Printed headline: Testing Structural Integrity
Detecting problems that cannot be seen with the naked eye—such as the extent of composite damage, which can be hidden—is the forte of non-destructive testing (NDT).
In the case of composites, “You can have internal damage in carbon-fiber composites that is barely visible on the outside, so designers have to ensure that no invisible damage will compromise the structural integrity,” says Robert Smith, professor of NDT and high-value manufacturing at the University of Bristol in England.
There are 10 or so NDT techniques in use by the aerospace industry—from ultrasonic testing to eddy current testing and magnetic particle inspection—depending on the material. “There is a move in the industry toward rapid diagnosis of issues without the need to use more complex equipment,” says Mark Summers, head of technology for structures, manufacturing and materials at the British Aerospace Technology Institute (ATI).
One of the questions exercising the minds of engineers at the University of Bristol, meanwhile, is whether the NDT process for carbon-fiber structures can be expedited.
“Aerospace engineers will match the technology to the material and defect type: You are trying to maximize safety while minimizing the cost,” Smith says. “Engineers want to find all genuine defects, as small as possible—but not to find things that turn out not to be defects—false indications, which are costly. You need a technique that clearly distinguishes between a defect and something that is just a structural artifact or system noise.”
Stefan Sahlen, a non-destructive testing specialist at test house Exova Materials Technology in Linkoping, Sweden, which is an NDT training center for operators throughout the country, says the key NDT technique for field testing of aircraft components is eddy current for aluminium airframes. Ultrasonic and X-ray techniques are also commonly used. “With more and more composites used on aircraft, methods are changing,” Sahlen acknowledges. “Ten years ago, eddy current was used more than it is today.”
But this method is still commonly used by airlines to assess the structural integrity of aircraft in service. For example, Scandinavian Airlines says 80% of its inspections are still carried out using eddy current technology. In eddy current NDT, a coil of conductive wire is excited with alternating current. This wire coil produces an alternating magnetic field around itself. The magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material—eddy currents. Variations in the electrical conductivity and magnetic permeability of the test object, and the presence of defects, cause a change in the eddy current. For example, if there is a crack in the aluminium, the eddy current will not flow, and the engineer can use an instrument to determine where the crack is.
With composites, there is greater delamination in the material if there is damage, and the defect is often much bigger underneath the surface of the structure. It is not always possible to see the results of an impact such as a bird strike or hailstorm on a composite structure. You must assess how great the damage is in the depths of the structure, Sahlen explains, and if a shockwave from impact has separated carbon fibers from the matrix.
Ultrasonic testing for composites has the advantages of high sensitivity, making it capable of detecting small elements of damage, and high penetration power, meaning it can detect flaws deep in the part. Magnetic particle testing is only used on ferromagnetic materials. It works by putting magnets on the surface of the component. A crack will cause magnetic powder placed over the component surface to reflect the shape of any defects. Fluorescent powder that is sensitive to UV light tends to be used today, says Sahlen: “You can see defects in metals very quickly. This method is most common—along with dye-penetrant testing—in the production environment.”
In fact, along with MRO, the production line and design office are important users of NDT for aerospace. Engineers are benefiting from advances in computer power and the impact this has had on the design and manufacture of aircraft. “NDT has transformed itself in the last 20 years,” Smith says. “NDT is more visual; there are more images—2D and 3D maps. However, traditional methods still predominate because it takes a long time for new technologies to make their way into use. But these days, it is standard to use scanning technologies to produce 2D images from the surface of a structure.”
“There has always been a need for NDT [methods], but their complexity has evolved as high-performance computing has evolved,” adds the ATI’s Summers. “As new techniques have become available, they have become embedded in the aerospace industry.”
These technologies are also feeding back into manufacturing processes for new aircraft. “During manufacture, future NDT will be about process verification; trying to verify that the manufacturing process you have designed is actually achieving exactly what is intended, and that you have conformance to design for every component manufactured. That increases confidence in the design, reduces risk and has the potential to subsequently reduce weight. It means you could achieve the same safety levels with a lighter structure,” says Smith.
Work in academia is looking at future NDT inspection techniques or new ways of implementing current methods. Most of the NDT research at the University of Bristol is focused on ultrasonics, but researchers also work with eddy current techniques and X-ray computed tomography. “The NDT research group at Bristol is quite diverse, but my own team specializes in NDT of composite materials,” says Smith. This is principally focused on supporting the manufacturing and design stages. “We are carrying out high-fidelity 3D characterization of the internal structure of the composites from ultrasonic data, allowing confirmation of conformance to design of as-manufactured parts,” he adds. “This produces a lot of information to supplement the mechanical testing results during design.”
Engineers are also particularly interested in the potential of embedding sensors throughout the airframe and other components that would transmit data on structural integrity, known as structural health monitoring (SHM). “SHM has been talked about for 30 years but hasn’t really happened on aircraft yet, despite significant research effort. That is going to be the real game changer,” Smith notes.
“There is a lot of interest in SHM,” Summers adds. “If you think about Rolls-Royce as an example, carbon-fiber fan blades might have embedded sensors that provide data on the structural status of components while in service, so you don’t need to inspect them.” This is likely to become a part of the increased digitization of the aero engine and “fourth industrial revolution,” he says.
But today modern aircraft have only a few structural health-monitoring sensors, Smith says. “Part of the problem is that sensors have to be built into the structure from the start, which makes the initial cost high," he says. “It is still not clear how you validate the capability of such a system. Also, if something happens and you have to do a repair, you may lose part of the monitoring system.”
The University of Bristol has launched a spinoff company called Inductosense that embeds wireless sensors in a structure, or attaches them later, potentially solving some of the problems associated with structural health monitoring. Passing a “wand” over the structure fires ultrasound waves into it; the signal is fired back wirelessly to the wand to indicate the structure’s condition.
The desire is always to use “less NDT, because it is expensive,” says Smith. The move to greater NDT in process verification is to prove that every component that is made is correct and conforms to the design each time. If aircraft manufacturers can prove that, it could be less necessary to carry out as many quality control inspections after manufacture, he says.
But it is likely that NDT is here to stay. “I suspect there will always be a requirement for monitoring the aircraft structure,” Smith says.