Figure 1: Fiber-reinforced composite with delamination.
Figures and image courtesy of Olympus

Lightweight and corrosion resistant, composites provide a high strength-to-weight ratio and can be designed with certain mechanical properties that make them ideal for flight.

With major manufacturers constructing some aircraft containing approximately 50% composites, inspectors face new challenges in detecting defects or discontinuities. The critical importance of solving these issues to keep the public safe has sparked a race within the industry to find new ways to efficiently inspect these materials.

Composite types

Aerospace composites are either honeycomb core or fiber-reinforced polymers. Honeycomb composites sandwich aluminum, fiberglass, aramid polymers, or structural foam between thin layers of skin. The type of core material used depends on the application. The skin can be constructed of aluminum, graphite, fiberglass, heat-resistant fibers, or hybrid materials.

Fiber-reinforced composites, on the other hand, are made up of multiple layers of fibers bound together with a polymer matrix. The mix of strong-but-brittle materials inside a polymer matrix combines the advantages of each type of material while reducing their disadvantages. Fiber-reinforced composites are used differently than honeycomb composites, and the most common type of defect found is delamination. (Figure 1)

Despite their advantageous mechanical properties, composites are not immune to discontinuities, which can be caused by impacts, lightning strikes, and manufacturing defects. Honeycomb composites are more susceptible to the following discontinuities:

  • Type A – Delamination between plies of outer carbon fiber-reinforced polymer (CFRP) skin parallel to the surface
  • Type B – Disbonding between the outer skin and the honeycomb core
  • Type C – Cracked honeycomb core parallel to the inspection surface
  • Type D – Crushed honeycomb core in a parallel area
  • Type E – Disbonding between the inner skin and honeycomb core
  • Type F – Fluid ingress in the honeycomb core (Figures 2 & 3)
  • Figure 2: Honeycomb composite showing examples of different discontinuities.
    Figures and image courtesy of Olympus
    Figure 3: A bond testing unit with a pitch-catch probe.
    Figures and image courtesy of Olympus

Inspection methods

Pitch-catch technique – Acoustic energy is emitted from one tip on a pitch-catch probe and received by the other tip. On a flaw-free part, most of the energy travels on the surface, but part of the energy is dispersed into the composite. When the probe is placed over a discontinuity, such as a disbond, more energy travels on the surface, increasing the amount of energy read by the receiving tip. Pitch-catch probes are used on honeycomb composites to detect disbonds and core damage. However, the defect, in some cases, must be as large as the distance between the two activated tips to be detected. Also, repaired honeycomb can appear as a discontinuity because the repaired area will alter the signal. An inspector can read such a result as a false positive.

Figure 5: An MIA probe showing a defect, a positive signal, and a repair negative signal.
Figures and image courtesy of Olympus

Mechanical impedance analysis (MIA) – Tests use single-tipped, dual-element probes to measure the stiffness of the part to detect smaller discontinuities. When the composite has no discontinuities, the probe reads a very high stiffness level, which is the base value. As the probe passes over a discontinuity, such as a disbond, the stiffness decreases. The change in stiffness is detected by the probe. This type of inspection works best on a stiff composite since the difference in stiffness between a discontinuity and flexible composite is not enough to be registered by the probe. (Figure 5)

With the right supporting instrument, pitch-catch and MIA probes can be used to generate a C-scan image of the material – an intuitive, color-coded image that makes it easy to visualize the discontinuities in the part. With an advanced instrument with bond testing and C-scan capabilities, inspectors can scan using several frequencies to increase the probability of detection and sizing performance. Every frequency recorded can be analyzed using the amplitude or phase setting, giving the inspector up to 16 C-scans to analyze. (Figure 6)

Figure 6: Bond testing array instrument with encoded buggy and a pitch-catch probe.
Figures and images courtesy of Olympus

Resonance technique – Special narrow-bandwidth sonic contact probes detect the change in impedance of a sharply resonant high-Q sonic probe when acoustically coupled to a material. (Q being the probe’s tuned, set resonance frequency after the probe has calibrated to the instrument.) The probe’s electrical impedance is affected by the acoustic impedance of the test sample, and the composite’s acoustic impedance is altered by any lack of bonding. A disbond acts as a thin plate that vibrates, generating a standing wave when the thickness is equal to odd number multiples (1, 3, 5, etc.) of the length of the acoustic wave in the plate. This method is used on fiber-reinforced composites.


In an adhesive-bonded joint, changes in the effective thickness caused by disbonding significantly affect the phase and amplitude of the signal at the resonance frequency of the probe. In a multilayered joint, the phase is related to the depth of the disbonded layer (Figures 7 & 8).

 

Figure 7: A multilayer disbond test using a resonance probe.
Figures and images courtesy of Olympus
Figure 8: Resonance signal shows different defects between layers.
Figures and images courtesy of Olympus

Conclusion

Testing composite parts for discontinuities using acoustic probes is an important part of maintaining safe air travel. Bond-testing multimode instruments and advanced flaw detectors with bond testing add-ons can help inspectors do their jobs quickly and efficiently to keep aircraft operating safely.

Olympus Scientific Solutions Americas

https://www.olympus-ims.com

About the author: Terence Burke is eddy current and bond testing product leader for Olympus NDT Canada, a subsidiary of Olympus Scientific Solutions Americas. He can be reached at terence.burke@olympus-ossa.com.