# Analysis of Acoustic Emissions for Determination of the Mechanical Effects of Scratch Tests
## 3 Results
### Single Grit Tests  
Figure 20 shows the raw data for the AE response of the four materials used in the SG tests, which display observable differences for the time-domain when considering SG and different material interactions. This is important in respect to the argument that different material interactions give off different ranges of acoustic energy.  
Figure 19: Example scratch from test 4, a steel sample.  
Figure 18: Indenter for test 4, showing no transferred material from the steel sample to the indenter.  
Figure 21 shows the STFTs of the AE from the SG tests. The four tests all show a clear period of activity, which must be from the scratch. As well as showing noise at 100 kHz, they each show a different frequency response, depending on the material: Inconel 718 around 200 kHz, CSMX-4 at 150 and 600 kHz, EN8 at 150 kHz, and MARM-002 at 150, 220, and 550 kHz. The AE waveform from the CSMX-4 test is notably longer and of greater amplitude than the other tests.  
The SG tests were originally carried out to look at the micro effects of cutting, ploughing, and rubbing [21]; however, at the same time they are controllable and repeatable tests in  
Figure 21: AE STFT for SG tests for Inconel 718, CSMX4, EN8, and MARM-002.  
Figure 20: AE SG events for Inconel 718, CSMX-4, EN8 & MARM-002.  
terms of loads and therefore provide a suitable test to display the differences of AE energy passing through different materials. The SG scratches are not a single phenomenon on the workpiece surface; instead, there are about 15 scratches in all where the fixed SG mounted to a steel disk is rotated at commercial speeds and 1\(\upmu\)m incremented towards the workpiece. As soon as the scratches approach, the workpiece is inspected for plastic deformation. Once contact is made, smaller scratches with rubbing exist and cutting predominately exists at the middle of the workpiece. The grit signatures displayed in Figure 20 display predominately cutting phenomena as opposed to rubbing and ploughing phenomena.  
This is important in respect to the argument that different material interactions give different ranges of acoustic energy and especially what these physical actions mean from an acoustic energy perspective when applied to different materials. Based on very slight inconsistencies with setup in terms of a totally flat surface, there may be more energy exerted with one test to another--i.e., where the amplitude increases as the interaction increases between the indenter and workpiece. However, the frequency bands do change dependently upon the material under test, and this is key for discussions and the underlying argument.  
Figure 22 shows that each Centroid Frequency for the various SG tests is significantly different from the other. There is no notable trend for each of the materials. Figure 23 shows the Shannon entropy for each of the tests. There is an increase in Shannon entropy with time for the EN8 and CSMX4, which rapidly drops to 0 as the AE event ends, whereas the Inconel 718 shows a gradual increase and decline across the duration of the AE event. The magnitude of the MARM-002 is notably smaller than the other three materials.  
Figure 22: Centroid Frequency of the SG tests for Inconel 718, CSMX4, EN8, and MARM-002.