# Analysis of Acoustic Emissions for Determination of the Mechanical Effects of Scratch Tests
## 3 Results
### Scratch Tests with Increasing Load  
The raw data of the scratch tests are given in Figure 6. Steel under increasing load exhibits multiple distinct AE events, whilst aluminium under increasing load does not show such distinct AE events. For these tests, the sensors were in orientation A for the steel sample (on the scratch face parallel to the scratch direction) and orientation B for the aluminium sample (on the \"edge\" of the sample perpendicular to the scratch direction).  
Figure 6: Raw signal of the scratch tests under increasing loads.  
For the scratch tests, the STFT spectrogram (Figure 7) shows dominant frequencies in a low-frequency region below 300 kHz were persistent throughout the duration of all the tests and are therefore believed to be machine noise, as this is seen in all tests. Moreover, the STFT spectrogram of the steel/steel tribopair does not appear to show any significant activity above 300 kHz. However, there appears to be some activity in the steel/aluminium tribopair, which has been attributed to greater interaction between the indenter and plate materials, as evidenced by the deeper penetration depth and higher measured friction shown in Figure 11.  
The mechanical data show the depth of cut increased linearly for both steel and aluminium, whilst the frictional force for aluminium increased slightly faster than for steel, Figure 8 (below).  
Figure 7: STFTs for scratch tests of tool steel indenter on steel sheet (**top**) and tool steel on aluminium sheet (**bottom**) under increasing load.  
The FFT analysis shows (Figure 9) substantial activity below 300 kHz, as expected from the noise seen in the STFT analysis, as well as three dominant peaks at approximately 700 kHz, 800 kHz, and 950 kHz for both plate materials.  
Figure 8: Mechanical data for tool steel indenter on tool steel sheet (test 1) and tool steel on aluminium sheet (test 2) under increasing load. Depth of cut (**left**) and friction force (**right**).  
Figure 9: FFTs of tool steel indenter on steel sheet (**top**) and tool steel on aluminium sheet (**bottom**) under increasing load for scratch tests. (**left**), sensor 1. (**right**), sensor 2.  
Figure 10 (left) shows that the mean frequency for the steel plate exhibits lower mean frequency than the aluminium plate with a gentle increase across the duration of the test. The aluminium also exhibits this increase in mean frequency with increasing load. Figure 10 (right) shows the mean frequency after a high pass filter at 300 kHz has been applied to the signal, which now shows the steel plate exhibiting a higher mean frequency (approximately 600 kHz) than the aluminium (500 kHz). The mean frequency for the steel appears to gently decrease with increasing load, whilst the aluminium gently increases with increasing load after this filter has been applied.  
The centroid frequency (Figure 11 top) shows the steel typically has a higher centroid frequency (600 kHz) than the aluminium (approx. 500 kHz) throughout the duration of the increasing load test. However, the centroid frequency for the steel remains almost constant, whilst the aluminium exhibits a very gentle increase in centroid frequency. However, Shannon entropy (Figure 11 bottom) shows an increase with load for the aluminium sample not seen in the steel sample.  
Whilst AE hits and counts were measured, the threshold for the hits was set too low such that noise was sufficient to trigger a hit. Moreover, there was little to no correlation between the counts and the scratch, such that the period where the scratch occurred could not be found within the counts data, so these results have not been shown here.