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  • Writer's pictureNed Patton

How to Know When It's Broke

For most things, it’s pretty easy to tell when it’s broke, because failures in metals are usually visible on the surface. While this can be the case for composites as well, especially when something has a catastrophic failure – like your brand new driver with the graphite shaft just broke right in the middle of the shaft. You can pretty much tell when that happens.

With composites, however, quite a bit of the time they fail or start to fail somewhere in the middle of the part or structure, and the damage is not visible on the outside of the part. Things like the wing root of an airplane that has some fatigue delamination buried inside the curvature of the wing connection to the fuselage. Or, a surface took an impact and there is little to no damage on the surface itself, but the matrix is cracked on the inside and starting to propagate a crack or delamination.

Ultrasonic Testing of a Composite Laminate

This is where the non-destructive examination or what is commonly referred to as NDT (Non-Destructive Test) comes in. The most common of these techniques is ultrasonic testing or imaging of the interior of the part that is suspected of potentially being damaged. This is a technique that is also commonly used with metals, so the equipment that is used is the same as it is for inspecting something like a welded joint. It is the interpretation of the data that comes from the ultrasonic inspection that is the hard part. Ultrasonic inspection works by injecting ultra-high frequency sound waves into the part being examined, and listening to either what bounces back from inside the part, or seeing what gets transmitted through the part. Both of these are illustrated in the figure here to the right. Since sound comes in waves, like light does, the waves act the same way that light waves do. X-rays are just ultra-high frequency light, so they work in much the same way as does ultrasound.

X-Ray pics of a bike frame

In other words, they either go through the part (transmitted) or they get reflected back, often not on the same path they went into the part (scattered). Ultrasonic NDT systems use either transmitted waves – called pitch-catch in the lingo of the business, or they get read as they scatter back toward the transducer – called pulse-echo.

Interpretation of the signals that either the pulse-echo or the pitch-catch method create is that sound has a tendency to scatter off of things that are a different stiffness and density – and thus sound speed – than the base material. In metals this is really simple, because a piece of weld slag for instance that gets buried in a weld has a different sound speed than does the metal of the weld. So, inclusions or holes are easy to spot. For composites, a little bit of the sound scatters off of every layer of the stackup and also off of every fiber or tow of fiber that is in the part. In other words, the signal that comes back to be read by either type of transducer is very noisy, and sometimes it is difficult to pick out the actual flaw signal from all of the noise. That is why inspection of composites using ultrasonic probes requires careful interpretation of the data. The way around this is to use a calibration block made out of the same composite material as the part, and the same general layup, but that has embedded flaws in it. What the operator of the ultrasonic device does is to calibrate their instrument to the known flaws in the calibration block before inspecting the part. This of course is somewhat expensive for something like the Boeing 787 because the range of thicknesses and part geometries where there could be flaws is rather large.

What can be done about this? One way is to do a computer simulation of the ultrasonic response of the structure or part in question, mimicking a flaw in the part, and then building one or two blocks and testing them to see if the simulation produces the same result as the blocks. Then when you go an inspect the actual airplane, you have some pretty high confidence that you can find flaws that could cause an eventual problem.

Both Ultrasonic Testing and X-Ray imaging are used extensively in the composites business to detect flaws that the naked eye can’t see. This is because since both are very high frequency, they easily penetrate the part and either get transmitted right through or get reflected or scattered back to the sending unit (transducer or transceiver).

One more technique that is used extensively with X-Rays is the CT scan. So, just like your doctor will do to you if they are looking for a problem with your bones, CT scanning of a part will give you a good idea of where you have flaws, porosity (like the pic to the left), delaminations, or even disbands near a joint. This is good for rather small parts that will fit within the chamber of a CT Scanner, but it isn’t appropriate for looking at the root of an aircraft wing for instance.

In my next post I am going to talk about how to keep your composite design from getting broke in the first place.It is a technique called “fault-tolerant design” and it is most commonly used for structures that are critical to either the function of the part or even injury or loss of life.


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