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.
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.
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 untl 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. Improvements in the functionality of video borescopes 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 information is a vital part of any inspection procedure, especially in aerospace, 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 addressed 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.
The 3D Phase Measurement provides accurate measurement "on-demand" by eliminating the need to change the probe tip to capture the measurement, streamlining the inspection process.