Figure 1

Introduction 
Capillary action has been used to test objects for centuries. It is unclear if early users understood what was happening and why. Clay jugs were fired to assure they would hold wine and were tested to see if they would weep. Towels have been used for drying, and wicks were employed to bring oil from a jar to be burned for lighting for even longer. Eventually, the railroads employed the phenomenon to locate cracks in wheels.

Refinement of the method continued; the problem was that the surface had to be bare for proper inspection. More recently, medical uses were developed. For example, tissue can be stained to make anomalies more visible. Microscopy aided in obtaining sharper views of the problem areas. The medical field also adapted the method to observe actions beneath the surface. The combination of these approaches formed the basis for material testing through paint, which proved to be a very useful approach for inspecting the hulls of ships, aircraft, and spacecraft (Reinhardt, 1997a).

In regard to the uses by the railroad industry, the liquid penetrant testing approach developed in the 1800s has remained for all practical purposes the same since. Users apply penetrating oil to a clean surface, allow it to sit for a time, and then wipe off the excess oil and apply a coating of powdered chalk that, by capillary action, absorbs the oil in any discontinuities. The oil shows through the coating either as a visible dye or one that would require excitation by ultraviolet light.

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The prime improvement over the years has been the development of better grades of penetrant. 

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The prime improvement over the years has been the development of better grades of penetrant. Most of the developments involved fluorescent materials because the contrast was superior to normal dyes. They were designed to find smaller discontinuities and increase the speed with which the inspection process could be completed. Production lines could be set up with photodetectors that would identify units for manual review that exhibited too much fluorescence. But the drawback of needing a bare surface has remained.

In the 1980s, confocal microscopy became very useful due to the ability to detect fluorescence emissions from a sample. This approach was proven effective in the inspection of tissues and cells for the dental and ophthalmology fields; additionally, it showed that 3D imaging is possible (Prasad, 1999; Prasad et al., 1997).

The 1990s brought many improvements to the system. The normal process of using single photon excitation by ultraviolet laser light on a sample resulted in an emitted fluorescence of a longer wavelength than was the ultraviolet itself. By focusing the microscope at different depths and scanning, a 3D image could be created as shown in Figure 1 (Prasad et al., 1997).

The introduction of two photon excitation was demonstrated on both biological and material samples. In this method the laser light has two photons with nonlinear absorption and is beamed at the dye, which fluoresces. The laser beam is of a longer wavelength than that used in the single photon unit. Its wavelength is toward the infrared spectrum, which results in a single high energy photon being emitted. This gives better depth resolution without the use of the confocal aperture. The main difference between the single photon and the two photon equipment becomes more apparent when looking for fluorescence below a coated surface and at the substrate surface. In 1996, fluorescent dyed polymer blocks revealed images at a depth of 200 mm (8 ¥ 10^-3 in.) with submicron resolution. With good penetration of the excitation light, the surface can readily be examined for various discontinuities. Excellent images can be obtained with the use of newly developed dyes (Reinhardt, 1997b). Dyes designed, synthesized, and developed at Wright Patterson Air Force Polymer Research Laboratories by the late Bruce A. Reinhardt are the basis of this step forward in penetrant inspection. They have 100¥ greater two photon absorption (Reinhardt, 1997a).

The practical side of this development is the fact that this is a new approach to performing penetrant inspection of a substrate of aluminum, or other material, through a painted surface. A sample was given three layers of epoxy paint. The top and bottom layers had the newly develop dyes mixed in. They both absorb 800 nm (3.1 ¥ 10^-5 in) light, induced by two photon absorption (Reinhardt, 1997a). However, due to the different chemical makeup of the dyes, they fluoresce at different wavelengths (460 nm and 520 nm [1.8 and 2 ¥ 10^-5 in.]). The cover coat will fluoresce blue and the primer coat green. This allows photosensors to establish the specific depth being viewed. The intermediate coating had no dyes. The thickness of each layer of paint can be measured, the bond between layers can be checked for uniformity, and the substrate surface profile can be studied. The substrate can be checked for fatigue cracking and or corrosion with reasonably good results. Rough or smooth surfaces are easily distinguished. Negative and positive contrasting can be performed depending on what is required. When checking for the substrate smoothness, using the negative contrast mode, aluminum is dark, while painted areas fluoresce (Reinhardt, 1997a). With the two photon confocal microscope recording the fluorescent indication information noted, a computer generated 3D map of the surface is rapidly created. Details of the material structure are immediately available for review and evaluation. This method is adaptable to individual part inspection as well as large surface areas.

 

References
Reinhardt, Bruce A., "Detection of Corrosion, Under Paint," NTIAC Newsletter, Nondestruc-tive Testing Information Analysis Center, Volume 22, No. 3, September 1997a, pp. 1-3.

Reinhardt, Bruce A., private communications with the author, Polymer Branch, US Air Force Wright Laboratory, Dayton, Ohio 45433, November 5, 1997b.

Prasad, Paras N., private communication with author, Photonics Research Laboratory, State University of New York at Buffalo Buffalo, New York 14260-3000, December 8, 1999.

Prasad, Paras N., J.D. Bhawalkar, J. Swiatkiewicz, P.C. Chang, S.J. Pan, A. Smith, J.K. Samarabandu, and Bruce A. Reinhardt, "Nondestructive Evaluation of Polymeric Paints and Coatings Using Two-Photon Laser Scanning Confocal Microscopy," Polymer Communications, Vol. 38, No.17, 1997, pp. 4551-4555.

 

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