**6. Results of the high impact tests on aramid/PP composite material**

A comparison of ballistic behavior was carried out between aramid/PP consolidated and independent laminates with 2 to 6 layers, both with the same fiber volume fraction (64%). Energy absorption is calculated with the projectile mass and at the ballistic limit of the laminate, which determines the kinetic energy that the composite material is capable of absorbing before failure. An increase in the number of consolidated laminates led to a nonlinear increase in absorbed energy (Fig. 15). This non-linearity is a consequence of the rigidization undergone by the material as it becomes thicker, thereby causing a restriction in its transversal deformation.

The independent laminates showed an approximate increase of 13% in energy absorption in comparison with their counterpart, consolidated laminates (Fig. 15). The amount of energy absorbed by each laminate is dominated by the rigidity of the material, which increases with the number of layers, resulting in a reduction in the slope of the curve. Composite materials based on epoxy resin show an abrupt change in the slope at four layers, in contrast with the thermoplastic composites in this study which did not register this behavior, even at six layers 20.

Fig. 16 shows energy absorption normalized as a function of the number of layers, from this it can seen clearly that an increase in the number of layers in an arrangement reduces its capacity to absorb energy; however, the reduction was lower for independent aramid/PP arrangements.

Advantages of Low Energy Adhesion PP for Ballistics 207

Restriction of adjacent layers

Fig. 17. Laminates subjected to impact, a) with restriction of deformation, b) without

this energy, making the process of energy absorption less efficient.

distance penetrated in the ballistic gelatine subjected to direct impacts.

y = 0.1636x - 5.6258

Fig. 18. Calibration curve for ballistic gelatine Bloom 250 at 10% weight

The results demonstrate how important rigidity of fiber-reinforced composite material is in the process of energy absorption during high impact; the mechanism by which a flexible laminate absorbs energy is completely different, the higher the flexibility of the laminate, the better the dissipation of energy. A laminate with restrictions of deformation concentrates

In order to carry out the tests with ballistic gelatine, it was first necessary to generate a calibration curve for the gelatine in relation to impact velocity; this curve is shown in Fig. 18, with the projectile velocity *vs.* length penetrated by the projectile. Linear regression of experimental data corresponds to penetration velocity of the projectile as a function of

40 50 60 70 80 90 100 110 120

**Velocity (m/s)**

restriction of deformation

No restriction to deform

Restriction of adjacent layers

**length penetrated (cm)**

Fig. 15. Energy absorption in consolidated and independent laminates subjected to high impact

This phenomenon of rigidization in laminates was explained by Rohchoon 20, who noted that in composite material subjected to high impact, flexibility is a crucial factor, since this deformation undergone by the material increases the period of contact with the projectile, thus giving the fibers more time to dissipate impact energy. At this point, the velocity of sound propagation in the fiber must be elevated in order to dissipate the highest amount of energy in the least possible time 21. In the case of a laminate with many independent layers, the adjacent layers impede posterior deformation, restricting energy absorption and consequently reducing the ballistic limit of the laminate. Deformation in a consolidated laminate is restricted only to its thickness, this means that the flexure forces present in a flexible laminate become localized tension forces, concentrating impact energy in small areas, and increasing the pressure experienced in these areas 20. Fig. 17 shows an example of this situation with a one-layer laminate.

Fig. 16. Energy absorption as a function of the number of layers in consolidated and independent aramid/PP laminates

Consolidated Independent

Fig. 15. Energy absorption in consolidated and independent laminates subjected to high

Fig. 16. Energy absorption as a function of the number of layers in consolidated and

23456

Consolidated Independent

**Layers**

This phenomenon of rigidization in laminates was explained by Rohchoon 20, who noted that in composite material subjected to high impact, flexibility is a crucial factor, since this deformation undergone by the material increases the period of contact with the projectile, thus giving the fibers more time to dissipate impact energy. At this point, the velocity of sound propagation in the fiber must be elevated in order to dissipate the highest amount of energy in the least possible time 21. In the case of a laminate with many independent layers, the adjacent layers impede posterior deformation, restricting energy absorption and consequently reducing the ballistic limit of the laminate. Deformation in a consolidated laminate is restricted only to its thickness, this means that the flexure forces present in a flexible laminate become localized tension forces, concentrating impact energy in small areas, and increasing the pressure experienced in these areas 20. Fig. 17 shows an example of

23456

**Layers**

impact

**Impact energy(J)**

this situation with a one-layer laminate.

independent aramid/PP laminates

**Impact energy (J)**

Fig. 17. Laminates subjected to impact, a) with restriction of deformation, b) without restriction of deformation

The results demonstrate how important rigidity of fiber-reinforced composite material is in the process of energy absorption during high impact; the mechanism by which a flexible laminate absorbs energy is completely different, the higher the flexibility of the laminate, the better the dissipation of energy. A laminate with restrictions of deformation concentrates this energy, making the process of energy absorption less efficient.

In order to carry out the tests with ballistic gelatine, it was first necessary to generate a calibration curve for the gelatine in relation to impact velocity; this curve is shown in Fig. 18, with the projectile velocity *vs.* length penetrated by the projectile. Linear regression of experimental data corresponds to penetration velocity of the projectile as a function of distance penetrated in the ballistic gelatine subjected to direct impacts.

Fig. 18. Calibration curve for ballistic gelatine Bloom 250 at 10% weight

Advantages of Low Energy Adhesion PP for Ballistics 209

high impact tests on aramid/PP consolidated laminates with four layers, energy absorption falls from 48 J in laminates tested without witness material to 30 J when the witness material is included, a 20% reduction. Fig. 21 shows a laminate which was tested under these

Fig. 20. Impact depth of witness material: commercial plastilene Modelina

Fig. 21. Four-layer consolidated laminate impacted in presence of witness material

Fig. 22. Witness material used to test impact on an aramid/PP sample

Trauma depth in witness material for the aramid/PP consolidated samples was 8.2 mm, this measurement was carried out from the unaltered surface of the witness material to the deepest

8.2 mm

The aramid/PP independent laminates with four layers were tested under the same impact as the consolidated samples, obtaining a trauma value of 8.13 mm. In presence of witness material, the independent laminates showed the same trauma values as the consolidated

point of impact. Fig. 22 shows a frontal image and cross-section of the witness material.

conditions.

Fig. 19 shows energy absorbed (ܧ௦) by a compact laminate of four layers in relation to impact velocity of the projectile. ܧ௦ was calculated with Equation 2, from impact energy (ܧ (and residual energy (ܧ௦). Where ܧ was calculated with the impact velocity registered in the chronograph and the mass of the projectile, and ܧ௦ was calculated as a function of the length penetrated in the ballistic gelatine.

Energy absorption was calculated in tests on four-layer consolidated laminates, with velocities below the V50, where composite material absorbs all the energy; however, on reaching velocities slightly higher than the ballistic level, a change occurs in energy absorption; this has been mentioned in other studies 13,22.

A linear behavior is clearly observed before the ballistic limit, due to the fact that the composite material has not failed. However, at velocities above the V50 a pronounced reduction in energy absorption is registered. Many theories have been proposed to explain this phenomenon, such as thermal effects, others mention a phenomenon called dishing (an indentation in the form of a dish). One of the most widely accepted theories is that of a reduction in the absorption period of impact energy.

Fig. 19. Energy absorption curve with respect to impact velocity in four-layer samples

Fig. 20 shows an image of plastilene, commercial brand Modelina, after the impact of a steel weight calibrated to determine the corresponding force-deformation. The NIJ standard 0101.04 used to characterize bullet-proof vests, requires the use of a steel weight (1.043 Kg) to measure trauma at high impact in body armor, this weight should form an indentation in the witness material to an average depth (δ) no greater than 20 mm ± 3 mm, calculated from five impacts. Results showed an impact depth of 19.97 mm, indicating that the commercial plastilene Modelina is an adequate witness material for use in determining trauma caused by an impact on an aramid/PP composite material.

One important difference in the test procedures for ballistic gelatine and witness material is that the latter must be in contact with the back of the laminate being impacted and must not be perforated. The fact that it is in contact with the armor plating means that an important mechanism of energy absorption is limited, i.e. posterior deformation. Consequently, during

Fig. 19 shows energy absorbed (ܧ௦) by a compact laminate of four layers in relation to impact velocity of the projectile. ܧ௦ was calculated with Equation 2, from impact energy (ܧ (and residual energy (ܧ௦). Where ܧ was calculated with the impact velocity registered in the chronograph and the mass of the projectile, and ܧ௦ was calculated as a

Energy absorption was calculated in tests on four-layer consolidated laminates, with velocities below the V50, where composite material absorbs all the energy; however, on reaching velocities slightly higher than the ballistic level, a change occurs in energy

A linear behavior is clearly observed before the ballistic limit, due to the fact that the composite material has not failed. However, at velocities above the V50 a pronounced reduction in energy absorption is registered. Many theories have been proposed to explain this phenomenon, such as thermal effects, others mention a phenomenon called dishing (an indentation in the form of a dish). One of the most widely accepted theories is that of a

V50

Fig. 19. Energy absorption curve with respect to impact velocity in four-layer samples

Fig. 20 shows an image of plastilene, commercial brand Modelina, after the impact of a steel weight calibrated to determine the corresponding force-deformation. The NIJ standard 0101.04 used to characterize bullet-proof vests, requires the use of a steel weight (1.043 Kg) to measure trauma at high impact in body armor, this weight should form an indentation in the witness material to an average depth (δ) no greater than 20 mm ± 3 mm, calculated from five impacts. Results showed an impact depth of 19.97 mm, indicating that the commercial plastilene Modelina is an adequate witness material for use in determining trauma caused

250 260 270 280 290 300 310 320

**Impact velocity (m/s)**

One important difference in the test procedures for ballistic gelatine and witness material is that the latter must be in contact with the back of the laminate being impacted and must not be perforated. The fact that it is in contact with the armor plating means that an important mechanism of energy absorption is limited, i.e. posterior deformation. Consequently, during

function of the length penetrated in the ballistic gelatine.

absorption; this has been mentioned in other studies 13,22.

reduction in the absorption period of impact energy.

by an impact on an aramid/PP composite material.

30

35

40

**Absorbed energy (J)**

45

50

55

high impact tests on aramid/PP consolidated laminates with four layers, energy absorption falls from 48 J in laminates tested without witness material to 30 J when the witness material is included, a 20% reduction. Fig. 21 shows a laminate which was tested under these conditions.

Fig. 20. Impact depth of witness material: commercial plastilene Modelina

Fig. 21. Four-layer consolidated laminate impacted in presence of witness material

Fig. 22. Witness material used to test impact on an aramid/PP sample

Trauma depth in witness material for the aramid/PP consolidated samples was 8.2 mm, this measurement was carried out from the unaltered surface of the witness material to the deepest point of impact. Fig. 22 shows a frontal image and cross-section of the witness material.

The aramid/PP independent laminates with four layers were tested under the same impact as the consolidated samples, obtaining a trauma value of 8.13 mm. In presence of witness material, the independent laminates showed the same trauma values as the consolidated

Advantages of Low Energy Adhesion PP for Ballistics 211

The authors would like to express their gratitude to the Consejo Nacional de Ciencia y Tecnologia, CONACyT, for their support in the funding of this project, CB-2008-01-101680.

Instituto de Ciencia y Tecnología de Polímeros, Ciencia y tecnología de materiales poliméricos. Instituto de ciencia y tecnología de polímeros, Madrid; 2004, 41. Gay, Daniel,. Hoa, Suong,. & Tsai, Stephen. (2003). Composite materials, design and

Chiou, Minshon,. Ren, Jianrong,. Van, Zijl,. & A, Nicolas. (2001), Artículos Balísticos resistentes a la penetración, Oficina española de patentes y marcas, España Craig, Benjamin. (2005), High performance fibers for lightweight armor, Available from

Mark, Herman. (2004). Polyamides Aromatic, Encyclopedia of Polymer Science and

 < http://www.wiley.com/WileyCDA/Wiley Title/productCd-0471275077.html> Anand, Kulshreshtha,. & Vasile, Cornelia. (2002), Handbook of polymer blends and

/web/portal/browse/display?\_EXT\_KNOVEL\_DISPLAY\_bookid=2219>

Bryan, Cheeseman,. & Travis, Bogetti. (2003), Ballistic impact into fabric and compliant

Donald, Carlucci,. & Sidney, Jacobson. (2008), Ballistics: theory and design of guns and

Haruntun, Karian. (2003), Handbook of Polypropylene and Polypropylene Composite,

Lawrence, Drzal,. Pedro, Herrera-Franco,. & Hence, Ho. (2000), Fiber-Matrix Interface Tests.

Jorma, Jorma. (2004), Preparing ballistic gelatine - review and proposal for a standard

Paul, Wambua,. Bart, Vangrimde,. Stepan, Lomov,. & Ignaas, Verpoest. (2007), The response

Andrew, Merkle,. Emily, Ward,. James, O'Connor,. & Jack, Roberts. (2008), Assessing Behind

Gamboa, Ricardo. (2009), Estudio de la Absorción de Energía al Impacto en Laminados

S. Reid,. R, Zhou. (2000), Impact Behaviour of Fibre-Reinforced Composite Materials and

Human Torso Models, Lippincott Williams & Wilkins, Hagerstown

*Comprehensive Composite Materials*, *Anthony Kelly and Carl Zweben*, pp. 71-111, Oxford: Pergamon, Retrieved from <http://www.sciencedirect.com/science/

of natural fibre composites to ballistic impact by fragment simulating projectiles,

Armor Blunt Trauma (BABT) Under NIJ Standard-0101.04 Conditions Using

Fibroreforzados a Base de Fibras de Aramida y Matriz de Polipropileno, Instituto

composites, Rapra Technology Limited, Retrieved from < http://www.knovel.com

<http://ammtiac.alionscience.com/pdf/AMPQ9\_2ART02.pdf>

Technology, Wiley, pp. 558-584, Retrieved from

Registro Nacional de Armas. (2001), Chalecos Antibalas, Available from:

< http://www.aicacyp.com.ar/disposiciones\_legales/MA\_01\_seg.pdf>

composite laminates, Compos Structure, Vol. 61, pp. 161-173

method, Forensic Science International, Vol. 141, pp. 91-98

Composite Structures, Vol. 77, pp. 232-240.

Carolina Protect Ballistic. (2010), Ficha Técnica Comercial, 2010

Tecnologico Superior de Motul, Motul Yucatan

Structures, Woodhead Publishing

**8. Acknowledgement** 

applications, CRC Press LLC

ammunition, CRC Press

book/9780080429939>

Marcel Dekker

**9. References** 

laminates. However, in tests under the same conditions on body armor without polymeric matrix (four layers of aramid fabric), a trauma depth of 11 mm was observed, which is 27% greater than that of the laminates with polymeric matrix. Images of an impacted armor-plating without a matrix are shown in Fig. 23, where it can observe more clearly the diamond-shaped impact formation, as well as a more pronounced impression of the projectile.

Table 3 presents the values of impact energy and trauma depth generated in the materials. It is important to remember that the maximum trauma depth permitted by the standard is 40 mm. These results demonstrate that the presence of the PP matrix improves posterior deformation of a material even though its configuration allows flexibility, as in the case of independent laminates. In contrast, absence of the matrix increases posterior deformation by 27%. It is also important to take into consideration that the presence of a witness material in contact with the back of the laminate reduces the ballistic limit of the aramid/PP composite material by 20%.

Fig. 23. Four layers of aramid fabric without PP tested with witness material


Table 3. Ballistic properties of armor plating tested in presence of witness material
