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Disclaimers, The findings in this report are not to be construed as an official Department of the Army position unless. so designated by other authorized documents, Citation of manufacturer s or trade names does not constitute an official endorsement or approval of the. use thereof, Destroy this report when it is no longer needed Do not return it to the originator. Army Research Laboratory,Aberdeen Proving Ground MD 21005 5069. ARL TR 3038 August 2003,Patterned Armor Performance Evaluation for. Multiple Impacts,William S de Rosset,Weapons and Materials Research Directorate ARL. Approved for public release distribution is unlimited. Form Approved,REPORT DOCUMENTATION PAGE OMB No 0704 0188. 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August 2003 Final June 2002 June 2003,4 TITLE AND SUBTITLE 5a CONTRACT NUMBER. Patterned Armor Performance Evaluation for Multiple Impacts. 5b GRANT NUMBER,5c PROGRAM ELEMENT NUMBER,6 AUTHOR S 5d PROJECT NUMBER. William S de Rosset AH80,5e TASK NUMBER,5f WORK UNIT NUMBER. 7 PERFORMING ORGANIZATION NAME S AND ADDRESS ES 8 PERFORMING ORGANIZATION. REPORT NUMBER,U S Army Research Laboratory,ATTN AMSRL WM MD ARL TR 3038. Aberdeen Proving Ground MD 21005 5069, 9 SPONSORING MONITORING AGENCY NAME S AND ADDRESS ES 10 SPONSOR MONITOR S ACRONYM S. 11 SPONSOR MONITOR S REPORT,12 DISTRIBUTION AVAILABILITY STATEMENT. Approved for public release distribution is unlimited. 13 SUPPLEMENTARY NOTES,14 ABSTRACT, Patterned armor is characterized by an array of repeating cells such as tiled ceramic armor or reactive armor boxes. Performance characteristics of an ideal patterned armor with respect to multiple hits are discussed and the types of single shot. ballistic data needed to quantify that performance are presented An approach to use these data is developed to provide a. quantitative measure probability of nonperforation after a given number of impacts of the patterned armor performance against. multiple impacts This performance measure can then be compared to a well posed multiple hit criterion to assess whether the. patterned armor meets the criterion,15 SUBJECT TERMS. patterned armor armor performance evaluation limit velocity ceramic armor multiple hit criterion lightweight armor. multiple hit capability, 17 LIMITATION 18 NUMBER 19a NAME OF RESPONSIBLE PERSON. 16 SECURITY CLASSIFICATION OF OF ABSTRACT OF PAGES. William S de Rosset, a REPORT b ABSTRACT c THIS PAGE 19b TELEPHONE NUMBER Include area code. UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED 410 306 0816. Standard Form 298 Rev 8 98,Prescribed by ANSI Std Z39 18. List of Figures iv,Acknowledgments v,1 Introduction 1. 2 Patterned Armor Performance Characteristics 1, 3 Methodology for Assessing Patterned Armor Performance Against Multiple Hits 4. 3 1 Basic Methodology 4, 3 2 Patterned Armor Performance Comparison for Similar Designs 5. 3 3 Patterned Armor Performance Comparison for Different Armor Technologies 6. 3 4 Velocity Effects 7,4 Summary 15,5 References 16. Appendix General Formulation for the Probability of Nonperforation of a Patterned. Armor With Cells Capable of Withstanding More Than One Impact 17. List of Figures, Figure 1 Patterned armor cell array showing D and A 5. Figure 2 Probability of nonperforation P n given n impacts for two cell types 6. Figure 3 Performance comparison between tiled ceramic and B4600 Armor 8. Figure 4 Impact velocity vs range for the 12 7 mm AP M2 machine gun round 9. Figure 5 Possible outcomes from the first impact for N 3 10. Figure 6 Possible outcomes from the second impact for N 3 11. Figure 7 One third of the possible outcomes from the third impact for N 3 not all. configurations are represented in the figure 12, Figure 8 P 7 as a function of range indicating velocity effects 14. Figure 9 Performance comparison between a one impact cell and a two impact cell armor 14. Acknowledgments, The author is indebted to Dr Steven Segletes for his careful review of the draft manuscript and. his general formulation of the probability of nonperforation of patterned armor containing cells. capable of withstanding more than one impact which is used as the Appendix to this report. Dr Segletes provided the generalization to the combinatorial problem used concerning multiple. hits on a given cell, Dr Segletes would like to thank Dr Paul Tanenbaum of the U S Army Research Laboratory s. Survivability Lethality Analysis Directorate for discussing this problem at length and making. very helpful suggestions including that an inclusion exclusion approach was applicable to a. simpler though similar problem and should apply here as well Dr Tanenbaum s review of the. Appendix further corrected some deficient definitions and notations. INTENTIONALLY LEFT BLANK,1 Introduction, At the 14th Annual Ground Vehicle Survivability Symposium in Monterey CA on April 2003. LTG John S Caldwell called for armors to be designed that would be perforated 0 of the time. This is certainly the goal of all armor designers but in practice certain compromises must be. made between protection levels cost and vehicle weight such that this goal is rarely attained. This is especially true for lightweight air transportable fighting vehicles that the Army is. currently developing as part of the Future Combat System of Systems. The protection problem for lightweight armored vehicles is compounded by threats that feature. multiple impacts such as machine guns and fragmenting warheads Armors designed for these. types of vehicles must possess the capability to defeat a large number of closely spaced impacts. in a given burst or explosion Some of the armors being considered for lightweight vehicles are. modular in nature have a repeating geometric pattern or both These will be referred to as. patterned armor The module or repeating pattern element will be referred to as a cell. While specific cases of patterned armor performance have been examined in the past there has. been no general approach developed to evaluate patterned armor for lightweight armored. vehicles attacked by automatic weapons or bursting munitions Whatever level of performance. is set as the multiple hit criterion there has to be an accepted means or methodology by which. the results of single shot ballistic data can be used to estimate the performance of a given. patterned armor design against multiple impacts, Section 2 will contain a discussion of a possible testing strategy and the considerations that must. be made in conducting the ballistic tests Characteristics of effective patterned armor will be. included in this section, Section 3 will provide a proposed methodology that shows how the individual ballistic data. might be used to establish the performance level of the patterned armor against multiple impacts. It is based on a model recently published by de Rosset and Wald 2002 Several examples will. be presented that exercise this methodology and show its usefulness in evaluating trade offs in a. given armor technology trade offs between competing armor technologies and effects of. velocity on armor performance The final section will summarize the main ideas presented in the. 2 Patterned Armor Performance Characteristics, For the sake of clarity and purposes of discussion in this report it is necessary to distinguish. between the terms ballistic performance and multiple hit criterion Ballistic performance of. a patterned armor vs multiple impacts can be determined from an analysis of individual ballistic. results and is the subject of this report Multiple hit criterion is a specified level of performance. on a given armor cell or armor cell array This level of performance should be set high enough. to allow the armored vehicle to survive most realistic battlefield encounters For instance the. multiple hit criterion may state that the armor array must withstand perforation of a 10 round. burst from a 12 7 mm machine gun at a distance of 500 m with a probability of 90 The. weapon to target distance range and the weapon characteristics give the dispersion of the. impacts on the target as well as the striking velocity. The procedures used to evaluate patterned armor that are presented in this section are. straightforward and based on the nature of the armor being tested Most if not all of these. procedures have been and are being used by the U S Army Research Laboratory in testing. ceramic tiled armor arrays These procedures are to be distinguished from such tests as depth of. penetration DOP tests or other types of screening tests that are used in the early stages of. evaluating armor concepts and materials The procedures presented here are to be used for armor. arrays that have already undergone some initial tests and have high potential for actual fielding. on an armored vehicle, The main characteristic of patterned armor is that the armor is made up of repeating cells There. is generally a border or line of demarcation between the cells that is a potential weak point in the. armor array For instance the seam between ceramic tiles generally has lower ballistic. performance than the center of the tile It may also be that performance of an individual cell. depends on the hit location within the cell Reactive armor is a good example of this de Rosset. 1998 To ensure overall armor array performance an impact in one cell should leave adjoining. cells intact and capable of defeating a subsequent impact if it had not been previously struck. The properties of an ideal patterned armor can thus be summarized as follows. ability of an undamaged cell to defeat a single threat round at a velocity corresponding to. the specified range based on limit velocity as indicated next. uniform ballistic performance over the entire area of the cell. no loss of performance in the area between cells seam effect and. no effect on adjoining cells from impact on a given cell. The first step in the evaluation procedure is to determine the limit velocity of the threat against. an individual cell This has usually been accomplished though an initial screening process that. has determined suitable materials and design By suitable we mean that the armor material and. design produces a limit velocity that is high enough to meet the multiple hit criterion for a single. shot rather than the entire burst The limit velocity can be determined in either of two ways. First a plot of the residual velocity vs striking velocity will yield a limit velocity VL through. a Lambert Jonas Lambert and Jonas 1976 fit to the data Second a V50 can be determined. through standard test procedures Where there is little or no zone of mixed results the two. values are approximately equal For purposes of discussion this report will assume that there is. a single value for the limit velocity Above this velocity the armor is perforated by the threat. and below this value the threat is defeated This simplistic approach can be modified to take. into account a more sophisticated concept of probability of perforation However the essence of. the methodology presented in this report would not be affected with this more involved. The reduction in striking velocity as a function of range weapon to target distance is rather. steep for small arms ammunition It is conceivable that there is a range at which a given tile can. withstand two impacts due to the low striking velocity The range where two impacts can be. sustained by a single tile might vary slightly as a function of the distance between impacts on. that tile It is expected that with new ceramic tile encapsulation technology the probability of a. single tile sustaining multiple hits will increase The next section indicates how the evaluation. methodology can be adapted to take into account this increase in performance. The next step is to determine the uniformity or lack thereof of the ballistic performance over. the entire area of the cell Ideally the limit velocity is the same at all impact points within the. cell However if there is some falloff in performance in some particular area of the cell a. performance map of the cell needs to be generated as input data for the evaluation methodology. Clearly it is more advantageous to apply efforts to making the performance homogeneous over. the entire cell than to expend resources to show where the cell fails to meet the requirements. However there may simply be physical reasons why the performance cannot be uniform. The third step is to determine the performance of the patterned armor in the area between cells. For these tests multiple cells must be employed in an array that simulates to the greatest extent. practical the actual arrangement of the cells in the armor design Limit velocities should be. determined not only for straight edge seams but also for individual points where three cells. might meet triple point, Finally ballistic tests must demonstrate that an impact on one cell does not affect the. performance of an adjacent cell upon subsequent impact This is done by shooting at a target. with an array of cells put together similar to what is envisioned for the fielded armor design. Impacts on adjacent cells should be defeated consistently at the velocity specified in the. multiple hit criterion, Clearly it may not be possible to design a patterned armor with all the ideal characteristics. noted especially when there are limits to the allowable areal density and cost However a. sufficient amount of ballistic data must be gathered on the armor designs under consideration so. that the trade offs that may be required can be quantified to the greatest extent possible The. next section presents a methodology that can be used in this type of study. 3 Methodology for Assessing Patterned Armor Performance Against. Multiple Hits,3 1 Basic Methodology, The basic methodology for assessing patterned armor performance has already been presented in. a previous report de Rosset and Wald 2002 The basic concepts for ceramic armor are readily. transferable to patterned armor in general The methodology is based on the calculation of the. probability of perforating a given patterned armor array with one or more impacts The. dispersion of the impacts on the armor array is assumed to be in a bivariate normal distribution. and is related to the range dispersion characteristics of the weapon firing the ammunition The. report also shows that using a random distribution of impacts on the armor array gives similar. results with the largest differences in the calculations occurring for a small number of impacts. less than five at short ranges less than 250 m For simplicity this report will use a random. distribution of impacts on the armor array, Several assumptions go into this methodology First it is assumed that all the relevant ballistic. data have been obtained In particular any nonuniformity in ballistic performance over the cell. face has been determined Second it is assumed that the armor has been designed such that. impact on one cell will not affect the performance of adjacent cells This would appear to be a. reasonable requirement on a viable patterned armor Third it is assumed that when a cell is. impacted it is no longer capable of defeating a subsequent impact by the threat round This. assumption will be modified in a variation of this methodology later in the report We also take. a square with side length D as the cell geometry although other shapes can be used in the. analysis hexagons triangles and rectangles for instance The area A assumed to be a circle. over which the impacts fall on the target can be related to the range dispersion characteristics of. the attacking weapon, Consider the arrangement of square cells as shown in Figure 1 Each of the cells has uniform. ballistic performance and can defeat the threat round at the specified velocity i e the limit. velocity is above the threat velocity For computational simplicity the bullet diameter is taken. as 0 Thus it cannot impact two cells at one time The first round that impacts the target has a. 100 probability of being defeated i e no perforation The second round will have a finite. probability of perforating the target since it may land on the first tile struck If N tiles cover the. area A A N D2 neglecting corner effects the probability P2 that the second round is defeated. is given by,P2 P1 A D2 A P1 1 D2 A 1, Figure 1 Patterned armor cell array showing D and A. where P1 1 As subsequent cells are struck and become ineffective in stopping the threat. rounds the probability P n of the target not being perforated after n rounds have impacted it is. P n 1 D2 A 1 2 D2 A 1 3 D2 A 1 n 1 D2 A 2, De Rosset and Wald 2002 also showed that if the cell contained a zone at the edge of the cell. that could not stop the threat round the probability P n of stopping n shots was given by. P n 1 D2 A 1 2 D2 A 1 n 1 D2 A 1 2 D 2n 3, where is the width of the area of low performance Note that P n is not defined as the. probability that the nth shot is defeated, 3 2 Patterned Armor Performance Comparison for Similar Designs. Equations 2 and 3 can be used to determine which of two similar patterned armor designs is. better Suppose for instance that the armor designer is able to produce a patterned armor with a. cell of side length D that has constant ballistic performance across its face and stops the threat at. the specified velocity It was shown previously de Rosset and Wald 2002 that the performance. of a ceramic tile array could be increased with a decreasing cell size What if another design is. produced that has a smaller cell side length but has a small zone of low performance near the. edges Which design is better will obviously depend on the actual ballistic data For. demonstration purposes several input parameters will be chosen arbitrarily and fixed to. demonstrate how the methodology can be applied, Take A 2m2 corresponding to a dispersion of about 500 mm the large cell edge length as. 100 mm and the small cell edge length as 90 mm Figure 2 shows the results for a value of. 1 mm for the width of the low performance zone,0 5 10 15 20 25. n Number of Impacts,D 100 mm delta 0,D 90 mm delta 1 mm. Figure 2 Probability of nonperforation P n given n impacts for two cell types. The methodology shows the value of having no zone of weak performance The calculations. continue with smaller cell sizes but the same value of delta The difference in armor. performance grows due to the fact that as the number of cells increases in the impact zone area. A the negative effect of increasing vulnerable area overwhelms the positive effect of having a. smaller cell size The methodology can easily be applied to more complicated situations where. delta is nonzero and varies with cell size, 3 3 Patterned Armor Performance Comparison for Different Armor Technologies. Consider now a patterned armor with the following properties First it has a very small cell. size 3 actual bullet diameters on a side Of course the calculations still use a bullet with. 0 diameter Second ballistic tests show this particular armor can defeat the threat bullet 19 out. of 20 times That is the threat is able to perforate the target 1 out of 20 times even when. impacting an undamaged cell Third there is no zone of low performance 0 These. characteristics are purely hypothetical and the example is offered to show how the proposed. methodology might be used to evaluate a patterned armor where performance across the cell face. may vary in a way that is different from that generally associated with ceramic tiles The armor. concept will be referred to as B4600 Armor, The methodology is modified in the following way The cell performance is modeled by. assuming that there is a 5 probability that any impact on an intact i e undamaged cell will. perforate the cell We use a square unit cell and proceed as before If the impact area. corresponding to a given dispersion is A then the probability P1 that the armor stops the first shot. P1 0 95 N D2 N D2 0 95 4, where N an integer is the number of cells contained in A and D is the length of the edge of the. unit cell As before N D2 is taken as equal to A a good approximation for large A and small. The second round will see a target with one unit cell missing as well as a 5 probability of. perforating each of the remaining N 1 unit cells Thus the probability of stopping the second. impact will be given by,P2 0 95 N 1 D2 N D2 5, For n shots the probability P of defeating them all is then. P n P1 P2 P3 Pn,0 95 N 0 95 N 1 0 95 N 2 0 95 N n 1 D2n N D2 n 6. P n 0 95n N N 1 N 2 N n 1 Nn 7, The B4600 Armor can be compared to a traditional ceramic tile array using equations 3 and 7. As in the previous example choice of input parameters is arbitrary and the comparison is made. for demonstration purposes only For the B4600 Armor a cell edge length of 25 mm is selected. Using an area of 2 m2 gives N 3200 Figure 3 compares the performance of B4600 Armor to. that of a patterned armor with a square cell 90 mm on a side and a delta of 1 mm. For these particular input parameters the B4600 Armor has a slightly lower performance for. n 5 but then outperforms the ceramic tile armor for n 5 Different results would be obtained. with different input parameters,3 4 Velocity Effects. Based on the dispersion characteristics of the threat weapon the methodology presented to this. point takes into account the effect of weapon to target distance range through an increase in. impacted area That is as the range is increased A increases and the probability of no.
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