**3. Characterization of the composite material at high impact**

To characterize the material at high impacts, the standard STANAG 2920 established by NATO (North Atlantic Treaty Organization) was used. The purpose of this standard is to characterize at high impact any material with ballistic applications, a bulletproof vest, ballistic or combat helmets, or any kind of material produced with this purpose in mind. The projectile used may be a bullet, from which protection is required. For this, a non deformable, spherical, steel projectile (1.11 g) is commonly employed as it offers the highest ballistic limit in high impact tests in comparison to other shapes, compared to ogival, blunt or pointed projectiles 8. The parameter defined in these tests is the ballistic limit (V50), which is defined as the velocity at which a material fails 50% of the times it is impacted; this parameter, which is calculated with Equation 1, has a statistical origin due to the stochastic

mathematical analysis required for this parameter, no studies were carried out until the last decade; in those studies the only indications of this value are reported with techniques such as pull-out test, which has resulted in considerable controversy due to the lack of a standard

The methods for determining the interfacial shear stress can be direct or indirect. Direct methods are those which are analyzed from a micromechanics perspective, where a small representative sample of the unit is used. Some methods we can mention are fiber pull-out, fragmentation, single fiber micro-indentation and single fiber compression; however, due to the close relationship this parameter has with the mechanical properties of the composite, there are also indirect methods to determine interfacial shear stress, which are analyzed from a micromechanical perspective and where the behavior of the whole unit is analyzed in order to determine the levels of interfacial adhesion in the composite. These methods include the variable curvature method, slice compression, ball compression and bundle pullout. There are also some methods which are analyzed at a macromechanical level and by conventional tests; these are able to relate values of tension, compression or flexure at

As it has already established, interfacial adhesion plays an important role in the properties of a material; however, what role does it play in high impact properties? Studies focusing on improved interfacial adhesion in composite materials at tension, compression and shear abound in the literature; however, in high impact it does not appear to be particularly sought after. This is demonstrated in a study carried out in Rohchoon Park, where an aramid/vynilester composite material is characterized at high impact. The material received a superficial treatment to improve adhesion and it was possible to observe a reduction in the ballistic limit when this property is increased. This is precisely where PP can play a very particular role. In addition to all the properties found in PP, it also presents poor interfacial adhesion with practically any material due to its incapacity to generate covalent links. The aim of this work, therefore, is to demonstrate how a polymer with poor interfacial adhesion can be used in applications with high levels of energy absorption by taking advantage of precisely its inert character to dissipate the energy through other mechanisms which are more efficient at high impact, such as back cone formation and load transmission from

To characterize the material at high impacts, the standard STANAG 2920 established by NATO (North Atlantic Treaty Organization) was used. The purpose of this standard is to characterize at high impact any material with ballistic applications, a bulletproof vest, ballistic or combat helmets, or any kind of material produced with this purpose in mind. The projectile used may be a bullet, from which protection is required. For this, a non deformable, spherical, steel projectile (1.11 g) is commonly employed as it offers the highest ballistic limit in high impact tests in comparison to other shapes, compared to ogival, blunt or pointed projectiles 8. The parameter defined in these tests is the ballistic limit (V50), which is defined as the velocity at which a material fails 50% of the times it is impacted; this parameter, which is calculated with Equation 1, has a statistical origin due to the stochastic

three or four points with interfacial adhesion values in the composite 11.

**3. Characterization of the composite material at high impact** 

which can establish a specific methodology.

primary threads to secondary threads.

nature controlling ballistic events. The V50 is determined using the average velocity of six impacts, three which have totally perforated the armored plate and three which have partially perforated it with an interval not greater than 60 m/s between the six impact velocities. Other very important parameters determined in this type of tests are the relationship between impact velocity, absorbed energy and trauma depth.

$$\mathbf{V}\_{50} = \frac{\sum\_{i=1}^{6} \mathbf{V}\_i}{6} \tag{1}$$

The relationship between impact velocity and absorbed energy registers the amount of energy absorbed by the material at the moment of impact by a projectile at velocities equal or superior to its ballistic limit. The energy absorbed by the material (*Eabs*) is obtained based on the velocity at which the projectile impacts the sample and the velocity at which it exits at the back; these velocities are substituted in Equation 2, where *m* represents the mass of the projectile, ܸ the velocity at which the projectile impacts the sample and *Vres* the velocity at which the projectile exits the back of the material. The velocity at which the material is impacted is obtained with a Chrony chronograph which is capable of registering speeds between 10 and 2 134 m/s with a precision of 99.5%, conferring reliability to the readings. Residual velocity was obtained with the aid of a ballistic gelatine.

$$E\_{\rm abs} = \frac{mV\_{\rm imp}^2}{2} - \frac{mV\_{\rm res}^2}{2} \tag{2}$$

Ballistic gelatine is widely used in criminalistics due to the similarity of this material to the human body during high velocity impact. Due to the behavior presented by this material in response to a high velocity impact, it is possible to calculate the residual velocity with which the projectile impacted the material based on the depth of penetration. The velocity is obtained by characterizing the material during direct frontal impacts, where it is possible to generate an equation relating penetration length of the projectile with the velocity on entering the material. In some studies, such as the one carried out by Jorma Jussila 12 a more detailed procedure for this methodology is presented.

Once the relationship between impact velocity and absorbed energy is obtained, a phenomenon quite particular to this impact regimen emerges; a fall in energy absorption at velocities slightly higher than V50 with a subsequent recovery in absorption levels. A logical deduction could be that when a material presents a particular absorption of energy at perforation threshold during high velocity impacts with a 1.11 g projectile, for example 100 Joules, when this projectile impacts at 120 J one might assume that the material will absorb its corresponding part (100 J) and will allow passage of the projectile with a residual energy of 20 J. However, in reality, this does not happen. With projectile impacts at velocities slightly higher than the ballistic limit, what we find is a noticeable reduction in the capacity of the material to absorb energy. A study carried out by Paul Wambua *et al.* 13, shows a composite of natural fiber with a PP matrix, where it can be observe this phenomenon at velocities close to 250 m/s. Fig. 6 shows the curve obtained with this particular behavior.

Advantages of Low Energy Adhesion PP for Ballistics 201

ballistic limit, maximum deformation of the fiber has been reached and this has dissipated the greatest amount of energy, as can be seen in Fig. 7a; with an impact velocity higher than the ballistic limit, the energy is not able to dissipate throughout the material, which generates an area of less deformation (Fig. 7b). At this point, it is important to mention that the sensitivity to deformation of aramid falls below 2% when it is subjected to deformation velocities of 103 s-1. It is important to note that when body armor is in full contact with the user, the material does not have to be perforated to cause fatal injuries in the wearer, since the depth that this material can reach on impact without actual failing, is such that it can cause damage to internal organs. This phenomenon is known as trauma, and refers to the maximum depth of deformation undergone by a material on impact without reaching perforation. The National Institute of Justice Standard (NIJ standard 0101.04) 14 establishes a methodology which determines trauma depth by means of a material denominated witness material, this is a homogenous block of plastilene type ROMA 1 which is placed on the back of the material to be impacted (Fig. 8). The armor plate being tested is impacted at its ballistic limit as this is the point presenting maximum deformation. On impact, the projectile will produce a deformation (δ) which is measured from the unaltered surface of the plastilene block to the lowest point of the depression; the maximum depth permitted by the standard is 40 mm.

In order to carry out this process of high impact characterization, a test gun is required, for example, a laboratory gas gun. This equipment is based on the generation of pressure in a closed chamber; when the pressure is released, it channels the kinetic energy of the gas to accelerate the projectile towards impact on the target. Fig. 9 shows a general diagram of a high impact test on a sample; the storage tank contains pressurized gas, usually nitrogen or helium, which is released abruptly towards the gas gun containing the projectile, this is then accelerated along the trajectory of the barrel and passes through the chronograph which registers the velocity just before impacting the sample. A steel vice holds the sample in place

δ

Chronograph Barrel

Storage tank

Fig. 8. Measurement of deformation in the witness material

Trauma

and the witness material or ballistic gelatine is situated behind it.

Projectile trajectory Sample

Fig. 9. General diagram of a gas gun

Witness material or ballistic gelatine

Fig. 6. Energy absorbed by the composite material hemp/PP subjected to impact 13

ߝ ൌ ௌ ௌ ൌ ݔʹ ǤͷΨǡ ߝ ൌ ௌ ௌ ൌ ݔʹ ǤͷΨ

Fig. 7. Unitary deformation (ε) undergone by a laminate at the moment of absorbing impact energy, a) at the ballistic limit, b) above the ballistic limit.

This reduction in energy absorption can be explained, according to some authors, as a reduction in time of residence of the projectile in the material. One of the characteristics that define the ballistic limit of a material is the velocity at which sound can pass through it, the higher this value is, the greater capacity the material has to dissipate energy. Ideally, at the

V50

Fig. 6. Energy absorbed by the composite material hemp/PP subjected to impact 13

0 50 100 150 200 250 300 350

**Impact velocity (m/s)**

Fig. 7. Unitary deformation (ε) undergone by a laminate at the moment of absorbing impact

S

S + ΔS

This reduction in energy absorption can be explained, according to some authors, as a reduction in time of residence of the projectile in the material. One of the characteristics that define the ballistic limit of a material is the velocity at which sound can pass through it, the higher this value is, the greater capacity the material has to dissipate energy. Ideally, at the

ߝ ൌ ௌ

ௌ ൌ ݔʹ ǤͷΨǡ ߝ ൌ ௌ

S + ΔS

S

**Absorbed kinetic energy (J)**

ௌ ൌ ݔʹ ǤͷΨ

a) b)

energy, a) at the ballistic limit, b) above the ballistic limit.

ballistic limit, maximum deformation of the fiber has been reached and this has dissipated the greatest amount of energy, as can be seen in Fig. 7a; with an impact velocity higher than the ballistic limit, the energy is not able to dissipate throughout the material, which generates an area of less deformation (Fig. 7b). At this point, it is important to mention that the sensitivity to deformation of aramid falls below 2% when it is subjected to deformation velocities of 103 s-1.

It is important to note that when body armor is in full contact with the user, the material does not have to be perforated to cause fatal injuries in the wearer, since the depth that this material can reach on impact without actual failing, is such that it can cause damage to internal organs. This phenomenon is known as trauma, and refers to the maximum depth of deformation undergone by a material on impact without reaching perforation. The National Institute of Justice Standard (NIJ standard 0101.04) 14 establishes a methodology which determines trauma depth by means of a material denominated witness material, this is a homogenous block of plastilene type ROMA 1 which is placed on the back of the material to be impacted (Fig. 8). The armor plate being tested is impacted at its ballistic limit as this is the point presenting maximum deformation. On impact, the projectile will produce a deformation (δ) which is measured from the unaltered surface of the plastilene block to the lowest point of the depression; the maximum depth permitted by the standard is 40 mm.

Fig. 8. Measurement of deformation in the witness material

In order to carry out this process of high impact characterization, a test gun is required, for example, a laboratory gas gun. This equipment is based on the generation of pressure in a closed chamber; when the pressure is released, it channels the kinetic energy of the gas to accelerate the projectile towards impact on the target. Fig. 9 shows a general diagram of a high impact test on a sample; the storage tank contains pressurized gas, usually nitrogen or helium, which is released abruptly towards the gas gun containing the projectile, this is then accelerated along the trajectory of the barrel and passes through the chronograph which registers the velocity just before impacting the sample. A steel vice holds the sample in place and the witness material or ballistic gelatine is situated behind it.

Fig. 9. General diagram of a gas gun

Advantages of Low Energy Adhesion PP for Ballistics 203

Both molding conditions and stacking configurations generate 64% fiber volume fraction in the composite material. This value was obtained thanks to previous studies in which the percentage contained in the composite was varied, it also coincides with values reported in

The aramid/PP composite material was subjected to the following high impact tests; first, consolidated and independent laminates with two to six layers were characterized at high impact. Each batch per layer consists of six samples in order to determine the ballistic limit in each point and thus create a comparative curve between the ballistic limit and the number of layers between both laminate configurations, independent and consolidated. Subsequently, an intermediate point of four layers was used to carry out the following tests; trauma in four-layer consolidated laminates, trauma in four-layer independent laminates, trauma in aramid fabric arrangements without polymeric matrix, and the velocity curve of residual impact-energy in four layer laminates using ballistic gelatine. Fig. 12 shows a general representation of how a high impact test is carried out with the use of witness material and ballistic gelatine. One important difference in these two tests is that the witness material must be in direct contact with the sample being tested, while the ballistic gelatine may or may not be in contact, in this case it is not in contact with the

185 °C

878 PSI

20 min at 185 °C

Molding temperature

> Molding pressure

Processing period

the literature, where the fiber volume fraction recommended is 60% to 70% 17.

Fig. 12. Representation of an impact test on a sample with material placed behind it.

Holding vice

Table 2. Laminate molding conditions

sample.

Witness material or ballistic gelatine

> Sample to be impacted
