Unseen cracks and other hidden damage to composite materials used in aircraft—a safety concern recently noted at the federal level—may get a boost from new coatings being developed to reveal such problems under UV light.
|New coatings may make damage to composites visible at certain wavelengths, aiding in inspection of composites used in aircraft such as the A350 XWB, shown during assembly.|
Combining materials such as carbon fibers with epoxy, composite materials have been used in aircraft for decades, but the issue has gained new importance with the introduction of the Boeing 787 Dreamliner.
The 787 is the first mostly composite large commercial transport airplane. Because current safety standards are based on the performance of metallic airplanes, the General Accountability Office decided last fall to review the Federal Aviation Administration’s safety certification process for the new jumbo jet.
GAO found no problem with FAA’s specific review but noted four general safety-related concerns with the trend toward aviation composites:
• Limited information on the behavior of airplane composite structures;
• Technical issues related to the unique properties of composite materials;
• Standardization of repair materials and techniques, and
• Training and awareness.
“FAA is taking action to help address these concerns,” GAO noted. “However, until these composite airplanes enter service, it is unclear if these actions will be sufficient.”
The new coating, being developed by UK-based GKN Aerospace, is designed to reveal damage to composites that may be invisible to the naked eye on inspection. Because composites are so tough and durable, they may appear to have survived an impact without lasting damage. GKN is a supplier of aircraft composites to Boeing.
|Boeing’s new 787 Dreamliner has a unique one-piece composite barrel. Ironically, because these sturdy composites are made to “recover” from damage, they may not reveal problems below the surface.|
In an 18-month collaboration with Bristol University’s Advanced Composites Center for Innovation and Science, GKN’s researchers are exploring advanced coatings chemistries that would accurately indicate the existence and extent of damage below the surface of a composite structure.
“As the proportion of the airframe that is composite structure grows, the ability to reliably and swiftly identify damage and understand its extent becomes ever more critical to an effective repair procedure,” said John Cornforth, GKN’s Head of Technology. “However, composite damage can be hard to spot with the naked eye as this material has the ability to appear to ‘recover’ post-impact.”
GKN said the new coating “will contain smart microspheres, which will signal in the ultraviolet visible spectrum when damage has occurred.”
By carefully designing the microspheres, the coating will be able to “provide a signature reflecting different energy levels, indicating the extent and level of damage to trained maintenance staff.”
The research combines “smart coatings” expertise from GKN’s aircraft transparencies business with the company’s experience in complex composite structure design and manufacture.
‘More Intelligent Inspection’
The goal is a “more intelligent inspection capability,” a GKN aerospace engineer recently told Design News. “For example, when inspectors are looking at a wing structure, they could look in the right wavelength range, and any areas that have damage above a certain preset threshold will clearly display that damage."
Said Cornforth: “We commenced this research activity with Bristol University just last year, and already results are very promising. We believe this coating has the potential to make damage detection far easier across all types of composite structure and in difficult, real-world, operating conditions.”
Experts expect to have the coating chemistry nailed down by the end of the study period. Flight trials may come 24 months after that.