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Appendix I Nondestructive Evaluation for Armor rastered full area or C scan3 to generate mapped images of Various nondestructive methods have historically been sample properties. Brennan et al.4 illustrate this technique’s used to rapidly locate and identify anomalous internal flaws within dense armor materials; these methods have included ability to determine how properties vary as a function of resonant ultrasound spectroscopy, high-frequency ultrasound distance from the sample edges. Elastic property maps serve C scans, infrared thermography, and microfocus x-ray com- as a visual representation of density variations throughout a puted tomography (XCT). Testing before the materials have material. been used in their particular applications can be further Ultrasound C scans of acoustic energy loss can map changes in sample composition.5 This nondestructive evalu- subdivided into tests on individual armor materials and tests on arrays of tiles or body armor plates assembled with other ation (NDE) technique is founded on an understanding of confining materials. how a material’s microstructure attenuates an acoustic wave Resonant ultrasound spectroscopy has recently been as the wave interacts with grains, inclusions, and porosity. shown to demonstrate excellent potential for rapid go/no-go This technique can identify anomalous defects as well as testing of armor materials.1 In this technique, a tile of armor more subtle compositional variations throughout a SiC tile. material is held at the corners and struck to create a set of Interpretation of maps of acoustic energy loss result in an vibrations at the tile’s harmonic frequencies. Each peak in understanding of how mean grain size and inclusion con- the spectrum is determined by the material’s geometry, elas- centration vary, aiding in an assessment of the material’s tic properties, and microstructure. Shifts in expected peak suitability for armor applications. Acoustic spectroscopy, positions can identify the presence of internal flaws such as the analysis of the frequency dependency of acoustic loss, cracks, anomalous inclusions, and large porosity. Spectra are can be used to estimate distributions of bulk inclusions and used to identify quickly whether the component is suitable mean grain size. for armor applications. Since a single spectrum is measured Although ultrasound C scans provide additional in- for the entire sample, determination of the location within the formation regarding sample homogeneity, this information material where flaws exist is not currently possible. comes at the price of increased testing time. Conventional High-frequency ultrasound has been successfully dem- ultrasound testing requires approximately 10 to 20 minutes onstrated for quickly evaluating armor material homogeneity to characterize a 4-in. × 4-in. tile. Through use of ultrasound and measuring properties of interest.2 Ultrasound testing phased arrays, however, the time requirement can be reduced can be performed at individual points to measure acoustic by an order of magnitude. Phased-array probes contain an energy loss, elastic properties, and surface roughness. These assembly of several dozen ultrasound transducers, allow- measurements can be extended over the entire material in a ing for digital beam steering, focusing, and rastering, all of which increase the rapidity of testing. Phased-array probes 3A C scan is a nondestructive technique that uses ultrasound to inspect 1Ashkin, D., R. Brennan, J. Campbell, S. Klann, R. Palicka, and R. materials. 4Brennan, R., R. Haber, D. Niesz, G. Sigel, and J. McCauley. 2009. Sisneros. Resonant ultrasound testing of hot pressed silicon carbide. Proc. 2010 International Conference and Exposition on Advanced Ceramics and Elastic property mapping using ultrasonic imaging. Advances in Ceramic Composites. Armor III: Ceramic Engineering and Science Proceedings 28(5): 213-222. 2Brennan, R. 2007. Ultrasonic Nondestructive Evaluation of Armor 5Portune, A., and R. Haber. 2010. Microstructural study of sintered SiC Ceramic. Ph.D. Dissertation, Publication Number AAI3319593. New via high frequency ultrasound spectroscopy. Pp. 159-170 in Advances in Brunswick, N.J.: Rutgers University. Ceramic Armor V. J. Swab, ed. Hoboken, N.J.: John Wiley & Sons. 148
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149 APPENDIX I damage in confined armor materials.9,10 The XCT reconstruc- can thus characterize specific material layers within armor assemblies. Whereas conventional ultrasound can effectively tion can be used as a damage diagnostic for understanding test materials before their inclusion in final pieces, phased- crack-propagation behavior and the extent of damage spread. array techniques can evaluate materials both before and XCT can be performed on an armor piece assembled from after assembly.6 Although phased-array instruments have multiple tiles and used to illustrate how this configuration advanced capabilities, they currently exhibit significant minimizes the spread of damage to surrounding areas. Ad- hardware limitations and increased costs. ditionally, since testing can be performed without changing XCT has proven to be a powerful tool for evaluating the sample state, it is possible to visualize residual projectile armor integrity and visualizing compositional variations in fragments. three dimensions. Layer, or X-ray slice, data are generated by Each NDE technique acquires different kinds of infor- an x-ray source rotating around an object; x-ray sensors are mation about the armor material. No single technique has placed on the other side of the circle from the x-ray source. been shown to be sufficient for full sample characterization. Testing is then repeated until the entire material has been XCT provides excellent visualizations of damage incurred by characterized. By assembling these layers with a computer, materials and can map large compositional variations, but it three-dimensional images are created. XCT is used to evalu- cannot provide the level of microstructural information pos- ate samples prior to assembly to map variations in sample sible through ultrasound spectroscopic analysis. Ultrasound density and to locate anomalous flaws or microcracks. C scan testing provides excellent maps of fine microstruc- One benefit of XCT is its capability for rapidly assessing tural variations in a material, but it requires more time than sample homogeneity in armor assemblies. Devices have been other techniques do and may be unsuitable for the rapid created that can quickly examine armor in the field prior to testing of full sample lots. Resonant ultrasound spectroscopy engagements.7 Inspection devices for use in the field can be provides rapid go/no-go tests, but it cannot identify where optimized toward a single expected part geometry, increasing flaws exist in a material, as only a single curve is measured the speed by which crucial parts of the armor composite can for the entire sample. A separation therefore exists between be identified and characterized. An example is a device to using NDE for studied characterization and using it for rapid characterize a small-arms protective insert plate and identify identification of a material’s suitability for use. an internal crack.8 Rapid characterization is necessary in Many challenges exist for the future development of the field because flaws in armor that were not present after NDE for armor. Ideally NDE would be employed in produc- production or assembly may be introduced during handling. tion lines for all armor materials. However, the assessment Nondestructive tests are also used to characterize dam- of individual components requires the standardization of age incurred by armor materials after destructive testing. test techniques and the integration of testing equipment. The NDE is an excellent tool for this purpose as it does not characterization of armor material microstructures through introduce further damage to the material or change the dam- NDE could be improved through the study of defined stan- age state that already exists. To date, XCT has proven most dards. The use of standard sample sets that could be used efficient at this task because it can provide three-dimensional across industry, in governmental institutions, and in research images of damage zones. facilities would benefit this process. It is clear that there is XCT has also been applied to the characterization of room for improvements: The characterization of damage and defects can still be made faster and more robust, as many defects beneath a critical size currently go undetected. Finally, any future improvements in test equipment and soft- ware need to decrease the time required to perform analyses, increasing the feasibility of the use of such analyses outside 6Steckenrider, S., W. Ellingson, E. Koehl, and T.J. Meitzler. 2010. dedicated laboratories. Inspecting composite ceramic armor using advanced signal processing together with phased array ultrasound. Advances in Ceramic Armor VI: 9Wells, J., and N. Rupert. 2009. Ballistic damage assessment of a thin Ceramic Engineering and Science Proceedings 31. J. Swab, S. Mathur, and compound curved B4C ceramic plate using XCT. Advances in Ceramic T. Ohji, eds. Hoboken, N.J.: John Wiley & Sons. Armor IV: Ceramic Engineering and Science Proceedings 29(6). L. Franks, 7Haynes, N., K. Masters, C. Perritt, D. Simmons, J. Zheng, and J. ed. Hoboken, N.J.: John Wiley & Sons. Youngberg. 2009. Automated non-destructive evaluation system for hard 10Wells, J., N. Rupert, and M. Neal. 2010. Impact damage analysis in armor protective inserts of body armor in Advances in Ceramic Armor IV: a Level III flexible body armor vest using XCT diagnostics. Advances in Ceramic Engineering and Science Proceedings 29(6). L. Franks, ed. Hobo - Ceramic Armor V. J. Swab, D. Singh, and J. Salem, eds. Hoboken, N.J.: ken, N.J.: John Wiley & Sons. John Wiley & Sons. 8Ibid.