Infrared imaging aids nondestructive testing

NDT system detects defects in metal, ceramic, and composite parts using IR-imaging and ultrasound-excitation technologies.

By Lawrence J. Curran,Contributing Editor

A California company has integrated infrared-imaging and ultrasonic-excitation technologies in a nondestructive-testing (NDT) system that can detect cracks and other defects in metal, ceramic, and composite-material parts. What's more, a major application of the integrated system has provided such impressive results that the company may use it to replace its conventional NDT methods, which include eddy current and dye penetration.

Test sequence shows infrared images of a ductile iron disk-brake holder. A barely visible crack appears (left) just before ultrasonic excitation. If the crack intersects the part's surface, it looks like a heat source and a line (middle). During ultrasonic excitation, the line then blurs and broadens into a diffusely heated region surrounding the original line (right).

The system integrator, Indigo Systems Corp. (Santa Barbara, CA), has merged one of its Merlin-Mid cooled focal-plane-array infrared (IR) cameras with an ultrasonic-excitation source and custom pneumatically actuated hardware into a test station called ThermoSoniX. Another virtual instrument functions as the system's graphical user interface (GUI). Image Therm Engineering Inc. (Sudbury, MA) developed the GUI instrument using LabVIEW software provided by National Instruments Inc. (Austin, TX). National Instruments also contributes two boards to the test station—an IMAQ machine-vision and image-processing board and a DAQ data-acquisition board.

Siemens Westinghouse Power Corp. (Orlando, FL), a manufacturer of steam- and gas-turbine power plants, is evaluating the test station in a research-and-development mode to reduce the cost and time required to inspect turbine components. The complete system generates moving thermal images that readily show cracks and other defects in parts without the need for interpreting nongraphic data and without dismantling the tested parts. The images are stored on disk and played back for a detailed analysis of test conditions and results.

Paul Zombo, principal engineer at Siemens Westinghouse, says the company has been working with the ThermoSoniX system for about four months, "and we expect significant cost, time, and rework savings once it's installed on the shop floor." He's confident the results he's seen so far will be reproducible there.

"This method is highly intuitive," Zombo says. "You see a picture of a part and the defect in the part. You get a full-field view and you get it quickly. You can look at the whole part or a large area of the part at once. Other methods don't allow that," he adds. "Most NDT methods, including eddy current, radiation, and ultrasound alone, aren't intuitive, nor do they provide a full-field view."

The eddy-current method, for example, usually provides a scope display that shows a blip corresponding to a defect. The inspector either must know that the blip is a defect or has to interpret the blip by comparison to reference standards. In this manner, the inspector determines whether the signal shows a defect and if that defect is sufficient to cause a part to be rejected.

An applications engineer at Indigo Systems, Austin Richards, further assesses some conventional NDT practices. He notes that eddy-current testing does find cracks. However, this test method is slow, uses a small probe moved over a small area, and scans like a portable metal detector. Another test method, x-rays, can detect voids, but it might not be able to find cracks, especially compression cracks. These types of cracks, which may not show up initially, but "grow" under stress, are easily detected by ThermoSoniX. Furthermore, x-ray inspection involves hazardous radiation and generally requires costly and consumable materials such as film and development fluids. Richards says Siemens Westinghouse is impressed with the test station because it is an authentic NDT tool that can be used in production to inspect turbine blades without removing the parts from the turbine rotor, which would be costly in time and labor.

He adds, however, that ThermoSoniX is not yet a certified NDT technique, although the certification process has begun. "But ThermoSoniX can be combined with existing certified NDT methods to increase the speed of certifiable inspections substantially," Richards contends. For instance, painted aluminum aircraft parts often are inspected with eddy-current techniques to locate both surface and subsurface cracks. But this method is slow because the probe must be swept over the entire surface area of the part.

FIGURE 1. Indigo Systems Merlin-Mid camera captures infrared (IR) images of a part under test as an AVI movie for insertion into Microsoft PowerPoint or as single bitmap images. The digital image data are linked via an RS-422 link to a PC that houses National Instruments' PCI-1422 IMAQ and PCI-6052E DAQ boards. Computer commands are relayed to the IR camera over an RS-232 link. The ultrasonic head excites the part under test, provides analog outputs for applied power and excitation amplitude, and accepts computer commands. The test-station base holds clamped parts and receives analog outputs for system air pressure and tip force from the DAQ board.

Liquid penetration methods are also slow because the test part must be stripped of paint, cleaned, etched, inspected, and then repainted. "A much- faster method for achieving a certifiable inspection is a combination of ThermoSoniX and eddy-current techniques," Richards says. "ThermoSoniX quickly determines the location of any cracks, displays them in a wide-field image sequence, and records their positions. Eddy-current testing then confirms the existence of the cracks and quantifies them. This combined approach would be especially advantageous for checking parts with complex geometry. In this application, eddy-current testing is especially slow and requires special probe geometry," explains Richards.

How it works
Richards points out that when the part being tested with ThermoSoniX is exposed to a 50- to 200-ms pulse of ultrasonic energy at a frequency of 20 to 40 kHz, "cracks or defects usually vibrate differentially, inducing localized frictional heating." That condition arises because the two surfaces of internal defects do not move in unison when sound propagates in the part.

"For example, the facing surfaces of a closed crack will act as a planar heat source, and our IR camera will capture images of this induced heating, which typically is only a fraction of a degree," says Richards. When the sound pulse is turned off, "the resulting temperature pattern decays according to the principles of thermal diffusion. Precise synchronization and control of the ultrasonic excitation and infrared imaging lead to a sequence of thermal images that are linked to the excitation amplitude and power as a function of time," he adds.

Image processing then highlights the cracks and other defects in the part under test. "The process takes just a fraction of a second and enables high-speed, automated defect inspection," which exposes compression cracks, open cracks, poor bonds, and delaminations, Richards explains.

The test station incorporates several key elements: ThermoSoniX system software developed by Image Therm Engineering; National Instruments' IMAQ Vision and DAQ data-acquisition software; and Indigo Systems' Merlin-Mid indium antimonide midwavelength (3- to 5-µm) IR camera with an RS-422 digital image output and RS-232-based remote-control commands (see Fig. 1). The camera is connected to a National Instruments PCI-1422 IMAQ machine-vision and image-processing board and a PCI-6025E DAQ data-acquisition board, both of which are housed in a portable PC made by Broadax Computers (City of Industry, CA).

FIGURE 2. The infrared imaging system's virtual instrument or graphical user interface is used to govern test functions via control knobs for pulse and acquisition times and background image intensity. It also offers various waveform displays for power, force, and pixel intensity, as well as virtual gauges for static tip force, air pressure, and amplitude.

FIGURE 2. The infrared imaging system's virtual instrument or graphical user interface is used to govern test functions via control knobs for pulse and acquisition times and background image intensity. It also offers various waveform displays for power, force, and pixel intensity, as well as virtual gauges for static tip force, air pressure, and amplitude.The remaining ThermoSoniX hardware comprises the test-station base and channel-plate surface, which incorporate hardware and software safety interlocks, analog outputs for system air pressure, and ultrasonic tip force connected to the DAQ board. The ultrasonic excitation source, which is furnished by Branson Ultrasonics Corp. (Danbury, CT), also has analog outputs for ultrasonic power and excitation amplitude. It is driven by a 400-W power supply via RS-232-based remote-control commands from the PC. The system uses the National Instruments real-time system interface (RTSI) bus to link the image-processing and data acquisition boards for synchronized acquisition and dynamic control.

Richards says the Image Therm application software implements a virtual instrument. This instrument displays a GUI that presents knobs and dials to an operator for controlling test sequences (see Fig. 2). Controls are included for ultrasound-pulse and image-acquisition times and for storing infrared images in memory in the form of a looping movie. This audio-visual interleave (AVI) movie can be inserted into Microsoft PowerPoint or used as single bitmap images. Images in either format can be accessed for later analysis.

Test sequence
To initiate a test on a part that has been clamped down on the test-station-base channel-plate surface, the test technician first sets the pulse and acquisition times on the virtual instrument's GUI. Pulse time—the length of time the ultrasonic source is turned on to excite the part under test—can be set for 0.1 to 1.5 s. Acquisition time—the total time needed to acquire true 12-bit digital infrared video—can be adjusted from 0.5 to 4.0 s. The set times depend on the composition of the part and the anticipated defects.

Then, the operator simultaneously presses two interlocking buttons on the side of the test station. Triggering these buttons lowers the ultrasonic device tip to contact the clamped part and applies the programmed tip force. Next, the system acquires and averages presonic images that serve as a static background image; these images are used later to highlight detected cracks or other defects. The system synchronizes an RTSI trigger on the DAQ and IMAQ boards to begin acquiring images, acquires power and tip-force signals at the IR camera's 60-Hz framing rate for the programmed acquisition time, and simultaneously turns on the ultrasonic-excitation source for the prescribed pulse time.

Finally, the system collects measurements of the applied energy and peak power from the excitation source via an RS-232 link, displays them on the GUI front panel, and records the test data. "When the test is completed, the software enables the user to play back the acquired images and linked DAQ data using intuitive VCR-like controls and false-color palettes," Richards explains.

He adds that the front-panel blue background-intensity control allows the user to adjust the amount of effect that the presonic background image has on the images captured while the excitation tip is activated. "This feature is especially useful for highlighting the detected defects and cracks against a 'ghosted' background image that shows the basic physical features of the part," Richards says. A value of 25% usually produces adequate contrast to clearly show excitation-enhanced defects against the edges of the part derived from the background image.

The DAQ board provides both digital and analog I/O—a logic pulse or an analog signal ranging from -10 to +10 V for use as desired. But the test station's output is fully digital: Richards says, "We send digital focal-plane data [from the camera] directly to a digital frame grabber with no need for analog-to-digital conversion. In this manner, we get a pure 12 bits from the camera." System cables can run 50 to 100 ft to the IMAQ board with no signal losses.

Taking a look
Images were collected during a test sequence on a ductile iron disk brake holder (see p. 24). Richards stresses that these images have not been enhanced for contrast other than using the rainbow false-color palette for display only. He points out, "The superb temperature sensitivity of the indium antimonide detector used in the IR camera causes a temperature rise of a fraction of a degree to stand out in sharp contrast to the surrounding material."

At Siemens Westinghouse, Zombo says, the company still uses eddy-current, dye-penetration, and magnetic-particle inspections of turbine castings but seeks to replace those methods with ThermoSoniX technology. He cites that these three inspection methods use liquids that are environmentally unfriendly and sometimes flammable, "and all three are much more labor-intensive and time-consuming [than ThermoSoniX] by an order of magnitude."

But Siemens Westinghouse wants more experience with and more data from the test station. Zombo comments, "We don't yet fully know the principles of operation; we just know it works, but that's not good enough. We need to know the physics of ultrasound and thermal energy that provide the underlying technology of ThermoSoniX. In The NDT field, methods have to be qualified and personnel have to be certified. This [test] method is new, and no prior work can be referenced for qualification. And no programs exist yet for personnel training. We need to conduct systematic studies of the device for a few months as part of the qualification process," he adds.

Broadax Systems Inc.
City of Industry, CA 91748
E-mail: [email protected]

Image Therm Engineering Inc.
Sudbury, MA 01776
Web: www.imagetherm.com

Indigo Systems Corp.
Santa Barbara, CA 93111
Web: www.indigosystems.com

National Instruments Inc.
Austin, TX 78759
Web: www.ni.com

Siemens Westinghouse Power Corp.
Orlando, FL 32826
Web: www.swpc.siemens.com