Ultrasonic Flaw Detection
We all know how the Marine Industry has been following Defense and Aerospace in developing lighter, stronger composite parts, primarily through the application of carbon fiber. Well, along with these amazing gains in strength to weight ratio are some inevitable trade offs.
The same deep black fibers that have become a fashion statement for leading edge technology effectively hide potential defects or damage beneath the surface. The stiffness and density of these laminates tend to defy the old school method of tap testing the laminate to listen for voids. At the same time, now that the parts cost five digits and up, it's awful hard to tell the client that you checked it out with a $20 plastic hammer.
The Non Destructive Inspection industry has been scrambling to meet the needs of the big ticket clients like the Navy, the Airforce and Boeing. I've spent a number of years trying to figure out who has won the competition for the most accurate inspection gizmo arms race and finally chose ultrasound. Here's what the technology has to offer:
-Ultrasound has been around since the end of World War II and so is not a fly by night idea. It is a proven technology that has continued to grow and has been adapted to many fields of inspection.
-The results are repeatable and are based on well known physical properties. This point shouldn't be underestimated. There's a saying in the UT business that if, "you can punch a signal through it, it will tell you information."
-The dense, crystalline structure of vacuum bagged solid carbon laminates transmits sound extremely well, which helps to give crisp, clear wave forms.
-While thick, hand laid fiberglass laminates tend to disperse signals far more than carbon, the technology can be a rapid, and non destructive option in determining if there is a serious bond issue deep in the laminate stack.
-The tools are compact, weather resistant, and work well in the field.
-Inspection cost is relatively reasonable. I don't charge an added premium for the tool. I only bill my normal shop rate.
As you might expect, there are limitations to any technology. I'm the first one to tell you that UT cannot tell you everything. I'd rather be up front and pass up a job rather than promise the world and not deliver. I found out the hard way that many tech companies selling the latest and greatest whiz bang tool will tell you it will cook your breakfast so long as you buy it.
-Ultrasonic signals don't like traveling through air. As a result, foam core acts as a significant barrier to signal penetration. If you are concerned with detecting water intrusion into core- use a moisture meter. If you would like a global image of water intrusion into a cored hull, then thermal imaging is your best bet.
-Low tech laminates like thick, hand laid hulls fiberglass tend to have many tiny air bubbles that cumulatively tend to disperse a UT signal. In some cases, UT is like using a scalpel when a blunt instrument would suit you fine. There are times when you just need to break out the hammer and grinder, mate.
-UT sends out a signal that is a beam. The thicker the laminate, the lower the transducer frequency and the wider the beam. This can limt the ability send signals into tight spots such as inside corners.
-The A-scan UT that field units offer cannot produce a pretty, global, color picture of the entire part. The only way to get that is with a monstrous jig and a robotic arm such as you would see in a production setting. The accuracy and information is the same- it's just that the individual wave forms cannot be collated and plotted into a single graphical image.
-UT records separate waveform images as you scan the part. You need to be realistic in deciding what size defects are significant as well as what kind of inspection grid you need to set up on the boat. You can't expect to ferret out every 1/4" bubble in the hull of an 80' boat. Well, you could...but you wouldn't want to pay me the time it would take to do it!
There is no single magic inspection tool that is ideal for all situations. Ask anyone in the composites industry and they'll tell you that marine structures can be horrendously difficult to inspect when compared to more conventional production laminates. UT is particularly adept at assessing the following situations:
-Solid vacuum bagged carbon spars. The same clean, dense laminates that make it virtually impossible to detect subtle damage with a hammer are easily scanned with UT. Ultrasound will not only tell precise overall thickness, but also the depth of the defect/anomaly. Poor consolidation (loss of bag pressure), dry fibers, and voids are all good examples of detectable flaws.
-Bond line issues. Ultrasound can pick up the transition between skin coat and structural laminate. A clean line will transmit the signal, where a delamination or "never bond" will show a clear interuption on the waveform.
-Secondary bond damage. As noted above, UT can pick up delamination fairly quickly. There's no need to tent off and demolish the interior of the boat in order to determine the extent of damage.
-Quality Assurance. A new part can be scanned to check for defects and laminate consistency. Critical areas of the part can be scanned and the readings can be archived so that, in the event of future trauma, the part can be scanned again to see if there are any anomalies compared to the "as built" baseline readings.
This one's simple: I charge my normal hourly rate plus expenses. There are no hidden charges for the machine or my certification. Depending on the results and if you don't require a report, that will save the cost of my time in the office. I'd like to make this inspection as reasonable as possible to encourage its use in the field.
Ultrasound is a proven and reliable technology for use in non destructive inspection of composites. However, it's not for everybody. The technology is not "plug and play" in terms of gathering and interpreting results. You can't just run out and buy an inexpensive ultrasonic thickness gauge, slap it on the part, and expect to get meaningful results. I've done lots of research on the subject, attended technical seminars, received my certification for UT testing, and spend significant time making up reference test panels to constantly test the limits of the unit.
Give me a call and we can discuss whether UT can help you get a deeper look into your boat.
Ultrasonic signals are pulsed from a transducer into the part. When the signal reach the back of the part, it reflects back to the transducer, then to the unit's clock, is processed, and then converted into a digital wave form. In the example below, there is a sharp spike, or "backwall echo",at .248 inches, confirming the thickness of our 1/4" plate. The small spikes to the right are residual reflections.
In the next example, two quarter inch thick plates of carbon were glued together, leaving a void in the center that is not bonded. The void cannot be detected with a tap hammer. The upper screenshot shows the results of a scan along the side of the part where the plates are glued (A). The machine is set to display a readout of overall thickness (in this case, .459"). Note that there is a strong backwall echo. At the same time, there is slight signal reflection off the glue line. The important point here is that the backwall is strong and so indicates that the signal is travelling through the part.
When the trandsucer is shifted over the void (B), note how there is a sharp rise in the spike halfway through the part. At the same time, the backwall echo drops way down. This is telling us that the majority of the signal is reflecting off the back of the first plate and is not making it through the entire part- we've found a void!.
This exercise is a good example of how interpretation of ultrasound wave forms can tend to be comparative. We spend some time establishing what "good" looks like, then use this baseline to search for anomalies.