# Analysis of Acoustic Emissions for Determination of the Mechanical Effects of Scratch Tests
## 3 Results
### Scratch Tests with Constant Load  
Figure 12 shows the raw AE signal from the constant load scratch tests. Again, the steel plate exhibits several distinct AE events (as also shown in the increasing load tests), whilst the aluminium this time does show some AE events of smaller magnitude (than the steel) but more than the increasing load tests. For these tests, the sensors were in orientation C for both samples (on the \"edge\" of the sample parallel to the scratch direction). Figure 10: (**Left**) Mean frequency across test of tool steel indenter on tool steel sheet and tool steel on aluminium sheet under increasing load, including the sub-300 kHz region. (**Right**) Mean frequency of same increasing load tests with 300 kHz high pass filter applied. Figure 11: Centroid frequency (**top**) and Shannon entropy (**bottom**) for the increasing load tests after a 300 kHz high pass filter has been applied to the signal. Figure 12: Raw AE data from the constant load tests. The STFT spectrograms (Figure 13) for the constant load tests showed dominant frequencies in a low-frequency region (below 300 kHz), which were persistent throughout the duration of the test for all tests and are therefore attributed to machine noise. The steel/steel tribopair does not appear to show any significant activity above the noise (greater than 300 kHz) except for a few peaks at 17 s, 24 s, and 27-30 s, which correlates well with the AE events seen in the raw data, Figure 12. There appears to be much more activity in the steel/aluminium tribopair, which has been attributed to greater interaction between the indenter and workpiece/plate materials, as evidenced by the deeper penetration depth into the aluminium sheet and higher measured friction as shown in Figure 14.  
Figure 14: Mechanical data for tool steel indenter on tool steel sheet and tool steel on aluminium sheet under constant load. Depth of cut (**left**) and friction force (**right**). Figure 13: STFTs of tool steel indenter on tool steel sheet (**left**) and tool steel on Al sheet (**right**) sheet under constant load. The mechanical data show that the depth of cut was slightly greater for aluminium than steel, but remained constant throughout. The frictional force for aluminium increased sharply at the start of the test and then decreased across the duration whilst the steel remained constant, Figure 14.  
The FFTs (Figure 15), again show substantial activity below 300 kHz due to the machine noise, as well as some dominant peaks at approximately 400 kHz, 500 kHz, 700 kHz, and 950 kHz for both plate materials. When mean frequency is considered, Figure 16 (left) shows the steel plate samples exhibit a lower mean frequency than the aluminium plates across the duration of the test. It is difficult to see if there is a correlation between the constant load and mean frequency, as there are occasional peaks and troughs in the data. The overlap between two tests, Test 3 and Test 4, makes it difficult to comment on a difference in materials. However, once the mean frequency is recalculated after a 300 kHz high pass filter is applied to the data Figure 16 (right), the steel plate once again exhibits a higher mean frequency (600 kHz) than the aluminium (500 kHz) across the duration of the test; this is similar to what was observed in the increasing load tests. Figure 15: FFTs of tool steel on Al sheet (**top**) and tool steel indenter on steel sheet (**bottom**) under constant load. Left, sensor 1. Right, sensor 2. The centroid frequency (Figure 17, top) shows the steel typically has a higher centroid frequency (600 kHz) than the aluminium (500 kHz) throughout the duration of the constant load test, and the centroid frequencies for both materials remain almost constant. However, Shannon entropy (Figure 17 bottom) shows an increase with load for the aluminium sample not seen in the steel sample. Figure 16: (**Left**) Mean frequency of tool steel indenter on steel sheet and tool steel on aluminium sheet under constant load including the sub 300 kHz region.