Turbine Technology: Advances in Remote Visual Inspection

Seeing what cannot be seen by the naked eye is the basis of all nondestructive testing techniques. Ultrasound is used to identify flaws and corrosion in metals and especially in welds. Radiography is used to examine laminar surfaces for hidden cracking. Remote visual inspection (RVI) is widely used throughout the aerospace industry for examining the insides of a wide range of aircraft components and structures, from engines to airframes. Recent advances in RVI have seen significant improvements in the imaging and measurement capabilities of the technique and these are described in this article.

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, video borescopes now account for most of the RVI tasks. They find application in the military and civil sectors inspecting airframes, APUs, and engines. They are used as part of MRO routines, as well as by OEMs during engine build. They can be used to inspect for leaks, corrosion, and surface cracking and to check internal gaps, as well as identifying the reasons for blockages and detecting foreign objects. With such a wide range of potential uses and requirements, it is not surprising that video borescopes have developed considerably over recent years.

The evolution of RVI

Industrial endoscopy has its origins in the medical sector and the very first endoscope was developed in Austria in 1806 "for the examination of the canals and cavities of the human body." However, the procedure was disapproved of by the medical authorities of the time and it wasn't until 1822 that an endoscope was first introduced into a human.

Industrial endoscopy only began in earnest after World War II and early instruments consisted of a lens and illuminating light source, connected to a light transmitting extension which ended in a viewing eyepiece. These basic borescopes were used solely for visual inspection, with no means for measuring. As such, they found application in inspecting inaccessible locations or in locations where normal access was denied because of interfering structures or components.

However, image capture and measurement capability were introduced into industrial endoscopy in the '60s when 35 mm still cameras complemented eyepieces and these developments were followed by the introduction of fiber optics as the light transmitting mechanism and the adoption of video cameras as the preferred method of image capture. At the same time, there have also been significant improvements in the functionality of video borescopes with the introduction of on-board computing power. This has allowed borescopes to offer the capability to save and store video images in digital format. The capturing and storage of digital data was formerly achieved by floppy disk and videotape. This technology then advanced to include CD, DVD, flash media, and solid-state memory cards, so that files could be transferred to a PC for further assessment or storage.

Sharing inspection information is a vital part of any inspection procedure, especially in the aerospace sector, where safety and economics often require expert assessments of engine fitness-for-purpose. Consequently, the ability to share information is a feature which has been addresses in the latest generation of RVI instruments.

Handling the data

Introducing on-board PCs to RVI has also allowed the introduction of application software to ensure that the vast amount of generated data is efficiently managed. Such software tags images and arranges them in logical files to allow quick and easy access. Instruments using the DICONDE (digital imaging and communication in NDE format, a nonproprietary format, developed from DICOM used throughout the medical sector in radiography, easily communicates with software platforms, which can acquire, report, review, and archive data. It also features important application tools, including image enhancement, manipulation, and zoom.

In the aerospace sector, which often struggles to cope with ever-increasing amounts of inspection information, the archiving data is especially relevant. It is important for software platforms to accept images from any number of LAN-connected or remote workstations and store these using various compression techniques to save storage space without sacrificing image quality. Input and retrieval of information is quick and easy by virtue of the simple DICONDE tagging system. Chosen software should not only store the raw inspection data but also any enhanced images and allow for more efficient data searching, as all information from all workstations in the network is available at one central repository. A similar order of productivity improvement can also be achieved in post-inspection, as only relevant inspection data needs to be sent for further analysis.

Making it simple

Software is also available to standardize inspection procedures to ensure consistency of both inspection and presentation of inspection results. Menu-directed inspection (MDI) is a software solution that provides a guided inspection, where context is added automatically. For example, when inspecting an engine, a drop-down menu will first allow an inspector to select the relevant manufacturer and specific engine. All the identification data relevant to the task (inspector, site, date, etc.) is then inputted before the inspector carries out the inspection, in the manner specified for that engine and component. The data image file is then tagged with annotations and filed within the borescope's data capture system. Finally, a hard copy report is produced with a convenient "click-to-report" feature.

Measuring what you see

The measurement of flaws, discrepancies and clearances is nowadays often just as important as their detection and identification. To date, there have been three major measurement systems: comparison measurement, stereo probe measurement, and shadow probe measurement.

Comparison measurement is based on a known reference dimension in the inspection image, which is used to measure other objects in the same view and plane. (The reference dimension is often set in place by the instrument manufacturer or introduced with the probe.)

Stereo probe measurement uses a prism to split images, allowing the camera to capture left and right views with a precise angle of separation. The position of user-place cursors is then analyzed using a computer algorithm and triangulation geometry is applied to obtain accurate measurements.

Shadow probe measurement relies on a shadow triangulation of tip-to-target distance. A shadow probe projects a shadow across an inspection image and the position of the shadow in the image indicates the distance to the object. With this information, the shadow probe system can accurately calculate the size of user-selected features or defects. Typical measurements afforded by these measurement methods are depth, length, area, point-to-line, multi-segment length and circle gauge.

Toward better accuracy and improved imaging with phase measurement

Even with the current range of measurement techniques, measurement remains the most difficult aspect of using video borescopes. Operators must be highly trained and practiced to obtain reliable and repeatable results. This expertise level has been addressed as RVI is now professionalized as an official NDT discipline and is a module within ASNT's TC1A Level-III testing and certification process.

However, significant advances have been made recently in improving the accuracy, repeatability, and ease-of-use of video borescopes with the development of phase measurement.

Phase measurement is based on an existing optical metrology technique and involves projecting line patterns onto a surface, capturing the patterns in a video camera with high-quality viewing optics and processing the images using proprietary algorithms to produce a point cloud, 3-D map of the entire surface. This is then used in conjunction with measurement to obtain more precise information of the defect or object being viewed. Measurement itself simply involves the placing of cursors on the full-screen image, with no need for the point matching, shadow identification, or dot selection steps that can be challenging with other techniques.

An innovative feature of this new measurement system is that the 3-D scan can be rotated and zoomed to provide enhanced indication of the indication's size and shape. Further assistance in assessing an indication's size and shape is provided by the system's profile view feature. This is achieved when the user positions cursors on either side of an area of interest and the phase measurement system draws a line between them. Profile view is then selected and a cross section of the part along that line is displayed, helping to visualize the shape of a pit, crack, or corrosion area. At the same time, profile view can also be used to measure depth at points along the cross-section.

Increased 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, as mentioned earlier, 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 of the new technology 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 between tips and shrouds can reduce efficiency by allowing gas or air to leak into the downstream stages. Consequently, it is very 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. Application specific software such as MDI has helped to improve probability of detection, while reducing the occurrence of false calls. 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.

Thierry Laffont is with GE Measurement & Control Systems.