NONDESTRUCTIVE EVALUATION DEVELOPMENT

The purpose of this task Is to conduct nondestructive evaluation (NDE) developmer it directed at identifying approaches for quantitative determination of conditions (including both properties and flaws) In ceramics that affect the structural performance. Those materials that have been seriously considered for appllca.Ion In advanced heat engines are all brittle materials with fracture affected by structural features with dimensions on the order of the dimensions of the microstructure. This work seeks to characterize those features using high-frequency ultrasonics and radiography to detect, size, and locate critical flaws and to measure nondestructively the elastic properties of the host material, 

The 76-MHz flaw tests were conducted with the transducer focused about 3 mm deep in the sample, which is the maximum focal depth attainable with this unit. Using this configuration, no indications were detected within the total depth coverage of the transducer ( 1 mm). All tiles were inspected from both sides. These results are consistent with those obtained on most of the earlier tiles and are characteristic of hot-pressed material. 

A second condition, one perhaps more inimicable to the ultimate use of the tiles, was detected during the surface wave examination. All of the samples except No. A contained indications suggestive of large pores on one or both surfaces. These indications were coincident with visual white spots on the tile. A microscopic examination of these spots at 30X revealed no pores but showed what appeared to be small clusters of clear crystals approximately 1 mm In diameter. We have previously seen such crystals, which analysis proved to be silicon nitride, in a whisker-reinforced material. One of the tiles, 89-289-A, contained perhaps a score of these clusters within a 25-mm-diameter circle. For all of the affected tiles, the indications lay within a ring of visually lighter colored material near the center of the respective face. 

We have also received and ultrasonically examined 21 silicon nitride tensile specimens. These samples are cylindrical and have a diameter of 6.4 mm in the gage section, which was the only region inspected. The evaluation was conducted using a 75-MHz. spherically focused transducer. As we have mentioned in previous reports, the sharply curved entry surface of small-diameter cylindrical specimens introduces both severe astigmatism and spherical aberration in the ultrasonic beam distribution inside the solid and limits the depth at which critical flaw sizes can be detected to a few hundred microns. Our work has indicated that this limitation could be greatly reduced by aspherical shaping of the transducer lens, but the cost of such a transducer has thus far proved prohibitive. No indications were observed in any of the specimens although we could not perform a 100% inspection on any of the rods because of equipment limitations. 

A 50-MHz surface wave examination was also conducted on each sample. This evaluorion is sensitive to voids and cracks lying within about one wavelength (-120 im) of the surface and is not affected by surface curvature because of the small area of interrogation of the entry beam. As before, however, full coverage of the surface area of each specimen was not possible No indications were detected on any of the rods. 

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What is novel is the manner in which they are tied together. In IP, new nondestructive evaluation sensors are used to monitor the development of a materials microstructure as it evolves during production in real time. These sensors can indicate whether the microstructure is developing properly. Poor microstructure will lead to defects in materials. In essence, the sensors are inspecting the material on-line before the product is produced. 

Nondestructive evaluation (NDE) of composites will be an important area of future development. NDE refers to the use of inspection techniques which can characterize a material or component without damaging it during the inspection process. In general, the types of testing described above are... 

With the recent collapse of two posttensioned bridges in the United Kingdom and one in Belgium the impact-echo nondestructive evaluation (NDE) technique was developed to detect voids in posttensioned ducts. In addition, a complementary magnetic-based nondestmctive technique (NDT) for assessing section loss in... 

ANTHONY G. EVANS is the Gordon McKay Professor of Materials Engineering at Harvard University. His research interests include the mechanical properties of brittle materials particularly the fracture of ceramics under conditions of impact thermal and mechanical stress and failure prediction based on nondestructive evaluation. He is a recipient of the American Ceramic Society s Ross ColEn Purdy Award and has authored and co-authored several publications. Dr. Evans is a member of the National Materials Advisory Board and has served on several National Research Council committees. Dr. Evans was elected to the National Academy of Engineering in 1997 for his contributions to the development and understanding of structural materials. 

The nondestructive evaluation of CFRP is recognized to be very important and indispensable, because these polymeric composites are utilized for many industrial applications such as planes, spaceships and various load-bearing parts due to their toughness and lightness. Although several methods including X-ray and ultrasonic techniques have already been proposed for the evaluation of defects in CFRP, no adequate method exists, and it is still a difficult task to distinguish between defects and carbon fibers in CFRP. Therefore, the development of new effective methods was necessary. 

Because Si,N ceramics are partially translucent to light and the strength limiting flaws are normally within a shallow depth under fte sur ce, optical methods are effective to detect and characterize these types of subsurface flaws. Argonne National Laboratory (ANL) has developed and utilized several optical methods for nondestructive evaluation (NDE) of subsurface flaws in silicon nitride ceramics. In this study, three optical methods are presented and evaluated (1) laser backscatter, (2) optical coherence tomography (OCT) and, (3) confocal microscopy. The la.ser backscatter is a two-dimensional method while both (XT and confocal are three-dimensional methods. It is demonstrated in the following that subsurface flaws of various types, sizes, and depths can be identified and imaged by these NDE methods. 

Most techniques for the nondestructive evaluation of ceramic materials fall into two categories high-energy penetrating radiation (for example. X-ray) and high-frequency elastic waves (ultrasonics). Some NDE techniques have been developed for specialized applications, such as optical birefringence for transparent specimens and shearography for laminar composites. 

Nondestructive evaluation of ceramics is currently useful for process development and improvement, failure analysis, and detection of gross defects. At this time, there is no NDE technique to find critical volume flaws quickly in arbitrarily shaped technical ceramics. 

Kersey A D, Davis M A and Bellemore D G, Development of fiber sensors for structural monitoring . In Proc. SPIE - Nondestructive Evaluation of Aging Bridges and Highways, 1995, 2456, 262-8. 

Inspection techniques for the detection and measurement of corrosion range from simple visual examination to nondestructive evaluation. Significant technological advances have been made in the last decade. For example, the combined use of acoustic emission (AE) and ultrasonics (UT) can, in principle, allow an entire structure to be inspected and growing defects to be quantified in terms of length and depth. Advanced corrosion monitoring methods have been developed that have both online capabihty and the abihty to detect problems at an early stage. The oil and gas production and petrochemical industries have assumed a... 

Recent advances in nondestructive evaluation (NDE) technologies have led to improved methods for quality control and in-service inspection, and the development of new options for material diagnostics. 

The final example of new instruments for lacquer analysis uses IR spectrophotometry. Kenjo has pointed out how useful IR spectra can be when used for provenance studies (39,40). M. J. D. Low, of New York University, has recently developed an IR instrument that is nondestructive no sample taking or preparation is required (58,59). Use of this instrument will prove to be extremely important because of the value of some lacquer ware and the justifiable reluctance of curators to allow samples to be taken. We are currently evaluating this new instrument s performance with lacquer. 

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