# Analysis of Acoustic Emissions for Determination of the Mechanical Effects of Scratch Tests
## 2 Experimental Methods
### Acoustic Emission Sensors and Data Processing  
The AE apparatuses used for all tests were two contact Physical Acoustics WD AE sensors connected to a data acquisition (DAQ) unit with two pre-amplifiers. The amplification to each sensor was maintained at 20 dB, with a 35 dB threshold selected for the DAQ. Based on the sensor power spectrum, AE devices are suitable for a range of frequencies between 70 kHz and 1 MHz. The positions of the sensors were moved between tests to observe the effect of the positions of the sensors on the measured AE signal. These positions were on the scratch face parallel to the scratch direction, on the scratch face perpendicular to the scratch direction, on the \"edge\" of the sample parallel to the scratch direction. A schematic of the sensor  
Figure 4: Schematic of horizontal single grit test rig. Reprinted from [PERSON] 2015 [21].  
Figure 3: Single grit test set up. Reprinted from [PERSON] 2015 [21].  
positions is given in Figure 5. It must be stressed that only 2 AE sensors were used during any one test, as indicated by the sensor configuration described in Table 1.  
It is known from the literature that it is critically important to select the most appropriate AE parameters [25] to minimize the probability for error. It is important to select some parameters that are a function of the peak voltage (which may be influenced by the researchers' choice of AE setup) and some that are waveform-dependent and therefore independent of the AE setup [26].  
The study of the application of AE is quite advanced in the study of failure modes in fiber-reinforced plastics and structural health monitoring [25], where parameters such as frequency, amplitude, duration, rise time, peak amplitude, energy, counts, centroid frequency, weighted peak frequency, partial power, number of hits, and counts per events are commonly used, as described by [PERSON] [26]. Only a select few of these parameters have been used in the application of AE to galling wear [11; 16; 17; 19; 22; 27; 28].  
Therefore, in the scratch tests in this work, analysis of the AE signal was conducted using the AE parameters hits (hits were unable to be used, as the threshold was set too low, such that the background noise was enough to register a hit and therefore no useful information could be inferred), counts, and standard time-frequency techniques; fast Fourier transform (FFT) and short time Fourier transform (STFT) was calculated using MATLAB's toolboxes. Additionally, a novel technique, mean frequency (mean frequency cannot be calculated for the single grit tests, owing to the short duration of the AE burst), is compared against more established techniques, Shannon entropy and Centroid frequency. For reasons explained later, a 300 kHz high pass filter was applied to the raw AE data using MATLAB's built-in Digital Signal Processing (DSP) toolbox. Mean frequency was calculated by windowing the signal into windows of 0.5 s and calculating the mean in each window using MATLAB's meanfreq function (Mean frequency--MATLAB meanfreq--MathWorks  
Figure 5: Schematic of test sensor positions. A = on the scratch face parallel to the scratch direction (red markers). B = on the “edge” of the sample perpendicular to the scratch direction (green markers). C = on the “edge” of the sample parallel to the scratch direction (yellow markers). D = on the scratch face perpendicular to the scratch direction (orange markers). Note measurements from orientation D are not used in this work.  
Australia). The sampling rate for the FFT, STFT, and mean frequency was set to 2 MHz as a compromise between preventing aliasing and capturing an unmanageably large data set.  
For analysis of the SG AE, the signal was reconstructed using the MATLAB DSP Toolbox and all the short-burst high-frequency information was obtained. By using a Chebyshev Type II bandpass filter with a cut off frequency of 80 kHz to 1 MHz, most of the noise generated was eliminated. The sampling rate was set to 5 MHz to prevent aliasing.