Van de Wyngaert, Matthieu
[UCL]
Pardoen, Thomas
[UCL]
Bollen, Pierre
[UCL]
Water droplet erosion (WDE), or the form of material wear caused by high speed impact of water droplets, is a serious damage mechanism affecting the lifetime of leading edges of airplanes and wind turbines. This erosion can be simulated in testing machines in two main ways, either by accelerating the sample as in whirling arm rain erosion rigs (WARER) or by projecting the water droplets as in pulsating jet erosion tests (PJET). In the present work, a study of the erosion damage mechanisms using both techniques is performed on three different metallic materials: Al7075, Ti6Al4V and Ni bulk of commercial purity. Two main damage mechanisms are observed in the early stages of erosion, namely peening at the macro-scale and pitting at the micro-scale. Notable results include the influence of the initial roughness of the sample down to the sub-micron scale grooves created by polishing, since they appear to be pitting initiation zones. Using laser interferometry, it is shown that the arithmetic mean roughness Ra increases in the early stages of erosion as a linear function of the total received energy, defined as the cumulative kinetic energy of the droplets, for the PJET technique. The skewness parameter Rsk, which represents the asymmetry of the profile with respect to the mean surface elevation, seems to be the most representative parameter in the present work. It shows a decreasing trend, explained by the increasing number of pits and their deepening as erosion progresses. Finally, the evolution of the sharpness of the damaged zones, represented by the kurtosis Rku, depends on the droplet speeds but quickly stabilizes. Despite the difference in erosion rate, a correlation can be established between 18 hours of exposure in the WARER at 120 m/s and either 9 000 impacts at 225 m/s or 15 000 impacts at 180 m/s in the PJET under reference conditions. Based on roughness measurements, the performance of several metallic samples is compared and a material selection approach towards a performance index is taken. Most interestingly, the analysis of Al7075 alloys highlights the influence of increased hardness and ductility by age-hardening and friction stir processing on the resistance to water droplet erosion. While increased hardness leads to better performance, the ductility does not seem to influence the resistance to water droplet erosion in the early stages of damage. The two main macroscopic material properties studied in the present work are the yield strength and Young’s modulus. Most existing material performance indices assessing the resistance to water droplet erosion correctly identify the superiority of Ti6Al4V but fail to predict the behavior of Ni bulk which has experimentally slightly better performance than Al7075. Therefore, a new performance index adding the influence of density is analyzed and seems to correspond to the experimentally observed ranking for the materials tested in the present work. Still, other properties such as toughness, ductility or strain-hardening are likely to have an influence on the resistance to WDE in more advanced erosion stages, which are not studied here. The response under high strain rate should also be further studied to identify a potentially universal performance index.
Bibliographic reference |
Van de Wyngaert, Matthieu. Characterization of the behavior of advanced surface treatments to rain erosion : towards a material performance index. Ecole polytechnique de Louvain, Université catholique de Louvain, 2021. Prom. : Pardoen, Thomas ; Bollen, Pierre. |
Permanent URL |
http://hdl.handle.net/2078.1/thesis:30549 |