According to a peer-reviewed study of non-destructive testing methods for composites materials by Saeed Gholizadeh, (Structural Integrity Procedia 1 (2016) 050-057), ultrasonic testing – whether pulse echo or through transmission – is still one of the best methods to analyze composite parts.

Classic ultrasound techniques require a composite part to be immersed in fluid, such as water or gel, which is restrictive when evaluating large parts. Newer methods of ultrasound evaluation include dry and non-contact approaches that take images without loss of signal strength.

“DolphiCam is an ultrasound NDE system with a dry, silicone-rubber stand-off nose at the end of the probe. It does not require a coupling agent. The operator simply walks up to the part, compresses the nose to the part and takes the image,” says Dorworth. “When I was first introduced to DolphiCam, I plugged in the software, plugged in the instrument and a standard, and started inspecting, receiving green ‘go’ messages or red ‘defect’ messages within minutes.”

Another alternative to “wet” ultrasound comes from the Ultran Group. Ultran offers non-contact ultrasound systems particularly for porous structures or structures incorporating honeycombs or cells. The technology increases the efficiency of its transducers through a proprietary gas matrix piezoelectric (GMP) material for a non-contact environment.

Most recently, the company has optimized its ultrasound NDE system to support large and complex structures and very attenuated products, such as rocket payload fairings. Ultran’s latest applications incorporate interconnectivity and networking to partner with systems integrators. These partners bring robotics and multi-axis scanning capability to the projects.

“We fit the non-contact ultrasound system on the robot and establish a protocol to speak to the robot’s motion controller,” says Michael Whetzel, COO of the Ultran Group.” We must be aware of cable locations and deal with electrical noise from the robot’s motors while still maximizing acquisition of the data. There’s no standard protocol, and so each time, we work with the system integrator to either transmit our data to the robot controller or to use our software to present the data to the customer in a way that is easy for them to interpret.” Whetzel says the Ultran Group is one generation away from standardizing a communications protocol, however there aren’t enough projects to require that just yet.

Like Blake of Aligned Vision, Whetzel views big data as the key to refining the composites industry’s approach to defining a defect. “Right now, ultrasound can identify a part that is not uniform, but it can’t tell you whether that constitutes a defect. The customer must correlate non-uniformities and variation to interpret whether it is a defect in their world,” says Whetzel. “I believe that in the future, big data will automate the determination of defects so that a technician isn’t required.”

For now, says Whetzel, most one-off composite parts are constantly changing so there’s not enough data yet, especially in aerospace and rocket applications. But the automotive industry has volumes, which can facilitate data analytics in the future. “That will lead to closed-loop manufacturing that can detect and correct the process when it falls out of specification limits,” says Whetzel.

State-of-the-Art Thermography

While many NDE methods require the evaluation equipment to be close to or touching the part, Thermal Wave Imaging Inc. departs from that model with its Large-Standoff, Large-Area Thermography (LASLAT) system, which performs NDE of large composite structures using projection thermography and high-resolution cameras.

LASLAT operates at a 10- to 15-foot standoff distance from the part being inspected at rates as high as 8-square-feet per minute, depending on the material and minimum defect size requirement. Instead of moving the instrument along the body of the large part with a robot, gantry or drone, LASLAT is set in one position and its beam is automatically scanned over the target surface. At each defined subsection, the scan stops, projects heat via light and measures the rate of cooling to identify issues such as delamination, impact damage or water entrapment.

“LASLAT streamlines the inspection process so that time spent on ancillary tasks – such as positioning, marking and archiving – is reduced or eliminated,” says Steven Shepard, president of Thermal Wave Imaging. “It can be moved to different inspection stations for different applications, unlike fixed gantry or robotics, which are considerably more expensive. The inspector has more time to analyze the problem.”

The first LASLAT system, developed for the Naval Air Systems Command (NAVAIR), was delivered in 2018 to a Fleet Readiness Center in Cherry Point, N.C., to improve detection capabilities for V-22 proprotor blade inspections. Thermal Wave was recently awarded a Small Business Innovation Research Phase II.5 contract from NAVAIR’s Fleet Readiness Center in North Island, Calif., to improve detection capabilities for aircraft structures on the E-2 and F-18 platforms. LASLAT is now available commercially.

Embedded Sensor Technology

It would be great if wind turbine blades or aircraft wings could alert you when they had a problem. Embedded sensors offer a step in that direction. One such NDE method is the ODiSI 6000 Series, a fiber optic sensor system from the Lightwave Division of Luna that can profile temperature in-situ, measure 2D and 3D strain fields to validate FEA models and evaluate multi-material joining. Fiber optics are flexible, low profile, require no electrical source and are able to be embedded into or bonded onto any part geometry where other sensors cannot – in bends and around corners.

The optical fiber itself is a non-intrusive wire-length silica that is approximately .15 millimeters in diameter. The ODiSI 6000 system provides more than 150,000 measurement locations with 1,000 strain or temperature measurements per meter, providing high-definition data that can be used to map the contour of strain or temperature for a structure being tested, according to Luna.

“When a part is in service, we can remotely see damage as a function of a change in the strain signal,” says Matt Davis, R&D director for the Lightwave Division of Luna. Depending on the needs of the customer, ODiSI 6000 can be deployed for continuous monitoring via a network that sends data to a larger data management system. Or it can be used in the field for periodic inspections. According to Davis, the part data in its current state is compared to a historic record of the part within the customer’s quality control infrastructure. The software will recognize the part once the ODiSI is connected and then collect the data to determine if there has been a material change.

Davis says the area of measurement can reach up to approximately 150 feet, making it ideal for identifying the horizontal and vertical location of material changes in large parts. Visualization software color maps the magnitude of measurement to the location of the fibers. As an example, bright red indicates load tension while bright blue typically shows areas of compression. So, a strike might show blue where the hit was and red surrounding it, depending on the part geometry. The user can adjust the color scale to identify what they are most interested in identifying.

These advances in NDE – and others on the horizon – take advantage of technological innovations and show promise for the composites industry. “These new methods of performing NDE to revise design, to make corrections to the manufacturing process and to analyze parts in the field are often headed for use in Industry 4.0 platforms,” says Dorworth. “It’s quite interesting to see how these advancements in NDE are reducing problems for the FRP industry.”