In the aerospace and defense industries, critical components such as engine housings, wings and control surfaces undergo rigorous testing to ensure quality and safety. They require methods that won't damage or permanently alter the product during inspection. Historically, nondestructive testing (NDT) methods relied on X-rays to check the strength of welds and look for evidence of defects and faults in both large and small parts.
However, the use of complex shaped carbon fiber and other advanced composite materials require testing methods other than X-ray. Machine design companies such as Marietta Nondestructive Testing (MNDT), Marietta, Ga., needed to get creative in their approach to developing testing equipment.
Big Parts Means Big Challenges
Large parts made of carbon fiber are difficult to inspect with X-ray technology, which works well for other aerospace components. Even though we're working with large, complex parts, the limitations of X-ray is not a factor of part size. "X-ray is limited in its ability to determine the defect size and depth in composite parts from one shoot," says Curtis Cooper, director of engineering for MNDT. In order to be able to define a type of defect and the size of the defect, you would have to take multiple shoots of a part in different orientations.
"Some materials just lend themselves better to different inspection technologies," adds Garrett White, MNDT senior mechanical engineer. "In this case, composite parts can be inspected accurately using ultrasonic technology (UT)."
The most common automated ultrasound testing machines use immersion tanks filled with water as a medium through which sound waves travel. As parts became more complex, immersion tanks became impractical. Instead, technicians had to scan complex parts by hand — a slow and labor-intensive process that resulted in overlapping scans that could lead to inaccurate or inconsistent test results.
Challenges Require Innovation
MNDT's new ultrasonic inspection equipment is custom-designed for each client, based on the unique inspection specifications of their parts. The company's challenge was to design an NDT scanning system to accommodate larger, complex aerospace parts, reduce high costs of ownership and reduce maintenance needs, automate the system to increase speed and produce consistent results, and improve accuracy and minimize electrical interference.
To meet the challenges of scalability, reliability and speed, the engineers at MNDT designed the gantry style AG2 Overhead Scanner — a rigid, multi-axis, automated-testing machine capable of scanning large, sophisticated parts and intricate shapes without the use of immersion tanks (Figure 1). With a scanning envelope of up to 60x20x16 ft, the machine can be configured easily to test a wide range of parts for each customer, instead of being designed for one specific part. The scanner's ultrasonic scanning system uses two sets of squirter jets that face each other. During inspection, the jets stream water — the medium through which the sound wave travels — through the part.
The precise servo motion control of the system became a critical factor in the design. For the machine to offer multiple axes of motion, component synchronization had to be tightly controlled so the testing would be accurate. "Each nozzle is roughly 3 in. from the face of the part," Cooper explains. "Since the two nozzles face each other, they have to be lined up (Figure 2). We were able to make streams of water, each of which are manipulated by five axes of servo motion, concentric to within 0.02 in. for a 0.25 ft water column."
"We evaluated the applicable technologies of a lot of companies," Cooper explains. "We looked at A-B, Parker, Siemens and [what was then] GE Fanuc, and went through a whole evaluation process." For MNDT, limiting EMI noise was a critical determining factor. "Noise can be pretty easily picked up by the sensors, and that can make ultrasonics susceptible to types of electronic noise that can distort the image," White adds.