Quality Magazine

Unconventional NDT

December 30, 2008
This section for Boeing’s 787 Dreamliner is shown loaded into the nondestructive inspection machine. The machine includes two phased-array systems for ultrasonic scanning of the part, which are integrated with a motion-control system and diagnostic software. Source: Vought Aircraft Industries Inc.


With nondestructive testing (NDT) methods ranging from visual examination, liquid penetrant testing, magnetic particle inspection, ultrasonics and eddy current, there is likely an NDT method to fulfill the application requirements.

Liquid penetrant testing may be the most widely used NDT method, and is often used in aerospace applications. But just because a method is popular, does not mean it is right for every application. In order to get the most out of nondestructive testing, it is essential to choose the right testing method.

At its most basic, nondestructive testing is performed to reject defective parts. Conventional NDT methods, such as X-ray and magnetic particle inspection, identify defective parts by scanning for indications of a defect. In addition to these methods, process compensated resonant inspection and phased array, though not the most commonly used, may be the right choice for a particular application.

Resonant Inspection

Robert Nath, chairman, and Mike Giannini, business development manager of Magnaflux Quasar (Albuquerque, NM), say that when customers approach them with a specific problem, they look at the application and then make recommendations to the customer.

“Sometimes our recommendation is based on the physical properties required, sometimes it’s the economics of it, sometimes a combination,” Giannini says. “When it is possible to test between two different methods, our job is to recommend the most reliable means of testing and the most economical way for the customer.”

Also, the method used depends on the industry. Liquid penetrant is heavily used in aerospace, but not widely used in automotive, where magnetic particle and resonance inspection would be more common. If ferrous materials are involved, magnetic particle inspection may be a good fit. If the customer is looking at areas with no surface imperfection and the potential for failure in service, resonant testing would be a solution.

Though Magnaflux Quasar’s customers come from various industries, Giannini acknowledges that NDT is not widely known. “If you asked 10 people on street, ‘what is NDT?’ you would get blank stares,” he says. However, Giannini points out that almost everyone is affected by NDT technology. “Anybody who uses a vehicle for anything is made safer by these industries.”

One recent example was an application for a high-end automobile engine manufacturer in Europe, for which Magnaflux successfully implemented resonant testing equipment. While the company has offered a magnetic particle method and liquid dye penetrant for more than 70 years, it began offering resonance testing equipment when it acquired Quasar in March of 2007.

Unlike other testing methods, resonance testing equipment finds structural anomalies, and measures structural weaknesses and surface systems.

Imagine driving to work, Giannini says, and the steering knuckle of the car breaks. “This would not be a good start to your day,” he says. These types of structural defects can be found with resonant testing, and it would take two to three seconds to test the knuckle. These results cannot be found with any other method, Giannini says.

“What we’re doing is making sure you make it back and forth to work without the steering knuckle giving out on you,” Giannini says, as the company deals with safety critical and warranty critical parts.

According to the company, Quasar process compensated resonant testing (PCRT) provides effective resonance inspection in the presence of typical manufacturing process variations. Using PCRT, defective parts can be reliably rejected on the production line because the results correlate directly to part performance.

The resonant frequency of a part is based on its stiffness and mass. The Quasar method measures several resonances for each part and uses a proprietary pattern recognition algorithm to compensate for the acceptable process variations, according to Magnaflux.

Bercli’s Guillaume Neau uses a phased-array system to inspect T-section stiffeners in the central plane of a jet at Dassault Aviation. Source: Dassault Aviation Inc.

PCRT in Action

Vibrant NDT (Albuquerque, NM) was formed to commercialize PCRT for the aerospace and power generation industries, says Lem Hunter, president of Vibrant NDT. The company uses PCRT technology, which is licensed from Magnaflux Quasar, for customers in the aerospace and power generation industries.

This technology can measure new components as well as components in service, and determine if they should remain in service or not. PCRT can be used for inspecting new or old parts and tracking individual parts throughout service life. It also can measure continued service of a part.

Vibrant grew from Hunter’s previous company, Mechtronic Solutions Inc., which was a supplier to Magnaflux technology.

Vibrant’s major focus is engine and landing gear, Hunter says, and he has been applying the technology to turbine blade testing, along with aircraft wheels and landing gear. Currently Vibrant NDT has customers in engine, component and airframe manufacturing.

The strength of PCRT involves its sorting capability. This allows manufacturers to continue using standard NDT methods, but simplifies the first part of the process.

“What we do is give an excellent screening tool and separate parts into piles: parts that are like good parts, parts that are like bad parts,” Hunter says. “In aerospace, everybody wants to know the ‘why.’ What was it about the part that made it so different?” Hunter says. PCRT highlights parts that differ from good parts. After that, manufacturers can employ other technologies for an in-depth evaluation of parts.

In the past, Hunter says, testing 1,000 parts with dye penetrant would involve cleaning, treating and washing all of the parts and looking for bad parts, which might mean finding 10 bad parts out of 1,000 parts-something that is hard to do consistently.

With PCRT, the system can whip through all of them, allowing operators to instead use another method to focus on the 10 parts that failed, determine why they failed and then fix the process. Focusing on only the parts that failed is a tremendous labor savings for manufacturers.

While PCRT may replace some systems, it will not eliminate the need for other testing methods.

“I firmly believe that manufacturers, particularly in aerospace, are never going to give up their tools. Nobody is going to put anybody out of business-all the technologies have a use,” Hunter says. The idea is to determine which tools apply to which type of problems.

The technology allows manufacturers to get rid of bad parts earlier in the process, before they have added value. Previously, manufacturers might add $1,000 components to a jet engine and only then find cracks.

The technology does not do everything, however. “Everyone wants to know where the defect is, that’s not our strength,” Hunter says. The system also requires a little more engineering on the front end to teach it what is a good or bad part. This also may prompt disagreements on exactly what is a bad part. It is no plug and play replacement, he says. Finally, Hunter says that the technology has a tougher time with very large parts and individual pieces that weigh more than 400 pounds. Because the technology gets slower as parts get bigger, such testing would not be ideal for applications with parts greater than 1,000 pounds, Hunter says.

Phased Array

Phased arrays may be the answer for manufacturers looking for a big increase in speed that also provides high-resolution images of defects or flaws, says Deborah Hopkins, CEO of Bercli LLC (Berkeley, CA), a three-year-old company focused on ultrasonic phased-array systems, along with training and consulting. “In addition to sales, we do a lot of simulation studies, help with probe designs, help people see if phased-array technology will help them,” she says.

Each company has different drivers-some are focused on speed, others on resolution or flexibility. In the NDT world, detection and sizing requirements vary, Hopkins explains. Some manufacturers only need to detect a flaw, and that is all. Others may have more complicated requirements-for example, some manufacturers can have a certain number of flaws in a certain area, or not more than a certain number of a certain size.

Phased arrays allow operators to size defects precisely despite a structure’s complexity. For example, does it have a honeycomb structure or layers? The beam-steering capability of phased arrays can make it possible to mitigate difficult access conditions, and flexible arrays allow inspection of parts with contoured surfaces having complicated geometries. Conventional ultrasound usually requires being right on top of the area to be inspected (or use of a wedge), which can be difficult in some applications. But with phased arrays, operators can focus and steer the beam-greatly improving access.

Inspecting large, flat sheets of aluminum is a relatively easy application in terms of access, but phased arrays can greatly increase inspection speed because of the ability to electronically scan the ultrasonic beam while also improving resolution. For aerospace, planes with curved parts are a natural fit for phased arrays, as are some automotive applications.

The automotive industry uses complicated parts, often with different kinds of steels and several different thicknesses on a single part-the result of trying to remove metal wherever possible to reduce weight. The ability to focus and steer the ultrasonic beam can make it possible to inspect welds that would be difficult to access otherwise.

In aerospace, when examining a rivet, some testing methods require taking the rivet out in order to perform the inspection. This can be extremely expensive and the operator may risk causing damage, Hopkins says. A phased array allows a beam to come in from the side, allowing operators to form and steer the beam to scan down the hole without removing the fastener, yielding a huge cost savings, she says.

Another common application involves a boiler with one large header pipe and many smaller pipes welded into the header. Operators cannot usually place the probe directly on the welds, so beam steering using a phased array allows them to scan the weld to produce high-resolution images.

“What phased arrays give you is the ability to focus the energy, steer the beam and do electronic scanning,” Hopkins says. “It provides the ability to create high-resolution images and consistent resolution with depth-all things that you can’t do with conventional ultrasound. It all translates into improved detection and sizing capability.”

Though it is a high-powered solution, the drawback is that it is more expensive, Hopkins says. In order to determine if it is worth it, customers should compare and quantify differences in speed, detection and sizing accuracies, coverage and flexibility between phased arrays and conventional ultrasound. For example, because of the ability to form and control the ultrasonic beam, a single phased-array probe can replace a drawer full of conventional probes and wedges.

“In high-volume manufacturing, time is everything,” Hopkins says. “Anything that slows them down is a big issue.” In this case, inspection must keep pace with production rates, which means inspecting large numbers of parts per minute. “Aerospace is the opposite-volumes are low and safety is everything,” Hopkins says. Phased arrays may be used to obtain high-resolution images and sizing accuracies that are not possible with other methods.

Because of its versatility, phased arrays are not just appropriate for one industry. Bercli gets calls from people in aerospace, medical imaging, pipelines, power generation and bridges, to name a few, as well as people who have heard of phased arrays, but do not know much about them. Hopkins says she aims to help them understand it, and determine whether or not it is useful to them. Though it may not be a fit for their current application, it does offer a lot of functionality and may be used somewhere down the road. In other words, it does not hurt to learn more about it.

Therefore, while a company’s current NDT methods may be adequate, it is always a good idea to know what is out there and then possibly learn more about the industry. While it may not lead to a quick adoption, it may prove useful in a future application. Q

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Tech Tips

  • In order to get the most out of nondestructive testing, it is essential to choose the right testing method.

  • Each company has different drivers-some are focused on speed, others on resolution or flexibility.

  • Unlike other testing methods, resonance testing finds structural anomalies and measures structural weakness.

  • Phased arrays allow operators to size defects precisely despite a structure’s complexity.