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Wall Thickness of Turbine Blades


turbine bladesApplication: Measuring the wall thickness of hollow metal turbine blades.

Background: Many turbine blades used in aircraft engines and other high-performance systems are hollow to permit the circulation of coolant within the blade. Core shifts during casting, faulty machining, or normal surface wear during operation can cause blade wall thickness to fall below acceptable limits. Mechanically measuring wall thickness is usually not possible without destroying the blade. However, with proper transducers and instrumentation this measurement can often be done ultrasonically.

Equipment: Precision thickness gages are most commonly used for turbine blade measurements. These include Models 38DL PLUS and the 45MG with Single Element softwareand waveform display option. They are typically used with immersion or delay line transducers that are selected according to the thickness and geometry being measured. The gage's waveform display allows a trained operator to monitor waveforms to help insure valid echo detection, and aids in insuring optimum transducer alignment.

The model 5072PR pulser-receiver with immersion or delay line transducers can also be used, reading pulse transit time from an oscilloscope or a PC-based digitizer system.

Procedure: The exact measurement system that is recommended for a given turbine blade application is defined on the basis of customer requirements and the acoustic properties of the blade. The following are basic considerations:

Transducer Type: Both delay line and immersion transducers are used in turbine blade applications. However, the curvature of small turbine blades can make it impossible to properly couple delay line transducers into the concave side. The 0.125 in. (3 mm) delay line transducers (M203 and M208) generally couple adequately onto concave radii down to about 4 in. (100 mm). With contouring of the delay line it is possible in some cases to couple onto sharper radii, but in general, sharply curved surfaces, particularly the leading edge of blades, are better measured with immersion transducers. The 20MHz V316-B in a B-120 bubbler provides a convenient handheld assembly for immersion transducer measurement of blades. In many cases it is also possible to use the V260-SM Sonopen focused delay line transducer to couple onto concave surfaces that cannot be measured with conventional delay lines.

In addition to the standard line of immersion and delay line transducers, we offer three special low-profile 20MHz delay line transducers for measurement of turbine blade thickness in multi-blade assemblies where the space between blades is limited and access is difficult. The M2054 is a 20MHz delay line transducer just 0.27 in./6.75 mm thick on a 3 in./75 mm handle. The M2055 is similar with a transducer/delay line assembly that is 0.40 in./10 mm thick. The V2034 has a 0.40 in./10 mm head on a 6 in./300 mm angled handle. Outline drawings of these transducers are available on request.

Measurement Mode: Using delay line and immersion transducers, thickness measurements may be made in either Mode 2 (interface to first back echo) or Mode 3 (echo to echo following interface). Mode 3 provides better thin material resolution than Mode 2, but it is possible only if the points to be measured on the turbine blade produce multiple backwall echoes. If there is only a single useable backwall echo (due to curvature or attenuation) measurement must be done in Mode 2. The Model 38DL PLUS and 45MG gages can operate in either Mode 2 or Mode 3. Establish the optimum setup for a given turbine blade application using the aid of reference standards representing the range of thickness and geometries to be measured.

Thickness Range : In typical metal blades the minimum resolvable thickness for a 20MHz delay line or immersion transducer is approximately 0.006 in. (0.15 mm) in Mode 3, and 0.020 in. (0.5 mm) in Mode 2. If thinner measurements are required, higher frequencies can be used with pulser-receivers. Most turbine blade measurements are done at 10MHz or 20MHz.

Dead Spots: Hollow turbine blades frequently contain assorted structures inside the blade to direct the coolant flow or add strength to the blade. Generally, it is not be possible to obtain a backwall echo from points where these vanes or ribs are located because the structure disrupts the smooth inside surface necessary for a good reflection. In cases where these structures are closely spaced, a focused immersion transducer with a small spot size produces better backwall echoes than a delay line transducer. There are also some cases where sharply tapering blade thickness can create a situation where inside walls and outside walls are significantly non-parallel, which can cause echo distortion and potential measurement errors.

In all cases, determine the combination of transducer and instrument on the basis of tests on actual product samples. The wide variation in turbine blade geometries makes sample evaluation important.

Figures 1 - 3 show typical waveforms associated with turbine blade measurement performed with a Model 38DL PLUS gage. The gage display screen instantly provides the live ultrasonic waveform with gates and thickness readings. This is ideal for difficult applications or when setup parameters need closer examination.

- Mode 3 Measurement of a Turbine Blade
Figure 1: Mode 3 Measurement of a Turbine Blade

Figure 1 shows a Mode 3 measurement of a 0.041 in. (1.05 mm) thick concave turbine blade trailing edge using an M208 transducer. The curvature is of sufficiently large radius at the measurement point that the M208 couples well into the material, and the clean multiple echoes permit a Mode 3 measurement.

Mode 2 Measurement of a Turbine Blade
Figure 2: Mode 2 Measurement of a Turbine Blade

Figure 2 shows a Mode 2 measurement of a 0.070 in. (1.8 mm) thick convex turbine blade wall, also using an M208 transducer. Here the presence of internal ribs damps the echoes somewhat, and in the absence of clean multiple echoes a Mode 2 measurement is recommended.

Immersion Technique to Measure Curved Convex Blade Section
Figure 3 : Immersion Technique to Measure Curved Convex Blade Section

Figure 3 shows an immersion technique using a V316-BB 20MHz immersion transducer with a B-120 handheld bubbler to check a thicker (0.051 in./1.3 mm) sharply curved (1 in./25 mm radius) convex blade section, where the sharp convex curvature made coupling a delay line transducer difficult. In this type of measurement, especially on concave surfaces, it is essential for the operator to monitor waveforms to insure optimum transducer alignment.

Note: Certain tubine blades cast from large-grain alloys can exhibit significant velocity variation from point to point due to anisotropic grain structure. In these cases the accuracy of ultrasonic thickness gaging will be limited by the degree of velocity variation. This limit must be determined experimentally for specific cases.

Olympus IMS

Products used for this application


45MG

The handheld 45MG ultrasonic thickness gage is packed with measurement features and software options. This unique instrument is compatible with the complete range of Olympus dual element and single element transducers, making this gage an all-in-one solution for virtually every thickness gage application.

38DL PLUS

The 38DL PLUS is an advanced ultrasonic thickness gage. Uses dual element transducer for internal corrosion applications, and has features that include THRU-COAT technology and echo-to-echo. Uses single element transducers for very precise thickness measurements of thin, very thick, or multilayer materials.
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