# Experimental Estimation of Energy Absorbed and Impact Strength of Kevlar/Basalt-Epoxy Interwoven Composite Laminate Added with Al2O3 Nanoparticles after High-Velocity Bullet Impact

Authors: J. Jensin Joshua; Dalbir Singh; P. S. Venkatanarayanan; Ch. Sai Snehit; A. Bipin Sai Eswar; Melaku Desta
Journal: Journal of Engineering
 (2023)

---

## Abstract

The use of composites has increased exponentially in most industries, especially in the aerospace industry; the usage of composites has increased by almost 40% in the last 15 years. Aerospace components are often subject to high-impact loads, so it is important to use impact-resistant materials. Kevlar is known for its high-impact resistance and is commonly used as a fiber with various resins to achieve the desired properties. This paper mainly focuses on weaving Kevlar with basalt fiber in plain weave type by the traditional handloom weaving method. The epoxy resin, along with the harder, is then mixed with the aluminum oxide nanoparticles (Al2O3) in different percentages [1%-KB1, 3%-KB2, and 5%-KB3] to form the matrix. The laminate fabricated is then subjected to the high-velocity bullet impact of around 250 m/s. A steel hemispherical head projectile was used in the testing, and it was found to be penetrated through all the specimens making holes. The impact strength for all the different specimens was then estimated by calculating the bullet damage area and energy absorbed. The plate added with 3% Al2O3 nanoparticle was found to have improved impact strength when compared with the other two specimens. This plate can be used for bulletproof and other high-velocity impact applications.


---

## Body

## 1. Introduction

A composite is a mixture of two or more chemically different constituent materials combined macroscopically or microscopically to yield a useful material [1, 2]. It is a mixture of fiber/reinforcement and matrix/resin. Composites are categorized based on the type of fiber and resin being used in the fabrication process; they can be naturally occurring or manmade (synthetic). Some examples of natural composites are wood (which contains cellulose fibers and lignin), bone, granite (which has the particulate type of fiber), and manmade composites are concrete, and plywood. [3]. Macro-fiber composite (MFC) is used for energy applications using unimorph and bimorph cantilever beams [4]. Composites have proven as a good replacement for alloys because of their high stiffness/weight ratio which also helps in fabricating lightweight materials [5, 6]. During the flight take-off, cruise, and landing the aircraft surfaces such as wings, engines, turbines, and nose experience high-velocity impact loadings. One of the major impacts is due to the bird strikes because there will be high relative velocities between the birds and the aircraft [7]. The defense industry also uses composite laminated bulletproof armor to prevent the entry of bullets into the protected area; however, armor with multiple fiber layers was also developed in the past few decades which gives improved penetration resistance for the bullet [8]. Today, composites are thought to have outstanding potential applications as primary load-bearing structures in many industries; therefore, studies on the impact behavior of the composites have attracted a lot of attention and become a burning research interest in the field of composite materials [9]. Composite structures are open to different load conditions during their useful life. An important requirement for the effective use of composites as a protective structure is resistance to ballistic impact loads from foreign objects. Such applications require a clear understanding of the damage mechanism associated with the conditions under which the projectile penetrates the composite target. Residual strength after ballistic impact is also an important consideration. Impact loads can be divided into three categories: low-speed impact, high-speed impact, and hyperspeed impact. The reason for this classification is that the energy transfer and damage propagation mechanisms between the projectile and the target change dramatically as the speed of the projectile changes. One way to raise the ballistic limit is to use fiber composites [10].Kevlar has more fascinating properties than other natural and synthetic fibers, resulting in it being in a key position in various load carrier applications. The most significant important property of Kevlar is its high tensile strength-to-weight ratio, which means that when compared to regular material, it has a higher tensile strength at a lower weight [11]. Kevlar fabric has a tensile strength of 3620 MPa (525,000 psi) and a very low relative density of 1.44, which is suitable for ballistics applications where the strength-to-weight ratio is paramount [12]. As a fibrous polymer, Kevlar has been shown to withstand high temperatures. Its tension/tensile strength depletes by 10–20% at elevated temperatures, which continues to fall within a few hours. When Kevlar is molded, the fibers formed have a tensile strength of approximately 3620 MPa and a relative density of 1.44 [13].Basalt fibers are prepared from basalt rocks which are obtained from volcanic magma that is solidified in the open air after flowing as a hot fluid. The general composition of basalt rocks is, SiO2 at 51.6–59.3%, Al2O3 at 14.6–18.3%, Fe2O3 at 9.0–14.0%, and CaO at 5.9–9.4% [13]. Comparing the thermal properties of glass fiber with the thermal properties of basalt fiber, the tensile modulus of basalt fiber is about 60.4 ± 18.9 GPa, and the tensile strength is 568 ± 267 MPa [14]. Basalt has several properties that make it an excellent fiber, such as chemical resistance, heat resistance, low water absorption, and heat insulation. When mixed with other matrices, basalt is a very good reinforcement [15]. The mechanical properties of basalt fiber-reinforced composites have been completely investigated [16–18]. The properties of Kevlar and basalt are given in Table 1.Table 1
Properties of Kevlar and basalt.
PropertiesKevlarBasaltDensity (g/cm3)1.442.66Tensile modulus (GPa)12493Tensile strength (MPa)36204500Elastic modulus (GPa)7657Elongation at break (%)2.03.1The properties of Kevlar fiber composites are dominant in the axial to the transverse direction. In order to improve mechanical properties even in the transverse direction, hybridization is considered as the way to construct [19]. The transverse mechanical properties can be improved by weaving with other fibers such as basalt, glass, and carbon. [20]. Recently, basalt fibers have acquired particular importance due to their high compound stability, heat resistance, etc. [21]. The purpose of hybridization is to obtain a better-reinforced structure.Hybrid composites can contain at least two types of fiber aggregates, helping to integrate the properties of the two materials and reducing costs. 3D textile composites, as shown in Figure1 [22], are well known for their resistance to ballistic forces.Figure 1
Plain interwoven basalt/aramid hybrid composite [22].The mechanical properties of hybrid composites such as basalt-carbon have been studied under ballistic impact. The results of these experiments demonstrate that the hybridization of basalt with Kevlar fibers significantly improves the performance of reinforced composites [23]. Bandaru and others studied the behavior of Kevlar–basalt composites under low-velocity impact. The basalt sample was found to have the highest peak force, and it was also found that the hybrid samples absorbed more energy than the individual samples [24].In many studies, researchers have explored traditional fibers such as carbon-based polymer composites, glass, and Kevlar fibers using computational and experimental methods to characterize the properties of composite materials. The energy absorption, ballistic impact performance, and penetrating behavior of these conventional fiber-based polymer composites when subjected to dynamic or impact loads have been investigated. Delamination, fiber-matrix compression damage, matrix cracking, and tensile failure of yarns have been recognized as the primary failure mechanisms under impact load [25].Therefore, based on all these observations, in this present study, the properties of the basalt hand woven with Kevlar in a plain weaving [1 × 1] manner composite in fiber form with epoxy resin were investigated. A traditional handloom weaving process is used to hybridize Kevlar and basalt fibers, which, although this is a time-consuming but cost-effective process, can reduce production costs. Due to the attractive mechanical properties, optimum adhesion, strong chemical resistance, and wide range of coatings, epoxy resins are often used in producing polymer-based composites [26].


## 2. Materials and Methods

### 2.1. Nanofillers (Al2O3)

Recent trends demonstrate the use of dispersed nanoparticles in polymers to improve polymer performance, and these composites are called polymer nanocomposites. Various nanofillers such as aluminum, graphene, and titanium, are used as fillers inside polymeric epoxies and other polymers. Compared with pure polymers, polymer-based nanocomposites exhibit unique properties due to their structure. This has led to the widespread use of polymer-based nanocomposites in various engineering fields [27].The inclusion of different concentrations of alumina nanoparticles (Al2O3) into the polymer matrix increased both final stress parameters and percent elongation. The existence of nanoparticles in nanocomposites acts as a bridge between different polymer molecules to come closer together to build a similarity index between them [28]. Yang et al. [29] proved that the presence of up to 10% (threshold value) by weight of distributed nano-Al2O3 particles in the polymer matrix plays an important role in improving the chemical and thermal properties of the nanopolymer composites obtained. According to the study, Al2O3 nanofillers improve the mechanical properties, but the hardening effect is gradually reduced for Al2O3 content higher than 10% (above a threshold value) by weight. Zhou et al. [30], in their results, found that the thermal conductivity and mechanical properties increased, and the material increased with the addition of 80% alumina nanopowder, but the mechanical properties started to decrease by 55%, i.e., above the threshold value. So, based on all the previous observations, nanoparticles, especially Al2O3, are known to increase the mechanical and thermal properties of composites. Therefore, in this study, the nanoalumina powder was chosen as filler in the fabrication of Kevlar/basalt and epoxy composite and examined the subsequent changes in properties. Figure 2 shows the alumina nanoparticles, and Figure 3 shows the processing methods and testing of composites.Figure 2
Alumina powder.Figure 3
Flow chart for fabrication process and testing of Kevlar/basalt-epoxy interwoven composite.Most studies have carried out impact testing for Kevlar/basalt composites, the content of nanoparticles plays a vital role in the mechanical properties of a composite, but this aspect is least bothered, so in this study, nanoalumina powder (Al2O3) was added with different percentages into different samples because it can reduce the porosity, and the density of the material increases dramatically with a small percentage of the addition of nanosized alumina particles, tensile strength also increased although there was no difference in the ductility of the material is recorded [31]. The mechanical properties, flexural strength, tensile strength, and Young’s modulus or stiffness of the material will be increased. If the alumina content is too low, the intermolecular distances will increase, reducing the impact strength, while if the alumina content is too high, the close stacking of molecules can lead to the fragile property of the material [32].


### 2.2. Fabrication of Laminate by Weaving

The traditional weaving process takes place on a loom. This process requires two types of yarns or fibers: warp and weft. Here the warp threads are Kevlar, and the weft threads are basalt fibers, the warp running along the length of the fabric, and the weft running across the length of the warp. The woven fabrics are shown in Figure4.Figure 4
Woven fiber.


### 2.3. Hand Layup Process

The production of a composite laminate is carried out by the traditional hand layup method shown in Figure5. Epoxy resin and HY951 hardener are used in the fabrication process. Three different laminates are prepared by mixing aluminum oxide nanoparticles in three different percentages viz. 10.6 g of alumina powder, 32 g of alumina powder, and 55 g of alumina powder [1%, 3%, and 5%].Figure 5
Hand layup process: (a) mixing of hardener and alumina powder and (b) fabricated composite laminates.
(a)(b)After curing, the resulting composite panels (samples) will have a thickness of 3 mm (which can be achieved by adjusting the thickness of the mold), which is a mandatory requirement for high-speed impact testing, and samples are cut into 150 × 160 mm. Three test samples were obtained, namely, KB1 (with 10.6 g of alumina powder), KB2 (with 32 g of alumina powder), and KB3 (with 55 g of alumina powder).


### 2.4. High-Speed Bullet Impact Test

High-speed impact tests are typically performed by a projectile that will be ejected from a high-pressure pneumatic accelerator [33, 34]. A similar configuration was used in this study, and a mild steel hemispherical projectile with details is shown in Table 2. The projectile strikes the composite samples at 200–230 m/s.Table 2
Details of the projectile.
ParametersDetailsMaterialMild steelMass7.34 (g) or 0.00734 (kg)Length15.5 (mm)Diameter9.66 (mm)The schematic test setup for the bullet (projectile) impact test is shown in Figure6. Additionally, the high-speed camera was set to an appropriate frame rate to capture several frames at impact. The impact velocities and the residual velocities (velocity of the projectile after striking the specimen) are calculated from the recorded videos with the aid of phantom cine viewer software.Figure 6
Schematic setup of bullet impact test.Table3 shows the different velocities of the projectile before and after impact. It has been observed there is more loss of velocity or energy in KB3. The bullet impact before and after in composite are shown in Figures 7–9.Table 3
Velocities of the projectile.
MaterialAverage initial impact velocity of projectile (m/s)Average residual velocity (m/s)KB1222.33206.66KB2230208.33KB3224.33199Figure 7
KB1 placed for testing: (a) KB1 before the impact and (b) KB1 bullet after the impact.Figure 8
KB2 placed for testing: (a) KB2 before the impact and (b) KB2 bullet after the impact.Figure 9
KB3 placed for testing: (a) KB3 before the impact and (b) KB3 bullet after the impact.


## 2.1. Nanofillers (Al2O3)

Recent trends demonstrate the use of dispersed nanoparticles in polymers to improve polymer performance, and these composites are called polymer nanocomposites. Various nanofillers such as aluminum, graphene, and titanium, are used as fillers inside polymeric epoxies and other polymers. Compared with pure polymers, polymer-based nanocomposites exhibit unique properties due to their structure. This has led to the widespread use of polymer-based nanocomposites in various engineering fields [27].The inclusion of different concentrations of alumina nanoparticles (Al2O3) into the polymer matrix increased both final stress parameters and percent elongation. The existence of nanoparticles in nanocomposites acts as a bridge between different polymer molecules to come closer together to build a similarity index between them [28]. Yang et al. [29] proved that the presence of up to 10% (threshold value) by weight of distributed nano-Al2O3 particles in the polymer matrix plays an important role in improving the chemical and thermal properties of the nanopolymer composites obtained. According to the study, Al2O3 nanofillers improve the mechanical properties, but the hardening effect is gradually reduced for Al2O3 content higher than 10% (above a threshold value) by weight. Zhou et al. [30], in their results, found that the thermal conductivity and mechanical properties increased, and the material increased with the addition of 80% alumina nanopowder, but the mechanical properties started to decrease by 55%, i.e., above the threshold value. So, based on all the previous observations, nanoparticles, especially Al2O3, are known to increase the mechanical and thermal properties of composites. Therefore, in this study, the nanoalumina powder was chosen as filler in the fabrication of Kevlar/basalt and epoxy composite and examined the subsequent changes in properties. Figure 2 shows the alumina nanoparticles, and Figure 3 shows the processing methods and testing of composites.Figure 2
Alumina powder.Figure 3
Flow chart for fabrication process and testing of Kevlar/basalt-epoxy interwoven composite.Most studies have carried out impact testing for Kevlar/basalt composites, the content of nanoparticles plays a vital role in the mechanical properties of a composite, but this aspect is least bothered, so in this study, nanoalumina powder (Al2O3) was added with different percentages into different samples because it can reduce the porosity, and the density of the material increases dramatically with a small percentage of the addition of nanosized alumina particles, tensile strength also increased although there was no difference in the ductility of the material is recorded [31]. The mechanical properties, flexural strength, tensile strength, and Young’s modulus or stiffness of the material will be increased. If the alumina content is too low, the intermolecular distances will increase, reducing the impact strength, while if the alumina content is too high, the close stacking of molecules can lead to the fragile property of the material [32].


## 2.2. Fabrication of Laminate by Weaving

The traditional weaving process takes place on a loom. This process requires two types of yarns or fibers: warp and weft. Here the warp threads are Kevlar, and the weft threads are basalt fibers, the warp running along the length of the fabric, and the weft running across the length of the warp. The woven fabrics are shown in Figure4.Figure 4
Woven fiber.


## 2.3. Hand Layup Process

The production of a composite laminate is carried out by the traditional hand layup method shown in Figure5. Epoxy resin and HY951 hardener are used in the fabrication process. Three different laminates are prepared by mixing aluminum oxide nanoparticles in three different percentages viz. 10.6 g of alumina powder, 32 g of alumina powder, and 55 g of alumina powder [1%, 3%, and 5%].Figure 5
Hand layup process: (a) mixing of hardener and alumina powder and (b) fabricated composite laminates.
(a)(b)After curing, the resulting composite panels (samples) will have a thickness of 3 mm (which can be achieved by adjusting the thickness of the mold), which is a mandatory requirement for high-speed impact testing, and samples are cut into 150 × 160 mm. Three test samples were obtained, namely, KB1 (with 10.6 g of alumina powder), KB2 (with 32 g of alumina powder), and KB3 (with 55 g of alumina powder).


## 2.4. High-Speed Bullet Impact Test

High-speed impact tests are typically performed by a projectile that will be ejected from a high-pressure pneumatic accelerator [33, 34]. A similar configuration was used in this study, and a mild steel hemispherical projectile with details is shown in Table 2. The projectile strikes the composite samples at 200–230 m/s.Table 2
Details of the projectile.
ParametersDetailsMaterialMild steelMass7.34 (g) or 0.00734 (kg)Length15.5 (mm)Diameter9.66 (mm)The schematic test setup for the bullet (projectile) impact test is shown in Figure6. Additionally, the high-speed camera was set to an appropriate frame rate to capture several frames at impact. The impact velocities and the residual velocities (velocity of the projectile after striking the specimen) are calculated from the recorded videos with the aid of phantom cine viewer software.Figure 6
Schematic setup of bullet impact test.Table3 shows the different velocities of the projectile before and after impact. It has been observed there is more loss of velocity or energy in KB3. The bullet impact before and after in composite are shown in Figures 7–9.Table 3
Velocities of the projectile.
MaterialAverage initial impact velocity of projectile (m/s)Average residual velocity (m/s)KB1222.33206.66KB2230208.33KB3224.33199Figure 7
KB1 placed for testing: (a) KB1 before the impact and (b) KB1 bullet after the impact.Figure 8
KB2 placed for testing: (a) KB2 before the impact and (b) KB2 bullet after the impact.Figure 9
KB3 placed for testing: (a) KB3 before the impact and (b) KB3 bullet after the impact.


## 3. Damage Evaluation of Composite Samples Using Immersion Ultrasonic Testing

Ultrasonic scanning is the most popular nondestructive testing (NDT) technique principally used for flaw detection in different materials such as composites, plastics, and metals. The widely used formats for ultrasonic scanning are A-scan, B-scan, and C-scan [35]. In this work, an ultrasonic C-scan was used since it is the accurate and best method for the detection of impact in composite materials [36].


### 3.1. Evaluation of Absorbed Energy

The energy drawn up by the composites is given by the following equation [37]:(1)Eabs=0.5∗mp∗Vi2−Vr2,where Eabs is the energy absorbed by the composite. mp is the mass of the projectile or bullet. Vi is the impact velocity of the projectile or bullet. Vr is the residual velocity of the projectile or bullet.The fiber layers of the composites resist the penetration of the bullet through it converting most of its kinetic energy to absorbed energy [37]. The absorbed energy of composites is presented in Table 4. The values of Viand Vr can be obtained from Table 3, and the mass of the projectile is 0.00734 kg (from Table 3). The impact energy is given by equation (2), and the residual energy is given by equation (3).(2)Impact energy=0.5∗mp∗Vi2,(3)Residual energy=0.5∗mp∗Vr2.Table 4
Absorbed energy of the composite samples.
MaterialThe impact velocity of projectile (m/s)Impact energy (J)The residual velocity of the projectile (m/s)Residual energy (J)Absorbed energy (J)KB1222.33181.41206.66156.7324.67KB2230194.14208.33159.2834.85KB3224.33184.68199146.839.34


### 3.2. Calculating Impact Strength of the Composite Specimens

The impact strength of the materials can be calculated by using the following equation [38]:(4)Impact strength=Energy absorbedDamaged area.The damaged area can be predicted from ultrasonic-C scan images of the specimens, and the impact area can be calculated with the aid of Image J software.


#### 3.2.1. Ultrasonic-C Scan

This analysis aimed to find out the extent of bullet damage on composite specimens. Two separate tubes for channeling and receiving pulses are spaced equally on opposite faces of the composite sample, as shown in Figure10. The echoes received from the high-quality region are taken as the threshold value such that the receiver can detect any signal below the threshold value. In general, the pulse energy of damaged regions is small compared with good regions.Figure 10
Simplified diagram of the immersion scan system.The test setup of the Immersion ultrasonic testing system and transmission setup is shown in Figures11 and 12.Figure 11
Immersion ultrasonic testing system.Figure 12
Through transmission setup with the sample.


#### 3.2.2. Estimation of Damage Area through C-Scan

The impacted specimen is scanned with ultrasonic C scan nondestructive evaluation to estimate the damaged area. Impact or damaged area can be predicted using Image J software.The impact strengths of KB1, KB2, and KB3 are 2.75 J/cm2, 4.28 J/cm2, and 3.63 J/cm2, respectively, shown in Table 5 and Figure13. Table 6 shows C-scan, front and back side images of all the samples.(i)
It is evident from the values obtained that the impact strength of KB2 (4.28 J/cm2) is higher than KB1 (2.75 J/cm2) and KB3(ii)
In the case of KB1, the addition of alumina powder is very low (i.e., 1% or 10.6 gm); if the alumina content is too low, the intermolecular distances will increase, resulting in the low impact strength [32](iii)
In the case of KB3, the alumina powder content is very high (i.e. 5% or 55 gm); if the alumina content is too high, the close stacking of molecules can lead to the fragile property of the material, so the impact strength of the specimen is again low(iv)
But in the case of KB2, due to the moderate addition of the alumina powder (i.e., 3% or 32 gm), the resultant impact strength is higher [32, 38]Table 5
Impact strength of the samples.
MaterialsImpact area (cm2)Absorbed energy (J)Impact strength (J/cm2)KB18.9424.672.75KB28.1334.854.28KB310.8239.343.63Figure 13
Impact strength vs. composite material.Table 6
C-scan, front and back side images of all the samples.Figure 14
Absorbed energy vs. composite materials.


## 3.1. Evaluation of Absorbed Energy

The energy drawn up by the composites is given by the following equation [37]:(1)Eabs=0.5∗mp∗Vi2−Vr2,where Eabs is the energy absorbed by the composite. mp is the mass of the projectile or bullet. Vi is the impact velocity of the projectile or bullet. Vr is the residual velocity of the projectile or bullet.The fiber layers of the composites resist the penetration of the bullet through it converting most of its kinetic energy to absorbed energy [37]. The absorbed energy of composites is presented in Table 4. The values of Viand Vr can be obtained from Table 3, and the mass of the projectile is 0.00734 kg (from Table 3). The impact energy is given by equation (2), and the residual energy is given by equation (3).(2)Impact energy=0.5∗mp∗Vi2,(3)Residual energy=0.5∗mp∗Vr2.Table 4
Absorbed energy of the composite samples.
MaterialThe impact velocity of projectile (m/s)Impact energy (J)The residual velocity of the projectile (m/s)Residual energy (J)Absorbed energy (J)KB1222.33181.41206.66156.7324.67KB2230194.14208.33159.2834.85KB3224.33184.68199146.839.34


## 3.2. Calculating Impact Strength of the Composite Specimens

The impact strength of the materials can be calculated by using the following equation [38]:(4)Impact strength=Energy absorbedDamaged area.The damaged area can be predicted from ultrasonic-C scan images of the specimens, and the impact area can be calculated with the aid of Image J software.


### 3.2.1. Ultrasonic-C Scan

This analysis aimed to find out the extent of bullet damage on composite specimens. Two separate tubes for channeling and receiving pulses are spaced equally on opposite faces of the composite sample, as shown in Figure10. The echoes received from the high-quality region are taken as the threshold value such that the receiver can detect any signal below the threshold value. In general, the pulse energy of damaged regions is small compared with good regions.Figure 10
Simplified diagram of the immersion scan system.The test setup of the Immersion ultrasonic testing system and transmission setup is shown in Figures11 and 12.Figure 11
Immersion ultrasonic testing system.Figure 12
Through transmission setup with the sample.


### 3.2.2. Estimation of Damage Area through C-Scan

The impacted specimen is scanned with ultrasonic C scan nondestructive evaluation to estimate the damaged area. Impact or damaged area can be predicted using Image J software.The impact strengths of KB1, KB2, and KB3 are 2.75 J/cm2, 4.28 J/cm2, and 3.63 J/cm2, respectively, shown in Table 5 and Figure13. Table 6 shows C-scan, front and back side images of all the samples.(i)
It is evident from the values obtained that the impact strength of KB2 (4.28 J/cm2) is higher than KB1 (2.75 J/cm2) and KB3(ii)
In the case of KB1, the addition of alumina powder is very low (i.e., 1% or 10.6 gm); if the alumina content is too low, the intermolecular distances will increase, resulting in the low impact strength [32](iii)
In the case of KB3, the alumina powder content is very high (i.e. 5% or 55 gm); if the alumina content is too high, the close stacking of molecules can lead to the fragile property of the material, so the impact strength of the specimen is again low(iv)
But in the case of KB2, due to the moderate addition of the alumina powder (i.e., 3% or 32 gm), the resultant impact strength is higher [32, 38]Table 5
Impact strength of the samples.
MaterialsImpact area (cm2)Absorbed energy (J)Impact strength (J/cm2)KB18.9424.672.75KB28.1334.854.28KB310.8239.343.63Figure 13
Impact strength vs. composite material.Table 6
C-scan, front and back side images of all the samples.Figure 14
Absorbed energy vs. composite materials.


## 3.2.1. Ultrasonic-C Scan

This analysis aimed to find out the extent of bullet damage on composite specimens. Two separate tubes for channeling and receiving pulses are spaced equally on opposite faces of the composite sample, as shown in Figure10. The echoes received from the high-quality region are taken as the threshold value such that the receiver can detect any signal below the threshold value. In general, the pulse energy of damaged regions is small compared with good regions.Figure 10
Simplified diagram of the immersion scan system.The test setup of the Immersion ultrasonic testing system and transmission setup is shown in Figures11 and 12.Figure 11
Immersion ultrasonic testing system.Figure 12
Through transmission setup with the sample.


## 3.2.2. Estimation of Damage Area through C-Scan

The impacted specimen is scanned with ultrasonic C scan nondestructive evaluation to estimate the damaged area. Impact or damaged area can be predicted using Image J software.The impact strengths of KB1, KB2, and KB3 are 2.75 J/cm2, 4.28 J/cm2, and 3.63 J/cm2, respectively, shown in Table 5 and Figure13. Table 6 shows C-scan, front and back side images of all the samples.(i)
It is evident from the values obtained that the impact strength of KB2 (4.28 J/cm2) is higher than KB1 (2.75 J/cm2) and KB3(ii)
In the case of KB1, the addition of alumina powder is very low (i.e., 1% or 10.6 gm); if the alumina content is too low, the intermolecular distances will increase, resulting in the low impact strength [32](iii)
In the case of KB3, the alumina powder content is very high (i.e. 5% or 55 gm); if the alumina content is too high, the close stacking of molecules can lead to the fragile property of the material, so the impact strength of the specimen is again low(iv)
But in the case of KB2, due to the moderate addition of the alumina powder (i.e., 3% or 32 gm), the resultant impact strength is higher [32, 38]Table 5
Impact strength of the samples.
MaterialsImpact area (cm2)Absorbed energy (J)Impact strength (J/cm2)KB18.9424.672.75KB28.1334.854.28KB310.8239.343.63Figure 13
Impact strength vs. composite material.Table 6
C-scan, front and back side images of all the samples.Figure 14
Absorbed energy vs. composite materials.


## 4. Results and Discussion

(i)
After the high-velocity impact test, the results are as follows:(ii)
The bullet impact test revealed that the bullet pierced smoothly through the composite specimens in all three cases(iii)
Delamination, fiber-matrix compression damage, matrix cracking, and tensile failure of yarns have been recognized as the primary failure mechanisms under impact load [25](iv)
Composite failure is due to the high-velocity impact of the projectile, which generates a compression wave through the thickness of the sample and is reflected as a tension wave, causing fiber delamination or detachment between the lamina layers [39](v)
When the warp and weft threads break in the collision zone, friction between the projectile and target becomes the primary mechanism, continuing to slow the projectile [39](vi)
The impact area, impact velocity, and residual velocities were predicted using the Image J and Cine viewer software(vii)
Adding nanoparticles to composite materials results in astonishing changes in mechanical properties [25](viii)
The hybrid samples absorb more energy than individual samples [26](ix)
It was found that the affected area of KB3 (10.82 cm2) was larger, i.e., more damaged composite panel; this was because the addition of a higher percentage of alumina powder resulted in a small intermolecular gap resulting in a series of alumina particles in one place causing the epoxy bond to break and lead to brittle nature of the material [32].(x)
As the nanopowder content increases, the impact energy increases, and as the impact energy increases, so does the absorbed energy, so the KB3 (39.34 J) composite sample absorbs more energy than the other two samples, as shown in Figure14 [20, 41](xi)
In the case of KB1, the alumina content is too low (1%), so the distance between the molecules is too small, which does not help to create stronger bonds between the epoxy molecules, so the impact strength is reduced [40](xii)
The impact damage area is low in KB2 (8.13 cm2) because of the strong polymeric bond, closely packed structure, and the dense nature obtained by adding nanofillers [31, 41](xiii)
KB2 (4.28 J/cm2), as shown in Figure 13, gives good impact strength compared to KB1 (2.75 J/cm2) and KB3 (3.63). The mechanical properties of KB2 got heightened due to the modest content of alumina powder that provided proper dispersion of alumina powder which in turn provided more useable area for epoxy to create a strong bond between the molecules which increases the impact strength of the material [41](xiv)
The presence of alumina particles gives a connection between the epoxy molecules creating a polymer bond that gives resistance to the impact loads [28](xv)
The percentage of alumina (3%) present in KB2 helped in overcoming the viscous nature of the epoxy molecules and helped to create a strong polymeric chain that increased the impact strength of the specimen [41](xvi)
The impact strength increases by two times if the content of alumina powder is moderate (KB2), and the impact strength of the composite starts degrading once the nanopowder volume goes beyond the threshold value (KB3)(xvii)
From the conclusions and results made from [28–31], every material has its threshold value in bearing the nanoalumina powder; the mechanical properties of the material increase up the threshold value and then start gradually decreasing and finally leading to the brittle behavior of the material(xviii)
From the tests performed and the results obtained, it is clear that a high percentage of alumina powder can cause higher content of porosity and dispense of alumina particles, and the formation of microcracks [42]


## 5. Conclusion

In this work, the effects of different percentages of nanoalumina powder on the impact strength of Kevlar/basalt and epoxy composites were investigated experimentally and numerically. The fiber layers are made with the help of the weaving process, and the fabrication of the composite is performed by the hand layup method. The ballistic study has been carried out to understand the behavior of the Kevlar/basalt polymer composite with the addition of the alumina nanoparticles. Ultrasonic-C scan was used to find the damage in the subsurface layers and also to find the extent of the damage.As discussed, the impact area is low in KB2 (8.13 cm2) compared to KB1 (8.94 cm2) and KB3 (10.82 cm2). However, KB3 absorbed more energy (39.34 J) due to the gradual increase of impact strength from the threshold value of alumina powder. As per the results obtained, the impact strength started increasing with the increment of alumina content, and once the percentage increased above the threshold value (i.e., 3% in this study), the mechanical properties started declining. Hence the impact strength is higher in KB2 (4.28 J/cm2), and less in KB3 (3.63 J/cm2), and also the impact area is less in KB2 (8.13 cm2) but more in KB3 (10.82 cm2), this implies KB2 is more resistant to the high-speed ballistic loads due to the modest content of Al2O3 particles.Several experiments and tests are still being carried out to explore the various benefits of introducing different nanoparticles to composite materials. Therefore, it is not recommended to use too high alumina powder content (above a threshold value) if the aim is to achieve high-impact resistance; a moderate amount of alumina powder can provide high-impact resistance. Alternatively, the woven Kevlar–basalt/epoxy composites can be spiked with other types of nanofillers available in various proportions, and the resulting changes can be investigated after the impact test.



---

*Source: 2830575-2023-02-10.xml*