Aircraft, systems and major components increasingly are designed with an eye toward how they will be inspected and maintained. In other words, the aftermarket has a lot to say about production techniques.
Usually, this means aircraft systems or parts must be designed to minimize repair frequency and costs. But sometimes improvements in inspection and repair methods can dramatically alter design requirements in positive ways.
Nowhere is this more true and important than in the expanding use of carbon-fiber reinforced polymer (CFRP) composites in major aircraft structures. Boeing’s 787 and Airbus’s A350 are both at least half CFRP by weight, and regional and business aircraft are following this trend. Reduced weight, less fuel-burn and lower operating costs are the big gains from using composites. But CFRPs have other benefits, such as reduced maintenance and improvements in cabin comfort.
Ceramic matrix composites (CMC) in engine hot sections will reduce weight and tolerate higher temperatures, making engines more fuel-efficient.
But CMCs are very new, and even airframe composites are young in terms of intensive repair experience. Under present conservative design standards, composite airframe structures are about 20% lighter than their metal predecessors. Theoretically, they could be up to 60% lighter, if we understood their production, performance, inspection and repair completely.
But OEMs cannot reach that theoretical nirvana partly because they cannot be absolutely sure very small defects will not occur in the initial manufacture of composites, be missed in periodic inspections or persist after a repair has been made. So for both safety and certification purposes, composite parts must be designed to perform their functions—including very major load-bearing ones—even with such defects.
Europe’s Evita project, conducted by the Swiss Center for Electronics and Microtechnology (CSEM), and universities in Manchester, England, and Athens, Greece, as well as GMI Aero and Dassault, aims to change that. Evita would enable affordable and much more sensitive inspection of composite structures, spotting defects in the microscopic range that may now elude common ultrasonic inspections.
Evita uses X-ray grating interferometry to measure pixel-wise refraction angles and scattering of a beam through inspection samples. It can detect much more than simple X-ray absorption-based images.
Evita’s actual inspection is a quick but complex process. The current Evita demonstrator has more than 150 parts, five motorized axes, plus an X-ray detector and source. It can inspect parts up to 1 sq. meter (10.7 sq. ft.), but the basic Evita technology could be used on much larger structures, up to wing size.
The demonstrator produces images measuring 15 sq. cm. (2.3 sq. in.) in under 30. sec., depending on the sensitivity desired. Inspection time increases linearly with the area to be inspected; in the future, images four times as large should be possible. Actual inspection times will vary by area and purpose but are expected to be much faster than the several days now required to inspect a major composite structure by computed tomography (CT).
CSEM is now benchmarking Evita against other inspection techniques, including shearography, CT and ultrasonic inspection. Data will be published at the end of 2015. Senior engineer Vincent Revol expects Evita will be much more sensitive to defects such as porosity and will yield higher resolution of defect regions than low-cost ultrasonics. “It will be good at detecting micro-cracking, porosity and errors in composite-fiber orientation,” he says. The technique also should be efficient at detecting delamination, foreign objects and areas rich or poor in resin.
The approach appears to spot small defects much better than the affordable ultrasonics and much faster than CT. CSEM offers Evita inspections now as a service, but the next step will be industrializing the approach with a manufacturing partner. And CSEM wants to apply the technology to CMCs after its initial research on airframe composites is done.
How much will Evita help OEMs to realize the potential of composites to lighten airframes and improve engine performance? Evita coordinator Anna Madrigal says that is a question for OEMs.
CSEM will present its Evita findings at MRO Europe this October in London. Revol expects Evita will be in use within 2-3 years, at least on a small scale, and will be more widely deployed in 5-10.