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 Nondestructive Testing of Ceramic Materials

by Ali Abdul-Aziz*

 

As far back as the mid 1970s, we were reading articles in Scientific American regarding the ability of computers to reconstruct images taken with X-rays in the medical profession. The technology has now become relatively commonplace and here we see encouraging examples of it in NDT. Discontinuity sizing by this advanced technique is obviously very necessary for the applications outlined in this review article. Roderic K. Stanley Associate Technical Editor

 

Ceramic matrix composites are being considered as candidate materials for high temperature aircraft engine components to replace the current high density metal alloys. Ceramic matrix composites are engineered materials composed of coated 2D high strength fiber tows and a ceramic matrix. Matrix voids are common discontinuities generated during the infiltration process. The effects of these matrix voids are usually associated with a reduction in the initial overall composite stiffness and a decrease in the thermal conductivity of the component. Furthermore, the role of the matrix as well as the coating is to protect the fibers from the harsh engine environment. Hence, the current design approach is to limit the design stress level of ceramic matrix composite components to be below the cracking stress (Abdul-Aziz et al., 2003).

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In order to promote the expanded application of engineering ceramics, the use of appropriate NDT techniques is critical.

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In order to promote the expanded application of engineering ceramics, the use of appropriate nondestructive testing (NDT) techniques is critical. NDT is highly essential for process control, assurance of high quality products and reliable performance in service (Abdul-Aziz et al., 2003; Bray and McBride, 1992; Cartz, 1995; Kim and Liaw, 1998; Kim et al., 2003; Martin et al., 2003; Mix, 1987; Richerson, 1992; Schwartz, 1997). This paper presents a brief tutorial and summary of some available NDT techniques that are currently used in testing monolithic ceramics and ceramic matrix composites. The advantages and limitations of each technique are listed, as well as their applications. These procedures include ultrasonic testing, radiography, X-ray computed tomography and infrared thermography. In addition, other NDT techniques and approaches that are being used and developed are discussed in the technical papers featured in this issue.

 

X-ray Computed Tomography
The computed tomography technique was initially implemented in the medical field in the early 1970s, but has since gained recognition as an important tool in the material research area as a viable NDT technique. It is used to measure volumetric information, such as dimensions, voids, inclusions and density variations, within a solid and to provide pictorial views of the internal and external structure of materials (Abdul Aziz et al., 2003). The most important advantage of computed tomography is the possibility of an accurate 3D visualization and quantification of the internal structure of materials using a series of 2D cross-sectional computed tomography images.

A computed tomography system typically contains an X-ray radiation source and a radiation detector as well as a precision manipulator to scan cross-sectional slices from different angles. Typically, the X-ray source is collimated to form a thin fan beam. The fraction of the attenuated X-ray beam received is directly related to the density and thickness of the test object, the composition of the material and the energy of the X-ray beam. To obtain a full set of imaging data, the test subject, X-ray source or detector array rotates and a sequence of measurements is made from multiple incremental angles. The data acquisition system reads the signal from each detector in the array, converts the measurements into numeric values and transfers the data to a computer for digital reconstruction. The reconstruction process uses an algorithm to identify the point by point distribution of the X-ray densities in the 2D image of the cross sectional slice. Figure 1 illustrates the computed tomography scanning process.

Figure 1 - The relationship of an X-ray tube, detector, image reconstruction computer and display monitor in the computed tomography process.

 

Ultrasonic Testing
Ultrasonic testing is among the most common techniques used for quality control and service integrity testing due to its low cost and convenience of data collection. It is used to detect, locate and size material discontinuities. The ultrasonic region is generally defined as sonic waves of 20 kHz or higher frequency. In ultrasonic testing, beams of high frequency sound waves are introduced into materials so as to detect both surface and internal discontinuities. The sound waves travel through the material and are deflected at the interfaces or by discontinuities. The deflected beam is displayed in an oscilloscope viewport and analyzed to determine the presence of discontinuities. Most ultrasonic tests are performed at frequencies between 0.1 and 25 MHz. There are several ultrasonic testing techniques used in testing ceramic materials, including:

• an A-scan, which presents 1D discontinuity information (in the oscilloscope view, the A-scan signal displays the pulse and amplitude against time; the A-scan display is commonly used to measure material thickness and for point by point discontinuity detection)
• a B-scan, which displays a parallel set of A-scans with 2D data (the B-scan can also be used to test rotating tubes and pipes because it provides a cross-sectional view of discontinuity distribution)
• the C-scan, which is the most widely used scan mode (it provides a 2D presentation of discontinuity distribution and displays the size and position of discontinuities in an area parallel to the surface through the raster scan of two axes; turbine blade roots and attachments are common components typically tested via this technique).

Figures 2 and 3 show a representative sketch of an ultrasonic testing and a typical immersion ultrasonic setup. Figure 4 shows 1 MHz through-transmission ultrasonic test results of a reinforced carbon/carbon specimen from a test panel associated with the Return to Flight program at NASA Glenn Research Center. The figure demonstrates the difference between the preimpact and postimpact condition of the panel, where the black indicates the damaged or delaminated region that was not visually or optically detected (Martin et al., 2003).

Figure 2 - A schematic of the through-transmission ultrasonic testing setup (Kim et al., 2003).

 

Figure 3 - Typical immersion ultrasonic setup.

 

Figure 4 - A 1 MHz through-transmission test of a reinforced carbon/carbon specimen: (a) baseline peak amplitude image prior to impact; (b) peak amplitude image following foam impact.

 

Infrared Thermography
Infrared thermography is a powerful NDT method for the characterization of composite materials. Since composite materials possess relatively high emissivities depending upon the type of material, surface finish and so on, this makes them well suited for tests, with or without surface treatments (Kim et al., 2003; Martin et al., 2003). Generally, this technique involves the heating of a specimen with a short duration pulse of energy and monitoring the transient thermal response of the surface of the specimen with an infrared camera. The thermal energy on the surface travels into the cooler interior of the sample. In turn, there is a reduction of the surface temperature over time. This surface cooling will occur in a uniform manner as long as the material properties and surface condition are consistent throughout the specimen. Subsurface discontinuities that possess different thermal properties (for example, thermal conductivity, density or heat capacity) will affect the flow of heat in that particular region. This resistance in the conductive path causes a different cooling rate at the surface directly above the discontinuity, when compared to the surrounding material free of discontinuities. The change in the subsurface conduction is seen as a nonuniform surface temperature profile as a function of time. Analysis of thermographic data involves the examination of images based on the temperature/time data or derivatives calculated from the original data sets. Areas with discontinuities can then be identified based on deviations from the ideal cooling behavior.

Thermography can also be used to measure thermal diffusivity. A flash lamp provided a short heat pulse (2400 W in this investigation) to the front surface, and the infrared camera was used to record the temperature rise at the back surface after the pulse. The system uses Parker's method to calculate thermal diffusivity. Parker's method assumes no heat loss during the test. Although this assumption can generate a small systematic error (3 to 5%) in thermal diffusivity, this technique was chosen because of its simplicity, and more importantly because the main focus of the present research with the experiment was on the variation from point to point. Infrared images were acquired using a personal computer and thermal diffusivity was calculated pixel by pixel. Figure 5 shows a schematic diagram of infrared thermography testing to measure thermal diffusivity of a composite material (Kim et al., 2003).

Figure 5 - Mapping thermal diffusivity of SiC/SiC samples using the infrared thermography flash technique (Kim et al., 2003).

 

Figure 6 - Computed tomography image of the ceramic matrix composite (SiC/SiC) showing the observed material's imperfections and other structural discontinuities.

 

Figure 7 - Computed tomography, thermography and X-ray images of the ceramic matrix composite (SiC/SiC) showing the observed material's imperfections and damage due to impact.

 

Figures 6 and 7 show several results obtained via computed tomography, X-radiography and infrared thermography. Figure 6 illustrates slices of a ceramic specimen (Richerson, 1992) test section wherein several key findings such as voids and surface roughness are indicated. Figure 7 represents results obtained for a composite specimen subjected to an impact test using each of the techniques described. The agreement between all of them is apparent (Martin et al., 2003).

 

Conclusion
The NDT techniques discussed in this paper offer a descriptive summary of each technique and its application. The best approach is always the one that provides the most efficient results for a given application. A combination of two techniques often yields greater benefits, provided that it is possible. Each one of these techniques has advantages as well as limitations. The main advantages of ultrasonic testing are its ability to investigate relatively thick materials and that results can be obtained quickly. However, it requires water immersion or some other type of acoustic coupling.

Advancements in computed tomography scanning are continuing. Newer versions are being introduced which utilize flat panel area detectors and cone beams. Among the advantages of computed tomography is that it provides a cross-sectional view of the entire material. Infrared thermography provides a noncontact technique providing accurate and reliable data with the ability to image large areas at one time.

 

References
Abdul-Aziz, A., Louis J. Ghosn, George Baaklini and Ramakrishna Bhatt, "A Combined NDE/Finite Element Technique to Study the Effects of Matrix Porosity on the Behavior of Ceramic Matrix Composites," Proceedings of SPIE: Nondestructive Evaluation and Health Monitoring of Aerospace Materials and Composites II, Andrew L. Gyekenyesi and Peter J. Shull, eds., Vol. 5046, 2003, pp. 144-151.

Bray, D.E. and D. McBride, Nondestructive Testing Techniques, New York, John Wiley & Sons, 1992.

Cartz, L., Nondestructive Testing, Materials Park, Ohio, ASM International, 1995.

Kim, Jeongguk and Peter K. Liaw, "The Nondestructive Evaluation of Advanced Ceramics and Ceramic-matrix Composites," JOM, Vol. 50, No. 11, 1998, pp. 1-15.

Kim, Jeongguk, Peter K. Liaw and Hsin Wang, "The NDE Analysis of Tension Behavior in Nicalon/SiC Ceramic Matrix Composites," JOM, Vol. 55, No. 1, 2003, pp. 1-13, www.tms.org/pubs/journals/JOM/0301/Kim/Kim-0301.html.

Martin, Richard E., Andrew L. Gyekenyesi and Steven M. Shepard, "Interpreting the Results of Pulsed Thermography Data," Materials Evaluation, Vol. 61, 2003, pp. 611-616.

Mix, P.E., Introduction to Nondestructive Testing, New York, John Wiley & Sons, 1987.

Richerson, D.W., Modern Ceramic Engineering, second edition, New York, Marcel Dekker, 1992.

Schwartz, M.M., Composite Materials, Vol. 1, Upper Saddle River, New Jersey, Prentice-Hall, 1997.

 

* NASA Glenn Research Center, Optical Instrumentation and NDE Branch, 21000 Brookpark Rd., MS 6-1, Cleveland, OH 44135; (216) 433-6729; fax (216) 977-7150; e-mail <smaziz@grc.nasa.gov>.

 

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