Aging infrastructure has a major impact on safety, increasing the need to assess damage severity. Machinery, systems, and components such as airplanes, cars, pumps, and pipes in the oil and chemical industry are subject to varying cyclic service loading and environmental influences. Sometimes multilayered coatings are used, requiring a high-resolution inspection to confirm the presence of a defect such as a delamination, and accurately locate and quantify its size. Highly attenuating materials may significantly increase the inspection time while limiting defect observability. Guided waves have been recognized as having excellent potential for nondestructive inspection. However, the presence of viscoelastic coatings used for corrosion protection is one of the major obstacles for guided wave inspection.

Side cross-sectional schematic view of the composite multilayered structure of the plate/tube-like specimens used for the current investigation.

The presence of sound absorbing viscoelastic rubber-like materials in multilayered structures can cause significant challenges for conventional nondestructive inspection methods. During the current investigation, an ultrasonic guided-wave-based pitch-catch scanning system was developed specifically to detect internal delaminations in plate- and tube-like multilayered composite structures.

The goal of this study was to investigate the feasibility of applying ultrasonic guided waves to detect internal delaminations inside multilayered composite structures. A secondary objective was to develop light, low-profile interdigitated transducers (IDTs) for interrogating entire targeted sections of multilayered composite structures by selectively exciting only one mode of guided waves in these structures.

Ultrasonic measurements were performed initially on multilayered flat plate having aluminum and carbon fiber outer skin. These measurements were repeated on multilayered cylindrical half-tubes with the same aluminum and carbon fiber skin. The first symmetric mode S0 of Lamb waves in plates and tubes was excited selectively by means of specially designed IDT sensors. These IDT sensors were fabricated from thin wafers of piezoelectric lead zirconate titanate (PZT) substrates using a pulse laser micromachining process to etch interdigitated electrode patterns on the surface.

While successfully demonstrating that the presence of internal delaminations can be detected reliably by measuring changes in the energy of the received signals, it is estimated that the pitch-catch ultrasonic system developed for the current investigation can detect a delamination as small as 1 mm wide. Similarly, in the half-tubes, small delaminations were also detectable in both aluminum and composite structures.

To inspect the carbon fiber composite structures, approximately four times more energy was required due to a higher attenuation property of the outer composite casing layer. Additionally, the received energy was four times lower than the aluminum multilayered plate case. The portable system was found to be effective for both aluminum and carbon fiber composite structures, even though the carbon fiber composite plate exhibited higher signal attenuation. Yet, both defects in the first and second bondline interfaces were successfully detected.

An IDT sensor-based guided wave inspection methodology is thought to have a high potential as a field-deployable inspection tool for complex multilayered structures. Among many advantages, the main benefit is that they can propagate long distances with minimum distortions and decays. From the current investigation, it is concluded that guided wave signals are sensitive enough to detect the presence of delaminations at the bond lines of multilayered structures.

This work was done by Gheorghe Bunget and Fritz Friedersdorf of Luna Innovations Inc., and Jeong K. Na of Edison Welding Institute for the Air Force Research Laboratory. AFRL-0240

Aerospace & Defense Technology Magazine

This article first appeared in the December, 2015 issue of Aerospace & Defense Technology Magazine.

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