Significant effort and resources are being expended to develop
ceramic matrix (CMC), metal matrix (MMC), and polymer matrix (PMC)
composites for high-temperature engine components and other parts
in advanced aircraft. In addition, composite structural material
development is being actively pursued in other industries, such
as the automobile and sports equipment industries. A portion of
the development effort assesses nondestructive evaluation (NDE)
technologies for detecting flaws in these materials. Recent technological
advancements in infrared camera technology and computer power
have made thermographic (infrared) imaging systems worth reevaluating
as a reliable NDE tool for advanced composites. Thermography offers
the advantages of real-time inspection, no contact with the sample,
nonionizing radiation, complex-shape inspection capability, variable
field-of-view size, and portability.
The objective of this NASA Lewis Research Center study was to
evaluate the ability of a thermographic imaging technique for
detecting artificially created defects (flat-bottom holes) of
various diameters and depths in four composite systems (two CMC's,
one MMC, and one PMC) of interest as high-temperature structural
materials. In the thermographic imaging technique used, the heating
source and camera were on the same side. The holes ranged from
1 to 13 mm in diameter and 0.1 to 2.5 mm in depth in samples approximately
2- to 3-mm thick. Limits of detectability based on the depth and
diameter of the flat-bottom holes were observed for each composite
material. This work was done in cooperation with Bales Scientific,
Inc., a manufacturer of state-of-the-art thermography systems,
via a Space Act Agreement. It was funded by the High Speed Research
program, the Enabling Propulsion Materials (EPM) program, and
the High Temperature Engine Materials Program (HITEMP) of which
Pratt & Whitney and GE Aircraft Engines are major partners
and customers with NASA. This study was completed in 1996, with
results reported at EPM, HITEMP, and Quantitative Nondestructive
Evaluation (QNDE) technical meetings.
On the basis of the detectability results for the flat-bottom
hole samples, the following conclusions were drawn. For the SiC/SiC
CMC samples, defects with depths <1.8 mm and diameters >2.6 mm probably
will be detected as shown in the graph. For the SiC/CAS CMC samples,
defects with depths <1.8 mm and diameters >1.6 mm probably will
be detected with the thermography methodology used in this study.
For the SiC/Ti MMC samples, defects with depths <1.6 mm and diameters
>3.2 mm probably will be detected. For the graphite/polyimide PMC
samples, defects with diameters of ~3 to 12 mm and <1.8 mm in depth
probably will be detected. Depth appears to be the limiting variable
with regards to detectability in the PMC system. The thermographic
images were compared with ultrasonic and conventional film radiographic
images (see the images below) for the SiC/SiC composite samples.
For these SiC/SiC samples, radiography clearly shows all defects.
The ultrasonic pulse echo (PE) image shows very diffuse indications
of most defects because of the porous nature (~15 percent) of
SiC/SiC. The ultrasonic through-transmission (TT) image shows
clear indications of most defects. Thermography clearly shows
shallow and intermediate depth defects.
Probable detectability limits for the thermography method based on depth and diameter were defined for four composite systems that are of interest for use in high-temperature structural components. These baseline results allow material developers and component designers to determine whether this thermography method can detect a "critical" defect. Thermography images and detectability results were compared with those from conventional ultrasonic and radiographic imaging methods to highlight the relative strengths and weaknesses of the three imaging methods when applied to the composite systems used in this study.
Roth, D.J.; Bodis, J.R.; and Bishop, C.: Capability of Single-Sided
Transient Thermographic Imaging Method for Detection of Flat Bottom
Hole Defects in High-Temperature Composite Materials. Proceedings
From the 1996 Review of Progress in Quantitative NDE, Brunswick,
ME, July 28 - Aug. 1, 1996.
Previous articleLast updated May 5, 1997
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