Examining x-ray testing for aerospace (continued)
A continuation of our June Test Digest article, "Examining x-ray testing for aerospace."
Greg Reed, Contributing Technical Editor -- Test & Measurement World, 5/31/2007 11:59:00 PM
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Testing turbine blades
Turbine blades are precision castings with a complex inner structure. The increased demand for turbine efficiency means that they are exposed to constantly increasing turbine inflow temperatures. Because of this, more and better cooling is required.
In addition to looking for anomalies in the turbine blade itself, placement of fine laser bores must be verified. “These bores must be placed very precisely through the front wall of a cooling channel without entering the rear wall,” Robbins says.
During use, turbine blades are shortened by friction and narrowed due to erosion by pressure. Used blades are inspected to ensure there is enough residual material to support repair. Pores in welded-on material, foreign substances in the cooling channels, or incorrectly executed bores can be detected using X-ray inspection.
X-ray CT allows the precise analysis of inner structures within turbine blades, as well as the comparison to CAD data. CT has become an important nondestructive technique that not only provides data about material integrity, but also valuable volumetric data, which is finding applications in reverse engineering, rapid prototyping, process control, and 3-D metrology.
CT is widely used in the medical community and is receiving increased attention from industrial users. CT systems are usually configured to take many views of the object, often more than 100; this provides reconstructed images of good quality, with excellent density discrimination.
A measurement of turbine-blade wall thickness using ultrasonic technology is nearly impossible due to the turbulators or inner walls existing inside. CT enables a noncontact, reproducible measurement of all structures with high accuracy, even on an automated basis. Generated CAD data from the volumetric model makes reverse engineering of these complex products possible.
But CT has limitations too. “Collecting CT data requires an X-ray system providing sufficient flux at a high enough energy to penetrate the sample along all viewing directions,” Stupian says. “The dynamic range of the detector is also important. These requirements are not always easily met.”
Ducts, tubes and exposureBut ducts, air channels and hydraulic lines in aircraft are exposed to the greatest mechanical and thermal stress. That is why they are manufactured using special alloys. Also, elements like these are supposed to exhibit a construction design that is as compact as possible. That leads to complex geometric structures. Welding seams are unavoidable with this type of construction. X-ray technology offers the possibility to inspect them with certainty (Figure).
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| This X-ray shows flaws in an aerospace duct. Courtesy of YXLON International. |
Films are normally utilized as the imaging medium. However, state-of-the-art digital flat detectors have made it possible to increase inspection assurance with respect to defect recognition further, even when radiation must penetrate a double wall. This procedure leads to substantially reduced inspection times. Until now, the use of chemicals which had been necessary, including all subsequent costs such as chemical waste disposal, no longer exists.
With the right detector, honeycomb structures and newer lightweight materials such as carbon fiber-composites can be inspected for fiber misalignment, damaged filaments and density variations like inclusions, micro cracks and porosities.
Most all aerospace and aircraft structures made of metal or composites in vehicles or payloads are immune to X-ray’s irradiation. Most space programs employ engineers knowledgeable about radiation to consult with regarding sensitive structural areas. Stupian says in most cases a test plan involving extra shielding or minimization of exposure time is created.
X-ray inspection methods will continue to have a significant role in manufacturing and maintaining aerospace and aircraft vehicles. Robbins predicts digital imaging combined with precise material handling and image enhancement with continue to contribute to higher detection probability and faster inspection times. A typical application that takes one minute for film can be accomplished in about eight seconds per image. Through new technologies and process analysis, X-ray inspection’s flaw-detection reliability will increase and move into more aerospace and aircraft applications.
| The X-ray process The typical X-ray inspection process involves several steps. First X-ray images (either radiographic or radioscopic) of the structure under test are acquired. Second, those images are converted to a useful format and presented to a skilled individual for interpretation. Finally, the data and results must be documented, archived and communicated. Inadequate performance at any of these steps, from either a cost or performance standpoint, can lead to degradation of the overall all inspection process. |
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