Eddy currents, ultrasonic imaging and a huge blast of X-Rays

RANDOLPH AIR FORCE BASE, Texas – It takes more than oil changes and fresh windshield wiper blades to keep Randolph’s aircraft aloft. One needs to look closer, beneath the skin.

The 12th Flying Training Wing’s Maintenance Directorate uses non destructive inspection techniques to find cracks and wear, which keeps the aircraft safe as well as driving engineering and design improvements.

NDI has the ability to use certain test methods to examine an object, material or system without impairing its future usefulness,” said Master Sgt. Robin Brown, Nondestructive Inspection functional manager. “In other words, NDI is used to investigate the material integrity of the test object.”

“We find the cracks when they’re very small,” said John Schwarting, 12th FTW NDI Lab Supervisor. “The things we look at, no one else checks but us.”

Schwarting said most of their work comes from the T-38 Talons - about 85 percent - but they also inspect cranes, hoists and other load-bearing machinery.

“It’s not just for aircraft, anything that has loading and unloading,” like bridges, concrete structures and trains have this kind of inspection, he said.

Some of the cracks they find are about the size of a lower case “I” on this page, or smaller. They are in tight areas, under screws and other fasteners, in the connection where the wing meets the fuselage. Cracks can be just about anywhere in the aircraft. They find small cracks before they turn into large cracks, which can lead to a catastrophic structural failure.

To find these small cracks, the inspectors have five basic methods, which use a variety of tools, depending on what is being inspected. The methods are X-ray, ultrasonic, eddy current, magnetic particle and fluorescent penetrant.

The methods were “developed by structural engineers,” Schwarting said.

Ultrasound uses the same technology as a prenatal ultrasound, only more focused. It’s used to find cracks under screw heads and other fasteners, such as what holds the wing of a T-38 to the fuselage.

“A lower wing skin fastener hole inspection uses a 10 megahertz ultrasonic transducer,” he said. “We orient the sound around the fastener hole at a 70 degree angle. It’s like SONAR.

It uses a hand-held device with a quarter-inch wide transducer affixed in the proper position. The device has means to keep the mechanism aligned and calibrated. It’s connected to an electronic machine with a display about the size of a modern smart phone. It’s doesn’t show an image like a prenatal ultrasound, rather it displays a jagged graph showing when the transducer is over the crack.

The eddy current method uses a small magnetic field to detect cracks in bore holes and other similar areas. Like the ultrasonic method, the eddy current method uses a small hand-held device attached to an electronic machine which shows a graph on a smart phone-sized display. Around the screen there are buttons, knobs and a jog and shuttle wheel to adjust the system’s sensitivity and frequency output.

The end of the hand-held device has a 26-gauge wire coil surrounded by graphite. This small assembly is at the end of a small shaft about the length of an average index finger and half the diameter of a pencil, but can vary depending on the size of the bore hole. There is a size and shape for all of the bore holes on the T-38.

The device emits a magnetic field into the bore hole. The artificial magnetic field from the device causes small, alternating currents of electricity to flow in a circular motion in the metal around the hole \(this only works in metallic structures\), which is an eddy current. The tendency of this kind of current to stay near the surface of a conductor is called the skin effect. These eddy currents create a magnetic field opposite but equal to the magnetic field introduced from the device, keeping the flow of electrons in balance. If there is a crack in the metal, it will disrupt the flow, causing an imbalance. The device reads the imbalance, allowing the user to pinpoint very small cracks very accurately.

For larger parts like wheels where they need to look at the whole part and other inspection means won’t work, there is a different and more colorful method.

“This is the messiest one,” Schwarting said, opening a cover to a stainless steel tank full of a florescent yellow liquid, resembling the color an energy drink, with the consistency of laundry detergent.

“The penetrant is the only way to get 100 percent coverage,” Schwarting said.

“The parts are immersed in highly-refined, fluorescently-dyed penetrating oil and placed on the dwell station for 30 minutes,” he said.

Through capillary action in the material being tested, the soaking process allows the oil to seep into any cracks and small voids. Capillary action is a natural physical process where liquid is drawn to narrow cracks, against forces of gravity. It’s similar to how a gravy stain almost permanently embeds itself in a new shirt.

After the bath in the bright yellow oil, the parts are immersed in a hydrophilic emulsifier, a soap-like liquid, and then washed with water to remove the oil from the surface of the parts. Soap, properly called a surfactant, allows oil to mix with water. Surfactants are also in photographic chemicals and detergents.

“The parts are dipped into a soluble talc developer solution that covers the entire part,” he said.

This solution begins the process of drawing the oil out of the parts, leaving the dye behind.

After the talc developer bath, the parts are left to dry so any remaining developer can drain from the surface. After the initial draining process, the parts are dried in a 140-degree Fahrenheit dryer. When the parts are dry - usually after a 15-minute drying time – they sit a little longer on another dwell station for the developer to draw out any remaining oil from any fatigue cracks.

“The parts are then inspected under an ultraviolet light,” Mr. Schwarting said. “The cracks will be indicated by fluorescent linear indications.”

To find cracks on steel parts, the NDI shop has a very attractive way of looking.

“The magnetic particle inspection is used exclusively on ferrous parts,” Schwarting said. “The parts are magnetized using either \[alternating current\] or \[direct current\] electricity in either a coil or contact blocks. ”

When an electric current is fed through the part, it creates a magnetic field “of the proper orientation to find fatigue cracks,” he said.

Any cracks in the part will distort the magnetic field, which attracts the fluorescently-dyed magnetic particles used in a similar manner as the fluorescent penetrant. The inspectors use a form of deodorized kerosene to carry the particles into the cracks by allowing the liquid to flow over the entire part.

“The particles are attracted to the field leakage and form a linear indication when viewed under an ultraviolet light,” he said.

To hunt for cracks in larger areas like wings and \[rear wings\] they use an aptly larger process, which takes the entire hangar. They use X-Rays, much like in a hospital to find broken bones in people and animals. They’re not looking for cracks in bones, rather cracks in structural members.

A cylindrical device a little larger than a big loaf of white bread is placed on the floor below the area to be X-Rayed. If a wing is to be X-Rayed, for example, technicians place the tube under the wing. A sheet of X-Ray film is placed on the top of the wing, over the area in question.

The tube blasts X-Rays through the wing from underneath where they expose the sheet of film on top. It is the same process as a dental X-ray where a sensor or small piece of film is held in the teeth.

In addition to the sheet of X-Ray film, the inspectors use a large clear vinyl sheet with numbered tags for identification purposes on top of the wing, near the X-ray film. The tags have lead numbers so they can be seen in the final image. This is to identify the part in the wing and its location.

After the film is exposed and the room is free from radiation, a technician picks up the film to bring back to the control area to be processed. Large cables connect the tube to a control device in the control room.

Schwarting said they still use traditional film instead of the digital means adopted by most of the industry because of technical limitations with the image and the process. Following appropriate technical orders for safety, there are red flashing lights in the control area on the other side of that big, sliding door. The walls have colorful signs warning of the dangers of radiation and just inside the door; meters monitor any radiation which may seep through. Schwarting said they have all of the lights and the two meters in place to comply with technical orders. They also have warning signs on the roof and more red lights because upward, conical beam of x-Rays. When the inspectors find a cracks, they send the damaged parts to engineering to look for signs of crack propagation growth, or how the crack spread or grew, how long it took and what kind of force caused it. With the use of electron microscopes and other means, they can see the actual crack at a microscopic level, which can yield a great deal about what caused the cracks. It effects inspections and design of new or upgraded parts. “It drives the frequency of the inspections,” Schwarting said. “It’s all about metal fatigue.”
“When pilots fly they put G-loads on the aircraft, which stresses it in different areas,” he said. “As the aircraft ages, our job becomes more important.” Schwarting compared the stresses from flight to bending a paper clip very slightly. Eventually the paper clip will break, gradually weakening.