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Protection Against Secondary Fragmentation From AP Mines Based on Natural Fibre Composites

The 1997 Ottawa Convention1 defines a mine as "a munition designed to be placed under or near the ground or other surface area and to be exploded by the presence, proximity or contact of person or a vehicle." An AP mine is defined as "a mine designed to be exploded by the presence, proximity or contact of a person and that will incapacitate, injure or kill one or more persons."


Figure 1: Typical secondary fragmentation.5
c/o Andy Smith
AP mines are either fragmentation or blast types and are equipped with high explosives (chemicals that can detonate2). Fragmentation mines (such as POMZ [75 g TNT]) are normally triggered by a tripwire and project metal shards at very high speeds toward the victim. Ballistic threats to personnel are caused by fragmentation as opposed to blast effects.3 Blast mines (e.g., PMN [40 g TNT]) cause injuries through direct or indirect blast effects. Direct blast effects are those involving changes in environmental pressure due to the occurrence of an air blast. Blast waves may therefore cause injuries to a deminer through overpressure. On the other hand, blast effects can be subdivided into secondary effects, tertiary effects (whole body displacement) and miscellaneous effects (e.g., dust).4

Secondary effects include secondary fragmentation from mine blast casing, inner mine components, scree, surrounding dirt, gravel, fragmented demining tools (such as those depicted in Figure 1), etc., all of which are blasted at the victim at different speeds depending on the mass of the projectiles. The extent of injury depends on the mass, velocity, shape, density and angle of impact of the fragments.4

The level of protection provided by an armour composite material depends on its energy absorption capability, which is in turn influenced by the type of reinforcing fibres and fabrics, number of fabric layers, areal density6 and compressive strength.7

The current protective clothing (flak jackets) and rigid armour for deminers are manufactured from high-performance fibres such as aramid (Kevlar®, Twaron®) fibres and their composites respectively. The envisaged benefits of composite materials may not be attained with just one type of fibre. Instead, a hybrid system consisting of different fibre types and/or non-fibre materials such as metals and ceramics may be used to achieve the desired properties. Previous writers have pointed out the benefits of hybridisation.8–12

Natural fibres are abundantly available in developing countries. These fibres are cheap and come from renewable resources.13 This paper investigates the possibility of developing a cheap AP mine-protective composite plate that can be manufactured using locally accessible materials and technology in developing countries. The research focuses more on the threat of secondary fragmentation (primary fragmentation results from fragmentation mines) caused by blast mines because in the countries where demining work is concentrated, most injuries are caused by AP blast mines.

Materials and Sample Preparation

Plain woven flax fabric (areal density 280 g/m2, 10 ends and picks/cm) was purchased from Libeco, Lagae, Meulebeke and Belgium, while polypropylene in sheet form was supplied by Japan GMT Co. Ltd. Details of other natural fibre composites tested in the ballistic study can be found in a paper titled "The Response of Natural Fibre Composites to Ballistic Impact by Fragment-Simulating Projectiles," submitted for publication in Composite Structures.

The fabric and polypropylene sheet were cut into 30-cm square pieces that were stacked and wrapped in aluminium foil. The composite samples were processed by compressing the stacking in a compression moulding press at a pressure of 6.4 bar (0.64 MPa) on the material at 190 C for 15 minutes. The resulting fibre volume fraction varied from 46 to 58 percent by reducing the number of polypropylene sheets. The effect of steel was investigated by bonding thin (0.8 and 1.5 mm) mild steel plates onto the composites with epoxy glue.

Ballistic Testing in the Laboratory

The current standard for measuring the effectiveness of a material against ballistic fragmentation is the North Atlantic Treaty Organization Standardization Agreement (NATO STANAG) 2920.14 The aim of the STANAG 2920 is the determination of the so-called V50 performance. V50 is the velocity for which the probability of perforation of the chosen projectile is 0.5. For demining equipment, a V50 of 450 m/s related to a fragment-simulating projectile (FSP) of 1.1 g is standard.


Figure 2: Block Manometric Cannon Interchangeable (BMCI)

The weapon used was a BMCI (Figure 2) and the FSP (Figures 3a and 4) was chisel-nosed, had a diameter of 5.385 mm and was made of alloy steel with a Rockwell hardness of 30 " 2. The mass of the FSP was 1.1 grams. The propellant used was ball powder 0.50-inch blank and the twist was seven inches per revolution. The room temperature during the tests was maintained at 22 C.


Figure 3: (a) Fragment-simulating projectiles (FSPs), (b) cases (c) propellant powder

Figure 4: Dimensional details of the FSP. ØA = 5.385 mm, B = 2.54 mm and C = 6.35 mm.14

The natural fibre composite panels and composite steel hybrid structures were clamped on a mild steel stand placed 10 meters (10.94 yards) from the weapon. The mean velocity of the projectiles was calculated with the help of a chronometer that measured the projectile flight time between two measuring bases 2 meters (2.19 yards) apart. The projectile impact and residual velocities were required to calculate the amount of kinetic energy absorbed by the target. A Doppler radar antenna linked to a computer was used to determine the velocity of the projectile after perforation.

Results of Laboratory Tests and Discussion

Sample VF = 46% Sample Code Total Thickness (mm) Areal Density (kg/m2) V50 (m/s)
Flax composite F26 12.9 14.5 312
Flax steel faced hybrid F26S 14.4 26.3 466
Flax steel faced and backed hybrid SF26S 14.5 26.7 576
Plain steel PS 1.5 11.8 264
Table 1: Results of ballistic tests.

Initial ballistic tests. The results of the ballistic tests are shown in Table 1. Whereas the plain flax composites tested did not meet the criteria set by the NATO STANAG 2920 standard (V50 = 450 m/s), the composite mild steel hybrids attained a V50 of at least 466 m/s. Despite the low V50, the flax composite panels were earmarked for field tests since the envisaged threat and the secondary fragmentation from blast AP mines is considered less than that posed by primary fragmentation from fragmentation mines. Most of the secondary fragmentation is usually of much lower density and larger diameter than metallic primary fragments. The irregular shape and larger surface area presented to the armour material further decreases the possibility of its complete penetration of the material.

Fibre Volume Fraction (%) Flexural Modulus (GPA)
46 5.22 ± 0.13
52 5.83 ± 0.31
55 6.47 ± 0.18
58 5.71 ± 0.16
Table 2: Effect of fibre volume fraction on the flexural modulus of flax composites.

Optimised solution. The ballistic performance of composites can be improved by processing to high fibre volume fractions. Depending on the application, the most suitable material for ballistic protection provides a good balance between weight, comfort, cost and the level of protection. While it is possible to use very low resin contents in synthetic fibre (e.g., Kevlar, glass) composites, natural fibre composites presented wetting problems at high fibre volume fractions. A flexural (three-point bending) test was conducted to monitor the bonding at the interface so as to ensure the mechanical integrity of the composite panel. The flexural modulus of the flax composites increased with increasing fibre volume fraction up to Vf = 55 percent, then showed a decrease at Vf = 58 percent as demonstrated in Table 2. A fibre volume fraction of 52 percent was utilised in the processing of samples for the field tests. The increase in the fibre volume fraction resulted in a decrease in the composite thickness and areal density (which in turn cause a reduction in the V50). With the said fibre volume fraction (52 percent), the composites presented a good balance between weight, thickness and V50. Table 3 presents the parameters of the composite and composite/steel hybrid solution for this research work. These two materials were used for the field tests.

Code Material Flax Fabric Layers VF (%) Thickness (mm) Areal Density (kg/m2) V50 (m/s)
F26FT Flax composite 23 52 9.9 11.3 280
SF26SFT Composite/
Steel hybrid
26 + steel plates
(2 x 0.8 mm)
52 11.6 24.2 489
Table 3: Flax composite and composite/steel hybrids optimised for field tests.

Ballistic Field Testing


Figure 5: An illustration of the field test setup.

Figure 6: C4 explosives placed in a PMN mine casing.

Simulated AP mines containing C4 explosives were utilised in the field tests carried out at the NATO test zone at the Houthalen-Helchteren shooting field in Belgium.

Experimental setup. The panel was placed in front of a wooden support fixed to the ground as illustrated in Figure 5. The test was conducted using 35 g, 70 g and 150 g of C4 explosives (see Figure 6) to simulate the small, medium and large AP mines. The explosives were placed in the ground at a distance of 30 and 50 cm (50 cm only for plain composites) from the target and covered with different kinds of projectiles, such as stones, to increase the amount of secondary fragmentation and to simulate demining accidents. The results of the field tests are as summarised in Table 4.

Field Test Number Composite Type (flax) Test Setup Distance from Mine (cm) Mass of C4 Explosives (g) Result
1a hybrid fixed/no back support 30 70 slight debonding
1b hybrid fixed/back support 50 70 minor front scratches
1c hybrid fixed/no back support 30 150 thrown 5 m, slight debonding
1d hybrid fixed/back support 50 150 thrown 2 m, no debonding
2a composite fixed/back support 50 35 surface dents,  fibre fracture
2b composite fixed/back support 50 70 fibre fracture, surface dents, crack at rear
Table 4: Summary of field tests.

Composite/steel hybrids.
Two samples were tested at a time at 30 cm and 50 cm respectively from the simulated mine. The distances were measured from the face of the panels to the centre of the simulated mine. These distances were shorter than the representative field operating distances from a mine to the sternum derived from field measurements by deminers, i.e., 65–70 cm.16 The differences in the distances may produce significant blast effects (pressure has been found to fall as the inverse cube of the distance from the blast17–19), but it was assumed that the differences in the distances selected did not produce a significant change in the velocities of the secondary fragmentation.

The simulated AP mines used for composite steel hybrids contained 70 g and 150 g of C4 explosives to mimic common medium and large AP mines used in many countries. The explosives were placed in two casings of a PMN mine (see Figure 5), which is the largest AP mine, and covered. After detonating high explosive material, almost 100 percent of the energy liberated is transformed into blast energy,3 which propels secondary fragmentation at high velocities.


Figure 7: Complete debonding of the front steel plate after detonation of 150 g of C4 at 30 cm from the simulated mine.

Figure 8: Front face of the composite/steel hybrid after detonation of 150 g of C4 at 50 cm away.

Tests using 70 g of C4. Tests on sample SF26SFT, the composite/steel hybrid placed 30 cm from the simulated mine, revealed a small debonding between the front 0.8 mm mild steel plate and the composite at one corner. There was no noticeable damage on the front or rear surface of the material.

Apart from small surface scratches on the front side steel plate, there was no visible damage on sample SF26SFT tested at 50 cm from the simulated mine.

Tests using 150 g of C4. Sample SF26SFT, placed 30 cm from the simulated mine, showed complete debonding of the steel plate on the front side of the composite/steel hybrid system (see Figure 7). The debonded steel plate had small dents on the surface. A small debonding between the backside steel plate and the composite was seen at one corner of the composite/steel hybrid system. There was, however, no penetration on the material by a projectile.

Apart from surface scratches (Figure 8), Sample SF26SFT at 50 cm from the simulated mine did not show any penetration or debonding, possibly due to the reduced blast.


Figure 9: Front damage on the composites after detonation of 35 g of C4 at 50 cm away.

Flax fabric reinforced polypropylene composites—tests using 35 g of C4. Figure 9 demonstrates damage that occurred on the front side of the composite panel after detonation of 35 g of C4 explosives. Numerous surface dents and fibre fractures as a result of projectile hits are clearly visible. One projectile caused barely visible damage at the back side of the composite. No complete projectile penetration was observed.

Flax fabric reinforced polypropylene composites—tests using 70 g of C4. Visual observation after detonation of the 70 g of C4 explosives indicated fibre failure and numerous dents (sizes ranging from small to large) on the front side of the flax composites as shown in Figure 10 (below). Several areas of visible damage were observed at the back of the plate. Figure 11 (below) indicates a nearly transverse crack at the back of the composite; however, none of the projectiles was very close to the actual penetration.


Figure 11: Crack at the back of the composite panel after detonation of the 70 g of C4 explosives.

Figure 10: Flax composite front side damage after detonation of 70 g of C4 explosives.

These tests show that in order to protect a deminer against secondary fragmentation from small to medium AP blast mines, a natural fibre composite protective material with a V50 less than the standard 450 m/s may be sufficient. For large AP mines, a composite steel hybrid system may provide the required protection, but care should be taken to prevent possible injury from steel plates in case they debond.

Conclusions

In this paper, it has been shown that composites based on natural fibres can be alternative materials for anti-ballistic protection against secondary fragmentation in situations of detection and clearance of AP (blast types) landmines. The performance per areal weight, both in terms of V50 and critical absorbed energy at penetration, reaches the highest value when the natural fibre composites are covered at the front and back with thin (0.8-mm) steel plates.

It is most probable that even better solutions exist using high-tech aramid fibres or special ballistic steels, but these materials are very expensive for people in developing countries threatened by landmines. Composite materials based on readily available natural fibres and commodity polypropylene, faced with cheap mild steel plates, could be a possible alternative. Field tests have been carried out and the preliminary results support this conclusion.

Acknowledgements

It is with gratitude that I acknowledge the staff at the Laboratory of the Department of Weapon Systems and Ballistics at the Royal Military Academy, Brussels, for their assistance with the ballistic impact experiments and the staff at DOVO (service for demining and destruction of explosives), Heverlee and NATO Houthalen-Helchteren shooting field, Belgium, for helping with the field tests.

This work has been supported financially by APOPO (www.apopo.org), a Belgian non-profit organisation developing alternative technologies for detection of AP mines and the Research Council, Katholieke Universiteit Leuven, Belgium.

*All photos courtesy of Paul Wambua

Endnotes

  1. Convention on the Prohibition of the Use, Stockpiling, Production and Transfer of Anti-Personnel Mines and on Their Destruction. Ottawa, 1997; http://www.icbl.org/treaty/.
  2. Cooper, P.W. and S.R. Kurowski. Introduction to the Technology of Explosives. New York: Wiley-VCH, 1996.
  3. Smith, P.D. and J.G. Hetherington. Blast and Ballistic Loading of Structures. Oxford: Butterworth-Heinemann; 1st edition, 1994.
  4. Baker, W.E., et al. Workbook for Predicting Pressure Wave and Fragment Effects of Exploding Propellant Tanks and Gas Storage Vessels. NASA CR-134906, NASA Lewis Research Centre, vol. 1, 1996.
  5. Smith, A. "IMAS and PPE Requirements." Journal of Mine Action 2003; 7.1: 43–46. http://maic.jmu.edu/journal/7.1/focus/smith/smith.htm.
  6. Cunniff, P. "An Analysis of the System Effects in Woven Fabrics Under Ballistic Impact." Textile Research Journal, 1992; 62(9):495–509.
  7. Weatherall, J., M. Rappaport and J. Morton. "The Outlook for Advanced Armour Materials." Proceedings of the 22nd International SAMPE Technical Conference, Boston. 6–8 November 1990; pp. 1070–77.
  8. Larsson, F. and L. Svensson. "Carbon, Polyethylene and PBO Hybrid Fibre Composites for Structural Lightweight Armour." Composites: Part A 2002; 33: 221–231.
  9. Noton, BR. "General Use Considerations." Dostal, C.A. and M.S. Woods, Eds. Engineered Materials Handbook, Composites. Ohio: ASM International, 1993. Vol. 1: p. 35–37.
  10. Bhatnagar, A.L., L.C. Lin, D.C. Lang, H.W. Chang. "Comparison of Ballistic Performance of Composites." Proceedings of the 34th International SAMPE Symposium, Reno, Nevada, 8–11 May 1989. pp. 1529–37.
  11. Cheeseman, B.A., T.A. Bogetti. "Ballistic Impact into Fabric and Compliant Composite Laminates." Composite Structures, 2003:61; 161–173.
  12. Disselbeck, D. "Composites: A Lost Opportunity or an Exciting Future." Polyester: Tomorrow's Ideas and Profits, Manchester: The Textile Institute, 1993; pp. 258–261.
  13. Wambua, P., J. Ivens, I. Verpoest. "Natural Fibres: Can they Replace Glass in Fibre Reinforced Plastics?" Composite Science and Technology, 2003:63; pp. 1259–1264.
  14. NATO. Ballistic test method for Personal Armours, STANAG 2920, 1st edition, Nov. 1992.
  15. Pirlot, M. ERN ABAL AB811. "Eléments de balistique—Balistique Extérieure des Projectiles Classiques." Bruxelles, 2002; pp. 60–62.
  16. Makris A. and J. Nerenberg. "A Full Scale Evaluation of Lightweight Personal Protective Ensembles for Demining in Providing Protection Against Blast-Type Anti-Personnel Mines." Journal of Mine Action 2000; 4.2: http://maic.jmu.edu/journal/4.2/Focus/Fse/fullscale.htm.
  17. Bass, C.R. "Development of a Procedure for Evaluating Demining Protective Equipment." Journal of Mine Action 2000; 4.2: 18–23. http://maic.jmu.edu/journal/4.2/Focus/Bass/bass.htm.
  18. Nerenberg, J., J.P. Dionne and A. Makris. "PPE: Effective Protection for Deminers." Journal of Mine Action 2003; 7.1: 47–51. http://maic.jmu.edu/journal/7.1/focus/nerenberg/nerenberg.htm.
  19. Wambua, P., B. Vangrimde, S. Lomov and I. Verpoest. "An Anti-Personnel Mine Protection Based on Natural Fibres. Proceedings, 4th International Symposium on Materials from Renewable Resources, MesseCongressCenter. Erfurt, Germany, 11–12 September 2003, CD paper No S1-14.

Contact Information

Eng. Dr. Paul Wambua
Moi University
Department of Textile Engineering
P.O. Box 7074 Eldoret
Kenya
E-mail: paulwambua@yahoo.com 

Department of Metallurgy and Materials Engineering
Katholieke Universiteit
Leuven, Kasteelpark
Arenberg 44, B-3001 Leuven
Belgium

Department Weapon Systems and Ballistics
Royal Military Academy
30 Renaissancelaan
B-1000 Brussels
Belgium