# Capability of Advanced Ultrasonic Inspection Technologies for Hydraulic Turbine Runners
## 2 Experimental Procedure
### Total Focusing Method (TFM)  
_Total Focusing Method (TFM)_ is still part of the Ultrasonic Testing (UT). This is because it uses the same basic principles as PAUT as well as the conventional UT [29; 30]. In this study, the new phased-array technique called the TFM is included in the test matrix. Unlike the sectorial scan described previously, this technique is not a beamforming method. The TFM is an imaging technique that processes elementary A-scan data to extract the amplitude for a determined path from a firing to a receiving element [31]. After multiplexing all elements as emitter and receiver and delaying and summing all the required amplitude, a data point in the region of interest is assigned an amplitude. The algorithm is simple, but the memory and processing burden increases geometrically with the increase in the number of elements of an array and the size of the area of interest. As a result, it is simple to achieve the configuration of TFM because once the transducer, the wedge, and the mode are defined, the user creates a rectangular area of interest, which is subdivided into a grid. In this regard, the grid spacing is the critical variable, which is normally smaller than one-fifth of the wavelength. However, if the spacing is above this value, the measured amplitude fails to be within 2 dB of the true amplitude, hence, producing unreliable results. All experiments respected the wavelength criteria and opted for values that had a spacing of at most one-tenth of the wavelength [28; 29; 32].  
The TOPAZ 64 includes a beam simulation amplitude map function that helps to optimize all the parameters. Therefore, with the use of TFM, the region of interest covered the whole weld on the opposite side of the entry surface [29]. In this regard, the sample was to be tested from both sides of the blade in order to fully cover the weldment. The experimental work used a 64-element linear array probe at a nominal frequency of 5 MHz. The study used two wedges, one for generating shear wave and one for generating compression wave and performed both manual and encoded scans. The linear encoder scan is done with the wedge's front abutting the weld cap, which produces a scan index of \(-\)25 mm. For manually delivered inspection, the technician has the freedom to make transversal, lateral, and skewing movements to locate discontinuities and peak them. The T-joint was examined using T-T and L-L direct modes with linear encoded scans and manual inspection. The T-T and L-L modes were used in the experiments because they produce a good amplitude response from a volumetric discontinuity. In this study,  
Figure 4: Example of PAUT results (Side A, Side view).  
before starting the sample's inspection, the 3 mm SDH was used on the martensitic sample (same material properties) for TFM model calibration. From these results, we obtained the following conclusions;  
1. TFM results appear clearer, with fewer spurious indications and better-defined flaw geometries.
2. The Longitudinal-Longitudinal reconstruction mode seems to give slightly better results than the Transverse-Transverse mode.
3. We obtained a slightly better detection rate with manual TFM as compared to the encoded one. We suspect that this is due to the operator's ability to minutely adjust the probe's position to improve the signal to noise ratio on the testing unit display. An example of TFM results (with two different reconstruction modes) compare with 32-element PAUT result is observed in Figure 5.