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.
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.
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.