4

Clay and Backing Materials

This chapter discusses the role of the backing material as a recording medium, the properties and use of Roma Plastilina #1 (RP #1) modeling clay in body armor testing, and potential alternative backing materials and systems. 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. The first is “to simulate [some aspects of] the tissue response appropriately beneath the point of impact so that … ballistic data generated in laboratory tests can be correlated to the effects seen on the human body” (Prather et al., 1977, p. 7). The second purpose of the backing material is to mark the extent of backface deformation (BFD) during ballistic testing. Multiple materials are available to simulate a body; in fact, at the time it was introduced, modeling clay was recognized to only approximate tissue response, and empirical correlations were needed to develop a probability for lethality or injury. The chief advantage of modeling clay over other materials available at the time was that it better served the function of recording the BFD, because when impacted, it deforms plastically and a permanent cavity (also termed indent, impression, or crater) is developed under the point of impact. Correlations were developed between the geometry of the cavity and the probability of lethal injury. These results, however, do not predict a strain-rate dependence for the mechanical response of RP #1 and therefore increase the committee’s sense that obtaining direct measurement of the mechanical response of RP #1 in the strain-rate regime, corresponding to the development of the cavity in live-fire testing, should be a high-priority task.

The role of a backing material such as RP #1 is to serve as a recording medium. That is, the backing material must exhibit plastic deformation. Ideal plasticity, illustrated in Figure 4-1a, exhibits no deformation until a critical stress is exceeded, at which point it deforms irreversibly (Fung and Tong, 2001). Thus, a backing made of such a material would serve as a “contour gauge” that would perfectly preserve the locus of points that corresponds to the maximum BFD.



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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION 4 Clay and Backing Materials This chapter discusses the role of the backing material as a recording medium, the properties and use of Roma Plastilina #1 (RP #1) modeling clay in body armor testing, and potential alternative backing materials and systems. 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. The first is “to simulate [some aspects of] the tissue response appropriately beneath the point of impact so that . . . ballistic data generated in laboratory tests can be correlated to the effects seen on the human body” (Prather et al., 1977, p. 7). The second purpose of the backing material is to mark the extent of backface deformation (BFD) during ballistic testing. Multiple materials are available to simulate a body; in fact, at the time it was introduced, modeling clay was recognized to only approximate tissue response, and empirical correlations were needed to develop a probability for lethality or injury. The chief advantage of modeling clay over other materials available at the time was that it better served the function of recording the BFD, because when impacted, it deforms plastically and a permanent cavity (also termed indent, impression, or crater) is developed under the point of impact. Correlations were developed between the geometry of the cavity and the probability of lethal injury. These results, however, do not predict a strain-rate dependence for the mechanical response of RP #1 and therefore increase the committee’s sense that obtaining direct measurement of the mechanical response of RP #1 in the strain-rate regime, corresponding to the development of the cavity in live-fire testing, should be a high-priority task. The role of a backing material such as RP #1 is to serve as a recording medium. That is, the backing material must exhibit plastic deformation. Ideal plasticity, illustrated in Figure 4-1a, exhibits no deformation until a critical stress is exceeded, at which point it deforms irreversibly (Fung and Tong, 2001). Thus, a backing made of such a material would serve as a “contour gauge” that would perfectly preserve the locus of points that corresponds to the maximum BFD. -46-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION (b) Linear elastic—ideal plastic material FIGURE 4-1 A schematic illustration of the stress-strain curves for two idealized solids. The material corresponding to (a) exhibits ideal plasticity, in which there is no deformation until a critical stress (the yield point) is exceeded, at which point the material continues to deform at a constant rate until the stress is removed. The instant the stress falls below the critical value, such a material will stop deforming—that is, it exhibits no recovery. In contrast, linear elastic–ideal plastic material deforms elastically as the stress is applied before the plastic yield point. As before, the material deforms irreversibly when the yield point is exceeded. But in this case, upon removal of the stress, the elastic portion of the deformation is recovered as illustrated in (b). Real materials always exhibit some degree of elastic recovery. SOURCE: Fung and Tong, 2001, Copyright 2001, World Scientific Publishing Co. A contour gauge is a device familiar to craftsmen. It consists of a linear array of steel pins held parallel by a light clamping force. A typical device is illustrated in Figure 4-2. The pins are held in place with friction and therefore do not move until the application of stress. The relative motion in this case is caused by moving onto a shaped surface, but the principal is the same as in the armor test. In the latter case, the relative motion is the same, but it is the back face of the armor that moves into the backing material. If the backing material exhibited ideal plasticity, the resultant cavity would be a record of the maximum deflection of the BFD of the armor system, but this is manifestly not the case. As illustrated in Figure 4-1b, the deformation of real materials differs in important ways from ideal plasticity. The first distinction is that all real materials have a finite elastic modulus. 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. -47-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION FIGURE 4-2 A contour gauge in use. The parallel metal wires slide under the force that results from pressing the tool onto (or into) a shaped surface. The wires closely approximate plastic behavior in that they do not move until the applied force exceeds the frictional force produced by the clamping force. 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. SOURCE: Micromark, photo of a 5 in. metal contour gauge, found at: http://www.micromark.com/5-Inch-Metal-Contour-Gauge,9335.html. In some materials elastic recovery is so large that they do not store any memory of the event. Prather et al. (1977) noted that ballistic gelatin, for example, is a highly elastic material and exhibits nearly total recovery. Constraining his choices to low-cost readily available materials, Prather et al. identified an oil-based modeling clay, RP #1, as a material that exhibited sufficient plasticity to evidence post-test cavities with geometries that correlated to lethality probabilities (Prather et al., 1977). It must be noted that in a presentation to the committee, Mr. Prather indicated that the study results should be considered provisional (i.e., not final or fully worked out or agreed upon at the -48-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION time). He also noted that RP #1 was “convenient,” and this attribute seems to have dominated as it rapidly became widely used.12 As time passed and a wide range of investigators used RP #1, two sources of confusion emerged. First, many assumed that it was a simulant when it was not. Second, the Prather report’s description of RP #1 has been misunderstood. 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. It is not. As early as 1974, measurements of elastic springback were made using a modified Charpy impact tester (Aerospace Corporation, 1974). (A Charpy tester consists of a pendulum fitted with a weighted hammer that is allowed to swing into the sample material from a prescribed height, i.e., a given potential energy.) The difference in distance between the maximum point of the penetrator during its swing was compared to the size of the cavity in the RP #1, with this difference being the measure of displacement of the modeling clay during unloading. The results indicate that elastic recovery is in excess of 40 per cent and in some cases more than 70 per cent. That is, the differences are very large. Results from the Aerospace Corporation final report are shown in Table 4- 1. TABLE 4-1 Elastic Recovery in Modified Charpy Testing of Oil-Based Modeling Clay # of Expected* Plies of Max. Depth of Depth of Difference Difference Elastic Kevlar- Peak Indentor Cavity (apparent (apparent Recovery 29 Load, N (mm) (mm) recovery) (mm) recovery) (mm) 3 4671 37.34 18.54 18.8 50% 12.45 3 5449 39.37 10.16 29.21 74% 18.8 5 10453 41.66 24.13 17.53 42% 20.07 5 10787 47.24 24.38 22.86 48% 20.83 *Calculated by Aerospace using "punch formula" de=[(1−u)Pia]/G. SOURCE: Committee-generated, derived from data in Table II, p. A38 (Aerospace, 1974). 12 Russell Prather, Survice Engineering Co., “Prather Study Results” presentation to the committee on August 11, 2010. -49-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION Results of drop tests conducted by H.P. White Laboratory, Inc., also were consistent with significant elastic recovery.13 Furthermore, low-rate indentation experiments on plasticine, which is the same class of material as RP #1, indicate that recovery would be expected at high rates (Huang et al., 2002). Thus, the cavity in the RP #1 is not a record of the BFD. It is, as originally stated by Prather, “a readily available cavity formed during impact from which measurements can be taken” and to which correlations can be made (Prather et al., 1977). This is a critical point to recognize when considering either a replacement or the potential for improving the backing material performance by adjusting the formulation to produce a “ballistic grade.” Another very important point is that the relative degree of elastic and plastic deformation will be expected to vary as a function of strain rate. That is, the material must be characterized under conditions that are relevant to those under which tests will be performed. To the knowledge of the committee this has never been done. Although the properties of RP #1 have not been reported as a function of the strain rate, those of other candidate backing materials have been. For instance, the compressive properties of 20 per cent ballistic gelatin measured at 10ºC using a modified split Hopkinson bar as a function of strain rate over a range comparable to the range of interest (hundreds to thousands of reciprocal seconds) (Salisbury and Cronin, 2009). The compliance is observed to change by a substantial amount, with the gel perhaps 10 times stiffer at the high strain rate. Also showing the dependence on strain rate is a study that compared ballistic gelatin with physically associated styrene-isoprene triblock copolymer gels (Juliano et al., 2006). In sum, RP #1 was selected as a material of convenience rather than on the basis of well-determined engineering properties. It serves as a recording medium rather than a body simulant. The cavity that results from live-fire ballistic testing is related to the BFD of the armor, but it is not a true record of the maximum deflection. It remains unknown, therefore, how the dimensions of the cavity relate to the true BFD (and how such a relationship depends on the rate at which the cavity is formed). 13 Don Dunn, H.P. White Laboratory, Inc., “Commercial Body Armor Testing Perspectives,” presentation to the committee, August 9, 2010. -50-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION CHARACTERISTICS AND PROPERTIES OF RP #1 Behavior in Testing Column-Drop Test As standards have evolved, column-drop tests have been introduced to ensure that the modeling clay used for each test has well-defined behavior. The drop test consists of dropping a cylindrical steel mass with a hemispherical cap (44.5 mm in diameter) of defined mass (1 kg) from a height of 2 m. The mass is then removed, and penetration is quantified by measuring the distance between the original flat clay surface and the deepest point in the indent. As the deepest point lies on a highly regular hemisphere, it can be readily and reliably located by an operator using a digital caliper. The Phase I committee letter report (NRC, 2009) found that a digital caliper is adequate for this measurement because of the well-defined planar reference, the smooth and shallow indentation, and the ease of locating the center of the indentation.14, 15 To assess the appropriate methodology for measuring the dimensions of deformed RP #1, it is useful to review the general characteristics of prior observations of its deformation, and this is best done by reviewing the results of so-called column-drop tests. The introduction of the column drop test is another consequence of widespread adoption of Prather’s originally preliminary recommendation. “Roma Plastilina #1” is a trade name and as such does not embody a set of technical specifications. This was not an issue at the time as the recommendation was not expected to become a standard. However, it has been confirmed that the formulation of RP #1 has evolved over time. In part the evolution was in response to the primary customer base for clay (artists) making performance requests and in part it was due to the shifting availability of raw materials from different suppliers. While this may be commonplace for commercial products, it has had profound effects on the use of RP #1 as a backing material for live-fire testing of ceramic body armor. To quote Aberdeen Test Center (ATC) personnel, “The mechanical properties of Roma Plastilina #1 are dramatically different from the clay that was used in 1977.”16 14 Finding 3 of the Phase 1 letter report stated that “the digi tal caliper is adequate for measurements of displacements created in clay by the column-drop performance test: there is a well-defined reference plane, and one can visually see the surface of the clay, given that the depression is relatively shallow (approximately 22 to 28 mm) and fairly smooth” (NRC, 2009). 15 Finding 4 of the Phase 1 letter report stated that “The column-drop performance test (including the testing protocols, facilities, and instrumentation) is a valid meth od for assessing the part-to-part consistency of clay boxes used in body armor testing” (NRC, 2009). 16 Scott Walton and Shane Esola, Aberdeen Test Center, “ATC Perspective on Clay used for Body Armor Testing,” presentation to the Body Armor Testing Phase II committee, March 10, 2010. -51-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION One consequence of shifting composition of the clay has been that a need was recognized to find a way to calibrate the modeling clay that was compatible with use on a ballistic firing range. This led to the development of the so-called column-drop test. Although it is not possible to trace the history of the test in published documents, it appears to have been developed in response to testers noting that newer versions of RP #1 were stiffer than older versions. Given that fact and the fact that oil-based modeling clay is readily softened by heating led to the use of ovens to warm the clay so that it behaved similarly to the older (de facto reference) formulation. The column-drop test developed to assess the similarity of clay behaviors. 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) of defined mass (1 kg) from a height of 2 m onto RP #1 contained in a clay box. The mass is then removed, and penetration is defined by measuring the distance between the original flat clay surface and the deepest point in the indent. As the deepest point is determined by a highly regular hemisphere, it can be readily and reliably located by an operator using a digital caliper, and the depth at this point can be measured by any of a number of techniques. As noted earlier, the digital caliper is adequate for this because of the well-defined planar reference, the smooth shallow indentation, and the ease of locating the center of the indention. The three photographs in Figure 4-3 illustrate the drop test. The cavity resulting from the drop test is of a volume and shape that is qualitatively similar to the cavity from an armor test. Both craters are tens of millimeters in depth and width, and both are smooth, regular shapes. However, the deformation rate experienced by the clay is markedly different. As demonstrated to the committee, the weight impacts the surface of the clay slightly faster than 6 m/sec, whereas the back face of the armor system moves at a velocity nearly an order of magnitude greater, just over 50 m/sec.17 17 Ibid. -52-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION FIGURE 4-3 Column-drop test as performed at ATC. The overall setup is shown in (A). The weight, shown up close in (C), is held in place by an electromagnet at the top of the antiyaw tube. Upon release the weight accelerates under gravity and is implanted into the surface of the modeling clay. The weight is manually removed and the depth of the cavity (two are visible) is measured. Also visible are two thermometer probes used to track the temperature of the modeling clay. The results of a typical drop are shown in (B). Notable is the significant yaw (inclination with respect to the normal of the clay surface). 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. The criterion for test/no test is that the cavity resulting in this test is 25 ± 3 mm (ATC, 2008). Drop test results reviewed by the committee were all obtained using the standard clay box on which the clay appliqué is mounted for the live-fire testing of hard armor, as described in Chapter 2. Four characteristics typify the results: 1. Drop test results exhibit scatter even under nominally identical conditions; 2. The flow of RP #1 in response to load (rheology) depends on thermal history or heating; 3. The rheology of RP #1 also depends on prior working (shear history); and -53-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION 4. Drop test results depend weakly on location in the clay box. The effects of temperature have been systematically studied by both Army ATC personnel and an independent lab. The Army study employed the standard drop test. 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. Although temperature measurements were taken, they were not reported; instead, the variation with time was presented. These data (Figure 4-4) reveal two very important characteristics of the modeling clay with respect to this application: (1) there is substantial lot-to-lot variation (under nominally identical conditions different boxes yielded penetrations that varied systematically from 1 to 2 mm) and (2) the drift with time is significant compared to the allowed range for “calibration,” that is, ±3 mm. Over the 45 min of the test the average penetration in all cases was reduced by more than 4 mm. One implication of the latter characteristic is that the majority of clay boxes that are within calibration when removed from the oven can be predicted to fall out of calibration during the 45-min time window. A second result from the same study is given in Figure 4-5. In this figure drop tests results using weights of different geometries are presented. The information implies that there is no particular advantage of any one shape. The three different geometries that are tested reveal equally useful information. However, the results do make startlingly evident the magnitude of the scatter associated with drop test results; it is disturbingly large compared to the allowed calibration range. A qualitatively similar degree of scatter was observed in a study of drop test penetration as a function of radial position measured from the center of the box (see Figure 4-6) (Esola et al., 2010). In this study, there was a large box-to- box variation in drop-test penetration and substantial scatter under nominally identical conditions. Significantly, there was not a systematic trend with respect to radial distance from the center. In most boxes there was only a weak variation with distance, but there were some tests in which the edges were significantly less deeply penetrated and some in which the penetration was deeper near the extremities (see Figure 4-7). The results of this study can be summarized as follows:  There was only weak correlation between radial position and average penetration depth;  Variability of the average penetration depth under nominally identical conditions was significant;  Variance increased as a function of distance from the center; and  Well-used clay boxes exhibited behavior different from “dormant” boxes or from new boxes until they had been used for a while; however, the time constants for changes in behavior were not determined. -54-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION FIGURE 4-4 The results of drop tests on clay boxes allowed to naturally cool from 40°C to normal room temperature (roughly 23°C). Drops were made in a randomized 4 × 4 grid. The surface was not repaired between drops, and drops were intentionally separated to minimize potential interference. Four separate clay boxes were used, each represented by a different line on the graph. Each point on the graph is the average of two drops. Initial pairs of drops were made 3 min after removal from the oven, and subsequent data were taken in 15-min intervals. Although there is scatter, over the range investigated the slopes of the curves are all consistent with a decrease in average cavity depth of 1.5 mm every 15 min. The difference in the absolute values of the cavities resulting from the drop tests is attributed to lot-to-lot variation in the modeling clay and differing lengths of time in service. 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. -55-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION FIGURE 4-5 Drop test results using the standard Army right-circular cylinder with a solid hemispherical cap (44.5 mm [1.75 in.] in diameter with a mass of 1 kg [2.2 lb]), a similar non-standard double-length cylinder of the same diameter with the same type of hemispherical cap, and sphere with the diameter specified in the National Institute of Justice Standard (NIJ ), 63.5 mm (2.5 in.) in diameter. The two horizontal blue lines represent the upper and lower limit of the calibration range. The most striking feature of the results is the observed scatter – which appears similar for all three classes of weight geometry. Under nominally identical conditions, the scatter is a substantial fraction of the allowable range! This is particularly so when the temperature is in the range of that seen in typical practice. 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. -56-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION FIGURE 4-12 Road map showing suggested near-term actions, medium-term research needs, and a long-term goal to develop a more consistent backing material and a more reliable process for evaluating hard armor. The color coding shows “highest priority” items in red text with “high priority” actions in orange. Near-Term Actions The critical near-term need is for the development of a backing material that can be calibrated at room temperature. To reach that goal, actions are needed in four areas: 1. Characterization of the current backing material, RP #1; 2. Lab trials with alternative formulations from the current supplier; 3. Exploration of in-box conditioning methods; and 4. Study of improved calibration procedures. Activities suggested for each action are prioritized in sections below and in Figure 4-12 to help guide efforts. -81-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION Basic Characterization of RP #1 The highest priority in the near term should be to characterize the current formulation of RP #1. Armor testing requires that the backing material exhibit predictable behavior. Two classes of characterization are needed. Fundamental thermomechanical information for RP #1 is lacking either at room or elevated temperatures. Specifically, the relative recoverable (elastic) and dissipative (viscous) fractions of the response of RP #1 must be determined under three conditions: as manufactured; after mechanical working; and after extended service, say, 6 months. Measurements should cover the temperature range of 23ºC to 40ºC and include viscoelastic solid response prior to yielding; the equilibrium yield stress; and the thixotropic response, including time constants or the apparent equilibrium viscosity and shear modulus as functions of shear rate. 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. Similarly, no data on thermophysical properties are available and must be collected. These include density, thermal conductivity, and heat capacity (permitting calculation of thermal diffusivity). Such data permit the straightforward calculation of temperature changes with respect to both position in the clay box and time after removal from the oven. They must be known in order to quantitatively predict the impact of clay-box cooling during service on the range. Thermogravimetric measurements are needed to measure weight loss due to selective evaporation of components during service. The obvious need for this is underscored by the on-range observation of sulfur evaporation and condensation associated with annealing the clay boxes in the ovens. It also is plausible that low molecular weight hydrocarbons are lost. Differential thermal analysis should be done in parallel with thermogravimetric analysis to help determine the relative contributions of evaporation or chemical reactions (e.g., oxidation) to observed weight changes.25 Formulation In the near term, incremental improvements in modeling clay to allow it to serve as a backing material need to focus on a reformulation to permit calibration and live-fire testing at room temperature. One benefit of this is to simplify range practice, but the chief, and vital, benefit is to remove the substantial drift of clay 25 The committee noted that the ATC has a contractual relationship with Rutgers University to carry out rheological and thermophysical measurements, but the scope of measurements and the data were not available to the study group. Successful conclusion of this effort must be a high priority, along with any additional work to generate the requisite data set. -82-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION properties observed with time after removal from the oven (see Figure 4-4). A room-temperature system would eliminate the need for a post-test drop test. The committee is aware that Chavant, the manufacturer of RP #1, has been contracted to develop a replacement for RP #1 that will meet the historical calibration specifications at room temperature. As material is provided to the government, rapid turnaround in assessment is required to justify this investment of resources. The committee has offered two considerations that apply to the reformulation effort. The first is that the formulation be simplified by minimizing the number of ingredients, in particular, the fillers. As previously noted, RP #1 contains both zinc stearate and sulfur in addition to clay. It is unclear how this combination affects its performance as a backing material. The goal of simplifying the formulation, with concomitant tightening of specifications on raw materials, is to reduce the lot-to-lot (and therefore box-to-box) variability of reformulated backing material. The second consideration is related to the characteristics of the filler phase. The filler employed in the formulation should be equiaxed (i.e., roughly spherical) rather than platy or otherwise anisotropic. The goal of this selection is to reduce variation of mechanical response to shear history and direction or location in the box. 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. Given that the preliminary results suggest the feasibility of mechanical vibration as a method to achieve in-box working of RP #1, this work should be systematically extended. In particular, experiments should be conducted with controlled, well- characterized vibration conditions to determine the effects of frequency and amplitude on properties so that reasonable limits can be established for the conditioning treatments. The assessment of flow should be extended to the standard drop tests as well as to the high-rate tests discussed in the next section. Further, the relaxation behavior of clay after vibration needs to be systematically investigated. The time required for the clay to return to its previbration state needs to be characterized to determine if vibration will produce a state that is stable enough to allow for testing before relaxation becomes significant. If necessary, ATC should engage the research community to conduct further evaluations. In addition to the vibration conditioning treatment, ATC is advised to explore alternative methods for box filling and clay processing prior to box filling as well as alternative methods for in-box clay working. -83-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION Calibration It is important to stress there are two different functions of an improved calibration test. The first is to characterize the variability of cla y within a given box at a given time in a manner that is directly relatable to the BFD. The second, very important role is to use such a system to estimate the variation of BFD measurements both within a given box and between boxes, under realistic testing conditions using existing test protocols. The latter will help to provide information of use for the statistical analysis of armor testing results. Specifically, statistical analyses of the test protocols require quantification of how much of the observed variation in BFD is due to the clay medium (and the test protocol in general) and how much is due to variation in the armor plates. The actual plates cannot be used to answer this question because of the destructive nature of the tests, and the results could be confounded with variation in manufacture. Clearly, the conditions of the column-drop performance test are very different from those experienced by the modeling clay during the actual ballistic test of the armor. The Army has been developing a gas gun capable of directing a penetrator onto the surface of a clay box at velocities comparable to those of a BFD. The committee strongly feels that the gas gun needs to be deployed to probe the strain rate effect. The first advantage of employing high speeds is that impactors will penetrate to depths comparable to the BFDs in ballistic tests. In addition, a gas gun will in principle be able to deliver penetrators ranging from spheres to other specialty shapes. 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). An additional interesting possibility is to use small diameter spheres, because this would allow a high-density matrix of small impacts that may permit direct measurement of clay homogeneity (Weber, 2000). Priorities for Near-Term Actions Highest priority should be given to the following:  Determining the rheological properties of RP #1;  Determining the thermal properties of RP #1; and,  Formulating a replacement backing material derived from RP #1 that calibrates at room temperature. Other small batch clays should continue to be studied. High priority should be given to the following:  Defining the desired rheological behavior; -84-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION  Evaluating conditioning with vibration; and,  Deploying gas gun calibration. Finally, medium priority should be given to the following:  Determining the effect of mechanical compounding before filling the box; —Exploring alternative box-filling by pressing; —Exploring alternative means of in-box working;  Studying the effects of amplitude and frequency; and  Matching velocity and penetration depth in calibration. 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. Analysis of results from the near-term activities will also define some fundamental research tasks that are necessary for the development of a long-term replacement for RP #1 as well as (possibly) a revised set of calibration procedures. Clay Replacements Assuming that room-temperature use has been achieved in the short term, the overarching goal of medium-term research is to produce a backing material that retains the room-temperature characteristic and exhibits batch-to-batch consistency while delivering both substantially lower variance during plastic deformation and less sensitivity to shear history. Three possible replacements were discussed in the Phase II study (NRC, 2010). These include a revised formulation similar to RP #1; microcrystalline wax emulsions without inorganic fillers; and ballistics gelatin. The first two possibilities would be plastically deforming materials that serve as recording media; ballistics gelatin, by comparison, is a transparent, fully elastic medium through which transient deformation can be imaged. Each has advantages. The original rationale for selecting an oil-based modeling clay remains valid. Such clays are inexpensive and straightforward to implement in a ballistics range setting. The ability to image through an elastic material offers the possibility of collecting dynamic information, including true measures of the maximum extent of transient deformation. Two paramount issues need to drive the development of improved or new backing materials. First is the reliability of the data, which means the material must exhibit a deformation behavior that is characterized by low variance and by minimal dependence on temperature and processing history. The second key issue is practicality. This includes cost, service life, ease of processing and -85-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION handling, and cost and complexity of associated equipment (notably the cost of equipment to measure deformation). Plastically Deforming Media. As discussed previously in this chapter (see “Influence of Structure on Properties of Oil-Base Modeling Clay”), the principal relationships between composition, phase distribution, microstructure, and macroscopic mechanical properties are understood. Therefore the scientific basis for a development program is in hand. However, the gap between principle and practice may be large and may require uncommon expertise to bridge successfully. The committee believes that the Army should engage an industrial firm rather than start an in-house development program to develop, and possibly supply, a formulation. Expertise resides at large chemical companies such as those involved in the development of cosmetics, lubricants, drilling fluids, molding waxes, and the like. However, it is recognized that the small potential market may make this an unattractive research program for many companies. If a market survey proves this to be the case, the committee suggests that a highly focused university-based program may offer the best potential for development of a new class of plastically deforming recording media. The committee feels the Army should develop a set of criteria to be used to guide such a development process at this time. Further, the criteria should include the ability to restore the material to a well-defined standard state through a combination of working and heating. This is properly regarded as the equivalent of solution-annealing, working, and tempering of metals to achieve a well-defined condition. Elastically Deforming Media. 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. The first is if a low-variance, plastically deforming material has not been, or cannot be devised. The second is if the equipment to record images through a transparent medium is less expensive than the equipment to measure the geometry of a cavity at required resolution. And the third is if dynamic information becomes important for assessing injury probability. Therefore this will be an area of interest over the longer term, and the degree of interest will ultimately depend on the success of other ongoing and planned effects. In the event that elastically deforming materials become a high priority, the committee recommends connecting to ongoing research efforts on gelatin and its replacements. It should be noted that although ballistic gelatin is widely used, it too suffers from gaps in knowledge about its fundamental structure–processing– property relationships. Solid-State Instrumentation. The focus of the development of solid-state instrumentation appears to be biofidelity. The range of these developments is discussed in Chapter 8. The committee believes the Army should critically -86-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION examine whether or not biofidelity is important to, or even desired for, production range testing of armor. In the event that it is judged to be so, ATC should maintain an awareness of developments in the Army-funded programs and others and be prepared to rapidly adopt technologies for range practice as they become mature technologies. In the event that biofidelity is not held to be a part of range testing, the Army should look to migrate sensor arrays in a manner that is compatible with range practice. One hypothetical example would be to consider using a planar sensor array covered with a thick planar slab of polymer behind a modeling clay appliqué. Evaluation criteria for such an approach should include cost, reliability, needed electronics, and availability of suppliers of modules. Such incremental inclusion of sensors would offer the best of both worlds—easy processing of appliqué with highly workable clay, but none of the processing, handing, boundary constraint issues, and history effects of the clay box. However, the committee wishes to reiterate that sensor development must supplement rather than replace the near-term actions outlined above. Revised Calibration In parallel with the clay replacement, improved calibration procedures are needed. 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. ATC must work with other testing organizations and industrial practitioners to devise a calibration methodology that balances the needs of the various constituencies. A manifest goal to this collaboration should be the definition and adoption of a single testing methodology. If promising backing materials other than clay are identified in the future, then calibration procedures for those materials that balance the needs of the various constituencies will also have to be developed. Summary of Medium-Term Research Needs The highest medium-term priorities are to develop backing materials that can be calibrated at room temperature and are less sensitive to work history and cost effective and to develop a robust calibration process that is well suited to the alternative backing materials. Initial research actions include these: • Accomplish basic research on clay alternatives —Improved or new plastically deforming backing material(s), —Ballistics gelatin or other transparent elastic media, and —ATM and/or sensor arrays. -87-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION • Have potential suppliers formulate test materials. • Direct basic research on alternative calibration processes. The desired outcomes are to develop replacement candidates for RP #1 and issue a revised calibration procedure. Long-Term Goal The ultimate goal is to develop a consistent, robust protocol for ballistic testing of hard armor plates that is also cost-effective for manufacturers and other testing laboratories. Selecting or developing a well-characterized high- performing replacement for RP #1 and devising an improved calibration procedure will enable development of a standard testing configuration and procedure. The success of the near- and medium-term activities could lead to a single, uniform test standard that could be used by all of the members of the testing community. Recommendation 4-2: The Office of the Director, Operational Test and Evaluation, and the Army should provide resources and execute the road map described in this chapter and graphically shown in Figure 4-12 with the objective of developing a standard ballistics backing material for testing body armor. The properties and behaviors of the material should be well understood; it should exhibit minimal variability due to temperature, working, and aging and require simple calibration techniques and equipment; and it should enable reliable and accurate recording of body armor test results. REFERENCES Abu-Jdayil, B. 2003. Modelling the time-dependent rheological behavior of semisolid foodstuffs. Journal of Food Engineering 57(1):97-102. Aerospace Corporation. 1974. Equipment Systems Improvement Program– Protective Armor Development Program–Final Report, volume 3. ATR- 75(7906)-1. El Segundo, Calif.: Aerospace Corporation. ATC (U.S. Army Aberdeen Test Center). 2008. Test Operations Procedure (TOP) 10-2-210 Ballistic Testing of Hard Body Armor Using Clay Backing. Aberdeen Proving Ground, Md.: Aberdeen Test Center. Barnes, H. 1997. Thixotropy—a review. Journal of Non-Newtonian Fluid Mechanics 70(1-2):1-33. Barry, B., and A. Grace. 1971. Structural, rheological and textural properties of soft paraffins. Journal of Texture Studies 2(3):259-279. Benna, M., N. Kbir-Ariguib, C. Clinard, and F. Bergaya. 2001. Card-House microstructure of purified sodium montmorillonite gels evidenced by filtration -88-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION properties at different pH. Progress in Colloid and Polymer Science 117:204- 210. Blagin, V.I. 1966. Compacting properties of powder mixtures. Powder Metallurgy and Metal Ceramics 5(6):454-457. Bonn, D., and M. Denn. 2009. Yield stress fluids slowly yield to analysis. Science 324(5933):1401-1402. Borwankar, R. 1992. Food texture and rheology: A tutorial review. Journal of Food Engineering 16(1-2):1-16. DoD (U.S. Department of Defense). 2008. DoD Test Method Standard for Performance Requirements and Testing of Body Armor. MIL-STD-3027. Arlington, Va.: Department of Defense. Esola, S., B. Gillich, W. Boughers, and S. Meiselwitz. 2010. Clay Calibration: Radial Dependence of Calibration Drop Depths for Body Armor Testing. Presentation to the Sixteenth Army Conference on Applied Statistics, October 20-22, Cary, N.C. Fackler, M., and J. Malinowski. 1985. The wound profile: A visual method of quantifying gunshot wound components. Journal of Trauma 25(6):522-529. Fung A.Y.C., and P. Tong. 2001. Advanced Series in Engineering Science— Volume 1: Classical and Computational Solid Mechanics. Hackensack, N.J.: World Scientific Publishing Co., Inc. Harvey, E., J. McMillen, and E. Butler. 1962. Mechanism of wounding. Pp. 143– 235 in Wound Ballistics. J. Coates, editor. Rockville, Md.: Office of the Surgeon General, Department of the Army. Hosseini, S., A. Fazlali, E. Ghasemi, H. Moghaddam, and M. Salehi. 2010. Rheological properties of a γ-Fe2O3 paraffin-based ferrofluid. Journal of Magnetism and Magnetic Materials 322(23):3792-3796. Huang, Z., M. Lucas, and M. Adams. 2002. A numerical and experimental study of the indentation mechanics of plasticine. The Journal of Strain Analysis for Engineering Design 37(2):141-150. Juliano, T., A. Forster, P. Drzal, T. Weerasooriya, P. Moy, and M. VanLandingham. 2006. Multiscale Mechanical Characterization of Biomimetic Physically Associating Gels. ARL-RP-134. Aberdeen Proving Ground, Md.: Army Research Laboratory. Lenk, R., and A. Ph. Krivoshchepov. 2000. Effect of surface-active substances on the rheological properties of silicon carbide suspensions in paraffin. Journal of the American Ceramic Society 83(2):273-276. Metker, L., N. Prather, and E. Johnson. 1975. A Method for Determining Backface Signatures of Soft Body Armors. EB-TR-75029. Aberdeen Proving Ground, Md.: U.S. Army Armament Research and Development Command. -89-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION Moy, P., T. Weerasooriya, T. Juliano, M. VanLandingham, and W. Chen. 2006. Dynamic Response of an Alternative Tissue Simulant, Physically Associating Gels (PAG). ARL-RP-136. Aberdeen Proving Ground, Md.: Army Research Laboratory. NIST (National Institute of Standards and Technology). 1994. Memorandum: Rheology of Clays. December 12, 1994. Gaithersburg, Md.: National Institute for Standards and Technology. NRC. (National Research Council). 2009. Phase I Report on Review of the Testing of Body Armor Materials for Use by the U.S. Army: Letter Report. Washington, D.C.: National Academies Press. NRC. 2010. Phase II Report on Review of the Testing of Body Armor Materials for Use by the U.S. Army. Washington, D.C.: National Academies Press. Peltsman, I., and M. Peltsman. 1984. Low Pressure Moulding of Ceramic Materials. International Ceramic Review 33(56). Pena, L., B. Lee, and J. Stearns. 1994. Structural rheology of a model ointment. Pharmaceutical Research 11(6):875-881. Prather, R., C. Swann, and C. Hawkins. 1977. Backface Signatures of Soft Body Armors and the Associated Trauma Effects. ARCSL-TR-77055. Aberdeen Proving Ground, Md.: U.S. Army Armament Research and Development Command Technology Center. Raftenberg, M. 2006. Modeling thoracic blunt trauma: Towards a finite-element- based design methodology for body armor. Pp. 219–226 in Selected Topics in Electronics and Systems, Volume 42: Transformational Science and Technology for the Current and Future Force, Proceedings of the 24th US Army Science Conference. J.A. Parmentola, A.M. Rajendran, W. Bryzik, B.J. Walker, J.W. McCauley, J. Reifman, and N.M. Nasrabadi, editors. Hackensack, N.J.: World Scientific Publishing Company, Incorporated. Reed, J.S. 1988. Introduction to the Principles of Ceramic Processing. Hoboken, N.J.: John Wiley & Sons, Incorporated. Roberson, C., W. Perciballi, D. Rogers, and D. Fleming. 2010. Consistently Depressed? Presentation to Personal Armour Systems Symposium 2010, September 13-17, Quebec City, Canada. Rothon, R. 1999. Filler manufacture and characterization. Pp. 67–107 in Mineral Fillers in Thermoplastics: Advances in Polymer Science 139. J. Jancar, editor. New York, N.Y.: Springer. Salisbury, C., and D. Cronin. 2009. Mechanical properties of ballistic gelatin at high deformation rates. Experimental Mechanics 49(6):829-840. Suwardie, H., R. Yazici, D. Kalyon, S. Kovenklioglu. 1998. Capillary flow behaviour of microcrystalline wax and silicon carbide suspension. Journal of Materials Science 33(20): 5059-5067. -90-

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PREPUBLICATION DRAFT—SUBJECT TO EDITORIAL CORRECTION Topolnicki, M. 2004. In situ soil mixing. Pp. 331–428 in Ground Improvement. M. Moseley and K. Kirsch, editors. Boca Raton, Fla.: CRC Press Taylor and Francis Group. Uzar, A., M. Dakak, T. Özer, G. Ögünc, T. Yigit, C. Kayahan, K. Öner, and D. Sen. 2003. A new ballistic simulant "transparent gel candle" (experimental study). Turkish Journal of Trauma and Emergency Surgery 9(2):104-106. Weber, D.J. 2000. An Indirect Method of Measuring Impact Velocity for Non- Lethal Weapons Blunt Trauma Studies. Aberdeen Proving Ground, Md.: U.S. Army Edgewood Chemical and Biological Center. Zorzi, J.E., C.A Perottoni, and J.A.H. Da Jornada. 2003. Wax-based binder for low-pressure injection molding and the robust production of ceramic parts. Industrial Ceramics 23(1):47-49. -91-