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March 2000

Finding Cracks and
Checking Out Walnuts

A novel nondestructive testing technique combines
infrared imaging technology and ultrasonics.

One of the biggest problems facing manufacturers of safety-critical parts is the existence of hard-to-find defects that can cause catastrophic failure of materials under stress. An automotive steering knuckle is an example of a component that can cause serious injury or death if undetected cracks lead to mechanical failure, as when a car is cornering at high speed. Unfortunately, conventional techniques for finding cracks in parts all have major drawbacks. For example, dye penetrant inspection relies on fluorescent dye that penetrates into cracks in a surface. When the surface is wiped off, small amounts of dye left in the cracks will fluoresce. The dye is toxic, however, and the process takes time. X-ray imaging can reveal cracks, although the technique is expensive and hazardous, and will not effectively detect some crack types such as compression cracks. A compression crack can form during manufacturing when a part cracks at high temperature and the crack closes up as the part cools, rendering the crack nearly invisible.

ThermoSoniX (thermal imaging and sonic excitation) is a novel nondestructive testing technique that uses high-frequency sonic excitation, together with infrared (IR) detection, to image surface and subsurface defects (patent pending, Wayne State University, Detroit, MI). This imaging technique uses a short (50-200 ms) pulse of high-frequency sound, typically 20-40 kHz, that is applied at some convenient point on the surface of the object under inspection to produce localized frictional heating at the defect. An IR camera images the heating of the surface resulting from the effects of friction or other irreversible internal surface interactions in the vicinity of cracks or disbonds. 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 appear as a planar heat source.

Figure 1. Crack detection in a ductile iron cast part.

A crack in a ductile iron part is shown in Figure 1, just before ultrasonic stimulation (above, left). If the crack intersects the surface, the heat source first appears as a line in the IR image, as shown in Figure 1 at center. The line subsequently blurs and broadens into a diffusely heated region surrounding the original line, as shown in Figure 1 at right. 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.

Note that these images have not been enhanced for contrast. The superb temperature sensitivity of the indium antimonide 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 ceramic, carbon, hard plastics, and even walnuts and pistachios. In fact, any hard material with a high emissivity in the mid- or long-wave IR bands can be evaluated.

Figure 2. ThermoSoniX test station.

Indigo Systems Corporation has developed a ThermoSoniX test station based on technology developed at Wayne State University and licensed by Indigo Systems. The test station, shown in Figure 2, is primarily intended for hand inspection of low-volume, high-cost parts such as aircraft turbine blades. It is also intended for research and development of fully automated ThermoSoniX inspection systems. The test station consists of the following subsystems:

  • a pneumatically actuated ultrasonic energy source coupled to a hard metal probe that makes contact with the part under test;
  • a mechanical structure that holds the actuator, ultrasonic probe, camera, and a platform for the part under test;
  • an Indigo Systems Merlin mid-infrared camera that images the part under test in the 3- to 5-micron waveband and transmits 12-bit digital image data to a digital framegrabber;
  • a computer hosting the data acquisition and control system. The test station is run by a LabVIEW virtual instrument (VI) that controls the pneumatic actuator, the ultrasonic source, a digital framegrabber, and the Merlin camera.

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. ThermoSoniX is not restricted to particular classes of materials, nor does it have the radiation or chemical hazards associated with x-ray imaging or dye penetrants, respectively.

This article was written by Dr. Austin Richards, applications engineer at Indigo Systems Corporation, and Prof. Xiaoyan Han of the department of electrical and computer engineering, Wayne State University, Detroit, MI. For further information, contact Indigo Systems at 5385 Hollister Ave., Suite 103, Santa Barbara, CA 93111; (805) 964-9797; fax: (805) 964-7708; e-mail: richards@indigosystems.com.

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