Significant effort and resources are being expended to develop ceramic-matrix composite (CMC), metal-matrix composite (MMC), and polymer-matrix composite (PMC) materials for high-temperature engine components and other parts in advanced aircraft. The development of composite materials is also being pursued actively in the automobile and sporting-equipment industries, among others. A portion of the development effort involves the assessment of nondestructive-evaluation (NDE) techniques for detecting flaws in these materials. Recent advancements in infrared-camera technology and computer power have made thermographic (infrared) imaging systems worth reconsideration as reliable tools for NDE of these materials. Thermography offers the advantages of real-time inspection, no contact with samples, nonionizing radiation, capability for inspection of samples with complex shapes, variable sizes of fields of view, and portability.
An experimental study sponsored by NASA Lewis Research Center was performed to evaluate the capability afforded by a thermographic imaging technique for detection of defects in four composite materials of interest as high-temperature structural materials. The materials studied were two CMCs, one MMC, and one PMC; artificial defects in the form of flat-bottom holes with diameters from 1 to 13 mm and depths from 0.1 to 2.5 mm into specimens (2 to 3 mm thick) of these materials. In the thermographic imaging technique used, the source of heat was a pair of xenon flash lamps that faced the same side of the specimen as that observed by an infrared camera.
In the experiments, limits of detectability based on the depths and diameters of the holes were determined for each specimen material. For a SiC/SiC CMC, it was found that defects with depths <1.8 mm and diameters >2.6 mm will probably be detected (see Figure 1) by the technique used in these experiments. Similarly, it was found that for a composite of SiC fibers in a calcia/alumina/silica matrix (SiC/CAS) defects with depths <1.8 mm and diameters >1.6 mm will probably be detected; for a MMC of SiC/Ti, defects with depths <1.6 mm and diameters >3.2 mm will probably be detected; and for a PMC of graphite/polyimide, defects with depths <1.8 mm and diameters of about 3 to 12 mm will probably be detected. Depth appears to be the limiting variable with regard to detectability in the PMC.
As part of the study, thermographic images and observations about detectability were compared with results from ultrasonic and x-radiographic imaging to highlight the relative strengths and weaknesses of each imaging technique as applied to the composite materials studied. For example, Figure 2 shows thermographic images along with film x-radiographic and ultrasonic images for SiC/SiC specimens. The x-radiographs clearly reveal all defects. The ultrasonic pulse/echo image gives very diffuse indications of most defects because of the porosity (about 15 percent) of SiC/SiC. The ultrasonic through-transmission image clearly shows all defects at shallow and intermediate depths.
Overall, this study has yielded baseline results that can be expected to enable material developers and component designers to determine whether the thermographic technique used can reveal "critical" defects. The technique is applicable to inspection of composite-material structures in any industry. Examples of materials and structures amenable to such inspection include tires, composite hulls of boats, composite frames of bicycles, and multilayer thick tapes.
This work was done by Don J. Roth of Lewis Research Center, James R. Bodis of Cleveland State University, and Chip Bishop of Bales Scientific, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category,or circle no. 180 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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