Unique Instrument Assesses Bonding Strength of Composite Materials Used to Join Aerospace Vehicle Parts

Commercial airlines make a substantial investment to ensure every plane is airworthy at every take-off. From the moment the aircraft door closes at departure to the time it’s re-opened upon arrival, an airline can expect to spend $2,550 an hour, according to an International Civil Aviation Organization conference report. Operations and maintenance accounts for $590, or approximately one-quarter, of this hourly expense.

Recertification is required whenever a plane undergoes repair, such as damage from a lightning strike or bird strike as well as on-ground scrapes and bruises. Researchers at the University of Virginia School of Engineering and the NASA Langley Research Center are working on ways to make re-certification simpler, more efficient and more cost-effective. 

Harold Haldren, who earned his Ph.D. in electrical engineering from UVA in 2020, developed a novel way to find weaknesses in the adhesives that hold together parts of aerospace vehicles. Haldren developed the method in collaboration with his advisor Mool Gupta, Langley Distinguished Professor of electrical and computer engineering, and Daniel Perey, William Yost and Elliott Cramer, who lead research projects and programs for NASA Langley’s Nondestructive Evaluation Sciences Branch.

“Few people realize that most if not all airplanes flying must be bolted and riveted together. This is because the industry lacks an approved method to certify the structural integrity of a bonded joint,” Perey said. Perey explained that plane parts are fabricated with composite materials that work better when glued or bonded; drilling holes into the materials weakens the material.

“This is the holy grail of the structural composite community—to verify the integrity of a bonded joint after it’s been in service so that we can forego secondary bolting and riveting,” Perey said.

two bonded specimens are inspected with an ultrasound transducer

In this lab test, two bonded specimens are inspected with an ultrasound transducer.  The oscilloscope in the background shows the waveforms produced by the instrument that help scientists determine bond strength. The wires in the lower right corner of the photo are part of the instrument itself.  Photo courtesy of NASA Langley Research Center.

Haldren conducted the research for his dissertation while serving as a NASA Space Technology Research Fellow. “I was fortunate to spend a significant amount of time at the (NASA Langley) Research Center” Haldren said. Haldren worked in the only structural inspection research laboratory in the entire NASA enterprise, and only one of two operated by the U.S. Government; the U.S. Air Force operates the other. NASA also has a vested interest in this research given its mandate to design spacecraft and re-certify experimental aircraft.

The team’s search for the holy grail involves ultrasonics, which is the second-most used method next to visual inspection. Whereas the conventional approach to ultrasonics measures amplitude, Haldren developed expertise in a more exclusive branch of ultransonics that exploits the phase information of the signal wave.

Perey draws a comparison to AM and FM radio signals. AM radio, which runs on amplitude, has more interference, noise, and dropped signals; FM radio is more precise and offers overall better quality. In ultrasonics, amplitude forms a visual image that separates good and bad bonds, but it cannot detail variations in chemical bonds of adhesives at the nanometer level.

Haldren developed an instrument that directs short pulses of ultrasonic waves to assess strength of the interface between the adhesive and the composite materials that form a joint. If the bond is strong, the sound wave bounces back quickly and cleanly; a weak or porous bond will soak up the wave, causing delay and distortion in the return signal.

“Thanks to Harold’s work, we can make use of this more exquisite area of ultrasonics,” Perey said. The branch where Perey works has received programmatic support, enabling hiring of more experts to continue the research and prototype instruments based on Haldren’s proof-of-concept. Beyond the aerospace industry, the future success of this research could benefit automotive manufacturers and other industries that rely on adhesive bonding and composites.

Following graduation, Haldren accepted a position at the Johns Hopkins University Applied Physics Laboratory, where he conducts research in radio communications for national defense.

NASA honored the team, which earned second place in the 2020 H.J.E. Reid award competition. The Reid award is the highest recognition for a scientific research publication authored by those who support space mission projects at the NASA Langley Research Center. The team’s paper, “Swept-Frequency Ultrasonic Phase Evaluation of Adhesive Bonding in Tri-layer Structures,” was published in the May 2019 issue of the Journal of the Acoustical Society of America.