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Opportunities in Protection Materials Science and Technology for Future Army Applications (2011)

Chapter: Appendix I: Nondestructive Evaluation for Armor

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Suggested Citation:"Appendix I: Nondestructive Evaluation for Armor." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
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Appendix I

Nondestructive Evaluation for Armor

Various nondestructive methods have historically been used to rapidly locate and identify anomalous internal flaws within dense armor materials; these methods have included resonant ultrasound spectroscopy, high-frequency ultrasound C scans, infrared thermography, and microfocus x-ray computed tomography (XCT). Testing before the materials have been used in their particular applications can be further subdivided into tests on individual armor materials and tests on arrays of tiles or body armor plates assembled with other confining materials.

Resonant ultrasound spectroscopy has recently been shown to demonstrate excellent potential for rapid go/no-go testing of armor materials.1 In this technique, a tile of armor material is held at the corners and struck to create a set of vibrations at the tile’s harmonic frequencies. Each peak in the spectrum is determined by the material’s geometry, elastic properties, and microstructure. Shifts in expected peak positions can identify the presence of internal flaws such as cracks, anomalous inclusions, and large porosity. Spectra are used to identify quickly whether the component is suitable for armor applications. Since a single spectrum is measured for the entire sample, determination of the location within the material where flaws exist is not currently possible.

High-frequency ultrasound has been successfully demonstrated for quickly evaluating armor material homogeneity and measuring properties of interest.2 Ultrasound testing can be performed at individual points to measure acoustic energy loss, elastic properties, and surface roughness. These measurements can be extended over the entire material in a rastered full area or C scan3 to generate mapped images of sample properties. Brennan et al.4 illustrate this technique’s ability to determine how properties vary as a function of distance from the sample edges. Elastic property maps serve as a visual representation of density variations throughout a material.

Ultrasound C scans of acoustic energy loss can map changes in sample composition.5This nondestructive evaluation (NDE) technique is founded on an understanding of how a material’s microstructure attenuates an acoustic wave as the wave interacts with grains, inclusions, and porosity. This technique can identify anomalous defects as well as more subtle compositional variations throughout a SiC tile. Interpretation of maps of acoustic energy loss result in an understanding of how mean grain size and inclusion concentration vary, aiding in an assessment of the material’s suitability for armor applications. Acoustic spectroscopy, the analysis of the frequency dependency of acoustic loss, can be used to estimate distributions of bulk inclusions and mean grain size.

Although ultrasound C scans provide additional information regarding sample homogeneity, this information comes at the price of increased testing time. Conventional ultrasound testing requires approximately 10 to 20 minutes to characterize a 4-in. × 4-in. tile. Through use of ultrasound phased arrays, however, the time requirement can be reduced by an order of magnitude. Phased-array probes contain an assembly of several dozen ultrasound transducers, allowing for digital beam steering, focusing, and rastering, all of which increase the rapidity of testing. Phased-array probes

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1Ashkin, D., R. Brennan, J. Campbell, S. Klann, R. Palicka, and R. Sisneros. Resonant ultrasound testing of hot pressed silicon carbide. Proc. 2010 International Conference and Exposition on Advanced Ceramics and Composites.

2Brennan, R. 2007. Ultrasonic Nondestructive Evaluation of Armor Ceramic. Ph.D. Dissertation, Publication Number AAI3319593. New Brunswick, N.J.: Rutgers University.

3A C scan is a nondestructive technique that uses ultrasound to inspect materials.

4Brennan, R., R. Haber, D. Niesz, G. Sigel, and J. McCauley. 2009. Elastic property mapping using ultrasonic imaging. Advances in Ceramic Armor III: Ceramic Engineering and Science Proceedings 28(5): 213-222.

5Portune, A., and R. Haber. 2010. Microstructural study of sintered SiC via high frequency ultrasound spectroscopy. Pp. 159-170 in Advances in Ceramic Armor V. J. Swab, ed. Hoboken, N.J.: John Wiley & Sons.

Suggested Citation:"Appendix I: Nondestructive Evaluation for Armor." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
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can thus characterize specific material layers within armor assemblies. Whereas conventional ultrasound can effectively test materials before their inclusion in final pieces, phased-array techniques can evaluate materials both before and after assembly.6 Although phased-array instruments have advanced capabilities, they currently exhibit significant hardware limitations and increased costs.

XCT has proven to be a powerful tool for evaluating armor integrity and visualizing compositional variations in three dimensions. Layer, or X-ray slice, data are generated by an x-ray source rotating around an object; x-ray sensors are placed on the other side of the circle from the x-ray source. Testing is then repeated until the entire material has been characterized. By assembling these layers with a computer, three-dimensional images are created. XCT is used to evaluate samples prior to assembly to map variations in sample density and to locate anomalous flaws or microcracks.

One benefit of XCT is its capability for rapidly assessing sample homogeneity in armor assemblies. Devices have been created that can quickly examine armor in the field prior to engagements.7 Inspection devices for use in the field can be optimized toward a single expected part geometry, increasing the speed by which crucial parts of the armor composite can be identified and characterized. An example is a device to characterize a small-arms protective insert plate and identify an internal crack.8Rapid characterization is necessary in the field because flaws in armor that were not present after production or assembly may be introduced during handling.

Nondestructive tests are also used to characterize damage incurred by armor materials after destructive testing. NDE is an excellent tool for this purpose as it does not introduce further damage to the material or change the damage state that already exists. To date, XCT has proven most efficient at this task because it can provide three-dimensional images of damage zones.

XCT has also been applied to the characterization of damage in confined armor materials.9,10 The XCT reconstruction can be used as a damage diagnostic for understanding crack-propagation behavior and the extent of damage spread. XCT can be performed on an armor piece assembled from multiple tiles and used to illustrate how this configuration minimizes the spread of damage to surrounding areas. Additionally, since testing can be performed without changing the sample state, it is possible to visualize residual projectile fragments.

Each NDE technique acquires different kinds of information about the armor material. No single technique has been shown to be sufficient for full sample characterization. XCT provides excellent visualizations of damage incurred by materials and can map large compositional variations, but it cannot provide the level of microstructural information possible through ultrasound spectroscopic analysis. Ultrasound C scan testing provides excellent maps of fine microstructural variations in a material, but it requires more time than other techniques do and may be unsuitable for the rapid testing of full sample lots. Resonant ultrasound spectroscopy provides rapid go/no-go tests, but it cannot identify where flaws exist in a material, as only a single curve is measured for the entire sample. A separation therefore exists between using NDE for studied characterization and using it for rapid identification of a material’s suitability for use.

Many challenges exist for the future development of NDE for armor. Ideally NDE would be employed in production lines for all armor materials. However, the assessment of individual components requires the standardization of test techniques and the integration of testing equipment. The characterization of armor material microstructures through NDE could be improved through the study of defined standards. The use of standard sample sets that could be used across industry, in governmental institutions, and in research facilities would benefit this process. It is clear that there is 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 software need to decrease the time required to perform analyses, increasing the feasibility of the use of such analyses outside dedicated laboratories.

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6Steckenrider, S., W. Ellingson, E. Koehl, and T.J. Meitzler. 2010. Inspecting composite ceramic armor using advanced signal processing together with phased array ultrasound. Advances in Ceramic Armor VI: Ceramic Engineering and Science Proceedings 31. J. Swab, S. Mathur, and T. Ohji, eds. Hoboken, N.J.: John Wiley & Sons.

7Haynes, N., K. Masters, C. Perritt, D. Simmons, J. Zheng, and J. Youngberg. 2009. Automated non-destructive evaluation system for hard armor protective inserts of body armor in Advances in Ceramic Armor IV: Ceramic Engineering and Science Proceedings 29(6). L. Franks, ed. Hoboken, N.J.: John Wiley & Sons.

8Ibid.

9Wells, J., and N. Rupert. 2009. Ballistic damage assessment of a thin compound curved B4C ceramic plate using XCT. Advances in Ceramic Armor IV: Ceramic Engineering and Science Proceedings 29(6). L. Franks, ed. Hoboken, N.J.: John Wiley & Sons.

10Wells, J., N. Rupert, and M. Neal. 2010. Impact damage analysis in a Level III flexible body armor vest using XCT diagnostics. Advances in Ceramic Armor V. J. Swab, D. Singh, and J. Salem, eds. Hoboken, N.J.: John Wiley & Sons.

Suggested Citation:"Appendix I: Nondestructive Evaluation for Armor." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×
Page 148
Suggested Citation:"Appendix I: Nondestructive Evaluation for Armor." National Research Council. 2011. Opportunities in Protection Materials Science and Technology for Future Army Applications. Washington, DC: The National Academies Press. doi: 10.17226/13157.
×
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Armor plays a significant role in the protection of warriors. During the course of history, the introduction of new materials and improvements in the materials already used to construct armor has led to better protection and a reduction in the weight of the armor. But even with such advances in materials, the weight of the armor required to manage threats of ever-increasing destructive capability presents a huge challenge.

Opportunities in Protection Materials Science and Technology for Future Army Applications explores the current theoretical and experimental understanding of the key issues surrounding protection materials, identifies the major challenges and technical gaps for developing the future generation of lightweight protection materials, and recommends a path forward for their development. It examines multiscale shockwave energy transfer mechanisms and experimental approaches for their characterization over short timescales, as well as multiscale modeling techniques to predict mechanisms for dissipating energy. The report also considers exemplary threats and design philosophy for the three key applications of armor systems: (1) personnel protection, including body armor and helmets, (2) vehicle armor, and (3) transparent armor.

Opportunities in Protection Materials Science and Technology for Future Army Applications recommends that the Department of Defense (DoD) establish a defense initiative for protection materials by design (PMD), with associated funding lines for basic and applied research. The PMD initiative should include a combination of computational, experimental, and materials testing, characterization, and processing research conducted by government, industry, and academia.

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