Rain Erosion Testing (RET) is an integral part of Leading Edge Protection (LEP) development and certification. It measures the durability of materials to rain droplet impacts. There are different methods to perform RET, including jet impacted stationary samples, solid projectile impact tester, and utilizing a whirling arm tester.
In this short article, the main focus will be whirling arm testing. The method consists of one or test specimens mounted on a rotating arm spinning at high velocities. The rotating specimens are exposed to a controlled rain field generated by the testing machine, where rain intensity and droplet size can be controlled. The damage progression is recorded through image acquisition over the complete duration of the test. The image material is then assessed by a specialist to identify the different stages and locations of defects. Finally, the data is visualised in a V-N curve, where V is impact velocity and N is the number of specific impacts. Another common resultant representation of a RET is the incubation period, defined as the time until first damage is observed on the testing sample.
There are 3 main failure modes observed during RET testing of LEP products. The first one is uniform erosion and is typical for older coatings. This defect type is characterised by gradual LEP material removal from the high-speed end towards the low-speed end of the testing sample. The second defect development mode is adhesive failure, where the interface between the LEP and the substrate fails due to insufficient adhesive strength. The third commonly seen defect type is local failure around a pre-existing imperfection from manufacturing. This is often the damage mechanism visible on newer LEPs, which are stronger and have improved adhesion to the substrate.
Due to the durability of modern LEP products, the amount of water impacts required to cause failure is significant. This leads to either very high rotation speed of the samples, or very long testing times to achieve meaningful testing results. If the rotation speed during RET is too high, it can lead to unrealistic defect modes or test results. This is mainly due to breaking of the droplets to smaller sizes, or vaporisation. On the other hand, longer tests are more expensive and create more data for analysis.
Wind Power LAB, in collaboration with R&D Test Systems and DTU – Technical University of Denmark, has developed an enhanced bespoke method for assessment of RET image data collected during testing. In the first stage of the project, which is now completed, we developed an annotation tool that enhances the process for the specialist looking at test images. In the next stage of the project, we will utilise computer vision and machine learning to automatically detect defects/damages on the test imagery. Our initial trials have shown significant potential for the success of such endeavor.
Stay tuned for more details on the testing solution software as it develops.
It is believed that through this innovation project, Leading Edge Protection coatings and other materials bound for Erosion Testing can be more efficiently and dutifully assessed. In turn, reducing the time to market through productive methodical research and development processes.
Hristo Shkalov, Senior Blade Specialist – Wind Power LAB
If you are interested in more blog post click here
