THE DEVELOPMENT AND MANUFACTURE OF FIXED- ULTRASONIC INSPECTION REFERENCE REFLECTORS AND TRANSDUCERS FOR COMPRESSOR BLADE DOVETAILS

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1 International Workshop SMART MATERIALS, STRUCTURES & NDT in AEROSPACE Conference NDT in Canada November 2011, Montreal, Quebec, Canada THE DEVELOPMENT AND MANUFACTURE OF FIXED- ULTRASONIC INSPECTION REFERENCE REFLECTORS AND TRANSDUCERS FOR COMPRESSOR BLADE DOVETAILS Ying XIONG, Dexiu DONG, Jiangang DUAN Liming Aero-engine Group Limited Corp., Shenyang, China Tel: ; Fax: ; ABSTRACT An Aero-engine compressor blade dovetail might crack during its use, a defect that affects its use, lifespan and even the safety of the aero-engine. It is essential to develop a nondestructive testing method to inspect and monitor compressor blade dovetail corner cracks. In this paper, fixed-ultrasonic inspection is suggested as a method to monitor compressor blade dovetails. Special ultrasonic reference reflectors and transducers were developed and manufactured, so that the application of special ultrasonic reference reflectors and transducers could be implemented and shown to be effective for fixed-ultrasonic inspection of compressor blade dovetail corner cracks. This ultrasonic inspection system will ensure the safe use of aero-engines. Keywords: Compressor blade dovetail cracks; Fixed-ultrasonic inspection; Ultrasonic reference reflector; Special ultrasonic transducer

2 INTRODUCTION When an aero-engine compressor rotor is running, the high-pressure compressor blade does work on the blade passage air, transforming the blade mechanical energy into air internal energy. And at the same time, the air density elevates, and the air velocity increases, thus enhancing the compressor air pressure as well as heat transfer efficiency; this provides an advantage for the inflation air to do its work. In the assembly compressor rotor, the compressor blade is joined to the compressor disc by a dovetail, and through this passes mechanical energy to the blade. If blade dovetail has a crack, it will mean that the blade cannot complete the function of transferring energy, so it is essential to develop a nondestructive testing method to inspect and monitor compressor blade dovetail corner cracks to ensure the safe use of aero-engines. MANUFACTURING ULTRASONIC REFERENCE REFLECTORS An ultrasonic reference reflector is a kind of work piece when used in a method to inspect specific test samples. It has the same or similar acoustic characteristics for testing materials containing significant explicit reference reflectors (even flat-bottom holes, slots, etc.). An ultrasonic reference reflector is used to adjust the ultrasonic instrument sensitivity and inspection scope and to guarantee that the scan sensitivity can sufficiently detect the requisite size and the orientation of defects, as well as compare the detected defect signals with the signal of fabricated defects produced in the work piece, thus evaluating the detected defects. [1] Blade Dovetail Fracture Configuration Analysis The macroscopic appearances of a high-pressure compressor rotor blade dovetail fracture section and fatigue strip are shown in Figure 1. The blade broke at the dovetail corner on the exhaust side 9 mm away from the dovetail bottom. As observed by an object magnifier, the fracture has the obvious fatigue source, expansion and the instant crack areas. The fracture divides into two colours: the fatigue area is smooth and light yellow, whereas the instant crack area is light gray. The fatigue fracture area comprises approximately two thirds of the entire fracture area. The expansion area fatigue arc is fine and clear. The fatigue begins at the dovetail corner and is a line source. When the blade is put into a scanning electron microscope to observe the blade dovetail fracture, there is the radial appearance within the 2 mm region, indicating that this region is the fatigue cracking area. The microscopic appearance of the blade dovetail fracture source area is shown in Figure 2.

3 Exhaust side of dovetail Dovetail back side Fig. 1: Blade dovetail fracture position and macroscopic configuration. Fig. 2. Fracture source area microscopically magnified. Dovetail Fracture Source Area Measurement A type JX11B Universal Measuring Microscope and a Makers Microscope were used in which the accuracy is (1 + L/100) µm (where L is the length of the defect) to measure the blade back dovetail crack length. The datum plane was adjusted to determine the X axis direction. Taking the right crack start point as 0 (see Fig. 3), the +X direction extends every 0.50 mm, covers a spot in the crack 3 mm in scope, and surveys each spot coordinate figure; the measurement results are given in Table 1. Fig. 3: Blade back dovetail crack view.

4 Table 1. Blade back dovetail crack measurement results Coordinate direction Measurement Spot Results (mm) X Y Definition of a Fabricated Defect According to the microscopic appearance size of the blade dovetail fracture source area of the fatigue outset region, a fabricated defect size is defined as 2.0 mm long (L) 2.0 mm wide (W), and the slot width is 0.20 mm. In order to make fabricated defect close to the microscopic appearance of the blade dovetail fracture source area, an electric spark method was selected to carry out the processing. DETERMINATION OF THE TRANSDUCER WAVE TYPE AND REFLECTION ANGLE Selection of the Transducer Wave Type Ultrasonic inspection is a non-destructive testing method. The ultrasonic wave (the commonly used frequency is 0.5 ~ 25 MHz) is disseminated by the medium, but if it meets a contact surface, it will be attenuated. Thus one can use this reflection characteristic to inspect the defects. The types of ultrasonic wave are: longitudinal wave, shear wave, surface wave and Lamb wave. The selection what kind of wave or the inspection technology primarily depends on the position and the orientation required to inspect the defects that are located in the work piece; the selection criterion is the requirement to obtain the best display of the defects. [2] According to the actual blade geometrical shape, and also the position of the blade exhaust side dovetail crack appearance (see Fig. 4), the transducer is put onto the blade platform and the oblique incidence method is selected to carry out the inspection. [3] Thereby a series of shear-wave transducers and small-angle longitudinal wave transducers are made separately to carry out the ultrasonic testing experiment for a blade with a fabricated dovetail defect. The preliminary test results are: 1. When using a small-angle longitudinal wave, the blade geometry shape and the special space relations of the dovetail corner with the scan surface can affect the inspection. This produces many wave modes, and therefore the mode conversion has many echoes, hence it is very difficult to obtain an accurate evaluation of the crack signal.

5 Fig. 4: Blade dovetail fabricated defect position and size. 2. When using a different angle for the shear-wave inspection, showing the echo signal position to be relatively fixed, by analysis and validation it was discovered that the blade exhaust side dovetail bottom R may form a permanent echo signal. This signal always appears together with the dovetail corner crack signal, so when one inspects the dovetail corner crack, it is easy to find the best coupling position between the transducer and the blade platform. Therefore this echo signal is called the control signal. In summary, it was decided to use the shear wave to carry out the inspection of dovetail corner cracks. Figuring Out the Scope of the Transducer Refraction Angle To observe the blade, it was discovered that the transducer-scanning surface (the blade platform), the fracture section and the dovetail bottom surface must be parallel, and the position becomes approximately an oblique line; thus the refraction angle must satisfy two basic conditions: 1. The refraction sound beam must be a pure shear wave. 2. The principal refraction sound beam must be able to cover the dovetail root profile, and the maximum refraction angle β must not surpass the dovetail exhaust side surface (see Fig. 5). Both sides of the dovetail pressure side and bottom surface of the clip angle δ plus β max should be equal to 90. Finally, one can work out that β max Cr yst al pl at e α Cr ack β δ Fig. 5: Calculation of the Maximum refraction angle.

6 The minimum refraction angle is determined according to Snell's Refraction Law. First one calculates the first critical angle α I, then second one calculates the pure shear-wave minimum refraction angle β s1 ; the results are shown in Eqs. 1 and 2. where, α I is the longitudinal-wave incidence angle (the first critical angle); β S1 is the minimum shear-wave refraction angle; C L1 is the longitudinal-wave speed of sound in the organic glass (2700 m/s); C L2 is the longitudinal-wave speed of sound in the titanium alloy (6100 m/s); C S2 is the shear-wave speed of sound in the titanium alloy (3200 m/s). In conclusion, one finds that the shear wave refraction angle scope should be within the range 32 ~38.7. Because the sound beam has a certain spread angle, the refraction angle may have a 1 ~2 fluctuation. A group of different angle shear-wave transducers was manufactured according to the refraction angle scope, which was obtained by computation. After experimental confirmation, when the transducer is placed on an appropriate position on the blade platform, the blade dovetail fabricated defect signal and the control signal are both clearly shown on the ultrasonic instrument screen, as shown in Figs. 6a and 6b. Fig. 6: Blade ultrasonic signals; (a) normal (on the left); (b) abnormal (on the right)

7 DEVELOPMENT OF THE FIXED-ULTRASONIC TRANSDUCER Transducer Design The compressor case is made of an outer case, a fore case, a stage 0 blade, a stageⅠ blade, and a stageⅠdisc. To implement a fixed-ultrasonic inspection, the transducer must pass the endoscopic hole (Φ8 mm) into the case, and through the blade s space gap contact to the blade, as shown in Fig, 8. Fig. 8: Blade dovetail assembly framework. It is well known that if there is thin layer of air between the transducer and the test sample when applying the ultrasonic inspection, the ultrasonic wave reflectivity is almost 100%. The very thin layer of air also possibly prevents the ultrasonic wave to spread along the test sample. If a coupling liquid is applied to fill the air gap between the transducer and the test sample, this enables the ultrasonic wave to spread along the test sample. In addition, the coupling liquid has lubrication and thus reduces the friction between the transducer and the test sample, preventing the test sample surface to hurt transducer, and makes the transducer move easily. Therefore, a soft coupling tube is tied together with the transducer. It is quite hard to put a Φ1 mm soft abrasion-resistant match tube inside the Φ8 mm transducer, and to adopt a medical way that has an intravenous drip to implement the coupling directly. Manufacture of the Transducer A conventional shear-wave transducer consists of a piezoelectric crystal plate, sound absorber, wedge, damping block, electric cable line and outer covering. In general, from the transducers handbook, the shear-wave transducer overall dimension is usually: length 24 mm width 14 mm height 22 mm. But this special transducer outer size is limited to Φ8 mm 10 mm (inside diameter size Φ7.6 mm 10 mm, which is about half of the smallest shearwave transducer size that is commonly used in the transducers handbook). This size is a serious limit for the transducer and causes the sound wave to produce too many reflected waves, so that it is very difficult to eliminate them completely. Thus one has to choose a very special sound absorber to reduce the noise. At the same time, there is a need to first manufacture a Φ7.6 mm 5 mm metal pipe in which to put the transducer. It is important

8 to keep the transducer outer metal coat and the metal pipe coaxial, and to pour into special a special solidification rubber to make sure the connection is secure. In addition, to enhance transducer sensitivity as far as possible, the match level was specially increased to process the crystal plate in order to enhance the sound absorption effect. CONCLUSIONS 1. One should select an ultrasonic inspection method to monitor a single blade that uses a reference reflector and the special transducer developed to detect blade fabricated dovetail defects, so that the crack signal and the control signal are easy to distinguish, and the inspection sensitivity is sufficient. 2. One should apply a fixed-ultrasonic method to inspect a simulation engine, in which five blades are loaded with individually fabricated defects. Then one can discover exactly the fabricated blade defects. After a series of experiments, the application of special ultrasonic reference reflectors and transducers was implemented and shown to be effective for fixed-ultrasonic inspection for compressor blade dovetail corner cracks. 3. As a result, in the service engine examination space, the working conditions and the superficial attachments, etc., in order to ensure that the fixed-ultrasonic inspection is effective, one not only needs to develop the special-purpose special transducer and the reference reflectors, but also one needs to match a stable specially-made fixture. Only in this way can the fixed ultrasonic inspection become an effective, accurate testing technology. REFERENCES [1] Paul Mclntirem, American Nondestructive Testing Handbook, Shanghai: Universe Book Publishing Company, [2] CHINA, Nondestructive Testing Integrated Knowledge, Second Edition. Beijing: Machine Industry Publishing Company, [3] CHINA, Ultrasonic Testing Training Book, Second Edition. Beijing: Machine Industry Publishing Company,