The NDE challenge

CW editor-in-chief Jeff Sloan discusses the aerospace industry's need for robust and reliable non-destructive evaluation technologies.

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I was on a flight recently and struck up a conversation with the man seated next to me. As we asked and answered the usual questions (“Where are you going?” “What do you do?”), I learned that he does oil and gas pipeline rehabilitation and maintenance. This, as you might guess, makes him a busy man, particularly in an industry where the transport of product (oil and gas) is money earned, and where lack of transport is money lost. (For a fuller understanding, Google “US oil and gas pipeline map” and you’ll see how complex and vast the challenge is.)

Anyhow, my new friend, it turns out, knows a little bit about advanced materials in general, including composites, and we got to talking about the carbon fiber composites used in the Boeing 787 and the Airbus A350 XWB aircraft. He had a lot of questions for me about where composites are used and how. And then he asked what would appear to be a simple question: “How are composites on an airplane repaired?”

“Ah. Well,” I said, stroking my chin sagely, “repairing a composite material can be tricky, but it’s very doable. The real question is how do you know if it is damaged at all?”

One advantage of an aircraft fabricated from aluminum or titanium is that damage, in the form of a dent, crack or corrosion, usually can be detected via simple visual inspection. Barring this, there is well-established nondestructive evaluation (NDE) tech- nology available to help technicians find and assess sub-surface cracks, thinning and corrosion. Much of the NDE technology used relies on aluminum’s and titanium’s conductivity, which enables the use of electrical signaling.

Aerospace composites offer well-documented advantages compared to aluminum and titanium, not the least of which is their ability to resist corrosion. Composites are also remarkably dent-resistant. But it is in the nature of their complex, layered composition to experience internal cracking and delamination, sometimes long before that damage becomes apparent on the part surface. That seriously limits the value of visual inspection. Further, although composites have been used in aircraft substructures for many years, there is not an established NDE regime in place to help maintenance personnel assess subsurface damage. This does not mean that there is no NDE technology available to check a composites structure for damage. Far from it. The NDE technology selection is vast, ranging from traditional C-scan to ultrasonic to adaptive beam forming to radio frequency to microwave. And more. On top of that, the user interface for each of these also varies, ranging from handheld units to larger freestanding or surface- mounted systems.

Even more problematic is the question of what to do with the gathered data. Many NDE systems currently on the market generate images and/or information that must be assessed by someone trained to interpret the results and able to recommend an appro- priate course of action.

What we know is that Boeing, Airbus and the airlines to which they’ve sold 787s an A350 XWBs are in the midst of figuring out which NDE technologies are most accurate and efficient, particu- larly for technicians and maintenance personnel in the field who are, in some cases, coming face-to-face with composites for the very first time and are on the front lines of damage assessment. This is not a trivial challenge — the relative newness of aerocomposites means that the performance of aircraft maintenance, repair and overhaul organizations (MROs) will face extraordinary scrutiny, and reliable NDE will be a necessary validation tool.

Ideally, NDE of an aerocomposite will involve a simple, robust handheld device that quickly generates easily understood results that help maintenance personnel make quick decisions regarding repair requirements. Such technology is not here yet, but it is close, and the next few years should prove revelatory.

CW will continue to track this technology for you, as well as the related issue of aircraft repair itself. Look for a full report on current commercial repair strategies next month in the April issue of CW. 

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