ThermoSoniX : A Novel 1 Infrared- and Ultrasonic-Based System for Non-Destructive Testing Built With LabVIEW, IMAQ Vision and DAQ

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1 ThermoSoniX : A Novel 1 Infrared- and Ultrasonic-Based System for Non-Destructive Testing Built With LabVIEW, IMAQ Vision and DAQ Category: Automotive Products Used: LabVIEW IMAQ, IMAQ Vision DAQ by Dino J. Farina, Dr. Austin Richards and Prof. Xiaoyan Han President, Applications Engineer, Professor Image Therm Engineering, Indigo Systems Corporation, Wayne State University The Challenge: To integrate and build an easy to use and cost-effective system for non-destructive identification of cracks and defects in cast parts. The Solution: Developing the ThermoSoniX Test Station using an Indigo Systems Corporation infrared camera, an ultrasonic excitation source and custom pneumatically actuated hardware with LabVIEW, IMAQ and DAQ hardware and software. Abstract This paper presents ThermoSoniX, a novel crack and defect detection system based on synchronized infrared imaging and ultrasonic excitation of ferrous and non-ferrous metals, ceramics, composite parts and other materials. The system design incorporated an Indigo Systems Corporation infrared (IR) camera, an ultrasonic excitation source, ThermoSoniX Test Station hardware, and a PCI-1422 digital IMAQ board and a PCI-6025E MIO DAQ board from National Instruments. The control, analysis and processing software of the system was fully implemented using LabVIEW, IMAQ Vision and DAQ software from National Instruments. The ThermoSoniX Test Station system is now available from Indigo Systems as a fully integrated system with either a PC or PXI chassis. Introduction and Background One of the biggest problems facing manufacturers of safety-critical parts is the existence of hard-to-find cracks and defects that can cause catastrophic failures of materials under stress. For example, an automotive steering knuckle can cause serious injury or death if undetected cracks lead to mechanical failure, as when a car is cornering at high speed. In composite materials, poor bonding between layers can lead to premature de-lamination and eventual part failure. In ceramic parts, hairline surface cracks can be nearly impossible to detect with traditional methods and can lead to a substantial loss in strength of the part and premature failure. The physical microstructure of all manufactured parts contains micro-scale cracks and defects. These cracks and defects can be altered by various manufacturing steps such as heat treating, melting, casting and cold-working. During manufacture and use, these micro-scale cracks and defects can change in shape and location within the part due to changing internal stresses being generated in the part. For example, when a part is heated a crack may grow and expand and upon cooling the crack can shrink and become hidden. These so-called compression cracks can be particularly troublesome for manufacturers because they are difficult to detect by visual inspection and can cause premature failures in parts without warning. Compression cracks can be difficult to detect with traditional NDT techniques such as x-ray or ultrasound that detect voids or gaps in a material. Dye penetrant techniques, described below, have many limitations that make them difficult to implement in a large-scale production line. 1 Patent Pending, Wayne State University.

2 The Dye Penetrant Method Dye penetrant inspection uses a chemical dye that penetrates and fills the cracks on a part surface. Typically, a part is etched, then coated with a dye that is wiped off, leaving dye trapped in surface cracks. For large cracks, visual inspection is used to locate dye trapped in cracks. Smaller cracks require a fluorescent dye and ultraviolet light source, creating glowing cracks that are easier to detect. Though simple in principle, the dye-penetrant technique suffers from many drawbacks. It is hard to document, subject to human error, slow, and can generate hazardous chemical waste, particularly in the case of fluorescent dyes. Worst of all, the dye tends to become trapped in tight corners, giving false crack indications. X-Ray Imaging X-ray imaging can reveal sub-surface cracks and voids in parts in the same manner as it finds cracks in human bones and other hard objects. However, implementing the technique in an industrial setting can be expensive and hazardous due to radiation safety concerns. Compression cracks are difficult to see on x-ray images, since the x-ray images measure column density through the material, and the column density of the material in the vicinity of a closed crack is almost precisely the same as in an undamaged sample of the same material. The ThermoSoniX Method The ThermoSoniX technique overcomes many of the limitations of these traditional NDT methods by synchronizing ultrasonic excitation with infrared imaging to identify cracks and defects in ferrous and non-ferrous metals, ceramics and composite parts. ThermoSoniX can detect both open and closed cracks in a host of materials in a rapid, easily recordable way. When exposed to a short pulse (typically ms) of ultrasonic energy (typically khz), cracks or defects in a part can vibrate differentially inducing localized frictional heating. These effects result from the fact that the two surfaces of internal defects do not move in unison when sound propagates in the object. Thus, for instance, the facing surfaces of a closed crack will act as a planar heat source. An Indigo Systems IR camera can be used to effectively image this induced heating which is typically only a fraction of a degree. When the sound pulse is turned off, the resulting temperature pattern decays according to the usual process of thermal diffusion. This entire process takes place in a fraction of a second, enabling high-speed automated defect inspection. Precise synchronization and control of the ultrasonic excitation and infrared imaging can yield a sequence of thermal images linked to excitation amplitude and power as a function of time. Subsequent image processing can highlight the cracks and defects in the part quickly and efficiently. Since ThermoSoniX relies on dynamic excitation of the part and a highly sensitive Indigo Systems IR camera, it has been proven to be quite effective for detecting various types of defects, including compression cracks, open cracks, disbonds and de-laminations. Hardware Implementation and System Setup The modular ThermoSoniX Test Station unit is shown in Figure 1 and was designed and built by Taylor Consulting Services (Santa Barbara, CA) using 3-D solid modeling software from SolidWorks (Concord, MA). The ThermoSoniX Test Station consists of the following components: ThermoSoniX system software built using National Instruments LabVIEW, IMAQ Vision and DAQ software. Indigo Systems Merlin IR camera with RS-422 digital output and RS-232 based remote control commands connected to a National Instruments PCI-1422 IMAQ board. ThermoSoniX Test Station hardware incorporating hardware and software safety interlocks, analog outputs for system air pressure and tip force connected to a National Instruments PCI-6025E DAQ board. National Instruments Real Time System Interface (RTSI) bus connection between the PCI-1422 and PCI- 6025E for synchronized acquisition and dynamic control. Ultrasonic excitation source operating between khz with analog outputs for applied power and excitation amplitude and RS-232 based remote control commands.

3 Figure 1. ThermoSoniX Test Station. National Instruments LabVIEW and IMAQ Vision were used to build the ThermoSoniX system software because they provided the best mix of image processing and data acquisition functionality and user interface development tools that would be needed to implement the final custom application. In addition, the built-in support for RTSIbased synchronization of IMAQ and DAQ events radically simplified the hardware implementation and cabling. The Indigo Systems Merlin camera family uses a modular electronics framework to give optimal performance in many different IR spectral bands while keeping the digital output on the cameras identical. This approach streamlines the interchangeability of the cameras in the system and makes it simple to optimally match the crack detection sensitivity of the camera to a wide variety of parts having different response characteristics. The ThermoSoniX Test Station hardware was designed for use in proof-of-concept testing and/or bench-top testing of relatively small parts. The design of the hardware incorporates safety interlocks that require two buttons located on either side of the unit to be depressed simultaneously in order to lower the ultrasonic tip and activate the ultrasonic excitation. This approach virtually eliminates accidental use of the system and makes it much safer to use by preventing the operator s hands from being near the tip during excitation. The Test Station electrical design provides a single connector for all the DAQ signals for simplified wiring and ease of setup. Finally, the modularity of the Test Station makes it well suited for custom integrated systems for production and online applications. General Setup In practice, a part is placed on the workspace area of the Test Station in line with the ultrasonic test head and secured down with the supplied clamps. System air pressure is then set to the desired level using the built-in regulator and pressure gauge. This air pressure is read out as an ultrasonic tip force. The higher the force, the greater the ultrasonic power applied to the part under test. The camera, Test Station and ultrasonic excitation source are connected to their respective acquisition devices using the supplied cables. Software Design and Implementation The software design of the ThermoSoniX system is inherently modular and is based on a LabVIEW state-machine architecture. This approach was chosen because it allowed us to rapidly develop the system using time-tested tools and functions and to leverage advances in hardware and driver software performance while keeping the overall software simple and easy to use from a user s perspective. We chose LabVIEW for ThermoSoniX system development because it offered us the best mix of rapid prototyping tools, built-in support for synchronized IMAQ and DAQ using the RTSI (real time system interface) bus and a modular framework that would enable us to offer cost-effective, custom integrated versions of the system. Figure 2 shows the main ThermoSoniX software interface incorporating intuitive icons for file I/O, camera control and test setup.

4 Figure 2. ThermoSoniX software interface. Running a ThermoSoniX Test To run a ThermoSoniX test on a part all that is necessary is to set the Pulse Time and the Acquisition Time on the software front panel and press the two interlocking buttons on the side of the Test Station simultaneously. The Pulse Time is the amount of time that the ultrasonic source is turned on and exciting the part under test. This value can be adjusted from 0.1 to 1.5 seconds. Lower pulse times are typically used on fragile parts such as ceramics, while longer pulse times are more appropriate for rugged parts such as large castings. The Acquisition Time is the total amount of time for the test. This value can be adjusted from 0.5 to 4.0 seconds depending on the nature of the part and its defects. Immediately after the interlock buttons are pressed, the system will automatically lower the ultrasonic tip into contact with the part under test and apply the prescribed tip force based on system pressure as described above. Next, the system will acquire and average pre-sonic images of the part that will serve as a static background image that can later be used to highlight the detected cracks and/or defects. Next, the system synchronizes a RTSI trigger on the DAQ and IMAQ boards to start acquiring images and power and tip force signals at the camera s framing rate (60 Hz) for the prescribed acquisition time while simultaneously turning the ultrasonic excitation source on for the prescribed pulse time. Finally, measurements of the applied Energy and Peak Power are collected via RS-232 from the ultrasonic excitation source, displayed on the front panel and recorded with the test data. When complete, the system software will allow a user to play back the acquired images and linked DAQ data using intuitive VCR-like controls and false-color palettes. The Background Intensity control on the ThermoSoniX software allows a user to adjust the amount of effect that the background (pre-sonic) image has on the images taken while the ultrasonic source was on. This feature is

5 particularly useful for highlighting the detected defects and cracks against a ghosted background image that shows the basic physical features of the part. As the Background Intensity level is increased, the effect of the background dominates the overall scene. Typically, a value of 25% will produce adequate contrast. The Image Histogram, Mean and standard deviation (StD) are computed and displayed for the current image for user visualization of the dynamic range of the image intensity. The software also incorporates automatic, histogrambased dynamic range compensation tools that can be enabled and controlled using the Best Fit and Span controls for optimal display contrast. Results A crack in a ductile iron disk brake holder is shown in Figure 3a, just before ultrasonic stimulation. If the crack intersects the surface, the heat source first appears as a line in the IR image, as shown in Figure 3b. The line subsequently blurs and broadens into a diffusely heated region surrounding the original line, as shown in Figure 3c. Note that these images have not been enhanced for contrast other than using the rainbow false-color palette for display only. The superb temperature sensitivity of the indium antimonide (InSb) detector used in the Indigo Systems Merlin Mid IR camera makes a fraction of a degree temperature rise stand out in sharp contrast to the surrounding material. Similar images have been obtained with damaged samples of aluminum, ceramic, carbon fiber, hard plastics and even walnuts and pistachios! Figure 3a-c. ThermoSoniX images of ductile iron disk brake holder. Summary For many practical applications, this new imaging technique has significant advantages over traditional nondestructive inspection methods. It is fast, wide-area, and sensitive to cracks with any geometrical orientation. Unlike magnetic particle inspection, ThermoSonix is not restricted to particular classes of materials, not does it have the radiation or chemical hazards associated with x-ray imaging or dye penetrant, respectively. References 1. Richards, A. and X. Han, Finding Cracks and Checking Out Walnuts, Photonics Tech Briefs, March, 2000, pp 14a-16a. Acknowledgments The authors would like to acknowledge the efforts of Mr. Peter Taylor of Taylor Consulting Services in Santa Barbara, CA for his expertise in mechanical design and Dr. Socratis Kalogrianitis of Image Therm Engineering in Sudbury, MA for his mastery of LabVIEW programming and expert software design.