# Capability of Advanced Ultrasonic Inspection Technologies for Hydraulic Turbine Runners
## 4 Finding and Discussion  
Our tests showed that conventional Ultrasonic results in the lowest detection rate among the different ultrasonic testing methods evaluated. Although the inspection is carried out to a well-defined standard, the detection rate is highly dependent on the  
Figure 11: CIVA Simulation Configuration.  
Figure 12: Simulation results from PAUT probe.  
operator. Fixed angle conventional ultrasonic testing (UT) provides less information for accurate assessment of actual flaw conditions. Detection capability increases with the divergence of the beam of a single element probe. Because a beam refracted at 70\({}^{\circ}\) is more likely to show greater divergence than one refracted at 60\({}^{\circ}\), the former has a better detection rate. PAUT allows one to use more angles and increases the probability of detection compared to conventional UT. PAUT sectorial scan is also well adapted to the T-joint sample's geometry. Low angles will be tangent to the weld cap radius, while the high angles will probe the center of the blade to the web region.  
The 16-element PAUT probe proves to be a good compromise for detection: Better resolution than the 12-element probe; Compared to the 32-element, a smaller wedge can be used, and for the same refracted angle, the beam exit point is closer to the edge of the weld; The size of the beam is adapted to the range of interest. With 32-element, focusing is needed to narrow down the beam emitted from a larger active aperture but did not lead to a better detection rate. Linear-encoded PAUT scans can distinguish two closely spaced flaws more easily than manual inspection.  
Among the different ultrasonic testing methods evaluated, manual TFM resulted in the highest detection rate. It is slightly better than encoded TFM due to the operator's ability to adjust the probe's position to improve the signal to noise ratio on display. TFM L-L reconstruction mode seems to give slightly better results than the T-T reconstruction mode. The reason for such a result is still under investigation. TFM by nature has better coverage than beam-forming PAUT, and that could explain the slightly higher detection rate. TFM has the advantage of a clear display with appropriate information on the actual distribution and geometry of the flaws (faceted, multi-faceted, sphere-like). However, using the T-T mode did provide more details of the ceramic bead interface and better estimate of the diameter because of the shorter wavelength. TFM requires a transducer with a large number of elements. Thus, a large contact area and good coupling for all the ray paths are needed to produce consistent and repeatable results.  
The best inspection strategy to inspect the trailing edge of a runner blade is to perform the inspection in two steps:  
Step1: Linear encoded scan with either PAUT sectorial scanning or TFM direct mode. PAUT shall use shear wave with the refracted angle from 40\({}^{\circ}\) to 70\({}^{\circ}\). TFM could use either L-L or T-T direct mode.  
Step 2: Manual sectorial scanning with a 12 or 16-element aperture and a small wedge footprint to inspect the trailing edge and the weld reinforcement. The small footprint wedge is essential on a real turbine because of the trailing edge's curvature to maintain good contact while scanning.