1. Benefits of Gelatin vs. Clay for Analyzing
Behind-Armor Effects in Body Armor Testing
Rob Kinsler
7 Oct 2008
APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED

2. Purpose
The purpose of this briefing is to illustrate the advantages of using ballistic gelatin instead of clay for body armor evaluation.

3. History/background on body armor testing
Overview
History/background on body armor testing
Early selection, use, and evaluation of gelatin and clay simulants
Blunt trauma testing
The momentum effect
V50 testing
Fragmenting projectiles and yaw
Costs
Summary and conclusions

4. History/Background
Armoring troops has always been a concern.

5. Problems With Animal Testing
Ethical/political concerns
Relatively high cost
Large amounts of time and labor
Varying responses
Species to species
Within species
Tissues within same animal
Animal testing provided the next best biofedelic response, but there were problems.

6. Historically Used Backing Materials
Some of the backing materials that have been used:
Water
Clay
Soap
Several densities of ballistic gelatin
In the 1940’s several alternative backing materials were examined.

7. Reasons For 20% Gelatin Selection
The similarity between bullet retardation in gelatin and muscle tissue
The homogeneity/reproducibility of the gelatin response to bullet penetration
Optical clarity of gelatin
The biophysics lab finally decided that 20% by weight ballistic gelatin provided the best homogeneous muscle tissue simulation.

8. ARL Input to Evaluation Standard
National Institute for Law Enforcement and Criminal Justice (NILECJ) wanted easier way to compare body armors
Prather compared response of 2 types of clay, gel, and live tissue
Prather determined that deformation depth vs. time of type 1 clay was similar to gel, which was similar to live tissue
But gelatin had it’s downsides. Relatively labor-intensive and required use of cumbersome equipment.

9. Time-deformation Data for Various Backing Materials
Baseline – Goat shots
Gel – 20% ballistic gelatin
Foam – Foam backing
1 – Type 1 clay
2 – Type 2 clay
Materials compared were goats (baseline), 20% gel, foam, type 1 clay and type 2 clay. Type 1 clay had response closest to gelatin.
Reference: Prather, Russell N., Conrad L. Swann, and Clarence E. Hawkins, “Backface Signatures of Soft Body Armors and the Associated Trauma Effects,” ARCSL-TR-77055, U.S. Army Chemical Systems Laboratory, November 1977

10. Clay and Gelatin Example
Photo of Prather’s original clay block and ball used in calibration.
Original Clay Block
Typical 20% Ballistic Gelatin Block

11. Prather’s Clay Caveats
Prather stated results were strictly provisional
Need to measure “base of cone”, in addition to depth to relate to incapacitation
Need to use formula consisting of aerial density of target, projectile mass, and velocity, in addition to depth and cone diameter to determine incapacitation
Prather never intended clay to be used as a standard.

12. Dr. Goldfarb thought 4.4 cm depth should be close enough for 6% P(I)
NILECJ’s Decision
Dr. Goldfarb thought 4.4 cm depth should be close enough for 6% P(I)
Dr. Goldfarb’s decision based on .38 caliber, 158 grain, lead projectile striking 7-ply, 400/2-denier Kevlar-29 armor at about 800 feet per second
Acknowledged that smaller limit might be appropriate for higher energy threats
NILECJ opted for simpler, conservative, uniform limit depth of 4.4 cm
Dr. Goldfarb (head of medical part of Bio-physics lab) was consulted contributed the 4.4cm depth that is considered the threshold depth to this date in the U.S.

13. Studies by Aerospace Corporation
Aerospace Corporation was sponsored by NILECJ
Metric not effecting backface signature:
Variation in momentum if K.E. kept constant
Metrics effecting backface signature:
Striking K.E.
Projectile mass
Projectile material type
Fabric denier
Number of fabric plies
Standoff distance
Aerospace Corporation, sponsored by NILECJ, tried to find correlation between clay, gel, and biological response of live tissue to blunt trauma.
Reference: The Aerospace Corporation, “Final Report, Protective Armor Development Program Volume II – Technical Discussion,” Prepared for National Institute of Law Enforcement and Criminal Justice, December 1974

14. Aerospace Corporation’s Conclusions
Clay deforms plastically
Greater energy absorption than air
Gelatin simulates human tissue better than either air or clay
Deforms elastically
Energy absorption characteristics between air and clay
Notable deficiencies in perfomance of clay
Considerable (~30%) elastic springback
Consistently smaller depths of penetration
They compared the effects of targets with air, clay, and gelatin backings.

15. Sample of Blunt Trauma Metrics Currently Available
Problem
Metric
MVWD
(Bio-Physics Lab)
Viscous Criterion
(Automotive testing)
CDF
(Aerospace Corporation)
mass of projectile, impact velocity of projectile, body mass of animal, and diameter of projectile
Viscous criterion
Conical Deformation Factor
Doesn’t account for dynamics (rate)
Not correlated to ballistic effects, neglects area of loading
Purely empirical, neglects area of loading
Some of the blunt trauma metrics measured in the past. MVWD – BioPhysics lab, Viscous Criterion – Automobile testing, CDF – Aerospace Corporation

16. Suggested Experimental Parameters for Blunt Trauma Analysis
Rate of deformation
Must account for shape of velocity curve
Must account for rate effects of impacted tissues
Depth of deformation
Must account for tissue interaction
Area of deformation
Must account for differences in dynamic response of armor
Mass of target involved in deformation
Used for determining K.E. or momentum (accounted for when determining area of deformation)
Basically, you need energy (Joules), depth, rate, and area.

17. Example of ½” Deformation Behind Hard Target
Results of 3 shots against hard target. Clay not able to
discern difference in momentum. Gelatin illustrates effect
of momentum. Notice that the armor plate didn’t move.
All three had ½” deformation on a hard target. The bottom 2 show the difference in how a 40 lb gel block responds.

18. V50 Testing
Gelatin can:
provide P(I|H)’s for penetrations
show fragmentation characteristics/deformation of projectile
show explicit path of projectile
Information from gelatin shots used as inputs to modeling and simulation
Clay can’t provide this information.

19. Perforation in Clay and Gelatin
Clay after perforating shot
Gelatin after perforating shot
Difference in response between clay and gel after a perforating shot.

20. Costs Gelatin Clay Materials – relatively small cost per block
Block reusable for blunt trauma
Man Hours – Half of time between shots than clay
Same time for Perforation or Blunt Trauma
Clay
Materials – larger initial cost, decreases with more shots
Block needs to be replaced more often with perforating shots
Man Hours – Twice time between shots than gelatin
Time increases for Perforations
Costs were one of the initial reasons the NILECJ selected clay. Since instrumentation costs have decreased, gel is now a viable option.

21. Relative Advantages of Gelatin
Attribute
Clay
Gelatin
Reusability +
Timeliness -
Logistics (material costs and preparation)
Shot Analysis
Collectable Data
Study of metrics for clay vs. gel.

22. capture/measure the primary blunt trauma effects.
Summary
Gelatin can be used to:
capture/measure the primary blunt trauma effects.
provide a single media for analysis of both blunt trauma and perforating injuries.
capture/measure the momentum effect in bullet-target interactions.
capture/measure kinetic energy deposition along the bullet’s path.
allow the explicit mapping of a projectile’s fragment path.
provide a dynamic photographic record of the event.
Reasons to use gel instead of clay.

23. Conclusions
Ballistic gelatin is better suited than clay for analyzing both blunt trauma and ballistic perforations
Ballistic gelatin is a cost-effective means to capture parameters vital in the analysis of body armor systems
More work is required in the future to map gelatin response to specific tissue response