May 5, 2011

ATC experiment looks at how clay performs in body armor testing

Photo credit Air Force Master Sgt. Demetrius Lester
A critical mission for the Aberdeen Test Center is the testing of body armor such as that worn by U.S. Army Spc. Aaron Franklin in this photo. ATC designed a unique experiment to characterize a special clay used to determine the back-face deformation that occurs when a bullet strikes such protective equipment during testing.
ABERDEEN PROVING GROUND, Md. - To support the Army Product Manager, Soldier Protective Equipment, the Army Developmental Test Command's Aberdeen Test Center recently conducted an experiment to characterize the clay backing material used for testing body armor.

The Army rigorously tests both hard armor plates and soft armor vests and their components, as well as helmets. It conducts this testing to verify the adequacy of a system's design and conducts further testing to ensure that the quality of the system is maintained during production.
One of the ways to characterize how these systems perform is to determine to what extent an armor system will deform itself in response to an impact from a projectile while preventing the projectile from completely penetrating the system. The crater or indentation formed by the armor giving into the backing material is called "back-face deformation." Roma Plastilina No. 1 clay is the backing material used to characterize how much of this deformation takes place.
Even if a bullet doesn't completely penetrate body armor, it can cause enough back-face deformation, or BFD, to result in catastrophic injury, so the clay that helps testers determine how much of this deformation takes place is critically important to the test process.

For test purposes the clay is built into boxes that are 24 inches by 24 inches by 5.5 inches, and these clay boxes must pass a calibration test to be declared ready for testing. The drop calibration test, or "drop test," is the standard way the Army validates the consistency of the clay blocks.
The drop test involves dropping a 2.2-pound weight that is 1.75 inches in diameter from a height of 6.5 feet. The clay blocks are considered to have passed this test if three drop indentations into the clay measure between 22 and 28 millimeters.
According to a report on the experiment released by ATC, the first objective was to "characterize the relationship between drop depth and radial distance from the center of the clay box." The second objective of ATC's experiment was to "characterize how that relationship may change as the clay box ages," and the third objective was to "characterize how those relationships may be affected by the position of the clay box in the temperature conditioning oven."
To do this effectively and ensure drop uniformity, ATC developed a template specifying 11 drop locations at varying radial distances from the center of the box. Testers selected three pairs of boxes from ATC's inventory, each pair representing a separate "age" based on when the boxes had been packed.

They used a six-position temperature-condition oven between experiments. Two dedicated crews conducted the testing, each working 12-hour shifts so the test could run 24 hours a day from March 8-14, 2010.
Between experiments, ATC personnel conditioned the clay boxes in a 105-degree-Fahrenheit oven for six hours. They opened the doors just once an hour to remove a box for the experiment and to return the box they had just used for that purpose. They conducted the 11 drops at a controlled rate of one minute between drops.
When they had finished conducting all of the drop tests, they measured the indentations in each box with a digital caliper and recorded the results. They then repaired the clay boxes by filling the indentations with clay and after an hour returned them to the oven.
ATC personnel conducted 22 replications of the drop test per box, so that each of the 11 positions marked out with the template on each box could experience each drop order twice in each box. The order of drop locations on the boxes was random for each drop replication. The boxes were rotated through each of the six oven positions at least three times. In all, ATC made a total of 1,452 individual drops, based on 22 replications of the 11 drops on each of the six clay boxes.

To examine the results of the experiment, ATC used a statistical method called analysis of variance. In layman's terms, this is an analysis of the outcomes of an experiment where the contribution of each source of variation under study is compared to the total variation to assess which factors, if any, are contributing at a rate greater than one would expect by chance alone; that is, if any factors are having a significant effect on the experimental outcomes.
The results of the study revealed the following: The most significant factors affecting the depth of indentations were the differences between one clay box and another based on box serial number rather than age, the location on the clay box where the weight was dropped, and the interaction between individual clay box by serial number and drop location, respectively. By contrast, oven position had no practical effect on the results at any drop location. And because the date the boxes of clay were packed showed no correlation with test results, it was learned that a better definition of clay age is needed.
ATC's analysis of the experiment further revealed that four of the six clay boxes showed no correlation between the drop depth and its radial distance while two showed a "moderate negative linear correlation," meaning there was some evidence that a relationship exists between the two variables and the shape of that relationship is described by a line. A negative linear relationship means that as one variable increases, the other decreases. In this case, as radial distance increased away from the center of the box, the drop depths for those two clay boxes tended to decrease in depth.
A most interesting finding was that the depth of indentation at the center of all the blocks remained consistently near 25 millimeters, despite the mixed results when dropping the weight at other locations.
Additionally, while there seemed to be a "marked difference" in the drop-test results between new and used boxes at the beginning of the experiment, by its conclusion all boxes experienced similar variation of results, independent of box age. That is, as more drops were performed, the clay became softer, judging from indentation depth, and testers observed less variation.
ATC's Barbara Gillich, who helped design and analyze the experiment, said it was a very worthwhile project.
The experiment was seen as an "excellent" effort between ATC and the Product Manager, Soldier Protective Equipment, designed to increase the knowledge of Roma Plastilina No. 1 clay for the entire body armor community. Although ATC was able to answer the three specific objectives of this experiment, quite a bit of work remains to be done on the subject of clay and its use in body armor testing.
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