes, such as turning, milling and shaping, except that many grains are in action simultaneously. The last three sources are more or less unique for the grinding process. Each of the sources may generate longitudinal and shear acoustic waves, which may become other types of waves, such as rayleigh waves, after they hit the surface boundaries. Some of the acoustic emission sources mentioned above may generate pulses or burst signals because of grain crack or disbonding. Others may generate continuous signals because of friction and plowing. Although the acoustic emission energy generated by each individual source might be very small, because of the tiny grain size and volume cut by each grain, the signal detected by the transducer is actually a bulk interference of the acoustic emission waves generated by numerous sources and therefore is very significant. Acoustic emission during grinding is generated by various means, such as grain fracture, bond cracking, chip formation and breakage, plowing, sliding friction, plastic deformation and elastic deformation. These mechanisms generate either a pulsing or continuous signal whose frequency characteristics are very rich in information. 17,25 It is believed that the acoustic emission generated during grinding generates a very wide frequency bandwidth ranging from tens of kilohertz to a few megahertz. The most interesting frequency bandwidth would be between 0.1 and 1.0 MHz because there might be a lot of mechanical and environmental noise below this band and big signal attenuation above it. Transducers with both resonant and flat band frequencies can be used for grinding applications. Transducers appropriate for grinding applications are discussed below. There are two types of acoustic emission signals in terms of waveform shape and style. One, called burst acoustic emission, exhibits pulse waveforms. The other is called continuous acoustic emission, where there is no clear end to the signal. Theoretically, each cutting grit generates a burst signal when it cuts through the workpiece. When numerous grits cut through the workpiece in such a way that the interval of two consecutive cuts is much shorter than the decay time of each burst signal, then continuous acoustic emission is formed. It can be inferred that wheel/workpiece contact consists of two steps. The first step is when the grinding wheel has intermittent contact with the workpiece. It can be called the grit contact step: only a few grains may be in contact with the workpiece at the beginning because of the 252 Acoustic Emission Testing F IGURE 27. Acoustic emission sources in grinding. Legend 1. Chip formation. 2. Chip breakage.
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