# Analysis of Acoustic Emissions for Determination of the Mechanical Effects of Scratch Tests
## 4 Discussion  
All scratch tests under both loading conditions, irrespective of plate material (aluminium or steel alloy SA52210), showed significant activity believed to be noise below 300 KHz. The FFT analysis consistently shows activity at 400 kHz, 500 kHz, 700 kHz, and 950 kHz across all tests (Figures 9 and 15).  
In the STFT spectrograms (Figures 7 and 13) for both increasing and constant load, there appears to be a difference between the steel/steel tribopair and the steel/aluminium tribopair, with the latter (steel/aluminium) showing much more activity in the frequency range above 300 kHz. In comparison to the SG tests, the scratch tests exhibit much more noise.  
The Shannon entropy showed an increase with increasing load for the aluminium sample. This increase is not seen in the corresponding increasing load test of the steel sample. It is interesting to note the large difference between the two sensors for both samples when under constant load.  
[PERSON] et al. used the mean frequency technique to show a difference between worn and unworn components, but not between materials [16; 19]. [PERSON] et al. reported that mean frequency outperformed other time-frequency techniques for showing signs of early wear. The results of our work confirm this conclusion of [PERSON] et al. [16; 19], in that mean frequency apparently outperforms Fourier analysis by discriminating between materials not seen in the Fourier analysis.  
Aluminium, as a softer material than tool steel (aluminium had a greater depth of cut and greater friction between indenter and plate for all tests), exhibits a lower mean frequency for all tests as well as a lower centroid frequency. This is believed to be due to the differing crystal structures of the materials, as microscopy showed no galling took place. Differing frictional forces between the counter face and samples attributed to the greater ductility of the aluminium alloy in comparison to the steel alloy used. In other words, the steel alloy resisted the deformation better than the aluminium, as expected, and this is reflected in the AE signals.  
The SG tests seem to show greater intensities for materials with greater levels of hardness, Figure 21. EN8 steel is the only material which does not have high-intensity peaks when compared to the Ni-based alloys. This is attributed to the higher hardness of the material requiring more energy to create dislocations and deform the material. The difference between CSMX-4 and the other Ni-based materials in these tests is attributed to a deeper (and therefore longer) cut in the material, resulting in greater plastic deformation. These results are in agreement with [PERSON] et al., [PERSON] et al., and [PERSON], whose works showed that the AE response of different materials should be different and distinguishable in the time-frequency region, and also in line with the work of [PERSON] et al., which states a drawback of AE is that several early phenomena (cracking, movement of dislocations, twinning, movement of grain boundary, break of cohesion bindings between different layers and inclusions in the material, flow of medium, cavitation, internal and boundary friction, impacts, growth of magnetic domains, condensation, and solidifying process of material or structures) are indistinguishable [13; 17; 20; 21].  
For the SG tests, the Centroid frequency and Shannon entropy techniques again appear to discriminate between some materials. For Shannon entropy, a difference in EN8 and CSMX4 is hard to spot; however, the Inconel 718 and MARM 002 are different in shape and magnitude. The Centroid frequency analysis reflects this, with the EN8 and CSMX4 appearing very similar to one another.