Aplicación del Seiso (Limpieza)

As the available flat lands from which to refract ultrasound into the inspection zones of the blade roots were so limited, there was no way of coupling the phased array probe efficiently to the component. Therefore a Rexolite wedge would be required to effectively 0dB

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

couple the probe for compression waves and also to provide the appropriate refraction angles for shear waves. The contact face of the wedges would be required to accurately match the radii and contours of the blade root platform and would therefore be difficult if not impossible to manufacture without taking account of the blade geometry. Positioning of such wedges would also be critical in order to ensure that refraction through the correct portion of platform was achieved repeatedly. The production of bespoke jigs was therefore necessary to accommodate the complex shape of the root platform to both position the phased array transducer accurately, and to produce the ultrasonic refraction required by the inspection.

The initial approach by the author was the development of Rexolite jigs which involved the production of jig blanks, modelled in a CAD environment and manufactured using advanced five axis CNC machines in the RWE workshops. Several blank wedges were produced with the negative profile of the convex side aerofoil and root platform radius, but with no specific profile for the transducer contact face, see Figure 4-5 and Figure 4-6.

Figure 4-5 CNC wedge layout

Blank Wedge Layout

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

Figure 4-6 Blank CNC wedges

The minimal flat land available on the root platform can be clearly seen on the jig blanks as highlighted in Figure 4-6, along with the exact profile of the aerofoil and the root

platform radius, giving a very accurate fit to the component. The next stage involved using the CIVA simulation tools to assess the ideal probe positioning for the shear wave

inspection of zone 2b, see Figure 4-7. It was determined that a skew angle of 45º to the side face was optimal with a mechanical wedge angle of 30.5º to provide a natural refracted angle of 45º. The compound angle produced by the skew and refraction angle was then milled into the blank wedge to provide a scanning surface on which to mount the probe. The wedge was coupled to a test sample containing EDM notches in the 2b position using sonagel couplant. The 20 element 10 MHz phased array probe was positioned and coupled to the surface of the jig and manoeuvred to optimise the responses from the EDM notch. The optimum position was marked and the probe mounted to the partially finished jig, see Figure 4-8.

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

Figure 4-7 CIVA model of shear probe positioning

Figure 4-8 Initial wedge angle and skew in blank wedge

The prototype wedge was tested on two blades containing defects of 3 mm and 8 mm through-wall respectively, emanating from the target area, see Figure 4-9. The resulting sector scans were recorded as seen in Figure 4-10 and Figure 4-11. As it can be seen, very positive results were attained with large measurable responses from the defects.

45º Critical Defect Area

20 element 10 MHz phased array probe

Compound wedge angle

Aerofoil/platform mating face Plan view

Probe and wedge Transparent view

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

Figure 4-9 MPI images of 3 mm and 8 mm defects to position 2b

Figure 4-10 Sector scan of 3 mm defect to position 2b

3 mm Defect 8 mm Defect Geometric response Defect response 0 cm 50 cm Scale 0 cm 50 cm Scale 0 5 10 15 20 25 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50 55 60

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

Figure 4-11 Sector scan of 8 mm defect to position 2b

The jig was finished by removing excess material to enable its insertion between adjacent blades. The final jig consisted of a small strip of flat refracting interface to make contact

with the root platform, the radius and profile of the aerofoil, see Figure 4-12.

Figure 4-12 Zone 2b shear wave inspection jig

Geometric Response Defect response 0 5 10 15 20 25 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50 55 60

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

The whole process was then repeated to create wedges to facilitate the inspection using compression waves in Zones 2, 3, and 4. Each jig matched the profile of the blade with sufficient flat surface to refract sound into the available land on the blade root platforms, see Figure 4-13 to Figure 4-15. Taking account of the outer hole centre being 25 mm deep and the inner hole centre being 50 mm deep the wedge heights were chosen which would result in the wedge delay response being imaged at around 35 mm deep to avoid masking around the holes. By taking the ratio of the velocity of rexolite compared to the longitudinal velocity of steel this was done by:-

Hi = vr/vs ×35 = 2320/5900 × 35 = 13.7 mm

where: Hi = height of the first element

vr = longitudinal velocity of Rexolite

vs= longitudinal velocity of steel

Figure 4-13 Zone 2a compression wave inspection jig

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Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011

Figure 4-15 Zone 4 compression wave inspection jig

Initial bench tials with multiple cracked blades were carried out followed by full validation on a live inspection rotor as outlined in Appendix C.

The bench trials concluded that all EDM notches were detected using the jigs in the various inspection zones as discussed in section C.1.1 and tabulated in Table C-1. Subsequent inspection trials on service rotors were further carried out as detailed in Appendix C and further discussed and analysed in section C.1.3, C.1.4 and C.1.5.