*2.3. Testing Case Results*

The coupons of LEP coatings, primer and filler materials for the impedance measurements were supplied by Aerox Advance Polymers [36] and the testing developed by Dolphitec [35]. The coupons prepared were of two geometries: a circular disc with a nominal diameter of 65 mm and thickness ranging for 5.5–6.3 mm on average and thin laminates of 400 μm on average, see Figure 2. The testing procedure was defined following the next steps:

**Figure 2.** (Left) Thin Coating LEP used for UT coupon, (Right) Example of how a Time of Flight measurement is used in a tested coupon.

For the 2.5 and 5 MHz probes, four locations were marked on each coupon and the thickness was measured at each of these marked locations. Marks/points (starting from #1) are made on various regions on the sample coupon.


Figure 3 shows the impedance measurements using the 2.5 MHz probes. The impendence for the coating LEP, primer and two different fillers were successfully measured in three different batches with 6 measurements developed on each material. All materials measured showed a minor reduction (5–10%) in the impedance values when measured with the 2.5 MHz probe frequency compared to the 5 MHz probe throughout all the materials tested. This would indicate a limited stiffness variation in order to develop the erosion lifetime performance analysis with Springer modelling, assuming a constant impedance value used as input data for each material and measured using the 5 MHz UT probe for all cases. Figure 4 shows the average speed of sound measurements for the 5 MHz probe.

**Figure 3.** Average impedance measurements with the 2.5 and 5 MHz probes.

**Figure 4.** Average Speed of Sound measurements with the 5 MHz probe.
