NANO-STRUCTURED COMPOSITES FOR ENHANCED IMPACT PERFORMANCE: A TECHNICAL OVERVIEW

NANO-STRUCTURED COMPOSITES FOR ENHANCED IMPACT PERFORMANCE: A TECHNICAL OVERVIEW Nelson Pinilla Department of Mechanical Engineering Stevens Institute of Technology ABSTRACT Local damage generated in composite aggregates leads generally to an enhanced overall compliance, or equivalently a reduced stiffness. Use of nanoscale material to enhance impact resistance of composites materials is reviewed in this paper. A technical overview of impact performance prior adding nanoplates, clays and carbon nanotubes to fabric nanoreinforced composites is also presented on this paper. Literature shows the evidence of enhance performance when nanoscale materials are added to composites. Nanoscale reinforcements seen enhance armor plates to prevent delamination, matrix cracking, shear plugging, and friction at penetration on laminates when subjected to impact loading. INTRODUCTION Structural composite applications in commercial and military industry are susceptible to projectile impact from bullets, fragments, or flying debris. Thus, penetration process of the composites by a projectile under ballistic velocities is taken importance experimental and analytical characterization. Ballistic is defined as the science dealing with a great variety of phenomena that occur from the moment an object or projectile is fired until its effects are observed on a target. On the other hand, ballistic impact is a low-mass high velocity impact by a projectile propelled by a source onto a target [18]. The effects on the target caused via high velocity are located near of impacted zone. During ballistic impact energy, transfer takes place from the projectile to the target. Among ballistic impact analysis, numerical, analytical, or experimental theories are based on empirical, semi-empirical or quasi-empirical data to define non-elastic strains as thermal expansion, phase transformation during impact, initial strains, plastic strains or misfit strains. In very early studies, the failure initiation model was defined as a stress or strain based criterion to onset the mechanical degradation of the material. Wilkins [23] investigated the penetration and perforation characteristics and main damage mechanism of the orthotropic composite alumina/aluminum target of a fired 7.62 mm projectiles defining that failure occurs when the material loses its ability to support any shear and/or tensile loads. Thus, susceptibility to impact loading of materials is studied to describe the ballistic impact behavior of a low mass high velocity impact caused by a propelling source capable to produce different damage during the ballistic impact [2]. Cheeseman et al [6], studied the deformation of the back face of the target, surface tension along the thickness, shear plugging and friction penetration, whether the energy transfer between projectile and target, energy

dissipation, and damage propagation mechanisms undergo drastic changes as the velocity change during the trajectory of the projectile at impact[24] (see figure 1). Thus, damage assessment on the laminates is measured by the impact loads, classified into three categories: low velocity, high velocity and hyper velocity impact. Considering low velocity impact when an object is dropped on a laminate surface, velocity range is less than 100 ft/sec. High velocity impact, a bullet fired from a gun, velocity between 100 to 800 ft/sec. Hypervelocity impact when velocities are up to 50,000 ft/sec. Regardless of speed, energy is transferred between projectile and target; energy dissipation and damage propagation mechanisms undergo drastic changes as the velocity of the projectiles change. Penetration mechanics concentrated on the determination of the ballistic limit, the penetration depth, the time variation of the penetration velocity and the target resistance. The studies have also identified many physical properties of the projectile materials that play important roles in penetration mechanics. Wilkins [22] and Walker [21], have found that the nose shape can have a significant effect in the penetration mechanics. Similarly, Tate et l [19], found that the change in length of the projectile could have an effect on the penetration efficiency. As a result, research areas began to focus on the ballistic limit, the penetration depth, the threshold of penetration velocity, the erosion of the projectile, the target resistance forces and the plastic flow in projectile and the target. 1. BALLISTIC IMPACT ON COMPOSITES With respect to their overall performance under ballistic impact conditions, advanced fiber-reinforced polymer-matrix composites are generally classified into two main categories [9]: First, high-strength/high-stiffness composites (carbon fibers), which are highly effective in deforming and/or fracturing the incoming projectile while having a very limited ability for absorbing the projectile’s kinetic energy, and second high ductility and toughness (glass or aramid reinforcements) whose properties are optimized with respect to absorbing the maximum fraction of the kinetic energy carried by the projectile. When a high-velocity projectile impacts a reinforced composite, the laminate slows down the projectile by creating a strong friction force; at the open hole, erosion conglomerated materials from the matrix are gathered by the excessive heat generated at the frictional surfaces, displaced by frictional forces and slide over each other inside of the hole surface. Melted and cracked zones are forming from heating generated from high-velocity friction and/or rapid plastic deformation. If the projectile pierces through and leaves, some spallation of

composite can be observed on the exit of the hole side; then, wearing mechanisms are different on the hole surface such as melting, adhesion and abrasion: on the projectile surface, where the mechanism is predominantly abrasive with some swelling. Different damage and energy absorbing mechanisms are present during the impact event, for example: cone formation on the back face of the target, deformation of secondary yarns, tension in primary yarns/fibers, delamination, matrix cracking, shear plugging and friction between the projectile and the target. During impact, three different events occur differing from each other on the amount of energy absorbed by the target during penetration: First, the projectile perforates the target and exits with certain velocity. Indicating that, projectile initial energy was more than the energy that the target can absorb. Second, the projectile partially penetrates the target. This indicates that the projectile initial energy was less than the energy that the target can absorb. Based on the target material properties, the projectile can either be stuck within the target or would rebound. Third, the projectile perforates the target completely with zero exit velocity. The entire energy of the projectile is just absorbed by the target. When the yarn has been impacted by the projectile, the momentum generates a wave that propagates on the longitudinal and transverse directions of the fibers. Naik [17]. At the impact event, fibers move from the initial position towards the vertex of impact; therefore, the fiber is moving from elastic to plastic. As a fixed plate, the wave would also move transversely, Ct. As for time dependency the complete event is divided into a number of incremental steps. The cone formation on the back of the composite plate can be explained in basis of transverse wave propagation during the impact. The shape of the wave at the impact zone is circular or quasilemniscate (an irregular circle) with radius equivalent to the surface radius of the puncture load. Yarns below the projectile are primary yarns, which are those that resist again penetration; the surrounding, the secondary fibers form the cone face on the plate during and after impact. Failure of all the fibers is indication of perforation of the projectile on the target (see figure 1). [8]

Figure 1: Damage mechanism at different projectile velocities, back cone formation, fiber breakage, and shear plug. Labels on the surface impacted with a V350m/s image represent (A) the local fiber shear failure (B) Matrix cracking, (C) Fiber failure and pull-out, and (D) Delamination. Labels on the surface impacted V650m/s are: (A) Delamination (separation of matrix), (B) Matrix cracking, and (C) Local fiber shear failure.[8]

1.1 LOW VELOCITY DAMAGE MECHANISMS Traditional engineering materials such as steel and aluminum experience low velocity impact, the energy is typically absorbed through plastic deformation. Although this deformation is permanent, it usually does not significantly reduce the load carrying capability of the structure. Graphite composites however experience very little or no plastic deformation during low velocity impact because of the low strain to failure of the fiber and brittle nature of the epoxy matrix. Therefore, the impact energy is absorbed through various fracture processes. It has been documented that the principal mechanisms for dissipating low velocity impact is through matrix cracking, delamination, and fiber failure (see figure 2). Depending on the dimensions of the test specimens [11], such as in long thin beams, a portion of the impact energy can also be absorbed through global bending of the composite; The extent of damage imposed by low velocity impact might affected by the geometry and laminate configuration of the composite. 1.2 HIGH VELOCITY DAMAGE MECHANISMS While most of the literature on the ballistic impact of composites is related to tougher and high strain materials such as aramids (Kevlar™), S-glass, and high performance polyethylene (Spectra™), a few important studies have been performed on graphite composites [12]. Since the contact time between the projectile and the composite is considerably less at higher velocities, the impact loading induces a localized response with no global deformation. It was also found that a specimen with high of mass ratio, the specimen boundary conditions have negligible effects on ballistic impact results[7]; the high velocity impact energy is dissipated over a smaller region; an additional damage mechanism is present at higher velocities known as the shear plug , (see figure 2). Due to the high stresses created at the point of impact, the material around the perimeter of the projectile is sheared and pushed forward causing a hole or “plug” slightly larger than the diameter of the projectile and increases as it penetrates the composite. The entire high velocity projectile penetration process at higher velocities involves a combination of fiber shear (shear plug), matrix crack growth, delamination, and tensile fiber failure [25] (see figure 1). Upon impact of the first ply, the projectile energy is sufficient to cut the fibers in shear. This shear process continues in successive plies until the impact energy of the projectile is lowered to the point that the fibers can provide some resistance to shear. When this occurs, the fibers in contact with the projectile are pushed forward. This causes a line of matrix cracks within that ply to generate outward between fibers on either side of the projectile. However, until the fibers are cut, the strip from first lamina loads transversely penetrates the second lamina along the length of the line of matrix cracks and pushes it forward also. This causes a separation (delamination) between the unloaded fibers of ply and the loaded fibers. Once the fibers are all cut in ply the process is repeated in ply. This

delamination process continues as the projectile makes its way through the composite. Material

Neat Nanophased(TiO)

Figure 2: Cone formation, fiber failure on the back of Twaron CT-716 at low velocity impact 200 m/s and high velocity impact 360m/s[27] 2 NANOADDITIVES Nano Additives have the potential to absorb energy during a ballistic impact event. Solutions have been scoped from nanoparticles to thin films, or nanotubes, to nanowires[4, 10]. A great deal of this research is not only reinforce to stop a projectile, but also to bounce it back. However, the interaction between woven fibers and nano-substances is still being studied with unconcluded research; furthermore, laminates can be tailored for desired mechanical properties with appropriate choices of materials, particle sizes and loading densities. This paper presents the latest studies to decrease damage on these composites using nanomaterials. 2.1 NANOPARTICLES In hybrid-armor walls and core construction, the insertion of nanoparticles on the core to provide dynamic behavior of the nanophase has been studied. Mahfuz and coworkers [13] assumed that, several energy-dissipating mechanisms like matrix shear yielding, crack front pinning or blocking play an important role in higher energy absorption in a nanophased sandwich. In one of the experiments impregnated SiC nanoparticles of 30nm diameter in continuous prepreg tapes. They achieved improved flexural strength and stiffness of approximately 32 and 20% respectively, for the nanophased laminate. These types of nanophased prepreg can be used in pultrusion and fiber placement technology for manufacturing nanocomposites with improved properties for ballistic applications. Results of the comparison of the reinforced woven epoxy composites and TiO are shown in table 1. Table 1: Experimental Data for core materials in High Velocity Impact Test Impacted (velocity 800 m/s) by a 13 gr fragment simulating projectiles (FSP) made from hardened steel ASTM 303.

Average Striking velocity (m/s) 802.0

784.2

Average residual velocity (m/s) 752.1 724.9

Gain

40.6

Ballistic limit per aerial density (m3/kgs ) 22.3

20%

12%

Average energy absorbed per aerial density (Nm3/kg) 48.8

24.9

2.3. NANO PLATELET Two types of nanoplatelet particle composites are reviewed in this article that can be used with woven composites: silicate clay minerals and graphite. Historically, the term clay has been understood to be made of small inorganic particles a part of soil fraction less than 2 mm, without any definite composition or crystallinity. The clay mineral, also called a phyllosilicate, is a layered type and a fraction of hydrous, magnesium, or aluminum silicates[15]. Every clay mineral contains two types of sheets, tetrahedral (T) and octahedral (O). Hectorite, saponite, and montmorillonite are the most commonly used smectite type layered silicates for the preparation of nanocomposites. Montmorillonite (MMT) has the widest acceptability for use in polymers because of their high surface area, and surface reactivity. It is a hydrous aluminosilicate clay mineral with a 2: 1 expanding layered crystal structure, with aluminum octahedron sandwiched between two layers of silicon tetrahedron. Each layered sheet is approximately 1 nm thick (10A ˚), the lateral dimensions of these layers may vary from 30nm to several microns or larger, depending on the particular layered silicate. 2.4. NANOCLAYS Using intercalative polymerization[26, 4]: Using this technique, polymer formation can occur in between the intercalated sheets. Polymerization’s technique is based on swelling of the layered silicate within the liquid monomer; chemically polymerization can be initiated either by heat or radiation, by the diffusion of a suitable initiator, or by an organic initiator. This procedure is been applied to nylon– montmorrillonite (nylon 6), and later it was extended to other thermoplastics. One obvious advantage of in situ polymerization is the tethering effect, which enables the nanoclay’s surface organic chemical, such as 12aminododecanoic acid (ADA), to link with nylon-6 polymer chains during polymerization. Exfoliation-Adsorption: determinate by a solvent system in which the polymer or prepolymer is soluble and the silicate layers are swellable. The layered silicates, owing to the weak forces that stack the layers together, can be easily dispersed in an adequate solvent such as water, acetone, chloroform, or toluene. The polymer then adsorbs onto the delaminated sheets and when the solvent is evaporated, the sheets reassemble. This strategy can be used to

synthesize epoxy–clay Nanocomposites, but removal of solvent is a critical issue. This process also includes the emulsion polymerization where the layered silicate is dispersed in the aqueous solution. Avila’s’ group present some results of how clays will disperse impact energy[3]. 2.5. NANOFIBERS Carbon nanofibers (CNF) fill the gap in physical properties between conventional carbon fibers with and average diameter of 10 m and carbon nanotubes of 10 nm. The reduced diameter of nanofiber provides a larger surface area with surface functionalities in the fiber. Typically CNF are not concentric cylinders; the length of the fiber can be varied from about 100 m to several centimeters, and the diameter is of the order of 100–200nm. The most common structure of CNF is the truncated cones, but there are wide ranges of morphologies like cone, stacked coins. The CNF have the morphology where they are hollow at the center and have a larger diameter than MWNT but the individual layers are not arranged in concentric tubes. Polymer nanofibers can be synthesized by a number of techniques, such as drawing, template synthesis, phase separation, electro spinning, self-assembly[14]. Oxidizing the surface of carbon nanofiber improve the tensile behavior in composites; flexural strength and modulus in epoxy-based composites [5]. 2.6. NANOTUBES Exist a large number of references of carbon nanotubes CNTs demonstrating improvement of mechanical characteristics of composites; however, few studies on ballistic applications [16]. Energy absorption capacity of different radii CNTs under ballistic impact has been reported in two extreme cases in which the bullet moved with constant speed. The composites materials show the ability to change their actuation direction from expansive to contractive, as greater imposed strain is applied. For a nanotube with one end fixed, the maximum load bearing bullet speed of the nanotube increases and the energy absorption efficiency decreases with the increase in relative heights at which the bullet strikes; these values have been found to be independent of the nanotube radii when the bullet hits at a particular relative height. Studies done using CNT on woven epoxy composites proof have improved (see figure 3) thermomechanical properties of conventional carbon fiberreinforced [20]. The observed thermal conductivity of MWNT is more than 3000W/mK at room temperature. In addition, carbon nanotubes are been filled with nanowires increasing heat resistant, adding sensing and actuation behavior. Similar results have founded on single (SWNT) or multiwall nanotubes (MWNT) in epoxy matrices. However, fabrication still easier for MWNT because their large diameter. Thus, these nanocomposites can be a smart material because the electrical and mechanical properties can be tailored to meet the overall

performance on ballistic applications given additional capability of monitoring loads and strains shown on figure 3.

Figure 3: Correlation of compressive strength with ballistic performance of MWNT-PC nanocomposites[1] CONCLUSION AND FUTURE OUTLOOK The ability to affect, in a positive manner, the mechanical strength, stiffness, and thermal properties of woven epoxybased nanocomposite has been demonstrated with experiments and characterization with these new components. Majority of the studies have shown that molecular weight and molecular structure of the polymer are important factors affecting interaction between the polymer and nanoaditives. Then, the wide range of possible chemical functionalities that can be introduced into armored shields are promising better performance on the battlefield. Thus, a fascinating research has been conducted to build intelligence in materials to provide structures that could learn, diagnose, and prevent failure. However, direct studies on ballistic of woven epoxynanocomposites need a detailed work on matrix cracking, delamination, and fiber failure and shear plug with nanoparticle systems. REFERENCES [1]Abdelkader M., Sennett M., Withers J.C., Loutfy R.O. and Moravsky A. (2002). "The investigation of carbon nanotubes for lightweith armor materials."US Army SBCCOM Natick Soldier Center. [2]Anderson C. Jr., B. S. (1988). "Ballistic impact: The status of analytical and material modeling." Int J Impact Eng (7)9(9): 9-35. [3]Avila A. F., Soares M. I. and Silva. A. (2007). "A study of nanostructured laminated plates behaviour under low velocity impact loadings." International Journal of Impact Engineering34(34): 28-41. [4]Avila A. F., Soares M. I.. and Silva. A.. (2005). "An experimental investigation on nanocomposites under impact loading." WIT Transactions on Engineering Sciences (49): 90102. [5]Beyer Frederick L. and Madison Phil (2002). "Morphological behavior of model, polystyrene based polymer

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