Testing of Body Armor Materials Phase III (2012) / Chapter Skim
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5 Instrumentation and Procedures for Measuring an Indent in a Clay Backing Material
5 Instrumentation and Procedures for Measuring an Indent in a Clay Backing Material
Pages 92-106
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From page 92... ...
Accordingly, this chapter discusses relevant criteria for test instrumentation and procedures, including fixed and variable costs, precision and accuracy, and human operator considerations. CONCEPTUAL STEPS TOWARD IMPROVEMENTS IN THE MEASUREMENT OF BFD It is informative to review past events to learn how the instrumentation and measurements of BFD relate to the overall methodology of the original animal tests, clay selection, selection of performance specifications, and instrumentation measurements.
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From page 93... ...
Army Aberdeen Test Center (ATC) has set the maximum acceptable BFD value at 44 mm for body armor plates tested using clay.
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From page 94... ...
INSTRUMENTATION PERFORMANCE BASED ON STATISTICAL ANALYSIS When discussing body armor testing and, particularly, the equipment required in the conduct of adequate tests, the question arises: How well must the BFD be measured? Put another way, what are the limits of acceptable error in BFD measurement?
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From page 95... ...
is necessary to ensure that the measurement device does not add any appreciable variation to the body armor testing process. This value is consistent with subject matter experts who expressed to the committee their intuition that measurement precision on the order of 0.5 mm is sufficient for the current testing process.
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From page 96... ...
In particular, three instruments have been used: the Coordinate Measuring Machine (CMM) , used as a reference instrument; the digital caliper; and a Faro laser scanning probe system.
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From page 97... ...
has designated the Faro Quantum Laser Scan Arm and Geomagic Qualify software for hard and soft body armor (referred to as "the Faro") as the device to measure BFDs (DOT&E, 2010)
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From page 98... ...
In addition, a laser scanning system has the ability to acquire substantial quantities of inspection data. Figure 5-2 shows the Faro Quantum Laser Scan Arm.
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From page 99... ...
Human Operator Considerations A number of practical human operator considerations have an impact on the measurement differences and variations associated with all measuring systems. These include subjective differences in human handling, process transparency, and the selection and settings of software.
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From page 100... ...
, which will result in the largest BFD measurement to ultimately protect soldiers.29 Manufacturers, on the other hand, feel that their armor may be unfairly penalized due to judgment decisions that depend on the smoothing setting the operator chooses.30 In statistical and testing terms, the choice of the smoothing setting directly affects the accuracy of the Faro. That is, the choice of smoothing settings can introduce a systematic bias into the measurements, a bias that can make the test either harder or easier to pass depending on whether the bias results in systematically larger or smaller BFDs.
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From page 101... ...
in the Overall Testing Process As described in Chapter 4, there is significant variability in the RP #1 modeling clay that has been used for decades as the backing material in the testing process. The RP #1 modeling clay was and continues to be designed for artists and not for the ballistics testing community.
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From page 102... ...
In other words, 6 mm of overall clay variability is accepted as, and perhaps understates, the noise in the testing process related to clay. A great deal of variation is introduced into the measurement system by RP#1.
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From page 103... ...
In the development of methods to measure BFDs, virtually no interlaboratory testing has been carried out to date to determine sources of interlaboratory errors as a consequence of test procedure differences or differences in the setup of the test equipment or of differences in the operation of equipment. Interlaboratory errors are often systematic, resulting in a constant statistical difference in BFD measurement from one measurement laboratory to the next.
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From page 104... ...
The di fferences between armor plate testing and helmet testing are considerable, and all operators interviewed agreed that a laser-based measuring tool was generally preferred for helmet testing due to the complex curves of the head form, on which the helmet BFD measurements must be made. 34 Committee site visit to the ATC, August 31, 2010; Rob Kinzler, Army Research Laboratory, "Improvements in Helmet Measurement," presentation, to the committee, October 13, 2010.
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From page 105... ...
The MicroScribe system could be used for testing both body armor plate and helmet BFD measurements (although a finer grid pattern is needed for helmet testing) and could significantly reduce the offset error currently seen with the caliper, which uses one preshot measurement.
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From page 106... ...
The list of best utility instruments should be shared with NIJ, international allies, and others, as appropriate, to promote measuring instrument standardization for body armor testing nationally and internationally. A formal gauge repeatability and reproducibility study is required to quantify accuracy and precision as inputs to the best utility analysis.
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