Advances in Remote Visual Inspection

Remote visual inspection (RVI) is a long-established inspection and nondestructive testing technique in the aerospace sector. Although conventional rigid borescopes are still widely used in the industry, using inspection ports built into aero engines...

Productivity through greater ease-of-use

Measurement using stereo and shadow can be time-consuming as well as requiring expertise. For example, with stereo measurement, it is first necessary to spot the defect using a nonmeasured optical tip. This tip must then be replaced with a stereo tip, the defect must be relocated, the image is frozen, the cursors are matched, and the measurement is taken. With phase measurement, the defect is located, the image is frozen, and measurement is carried out. There is no need to change the tip.

As there is no need for the point matching, shadow identification, cursor matching, or dot selection steps associated with other measurement techniques, 3-D phase measurement offers greater ease of use. This means fewer operator mistakes and more repeatable and accurate results.

More potential applications

An important application is the measurement of aircraft engine tip to shroud clearance. Aircraft engines and other axial flow turbo machinery are typically designed to minimize the radial gaps between the blade tips and the blade housing or shroud. Gaps can reduce efficiency by allowing gas or air to leak into the downstream stages. Consequently, it is important to check this clearance, both during manufacture and also during service as the gap changes during engine operation. (High operational rotating speeds and high temperatures can cause radial elastic growth of blades, as well as thermal expansion of the shroud.)

Historically, one method of measuring tip/shroud clearance has involved inserting a thin metal rod into an axially drilled bolt and attaching this assembly to the fan case so that the end of the rod is positioned where the blade tips should be. After the engine has been operated, the amount of wear on the rod is measured. Obviously, this is not a high accuracy technique and its execution often generates problems such as the liberation of metal from the rod, which can cause damage to the engine.

Phase measurement now offers a simple, noncontact, and high-accuracy technique for measuring tip to shroud clearance.

There have been many advances in RVI since its early days. Imaging quality has greatly improved as a result of fully digital data streams and improved optical and illumination technology. The integration of on-board processing has expanded RVI’s versatility and greatly facilitated data sharing through connectivity. Sophisticated data acquisition, review, reporting, and archiving software platforms allow the accumulated data to be organized efficiently and utilized to best advantage. And now it’s easy-to-implement. Innovative RVI measurement techniques can provide fast, accurate results with more comprehensive imaging to improve quality control during manufacture and to allow smarter, more efficient in-service inspection.

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