Sustaining jet engine blades requires maintainers to identify problems including geometric wear, poor surface quality, leading-edge wear, coating wear-off, and the location and size of cavities. Prompt repairs of these issues prevents engine failures and saves expensive assets. Yet each maintenance action increases costs. The DOD prioritizes maintenance expenditure reduction by its increasing use of condition-based maintenance (CBM+). To enhance materiel availability at the lowest possible cost, CBM+ minimizes unscheduled repairs, eliminates unnecessary maintenance activity, and employs the most cost-effective maintenance management techniques.1 CBM+ produces a holistic view of equipment conditions in part by conducting non-destructive inspections (NDI).
A CTMA project—Rapid and Agile 3D Scanner with Micron Accuracy for Non-Destructive Inspection (NDI)—has developed an affordable NDI system to decrease maintenance cycle time and costs for jet engine blades while increasing inspection accuracy. This initiative brought together the US Army (Letterkenny Army Depot), Navy (Fleet Readiness Center East), and Air Force (Oklahoma City Air Logistics Center), along with industry partners MDS Coating Technologies and Automated Precision, Inc. (API). Completed in 2018, the project has created an affordable, optical, one-cell NDI system for turbine rotor compressor blades. A one-cell system is defined as a single maneuverable platform that mounts the sample blade and enables optical sensors to provide inspections. This one-cell solution enables automation of sequential inspection processes seamlessly, where critical parameters are documented and shared as the system progresses through the processes.
The one-cell NDI system acquires precision dimensional data of blade profiles using API’s commercial 3D laser scanning technology, RapidScan. During inspection, three optical sensors are employed to provide three novel measurement techniques: 1) shadow imaging for cavity detection, 2) a laser line scanner for geometry, and 3) a time-to-spectrum mapping interferometer (TSMI) scanner for high-resolution scanning of blade edges and blade surface cavities. The instruments are integrated and aligned in a single sensor housing with the same standoff, which offers motorized rotary motion, motorized vertical motion, and semi-automated blade mounting. The platform applies the three sequential techniques, from the shadow imaging to the TSMI, seamlessly without readjustment of the blade sample’s location.
With the shadow imaging technique, the project team has created a new inspection process, “cavitation detection,” to identify the area of interest for blade surface inspection and locations of cavities with the most significant impairments. This technique combines a special lighting method with a high-resolution camera to emphasize shadows of cavities—it creates an image of the surface that highlights the contrast between a cavity and its coating boundary. The technology integrates a contrast-enhancing image processing and boundary detection algorithm into the hardware to enable real-time intelligent cavity detection. Users can apply a criterion to limit the number of most significant cavities to be scanned by the TSMI or simply to draw a go/no-go decision.
The line scanner has been designed to scan an area 50mm wide by 200mm high with a scanning resolution of 0.1mm. Based on the blade size information provided by the shadow imaging, the line scanning area is determined and scanned correctly without any readjustment of the blade sample’s position. The rotary stage allows the opposite surface to be scanned.
While the geometry measurement requires large area scanning on an entire blade surface with medium resolution, the TSMI provides 10 times finer resolution (0.01mm) and better accuracy (0.001mm) with programmable scanning at 120,000 point-measurements-per-second. This high-resolution accurate scanner is suitable for scanning microscopic-resolution surface topology on any impaired surface with feature size ranging from 0.005mm to 100mm or longer, with very long standoff of up to 300mm.
The TSMI scanner is used for two inspections: the cavitation topology scan and the blade leading-edge scan. For the cavitation scan, based on the given list of cavity locations, the TSMI points to each cavity and performs programmed scanning on the cavity’s surface. During the CTMA project, the typical cavity size was 3mm x 3mm square, which meant it would take 30 seconds to scan 10 cavities widely distributed on the surface of the blade sample. A configurable criterion is applied to the scan results to determine whether blades are repairable. The leading-edge inspection consists of two measurements. One profiles the edge and compares it with a nominal edge profile for estimating the extent of wear and critical chips on the edge. The second profiles the cross-sectional curvature of the edge and compares that with the nominal curvature for estimating the extent and progress of the wear. During the project, this process achieved a measurement repeatability of better than 0.001 mm (2 sigma).
Additionally, this project has developed user interface software to provide visual guidance for the inspection process without interrupting operations. All determined parameters and go/no-go decisions are displayed to allow an interactive process and final review. The software enables technicians to produce final reports with data that can assist with trend analysis and process improvements.
This rapid and agile 3D scanner’s ability to attain sub-micron level accuracy ensures that blades are inspected correctly and provides quantitative analysis of erosion and cavitation. The system can improve DOD maintenance by preventing hundreds of thousands of reusable blades from being rejected each year and by correctly rejecting faulty ones. Due to its portability, the scanner can be deployed at maintenance depots and at first- and second-line maintenance shops. Moreover, the scanner can be used for many similar inspections, including wide-area scanning of wings and other structural sections of aircraft, helicopters, heavy machinery, and vehicles.