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When the material deformation or defect occurs, the AE signal comes with the suddenly redistributed stress and transient elastic waves from a specific source within a material. This material’s stress can be generated from mechanical energies (e.g. cutting forces, component wears, thermal variation, machine structures, or tool breakages) and released in the form of the acoustic radiation into an electric signal. These materials’ voice can be received and interpreted by the AE sensor.

The most widely used AE sensor is the piezoelectric transducer that converts the mechanical energy into an electrical voltage signal. The AE sensor has the highest sensitivity in frequency response from 30 k Hz to 1 M Hz, which is significantly higher than that of microphone (20–100 k Hz), accelerometer, or strain gauge. Thus, this advantage makes the obtained signals less likely to be disturbed or attenuated by the mechanical structures or components. Figure 2.10 demonstrates the installation of an AE sensor on a tool and next to a strain gauge.

Figure 2.10 Installation of an AE sensor.

However, to acquire such high‐frequency data, a relatively high corresponding sampling rate is required. Therefore, a sensing system with large storage and strong computing power for the large volumes of data is indispensable. In addition, the high sampling rate also increases the difficulty of data preprocessing for raw AE signals.