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Report | April 2017

Ballistic protective garments' performance evaluation

Ballistic limit testing is intended to evaluate armor by testing either a single armor or multiple armor or multiple armor samples, aver Sakthi Bala Murugan G and Gobi Kannan T.

The main ballistic threats to military personnel are fragmenting projectiles rather than bullets. The projectiles originate from grenades, mortars, artillery shells, mines, and improvised explosive devices (IEDs). Other threats are low velocity bullets from hand guns, and high velocity bullets from rifles and machine guns. Compared to typical apparel, ballistic protective garments are made with specialized materials, innovative fastening systems, and unique designs. Ballistic Protective clothing systems are often complex and require user training. Protective garments are worn with auxiliary equipment and it is necessary to examine the comfort and performance of the entire ballistic protective clothing system. There are certain standard test methods like NIJ (National Institute of Justice) to evaluate the performance of ballistic protective clothing systems. The performance of ballistic protective garments classified according to their end uses. It can be mentioned as follows: ballistic resistance, ballistic V50 velocity limit, blunt trauma, fragment protective and residual velocity.

Procedures to assess performance of ballistic garments

V50 ballistic limit testing:Ballistic limit testing is intended to evaluate armor by testing either a single armor or multiple armor or multiple armor samples. The testing results are to establish a 50 per cent probability of penetration only. The full penetration curve not explored. V50 values are based on average equal numbers of velocities associated with complete penetration (witness panel penetrated) and partial penetration (witness panel not penetrated). To prevent skewing of the results, the lowest velocity associated with complete penetration and the highest velocity associated with partial penetration, must be included in the V50 calculation. If the lowest velocity associated with complete penetration is lower than the highest velocity associated with partial penetration, the arithmetic difference should be indicated in test results. In other hand, V50 procedures for helmets mainly requires that the helmet should be divided in to five sections-crown, front, back, left and right side.

Test procedure for V50: Figure 1 shows the experimental setup for V50 ballistic limit testing. In first step, place the triggering devices 2 and 3 m (6.6 and 9.8 ft), respectively from the muzzle of the test weapon as shown in Figure 1. Measure the distance between them with an accuracy of 1.0 mm (0.04 inch) and calculate the velocity of each test round by using the time of flight and distance measurements. After the specified test weapon has been supported, leveled, and positioned, fire one or more pretest rounds (as needed) through a witness plate to determine the point of impact. Place the test specimen in the support fixture and keep 5 m (16 ft) position from the muzzle of test weapon. Then position of un-perforated witness plate is 15 cm (6 inch) which is beyond the test specimen. Fire a test round and record the velocity of the bullet as measured by the chronograph. Examine the witness plate to determine penetration, and examine the specimen to see if the bullet made a fair hit. If no penetration occurred, reposition the test specimen and repeat the procedure with additional test rounds until the test is completed. Space the hits should keep as even as possible so that every portion of the test specimen is subject to test.

Ballistic resistance testing: Ballistic resistant testing is intended to confirm, compliance with minimum performance requirements of ballistic resistant materials. This testing may be conducted with the same or different ballistic threats, which used to conduct V50 testing of same material. It may conduct on backed or unbacked, flexible, or rigid, material. Ballistic protective garments are usually flexible as well as in rigid body armor; ballistic resistance testing of textile material is usually conducted with the armor sample backed with a deformable material, such as non hardening modeling clay, to more nearly simulate actual usage and to support blunt trauma evaluations. Ballistic resistance testing is the limited value to determine the relative performance of two or more candidate armors. Specifically, if two armor samples pass a ballistic resistant test, one does not know which of the two armor sample is having more resistance for ballistic penetration. Ballistic resistant testing of helmet requires a specialised rigid armor testing due to shape and standoff between the helmet and the skull. The head form used for this testing has been slotted –front-to- back and side- to-side –to accommodate a penetration witness panel at specified distance from the inside surface of helmet. The helmet are pre-marked to establish a test in five areas which includes crown, front, back, side, left and right side. Testing may include back face deformation, and penetration testing. Test procedure for ballistic resistant testing is same as v50 limit test. Figure 2 shows the experimental setup for ballistic resistant testing.

Blunt trauma testing: Blunt trauma testing, often refer to as back-face-deformation testing, is usually conducted same as ballistic resistant testing, and may performed in rigid or flexible material (like clay). The test samples are packed with a substance (usually non-hardening modelling clay) that will readily deformed by back face deformation of the material, from which deformation can be determined. Figure 3 and 4 shows the deformation non-hardening modelling clay pack.

Residual velocity testing: Residual velocity testing is not intended to determine the resistance of an armor sample to penetration, but it finds the potential lethality of a projectile after penetrated to the armor. It is conducted with V50 testing. The difference between residual velocity (vr) and striking velocity (vs) is used to assess the level of lethality [vs-vr] to be expected from ballistic threats whose velocity exceeds the level of protection of the material.

Fragment protective performance: In order to provide protection from fragmenting ballistic threats, combat body armor contains multiple layers of fabric is need to be utilised for testing. In current work, one or two layers of commercially available para-aramid fabric can be used which is having high fragment protective capabilities. The incorporation of such one or two layers of para-aramid woven fabric provides a level of fragment production with only a minimum associated increase in stiffness and mass. The use of one and two layer para-aramid woven fabric in clothing offers some protection against threats from fragments.

Test procedure: Figure 5 shows the experimental setup for fragment protective performance testing. The fragment protective performance of each specimen is measured by using fragment simulating projectiles (FSPs). These fragments typically have a mass of 0.15-0.25 grams and initial velocity of 1,500-2,000m/s. Velocity of small fragments are typically slower than 600m/s, which is most likely responsible for many of the injuries observed. Each FSP was mounted in a split polymeric sabot and inserted into 7.62 mm x 39 mm cartridge case (shown in a Figure 6). The barrel was located 1500 mm from this specimen. Fabric specimens were mounted on a plywood support (10 mm x 400 mm x 400 mm) which was clamped on G-clamps, and 0.5 mm thick alloy witness sheet was mounted 150mm begin the specimen. Velocity was manipulated by altering amount of gun power used in the cartridge case. Projectile velocity was measured by using a weibel fixed head Doppler radar (model w700). In between shots the specimen was adjusted, if required, and the G-clamps re-adjusted. Here 16 impacts were possible on each specimen. A barrel mounted laser aiming device was used to ensure the accuracy of impact point. A perforation of specimen can be defined from the perforation of witness sheet. Impacts which were not identified as perforations using this definition were recorded as non perforation. FSP perforated the witness plate were recorded and used to determine an estimated V50 for each specimen.

The relative energy absorbed data were calculated that is normalising the kinetic energy equivalent to the estimated V50 mass per unit area. As well as perforation and non-perforation evidence of fragment protective layer also assessed by witness sheet.

Conclusion The major goals of this paper were to identify the risks and levels of injury associated with velocity impacts to the wearer and determine a test method to evaluate ballistic materials in relation to levels of safety, and evaluate ballistic materials in relation to the levels of safety. Methods used today to evaluate the performance of ballistic protective garments like ballistic resistance, ballistic V50 velocity limit, and blunt trauma, fragment protective and Residual velocity. Here V50 is used to assess ballistic limit of the material, Ballistic resistance testing is used to confirm minimum performance requirements for body armour and Blunt trauma testing is used to assess the back face deformations along with ballistic testing. These test set up and the assessment of several ballistic materials provided a basis for future research into ballistic impact protection as well as evaluation of ballistic protective garments are offers unique capabilities to improve the design of body armour and improving technical performance, which can result in reduction of body armor failure and will save the peoples from injury or death.

References
  1. U.S. Army Research Laboratory, Test Method Standard for V50 Ballistic Test for Armor, 18 December 1997.
  2. Albert L. Chang, JTCG/AS Interlaboratory, Ballistic Test Program—Final Report, December 1997.
  3. Edited by Richard A. Scott, Textiles for Protection, first published 2005 at Cambridge.
  4. Int. J. Impact Eng, Characteristics of woven fabric under small projectile impact, 1995.
  5. U.S. Department of Justice National Institute of Justice, Ballistic Resistant Protective Materials.
  6. Paul V. Cavallaro Ranges, NUWC-NPT Technical Report Soft Body Armor, An Overview of Materials, Manufacturing, Testing, and Ballistic Impact Dynamics, August 2011.
  7. Body Armor Workshop NIST, Ballistic-Resistant Body Armor Selected Research Initiative programme, Gaithersburg, Maryland; Kirk Rice NIST- November 29, 2011.
  8. Sarah, Debra Carr & Ian horsfall, Fragment Protective Performance of One-and-Two-Layer Para-Aramid Woven Fabric, Textile Research Journal, volume-82 July 2012.
  9. DuPont, www.kevlar.com-Kevlar Aramid Fibre, 1994.

Sakthi Bala Murugan G and Gobi Kannan T are from the Department of Textile Technology, Bannari Amman Institute of Technology, Sathyamangalam - 638 401, Erode District, Tamil Nadu. Gobi Kannan T can be contacted at: Email ID: gobikannant@gmail.com, Mob: 91- 9894935446.

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