# Capability of Advanced Ultrasonic Inspection Technologies for Hydraulic Turbine Runners
## 2 Experimental Procedure
### Phased Array Ultrasonic Testing (PAUIT)  
The scan plans prepared to inspect the regions of interest were devised according to standards. These scan plans indicate the required refracted beam angle and the probe standoff positions to ensure appropriate volume coverage. Wherever possible, welds shall be inspected from both sides of the blade [19; 20; 21]. The ultrasonic system was calibrated using standard blocks to ensure the repeatability and reliability of experimental data [18; 21]. Phased Array (PA) transducers can be referred to as multiple small conventional transducers (elements) that are housed within a single unit [22]. Each transducer can be pulsed individually; by varying the delay between elements, the individual waves' constructive interference will occur at the desired angle, which enables the focus to be achieved. The phased array technique is a beamforming method that relies on emission delay to shape the beam in terms of refracted angle and position of the near field. A set of delays required to create a specific beam angle and focus point is called a focal law [23; 24; 25; 26]. By electronically changing the delays rapidly, the focal law group can produce an azimuthal scan with a starting angle, a final angle, and an angle step. For example, a focus group can use the following set of angles: starting at 40\({}^{\circ}\), an increment of 1\({}^{\circ}\), and stop at 70\({}^{\circ}\). A phased array probe has limits for beam formation; hence, both wave physics and array limitation must be taken into account. In the experiment, the wedge nominal refracted angle is 55 shear wave and the array of 5 MHz, with 0.6 mm element spacing, produces an acceptable beam profile in the range of 40\({}^{\circ}\) to 70\({}^{\circ}\) shear wave [27; 28; 29]. The PAUT inspection was carried out using TOPAZ 64, as shown in Figure 2. The transducers were all linear Phased  
Figure 1: The experiment setup overview. Figure 2: Simulation and experimental Setup. Array Ultrasonic probes operating at a center frequency of 5 MHz. The three models used in these inspections differ mainly in terms of the number of elements in the array, namely 12, 16, and 64 (Table 1). They were all used to perform manual or linear-encoded sectorial (Azimuthal) scans ranging from 40\({}^{\circ}\) to 70\({}^{\circ}\) from normal incidence, as shown in Figure 3. Data acquired along two different scan indexes (probe standoff positions) were combined to cover the whole volume of interest in a single file to analyze. The study developed an inspection configuration in accordance with the number of transducer elements and the focal group's capability to insonify the whole weld volume on the opposite face. The inspection strategy involves inspecting the direct skip to prevent beam spread and avoid ghost echoes caused by reflection from the weld cap's curved radius. With a direct skip inspection, the ultrasonic configuration is simpler by mainly focusing the beam for a metal path range of 10 to 50 mm. These permit the use of 16 elements per focal law and use small footprint wedges or a 64-element transducer's front elements to minimize the standoff distance. With these configurations, the focal group's high angle can hit the T-joint center where it meets with the web, and the lower angles can probe most of the weld cap radius. Only a small hidden area exists and cannot be covered with angle beam inspection from the blade. The only inspection technique that can provide 100% coverage of the weldment is from the web using a normal incidence wedge. Inspection from the web was not attempted because this surface is not accessible during an in-service inspection. The following conclusions can be drawn from the results obtained;  
1. Flaw resolution increases with the number of active elements used in the array. 2. However, the use of 32-element does not improve inspection performance. This finding could be attributed to the impact of a larger active aperture on the generated ultrasonic beam's characteristics.