Squirting Away X-Rays in NDT

Because complex-shaped carbon fiber and other advanced composite materials require different testing methods than the X-rays typically used to check welds and look for defects in parts, ultrasonic squirting may become the next non-destructive test of choice.

Critical components in the aerospace and defense industries, such as engine housings, wings and control surfaces, undergo rigorous testing to ensure quality and safety, but require methods that won’t damage or permanently alter the product during inspection.

Historically, X-ray methods were used in non-destructive testing (NDT) 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 different testing methods other than X-ray. Machine design manufacturers like Marietta Nondestructive Testing, LLC (MNDT; Marietta, GA), working in partnership with drive and control experts from Bosch Rexroth (Charlotte, NC) and its automation distributor, Livingston & Haven (Charlotte), needed to get creative in their approach to developing testing equipment.

Large parts present big problems, particularly large parts made of carbon fiber that are difficult to inspect with X-ray technology that had been used for other aerospace components. “X-ray is limited in determining the defect size and depth in composite parts. However, it can be accurately inspected using ultrasonic technology,” says Curtis Cooper, the director of engineering for MNDT.

The most common automated ultrasound testing machines employ immersion tanks filled with water as a medium through which the sound waves travel. As parts requiring this level of inspection became more complex, the immersion tanks became impractical. Instead, technicians used to scan complex parts by hand, a slow and labor-intensive process that resulted in overlapping scans, which could lead to inaccurate or inconsistent test results.

Complex challenges such as these require innovative solutions. MNDT’s new ultrasonic inspection equipment is custom-designed for each client, based upon the unique specifications of the parts being inspected. To overcome the challenges of scalability, reliability and speed, MNDT engineer 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.

With a scanning envelope of up to 60 ft x 20 ft x 16 ft, this machine can easily be configured to test a wide range of parts for each customer, instead of being designed for one specific part. The system’s ultrasonic scanning system utilizes two sets of squirter jets that face each other. During inspection, the jets stream water – the medium that the sound wave travels through – around the part.

The precise servo motion control of the system became a critical factor in the design. In order 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 five inches from the face of the part,” explains Cooper. “Since the two nozzles face each other, they have to be lined up. We were able to make streams of water, which are each manipulated by five axes of servo motion, concentric to within 0.020 of an inch.”

In selecting the components for precision and reliability that would ensure precise control, accuracy and reliability, MNDT used drive and control components from Bosch Rexroth. These components – including digital servo drives and controllers, profiled guide rails and pneumatic components – allow the machine to follow intricate path planning for scanning complex, curved objects with tightly controlled motion tolerance.

Distributor Livingston & Haven provided design and programming expertise for the new line of machines. “The high-quality controllers are reliable and easy to program. They can also accommodate the large number of interpolated axes of the machines,” notes Cooper. In addition, the controllers generate minimal overall electrical noise to minimize ultrasound interference that can distort the image quality of the scan. The pneumatic components include a vacuum generator to remove air from the nozzle to also improve the quality of the ultrasonic scans.

According to Ben Strong, an automation specialist at Livingston & Haven, the linear guide rails used in the AG2 also contribute to the overall rigidity and accuracy of the machine. “If the machine is not stiff enough to handle the squirter system, it will begin to vibrate and adversely affect the testing,” he says, adding that from a maintenance perspective, the linear guide rails are an ideal solution due to their longer lubrication interval, dual rail datums and interchangeable runner blocks.

In addition, the rails are plated with thin dense chrome to resist rust, a crucial consideration for a machine that incorporates water. Livingston & Haven also assisted MNDT in adapting the Rexroth control system to communicate with LabVIEW™, a robust platform and development package that design and control engineers use to automate measuring equipment. It’s commonly used for data acquisition, instrument control and industrial automation on a variety of programs, including Microsoft Windows and Mac OSX.

“We implemented a solution utilizing TCPIP sockets that provides real-time communication of position commands,” states Strong. “As a result, MNDT can present an image of the machine in real time.” This allows for remote viewing of the status of the system and for virtual programming of parts.

The result is a scalable machine with consistent, reliable testing capabilities and lower cost of ownership. “Because of the path planning and motion control, this machine is greatly improved over other inspection equipment,” adds Cooper. The new squirter-based machine can perform scans accurately and quickly with complete part coverage. “We can scan parts at about 25 inches per second, which increases our speed and output.”

Because the programming, software and mechanics are configurable, the machine can easily accommodate a variety of parts from the same customer. Before, customers had to order separate, custom-built systems to inspect each individual part. The functionality of the AG2 extends its usefulness and significantly reduces the cost of ownership for customers.

MNDT is currently working on the next set of improvements for the scanner by designing an interface to simplify the machine’s operation even further. Their engineers are currently integrating a CAD file importing feature to allow for automatic motion path planning. “The machine is specifically designed so users can update new part sets easily,” says Cooper. “We want operators to be able to simply log in and create their own scan plans.”

Bosch Rexroth Corporation, 14001 South Lakes Drive, Charlotte, NC 28273-6791, 800-322-6724, www.boschrexroth-us.com.

Marietta Nondestructive Testing, LLC, 530 Commerce Park Drive Southeast, Marietta, GA 30060, 770-528-9000, www.marietta-ndt.com.

Livingston & Haven, 11616 Wilmar Boulevard, Charlotte, NC 28273, 704-525-8802, www.lhtech.com.