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

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

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

History/Background Armoring troops has always been a concern.

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.

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.

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.

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.

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

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

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.

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.

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

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.

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

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.

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.

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.

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

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.

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.

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.

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