Above: Robots scan two areas of a large aerospace part simultaneously in TWI’s IntACom prototype robotic inspection cell.
Non-destructive testing is used extensively at every stage of aircraft production and operation, from raw material evaluation to in-service inspection. Automating the process promises to save time, cut costs and avoid potential production bottlenecks, particularly for composite structures.
Automated NDT systems are already widely incorporated into the production process of new airliner models. Airbus factories in France, Germany and Spain, for example, use ultrasonic systems from Areva NDE Solutions and its subsidiary intelligeNDT to inspect wing covers and other structural elements of the Airbus A350.
Phased array ultrasound transducers (PAUT) are the only option for examining such large panels at high throughput rates. The arrays’ multiple elements mean that the angle and focal depth can be varied by changing the pulse focal laws. For fuselage panels, which are up to 20m long and 6m wide, the center wing box, and keel beam parts, six-axis manipulator systems are used to position the PAUT instrument. For wing covers, dual six-axis tower systems are used: one system inspects the skin side while the other is synchronized and programmed to inspect stringers in a different position to avoid interference with water on the backside.
Above: Lufthansa Technik has developed a prototype robot to inspect Boeing 737 Classic fuselage panels
Clean Sky NDT
Exploiting the properties that make composites attractive – including improved stiffness to weight ratios and better resistance to corrosion, impact and fatigue – means fabricating increasingly complex parts that are correspondingly harder to inspect. For this reason, the pan-European Clean Sky research program that is pursuing dramatic reductions in the environmental impact of air transport includes work on NDT automation.
In Spain, Tecnatom and the Center for Advanced Aerospace Technology (CATEC) in Seville are collaborating on the €475,000 DIAAMOND (Development of Non-destructive Inspection Approaches for Automatic detection and Monitoring of Damage evolution) project. The aim is to develop new approaches to inspection and automated systems for the detection and online recording of CFRP damage during structural tests. The approach would shorten total test time and reduce the requirement for human presence, particularly for tests running continuously around the clock.
The resulting demonstrator will perform data acquisition and analysis automatically and transfer results to the structural test control system in real time. As well as avoiding the need for human intervention, the objectives are to reduce inspection time, improve inspection quality by providing NDT data positioning and recording, and simplify system automation. Relevant information will be extracted automatically by image processing and data fusion to support structural test follow-up.
DIAAMOND forms part of the Green Regional Aircraft platform. Under another Clean Sky initiative, Sustainable and Green Engines (SAGE), the €500,000 WELDMINDT project is looking at open rotor engine welded parts inspection using miniaturizable non-destructive techniques.
The SAGE 2 project focuses on a geared open rotor engine, which will feature rotating turbine frames. Because the frames are regarded as engine-critical components, they require extremely reliable design, manufacturing and inspection methods, and maintenance and overhaul procedures. NDT methods in particular need to be robust and accurate.
Welded joints are currently examined using techniques such as visual inspection, fluorescent penetrant inspection (FPI) and radiography. WELDMINDT aims to improve both detectability and NDT accessibility in interior semi-closed design elements such as hollow vanes and torsion boxes. It will mean developing non-contact techniques such as optical, infrared camera and others, with the potential for miniaturization and characterizing their capability.
Coordinated in Spain by Lortek, the project aims to integrate shearography, infrared thermography and ultrasonic inspection into a single inspection system, using a laser as the sole excitation source. The combination of multiple NDT techniques, along with the use of advanced signal processing, is expected to increase the defect detection rate and size precision by more than 20% while reducing the inspection time from minutes or hours – or days in the case of maintenance, which would otherwise involve dismantling the engine to gain access – to seconds.
Above: IntACom prototype robotic inspection cell at the TWI Technology Centre in Wales
IntACom project
Research is not confined to Clean Sky. TWI in the UK is a participant in the WELDMINDT project, and the TWI Technology Centre in Wales recently completed the first phase of its IntACom NDT automation project with the production of a prototype robotic NDT system.
Backed by Rolls-Royce, GKN, Bombardier and the Welsh government, the three-year project aimed to achieve a fourfold increase in the speed of inspection and analysis of geometrically complex composite components. And it has succeeded to the extent that the industrial partners have opted to back further phases of the project without waiting for a further round of Welsh government funding.
High raw material costs and labor-intensive processes make composite parts expensive to produce, and the requirement to inspect every part means that NDT can be a bottleneck. Exploiting the properties of composites to produce more complex parts exacerbates the problem and demands both faster and more advanced inspection techniques. The alternative
is manual inspections that can add variability and cost.
According to project manager Ian Cooper, the research addressed three areas. As well as automating the inspection itself and applying advanced PAUT, it uses advanced software techniques such as assisted defect recognition and scan display management. Automation means areas previously scanned by hand, with results transcribed on to the part and report drawings, are now scanned automatically using scanning paths generated from imported CAD data.
The heart of the automated inspection system is an inspection cell comprising two six-axis robot arms, capable of working independently and cooperatively. The robotic arms deploy end-effectors carrying ultrasonic array transducers that can scan wide areas of the part in a single pass. CAD data imported into the system is used to generate scan paths using commercial or in-house-developed off-line programming software. Mousing over the displayed CAD image enables the user to select surfaces to be inspected and assign tools to each surface, while live imaging allows programming of focal laws and other ultrasonic parameters. The tools and components are protected by detachable magnetic holders. Once the part has been defined, scanning is achieved by simply selecting the part from a menu and pressing the start button.
Above: The Morfi robot uses active thermography to examine the chemically milled pockets in 737 Classic fuselage skin plates
Ultrasonic arrays
The replacement of single element transducers with ultrasonic arrays enables curved surfaces, radii and other difficult areas to be scanned at much higher rates than was previously possible. Where possible, large PAUT probes were used to provide maximum area coverage in a single pass. Ultrasonic modeling was used to determine the optimum inspection techniques for difficult areas of geometry. For example, curved arrays were used to inspect internal and external radii in a single pass.
Complex curvatures are addressed by detecting the shape of the surface interface and adapting the focal laws to suit. This can be pre-programmed or done ‘on the fly’. Other challenges such as non-parallel front and back surfaces and highly varying thickness are addressed by a combination of relative positioning of transducers when working in through transmission mode, and post-processing of the data.
In enhanced scan imaging, the permanent data record is used together with CAD data to produce 3D imaging of parts and flaws using software developed as part of the project. “Acquiring high-resolution data is great,” comments Cooper, “but the job of the inspector can be made much easier if the data is displayed in such a way that flaws are easy to identify, characterize and size.”
The 3D images are fully scalable and can be manipulated intuitively by the user to show the area of interest or the whole part. Selectable views include the usual A- and B-scans and the 3D view, and features such as the display of the CAD image with scan data plotted on it. Clicking on the center mouse button normalizes the view at the cursor position on the surface for convenient sizing of flaws using the included suite of measurement and mark-up tools.
Cooper says the next phase of the IntACom project has already begun. So pleased were the partners with the work achieved that the industrial partners have provided funding support for another year, while a proposal for a larger program of work for a further three years is submitted.
In the short term, he says, work will concentrate on the integration of metrology equipment to provide automated part identification and verification: “This will allow parts to be positioned in approximately the correct position and the system will adjust scan paths to suit. It will also detect if parts do not conform to CAD data within set limits and either adjust scan paths or reject the part.” Other work will include the inspection of thicker components and the detection of damage under metal fasteners or erosion shields using full matrix capture (FMC) techniques optimized for use on composites.
Looking ahead to the larger project, a larger cell will be constructed with multiple robots on tracks and part translation systems aimed at the inspection of larger components such
as wing spars and skins, wind turbine blades and boat hulls. Development scope will be extended to include high accuracy inspection of complex metal parts such as those produced by the latest additive manufacturing methods such as laser metal deposition.
Fuselage inspection
Lufthansa Technik is developing the use of active thermography for inspection of carbon fiber composite structures. One project, looking at the optimization of NDT methods for composite materials, also involves industrial image processing specialist Automation Technology and Hamburg University of Technology (TUHH). Another, involving TUHH plus NDT systems manufacturer edevis and joining technologies consultant IFF, has applied the technology in an automated system that uses a robot to carry out inspections of aircraft fuselages.
Active – or heat flux – thermography uses a brief thermal pulse from a heat source such as a halogen lamp to heat the surface of the objects to be measured. As the heat moves from the surface to the interior of the material, any subsurface faults or defect areas with lower thermal conductivity prevent the heat from spreading. The surface above defective areas therefore remains hot for longer, enabling infrared cameras to make the temperature differences visible.
In composite materials, the technique enables concrete indications of damage or of defects such as delaminations to be detected in ribs and stringers. Its advantages include the ability to examine areas of up to 0.5m2 in a single pass. The four-year optimization project included the development of software for the automated detection of defects. And while the method is limited so far to materials 4mm or less in thickness, that already covers around 80% of the fuselage of a Boeing 787.
Bernard Fitzsimons is an aviation journalist specializing in the air transport business, technology and operations
