Zetec, Inc

Understanding NDT of Carbon Fiber

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Courtesy of Zetec, Inc

Carbon fiber has become an important material in the aerospace industry in recent years. It provides flexible and durable properties for civilian and military aircraft, but it’s susceptible to hidden cracks that stem from manufacturing defects or heavy stress.

Luckily, advanced NDT techniques such as ultrasonic testing of carbon fiber allow analysts to detect internal cracking and other indications on a volumetric level, while eddy current testing can conform to the thin profiles of carbon fiber on a surface and subsurface level.

Ultrasonic NDT of Carbon Fiber

With ultrasonic testing (UT), inspectors can assess bonding within honeycomb sandwich structures in addition to matrix porosity. Within complex materials, UT can also detect abnormalities like wall thickness variations or disbonds. For the most reliable results, carbon fiber composites are typically inspected using specialized UT techniques such as phased array ultrasonic testing (PAUT). PAUT instruments can provide the necessary scanning radius and flexible procedures to detect a variety of flaw types. 

With PAUT, you can find such deviations as:

  • Porosities
  • Foreign Bodies
  • Delaminations 

PAUT inspections of CFRP components are mostly performed with normal incidence (0LW) on the component surface. Due to the complex and variable geometries of the composite structures, multiple probe combinations are used. Straight parts are inspected using linear 1D probes while convex and concave surfaces are inspected using arc-shaped 1D probes. Inspections are performed in large water tanks with the scanning speed pushed to the maximum for productivity.

PAUT combined with a real-time adaptive UT inspection technique known as “Time Reversal” is especially useful applying surface profiling based on measurement and compensation of the surface echo positions. The focal laws are adapted to the profile of the component measured in real-time. While this technique can be used in large water tanks, recent PAUT innovations that include a powerful yet portable PAUT instrument, a free orientation scanner, flexible wedge, and local immersion probe provide a portable solution. The benefits of this solution include improved near-surface resolution without the need for an immersion tank, all with the confidence of constant tracking and complete coverage.

This Time Reversal Phased Array Ultrasound (PAUT) technique can improve signal quality when probe alignment with inspected component surface is not optimal. It provides real-time adaptation of the phased array beam with almost no impact on the inspection speed. This technique is supported with commercially available PAUT systems which can improve coverage and detection capability while maintaining a high inspection speed.

Disadvantages of UT for Carbon Fiber Inspections

Despite the advances in ultrasonic technology, there are some drawbacks analysts should consider when utilizing an ultrasonic testing approach. Using standard UT, in particular, runs a higher risk of overlooking numerous flaws and defects. 

Without the capabilities of the advanced PAUT techniques listed above, the UT method struggles to adapt to CFRP materials, such as aircraft skins. Probe misalignment with the component surface, water column variations, and surface geometry changes create significant inspection challenges.

Eddy Current Testing of Carbon Fiber

Despite the low conductivity of carbon fiber, some have claimed that eddy current testing can effectively inspect this material. However, the wide variability of results argues against the use of eddy current inspection techniques. Rarely, if ever, will eddy current testing be preferred over ultrasound for inspecting carbon fiber. 

Innovative NDT Techniques Ensure Carbon Fiber Integrity

Ultrasonic testing of carbon fiber gives analysts added flexibility and accurate data when compared to more traditional, less advanced NDT approaches. PAUT with Time Reversal is the best method for detecting deviations, thanks to custom wave beams and multiple angles. 

High-quality instrumentation will provide enhanced readings of flaws while shortening inspection times. The best equipment can greatly mitigate the traditional challenges associated with UT inspections. Moreover, advanced instrumentation and quality software simplify data gathering and analysis, helping analysts more easily conduct more thorough inspections with the greatest efficiency.

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