2.2 | Impact testing
Impact tests were performed on samples A-C using an electro-pneumatic
projectile launcher referred to as the Single Point Impact Fatigue
Tester (SPIFT) [25]. The SPIFT was developed to repeatedly impact a
target sample in a single point to be able to initiate and progress
damage at high impact speeds up to 170 m/s. The purpose of the device is
to mimic the impact speeds of rain droplets on the leading edge of wind
turbine blades to understand the damage mechanisms of the coating
materials at the relevant strain rates. Spherical nitrile rubber
projectiles with a diameter of 6 mm and mass of approximately 0.1 g was
used. The impacted surface was monitored continuously with a camera to
observe the damage. In the current work, samples A and B were tested in
the SPIFT until failure was visible on the surface, as seen in Figure
1(a,b). For sample C, the test was stopped before coating failure was
visible from the surface, so only a slight difference in the shading of
the coating is visible at the point of impact.
2.3 | X-ray CT reference scanning
XCT scanning was performed of the sample GZ3-01 for validation of the
OCT images. The scan was performed on a Zeiss Xradia 520 Versa with a
polychromatic X-ray beam with energies up to 60keV, generated by a
tungsten target. A total of 4501 projections with an exposure time of 10
s were acquired on a 2k × 2k CCD detector during a full 360° rotation
of the sample. Density maps were reconstructed by a Feldkamp algorithm
for cone beam reconstruction [26] to 3D pixel volumes with a pixel
size of 9.0 μm. The standard Zeiss Xradia procedure was followed for
selecting the appropriate energy and source filter to tune the X-ray
energy spectrum for the sample. The reconstruction of X-ray CT data
provides 3D density maps that allow detection of small density
differences in the bulk material, such as the presence of crack networks
or air bubbles. Figure 2(b) shows slices from different directions
through the reconstructed volume and a segmentation where air bubbles
are colored blue and the cracks are red. The segmentation was made using
a global intensity threshold value and visualized using Avizo 3D
software for materials science.