Testing of Body Armor Materials Phase III (2012) / Chapter Skim
Currently Skimming:
4 Clay and Backing Materials
4 Clay and Backing Materials
Pages 46-91
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 46... ...
It concludes with a road map for the body armor testing community to achieve reductions in the variability of clay as backing material for testing processes. USE OF BACKING MATERIAL AS A RECORDING MEDIUM As introduced in Chapters 2 and 3, the RP #1 modeling clay backing material used in armor testing has two important purposes.
|
From page 47... ...
The consequence of this is that the material deforms reversibly prior to the onset of yielding and will exhibit elastic recovery when the load is removed. In the context of armor testing, this means that the cavity that remains in the backing material after the armor system has been struck by the projectile will be smaller than the maximum BFD.
|
From page 48... ...
Given the high elastic modulus for the steel wires relative to the peak stress during sliding, there is effectively no elastic recovery when the tool and the surface separate. As discussed in the text, the backing material used in ballistic testing of armor is meant to serve an analogous role in that it should deform as the back face of the armor system moves and capture a permanent record of this transient event.
|
From page 49... ...
It stated that RP #1 was "a highly plastic material which undergoes viscous flow when deformed and exhibits little recovery, thus providing a readily available cavity formed during impact from which measurements can be taken." The qualitative assertion that RP #1 exhibits little recovery has been interpreted to mean that the level of elastic recovery is small enough to be safely neglected. This led to the assumption that the shape of the cavity is a record to the BFD.
|
From page 50... ...
13 Don Dunn, H.P. White Laboratory, Inc., "Commercial Body Armor Testing Perspectives," presentation to the committee, August 9, 2010.
|
From page 51... ...
is a valid meth od for assessing the part-to-part consistency of clay boxes used in body armor testing" (NRC, 2009)
|
From page 52... ...
Several variants of the drop test are currently employed. At ATC, the drop test consists of dropping a cylindrical steel mass with a hemispherical cap (44.5 mm in diameter)
|
From page 53... ...
SOURCE: ATC, 2008. Nonetheless, the column-drop test is what is used to determine if the clay box is what is termed "within calibration" and therefore can be used to test the hard armor plates.
|
From page 54... ...
A clay box was thermally equilibrated at 40°C (104°F) and subjected to serial drop tests over time as the clay box was allowed to cool, approaching room temperature.
|
From page 55... ...
SOURCE: Scott Walton and Shane Esola, Aberdeen Test Center, "ATC Perspective on Clay used for Body Armor Testing," presentation to the committee, March 10, 2010.
|
From page 56... ...
SOURCE: Scott Walton and Shane Esola, Aberdeen Test Center, "ATC Perspective on Clay used for Body Armor Testing," presentation to the committee, March 10, 2010.
|
From page 57... ...
PREPUBLICATION DRAFT -- SUBJECT TO EDITORIAL CORRECTION FIGURE 4-6 Spatial pattern used in a series of experiments to determine the effect of position on the size of the cavity produced during a drop test. The positions of the drops are given by the circled numbers (all dimensions are in inches)
|
From page 58... ...
represent the average and standard deviation for drops in different serial-numbered boxes. The ordering of the data is arbitrary -- it is not intended to show a trend but rather the magnitude of the difference that can be expected from one clay box to another (a difference of 2 mm could be associated with a box switch)
|
From page 59... ...
The calibration drop tests were performed to NIJ standards, which employ a sphere similar to that illustrated in the inset of Figure 4-5. Like the independent tests at the ATC, Roberson et al.
|
From page 60... ...
. Figure 4-8 shows the configuration that was tested and the indentations from calibration drop tests and sting ball experiments.
|
From page 61... ...
PREPUBLICATION DRAFT -- SUBJECT TO EDITORIAL CORRECTION FIGURE 4-8 A clay box used for .32-cal rubber sting ball testing. The large diameter impressions represent NIJ standard calibration drops.
|
From page 62... ...
Taken as a whole, the work discussed above indicates that when considering the methodology, preparing and working the modeling clay, or interpreting the results of ballistic tests, the following points must be taken into account: As a recording medium RP #1 is inherently highly variable, giving noisy results; The response of the oil-based modeling clay RP #1 is dependent on both shear history (or working) and thermal history; There is a paucity of data at strain rates approaching those experienced in real armor test conditions; and -62
|
From page 63... ...
Mechanical properties can be related to the structure, allowing a meaningful prediction of strain rate, shear history, and thermal history. Furthermore, properties such as thermal conductivity, diffusivity, and thermal expansion all can be deduced or modeled using appropriate mixing rules once the phase distribution has been well determined.
|
From page 64... ...
. However, many complex fluids exhibit thixotropic behavior -- or, more generally, rheology that is dependent on thermal and shear history.
|
From page 65... ...
In this schematic the dispersed phase is a nonspherical (platy) particle surrounded by a liquid.
|
From page 66... ...
The material no longer has a measurable yield stress. Particle diameter, 1.3 μm.
|
From page 67... ...
To begin with it is useful to consider the behavior of a mixture of microcrystalline wax, grease, and oil and in this context to make use of the structure-property discussion about the rheology of a model ointment that is a mixture of petrolatum, mineral oil, and microcrystalline wax (Pena et al., 1994)
|
From page 68... ...
The rheological properties are dependent, of course, on the relative fractions of crystalline solid and liquid elements of the microstructure. Of particular note in the context of the observed shear history dependence of modeling clay, a model all-organic ointment exhibits rheology that is dependent on both thermal history and shear history.
|
From page 69... ...
Furthermore, the principles of design are sufficiently clear that an alternative system with more favorable properties can be expected to be successfully developed. SHORT-TERM DEVELOPMENT OF AN INTERIM STANDARD CLAY FORMULATION FOR BALLISTIC TESTING The Army's protocol for ballistic testing of soft and hard body armor specifies RP #1 as the backing material (DoD, 2008)
|
From page 70... ...
For example, as previously noted, RP #1 contains sulfur, which has an unknown effect on its performance in ballistic testing. Minimizing the number of ingredients should reduce variability in performance over time and simplify attempts to characterize and model performance.
|
From page 71... ...
Recommendation 4-1: The Office of the Director, Operational Test and Evaluation, and the Army should continue to expedite the development of a replacement for the current Roma Plastilina #1 oil-based modeling clay that can be used at room temperature. Conditioning and Handling of Clay The conditioning and handling procedures associated with the use of modeling clay in live-fire ballistic testing of body armor can be reviewed in light of the above information.
|
From page 72... ...
The importance of such an effort is manifest in a statement by an ATC Protective Equipment Division science officer, who stated that in his opinion perhaps some of the most significant improvements to reduce variation in the testing process could come from the hand processing that goes into filling the clay boxes and working the clay before and after test firing.22 In-box mechanical conditioning alternatives were recommended in the Phase II report and offer the possibility of generating a mass that responds more predictably and uniformly (NRC, 2010)
|
From page 73... ...
Finding: In the short term, testing will continue to be conducted using the existing Roma Plastilina #1. As long as heating the clay is necessary, cooling will take place, and the post-test calibration drop test, as recommended in the Phase II Report (NRC, 2010)
|
From page 74... ...
As such, the column-drop performance test (including the testing protocols, facilities, and instrumentation) is a valid method for assessing the part-to-part consistency of the clay boxes used in body armor testing.
|
From page 75... ...
An additional appealing possibility is using small-diameter spheres because this allows a high-density matrix of small 23 Scott Walton, and Shane Esola, Aberdeen Test Center, "ATC Perspective on Clay used for Body Armor Testing," presentation to the Body Armor Phase II committee, March 10, 2010.
|
From page 76... ...
ALTERNATIVE BACKING MATERIALS AND SYSTEMS As stated earlier, the committee strongly supports DOT&E and the Army in developing a standard ballistic backing material that would be an improvement on the current RP #1 for body armor testing. The Phase II study was interested in learning more about possible mid- and long-term replacements for modeling clay.
|
From page 77... ...
that the Army should consider experimenting with ballistic gelatin and/or microcrystalline wax as a mediumterm solution to the issues associated with the current recording medium, RP #1. However, the information detailed above, coupled with an improved understanding of the microstructure-property relationships in oil-based modelingclay, calls for a general shift in emphasis.
|
From page 78... ...
Ballistic-Grade Clay In view of the finding above, attention must be directed at developing an improved ballistic-grade clay or an alternative plastic material to serve as a recording medium. As stated earlier the committee strongl y supports DOT&E and the Army as they work to improve the current RP #1 modeling clay as a backing material to test body armor.
|
From page 79... ...
ROADMAP FOR IMPROVING THE TESTING PROCESS The Army's protocol for ballistic testing of soft and hard body armor specifies RP #1 as the backing material (DoD, 2008)
|
From page 80... ...
. The purpose of this section is to propose a road map to guide development of a more consistent backing medium for ballistic testing of hard armor plates.
|
From page 81... ...
Exploration of in-box conditioning methods; and 4. Study of improved calibration procedures.
|
From page 82... ...
These results have direct value in interpreting the results of ballistic tests and the correlation to the calibration drop tests. These are the properties required for meaningful simulations that might be intended to relate indirect measurements to the mechanics of body armor deformation.
|
From page 83... ...
Conditioning Clay conditioning includes both thermal history and shear history. As discussed earlier in this chapter, a clay box that has been heated to 40ºC cools significantly during the time associated with ballistic testing and causes "drift," in the engineering sense of the word, in the results.
|
From page 84... ...
In particular, the committee suggests that the feasibility be assessed of using shaped impactors designed to reproduce the force distributions expected when a blunt trauma occurs as a projectile strikes hard armor. This option is particularly appealing as work progresses to measure the force distribution associated with armor testing (Raftenberg, 2006)
|
From page 85... ...
Medium-Term Research Needs Completion of the near-term actions will have several outcomes. First, the data gathered from these studies will enable selection of a short-term replacement for RP #1 as a backing material for ballistic testing of hard armor panels.
|
From page 86... ...
Several transparent elastically deforming media are available in addition to ballistic gelatin, including other polymer systems (Uzar et al., 2003; Juliano et al., 2006; Moy et al., 2006) , and the committee concludes that such materials may become of interest under three conditions.
|
From page 87... ...
Combining information attained by studying gas gun and other alternative calibration methods with a more consistent conditioning process (including the elimination of any conditioning steps) will enable development of a more robust calibration procedure that can be more readily and reliably extrapolated to ballistic testing conditions.
|
From page 88... ...
10-2-210 Ballistic Testing of Hard Body Armor Using Clay Backing. Aberdeen Proving Ground, Md.: Aberdeen Test Center.
|
From page 89... ...
2010. Clay Calibration: Radial Dependence of Calibration Drop Depths for Body Armor Testing.
|
From page 90... ...
2006. Modeling thoracic blunt trauma: Towards a finite-element based design methodology for body armor.
|
From page 91... ...
Aberdeen Proving Ground, Md.: U.S. Army Edgewood Chemical and Biological Center.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.