Abstract
Based on the current development and application background of bulletproof impact materials, this article mainly introduces the classification of new bulletproof composites. According to the different structural materials, it can be mainly divided into hard and soft composites, as well as soft and hard complex bulletproof composites. The bulletproof mechanism and advantages of these three types of bulletproof composites and new liquid bulletproof composites are discussed. Then, the main factors affecting the bulletproof performance of aramid fiber composites, ultra-high molecular weight polyethylene fiber composites, and shear thickening fluid/high-performance fiber composites of new liquid bulletproof materials were analyzed. Finally, common bulletproof standards at home and abroad were listed and compared.
Keywords
Introduction
War and peace are always hot topics in today’s society. Although on the surface the world is in a period of relative peace and stability, in fact, there are hidden currents, and armed conflicts in some countries and local regions occur from time to time. Terrorist attacks in various places are more frequent,1,2 such as the September 11, 2001, terrorist attacks in the United States, the Paris terrorist attacks in France on November 13, 2015, and the Russia–Ukraine military conflict that started on February 17, 2022. All have caused large numbers of innocent casualties.
These incidents all show that the current social development is not peaceful. With the continuous development of science and technology, weapons and equipment are constantly updated, and their killing power is also constantly enhanced. These weapons pose a huge threat to the personal safety of military and police personnel who perform their tasks. Therefore, human protection is of increasing concern to all parties, and the development of bulletproof materials has also ushered in a new chapter.
The development of bulletproof materials has experienced three stages of change. 3 At the beginning, rigid materials such as steel plate and aluminum alloy plate were used to resist the impact of bullets and shells, and the plastic deformation and local hardening of steel plate after being impacted by bullets were used to offset the impact,3,4 so as to achieve a bulletproof effect. However, because of its high density and heavy weight, to strengthening the bulletproof effect by increasing the thickness of the steel plate makes it difficult carry, and the steel can produce secondary fragmentation with the impact of shells, causing multiple injuries to the human body. Therefore, there is an urgent need to develop new materials with excellent bulletproof performance and low density.
In the second stage, soft bulletproof materials are mainly prepared using high-performance fibers. It mainly uses fiber fabric to absorb the kinetic energy of bullets, so as to achieve the purpose of reducing human damage. This kind of soft bulletproof jacket is light in weight, comfortable to wear, and will not produce the phenomenon of second fragmentation. Nylon fiber is twice as strong as cotton fiber and has the characteristics of high breaking strength and low density, so it is used as the lining of body armor in combination with steel plates. Later in the 1970s, DuPont Company in the United States accidentally synthesized para-aramid while researching tire materials and named it “Kevlar”. 5 Kevlar fiber has super tensile properties, heat stability, and impact resistance, and its absorption capacity of bullet kinetic energy can reach nearly twice that of nylon. Therefore, it has gradually become the main material of body armor.6,7
Because the high-performance fiber fabric mainly relies on the friction between the fiber interlacing points for support, the bullet impact will produce a great deformation resulting in non-inertial damage, usually needing dozens of layers of fabric to prevent this situation, which brings great inconvenience to the use of body armor, so for bulletproof materials, new developments are needed.
The third stage is the birth of high-performance fiber-reinforced resin matrix bulletproof composite materials. Such materials have great specific strength and specific modulus, their mass being only 1/5 to 1/7 that of traditional materials, 8 compared with the rigid molecular rebound of traditional steel plates, and the composite materials have good kinetic energy absorption capacity and shock absorption performance, at the same time not producing secondary fragmentation. In addition to its low density, there are various preparation methods, so in the development of lightweight bulletproof materials for use in a variety of complex environments, composite materials have unique advantages.
According to the types of structural materials, the existing bulletproof composites can be divided into rigid bulletproof composites and soft bulletproof composites, as well as a third generation of flexible protective materials in recent years: liquid bulletproof materials (TBS), 9 which is also called shear thickening fluid (STF). Hard bulletproof composite materials on high-speed bullet impact on the one hand, show material deformation and fragmentation, and on the other absorb the kinetic energy of the bullet, while using their own hardness to change the shape of the bullet or fragment, automatically bounce open the bullet and consume its kinetic energy, and play a bulletproof role.
The bulletproof mechanism of soft bulletproof composite material mainly has two aspects: on the one hand, deformation or fragmentation of the projectile body, so that it is bounced open by the material, and on the other, consuming the kinetic energy of bullet through tensile deformation and fracture of the bulletproof composite material.
Finally, the main component of TBS is a special “shear thickening fluid”, which is normally soft and deformable, just like any other liquid. When hit by a bullet, the liquid is instantly transformed into a hard material, preventing the bullet from passing through. 10 By combining it with other high-performance fibers to form composites, the material can significantly improve bulletproof properties, while making it lighter and thinner without limiting the soldier’s flexibility.
Research and Comparison of Commonly Used Bulletproof Standards
Due to the differences in caliber and ammunition structure of standard guns in different countries, the relevant bulletproof standards are formulated according to the different degrees of threat.
NIJ 0101.06 Standard for Body Armor
The National Institute of Justice (NIJ) NIJ 0101.06 body armor standard issued in 2008 is one of the most widely recognized body armor standards in the world at present. 11 Most of the body armor exported by China is tested and evaluated in accordance with this standard. The NIJ 0101.06 bulletproof standard divides the protection performance into five levels: Class IIA, Class II, and Class IIIA for resistance to pistol pellets, and Class III and Class IV for resistance to rifle pellets, eliminating Class I, the lowest level of protection in the NIJ 0101.04A standard.
NIJ 0101.07 Standard for Body Armor
In January 2018, the NIJ released the latest version of bulletproof standard for public comment: NIJ 0101.07 Standard for Body Armor.12,13 The new standard divides body armor into two categories: Handgun Protection Levels (HG) and Rifle Protection Levels (RF). The initial kinetic energy of the projectile ranges from 634 J, the lowest protection level, to 4124 J, the highest protection level. NIJ 0101.07 standard not only cancels the difference in bullet speed between new and old body armors, but also cancels the IIA in the NIJ 0101.06 standard. In addition, the new standard increases the number of types of rifle pellets tested.
GOST 34286-2017 Standard for Body Armor
As a military power, Russia has formulated and published many bulletproof standards, among which GOST 34286-2017 “Classification and General Specification for Body Armor”14 and GOST 34282-2017 “General Technical Requirements for Armored Protective Vehicles”15 are widely used. The GOST 34286-2017 standard divides the bulletproof capability of body armor into six grades: BR1–BR6. In the standard, BR1–BR3 is the protection grade against pistol threat. The three grades require that the depth of depression of the substrate material on the back of the tested sample after being hit is ≤17 mm, while BR4–BR6 is the protection grade against rifle threat. In the new standard, these three levels require that the target plate is not penetrated, and there is no requirement on the depth of depression of the substrate material on the back of the target plate.
HOSDB-2017 Standard for Police Body Armor
The Home Office Scientific Development Branch issued the first edition of the Standard for police body armor in 1993. After more than 20 years of modification and improvement, the latest standard for police body armor was released in 2017, 16 replacing the 2007 edition of the HOSDB Standard for Police Body Armor. The standard classifies four levels of protection for body armor, HO1 to HO4, and SG1 for shotgun protection. Two types of projectiles, .357 Magnum and SS109, are also classified as a special level. HO1 and HO2 levels of body armor are designed to protect against pistol pellets, which are of the same type and different speeds. HO3 and HO4 class body armor is used to protect against rifle pellets. Compared with the 2007 bulletproof standard, 17 the new standard HO3 grade adds an AK47 shot M43 projectile test.
SCHUTZKLASS Standard 2008 Edition
SCHUTZKLASS body armor Standard 2008 is one of the bulletproof standards widely used in Germany, which divides the bulletproof capability of body armor and insert plate into five grades: SKL–SK4. 18 This standard differs from other bulletproof standards in that there is no requirement for shooting the sample after sample immersion, but there is a requirement for a contact shooting test. Bulletproof performance requires that the projectile does not penetrate the sample, no visible penetration cracks appear in the crater site, and the depth of the bullet mark on the back of the sample is <42 mm.
GA 141-2010 Standard for Police Body Armor
Compared with European and American countries, China’s bulletproof standards started a little later, but after more than 20 years of development have been relatively perfected. The current version of the standard for body armor is GA 141-2010 Standard for Police Body Armor issued in 2010, 19 which divides the protection level of body armor into six levels, among which the test projectiles of levels 1–4 are pistol projectiles, and levels 5–6 are rifle projectiles. According to the standard, the bulletproof performance of the sample requires that all warheads do not penetrate the body armor, there are no visible penetration cracks, and the depth of the back mark is ≤25 mm.
GA 165-2016 Bulletproof Transparent Materials
The bulletproof standard of the new transparent bulletproof material is the same as the standard for the old bulletproof material. The bulletproof material is divided into two categories, A and B, according to the results of the shooting test. According to GA 165-2016, Category A is a bullet or shrapnel impenetrable bulletproof material that has splash on the back but does not penetrate the test card. Category B refers to bulletproof material that has not been penetrated by the bullet or shrapnel, and has no splash on the back of the bulletproof material and has not penetrated the test card. 9
IS 17051: 2018 Standard for Body Armor
Another important measure of bulletproof performance is the back face signature (BFS). BFS, also known as backface deformation, measures the depth of deformation of body armor caused by the bullet impact. 20
For all ballistic performance levels, the armor sample shall be removed from the backing material and the indentation depth (BFS) resulting from each test shot shall be measured from the top edges of the steel tray, using a similar apparatus to that shown in Figure 1. 21 Where the backing material is built up as in female bust shapes, BFS is not measured. Maximum permissible BFS limits shall be 25 or 44 mm. 22
Method of measuring the back face signature. 21
The V50 test is to shoot a helmet with a beveled cylindrical projectile of mass 1.1 g at different speeds within a specified distance (usually 5 m), in which 50% of the projectile breaks through the helmet, and 50% does not break through it. At this time, the average speed of the shot is the V50 of the helmet. 23 The higher the V50, the better the bulletproof performance of the helmet. The ballistic limit or V50 is measured after fulfilling the standard criteria for the BFS of an armor panel. As per Indian standard IS 17051: 2018, it is an option which is carried out during a lot of testing where the supply quantity is more than 500. 21
Indian standard IS 17051: 2018 mainly classifies threats faced by the armed forces, paramilitary, state police forces, and other law enforcement agencies into six threat levels listed in Table 1. 24
Threat levels in Indian standards. 24
Routine ballistic evaluation may use service ammunition where bullet weight is not considered. Bullet weight shall be considered for reloaded ammunitions. FMJ: full metal jacket; Pb: lead core; MSC: mild steel core; SI: steel insert; HSC: hard steel core; API: armor piercing incendiary.
Classification of New Bulletproof Composites
According to the different types of materials used, bulletproof composite materials can be mainly divided into hard, soft, soft and hard composite materials, and the shear-thickened composite materials studied in recent years.
Hard Bulletproof Composite Materials
Hard bulletproof composite materials usually use special steel plate, super aluminum alloy, and other metal materials, high-strength and high-modulus fiber and resin composite laminate materials, or silicon carbide and other ceramics and high-strength and high-modulus fiber composite laminate materials, and generally do not have softness.
New Composite Ceramic Bulletproof Plate
The research on ceramic armor in hard bulletproof composites is an important component of the development and application of new bulletproof composites. The ballistic protection effect of ceramic armor is better than that of ordinary armor steel. At present, passive armor and reactive armor are the most widely studied and applied. Nowadays, the United States, Russia, and other countries have used ceramics and composite materials to develop armor systems with better weight efficiency and have developed ceramic panel armor, which has been widely used.25–27
Performance Parameters of New Composite Ceramic Bulletproof Plate
Some major properties of ceramic materials are shown in Table 2. 28
Characteristics of ceramic material. 28
Ceramic materials have high specific stiffness, high specific strength, and chemical inertness in many environments, while their low density, high hardness, and high compressive strength make them more widely used compared with metals. High-purity aluminum has higher density, lower hardness, and fracture toughness, so its bullet resistance is poor. The structure of silicon carbide ceramics gives it high strength, high hardness, wear resistance, corrosion resistance, high thermal conductivity, and other properties. The elastic modulus of titanium diboride is higher. Boron carbide has high melting point and excellent hardness and mechanical properties; its density is the lowest among several common ceramic materials, plus it has a high elastic modulus, making it a good choice for military armor and space field materials.
The new composite ceramic bulletproof plate has incomparable advantages over the traditional bulletproof plate, as shown in Table 3. 29
This material can withstand the continuous impact of multiple bullets on the same surface at the same time, without overall breakage. It will only form an approximately circular bullet hole on the surface and this will not affect the bulletproof effect of other parts of the material.
This material has good structural designability. Composite ceramic plate can produce the corresponding angle bending deformation; deformation can be restored to the original shape and can be designed into a plane or curved surface or inclined surface and other shapes of composite ceramic bulletproof material.
The ceramic bulletproof plate can be reused after repair. After being hit by bullets, the round bullet holes on the surface can be filled in with ceramic bulletproof body, and the properties of the initial material can be achieved again by using bulletproof glue solution to reassemble and form them. 28
The material integrated use of high-performance ceramic plate, ultra-high molecular weight polyethylene (UHMWPE) plate, and TC4 plate bulletproof characteristics, making the bulletproof performance better than that of the monomer material, can effectively resist various specifications of pistols and related small and medium caliber perforating bullets.
The material has high technical maturity and strong designability. It has a quite mature production process, and it can be designed according to the actual needs and personalized requirements to meet different bulletproof needs.
Comparison between new composite ceramic bulletproof plate and traditional bulletproof materials. 29
Soft Bulletproof Composite Materials
At present, there are mainly the following kinds of soft bulletproof composite materials which are used more and have relatively good performance.
Aramid Fiber Composite Materials
Para-aramid fiber is a new high-tech synthetic fiber and has super high-strength, high-modulus and high-temperature resistance and other excellent properties. Its strength is 5–6 times that of high-quality steel; the modulus is steel or glass fiber 2–3 times that of steel or glass fiber; and the weight is only 1/5 that of steel. The aramid bulletproof composite material is mainly used for the protection against high-threat level bullets and is one of the most important items of protective equipment for military engineering, special forces, and police officers in the international market. With the need for weapon protection and defense against high energy and high penetration ability weapons in the military and police fields, aramid bulletproof composites play an increasingly important role in the field of personal protection and are beginning to gradually replace pure metal bulletproof materials and partially replace pure fiber soft materials. 30 This kind of composite material has the advantages of good bulletproof performance, a simple preparation process, high-temperature resistance, no bullet bounce, and so on. It is widely used in military engineering, personal protection, and other fields.
At present, the traditional preparation of aramid bulletproof composite materials is mainly through aramid immersion machine weaving and molding to prepare the bulletproof backplane, and then composite preparation with ceramics. However, the prepreg of aramid fabric will not only increase the number of working hours involved but also increase the cost of personnel and raw materials. At the same time, the backplane material prepared using aramid-impregnated rubber fabric is heavier, which ultimately makes the aramid bulletproof composite material heavier and affects the flexibility of fighters. Therefore, reducing the weight of the aramid bulletproof composite material is the focus of future research.
UHMWPE Fiber Composites
UHMWPE fiber is the lowest density high-performance fiber, with a density of 0.97–0.98 g/cm3, only two-thirds that of aramid fiber, half that of carbon fiber, and one-eighth that of steel wire 1/8. The extremely high molecular weight, high orientation, and high crystallinity molecular chain structure give UHMWPE fiber many excellent properties. The monofilament strength and specific strength of UHMWPE fiber are the highest of all fibers today, which is equivalent to 15 times that of high-quality steel wire and 1.7 times that of aramid fiber, and the specific modulus is much higher than that of other high-performance fibers other than carbon fiber. 31 The friction coefficients of UHMWPE fibers are lower than those of other high-performance fibers, and the impact resistance of UHMWPE fibers is strong, and it can still maintain good mechanical properties at low temperatures.
Compared with these advantages, UHMWPE fiber also has some shortcomings, mainly the following three points. First, due to the high crystallinity and non-polarity of UHMWPE fiber surface, its bond with resin is poor, the interface bonding force is insufficient, interfacial fracture easily occurs as well as debonding after stress, so that the mechanical properties of the composite materials are reduced. Second, the melting point of polyethylene material is low, around 130°C, and the thermal aging rate of UHMWPE fiber under tensile conditions is faster, which limits its application in high-temperature environments. Third, the main chain of UHMWPE fibers does not contain side groups, and the intermolecular force is weak. Under long-term stress, slippage between molecular chains easily to occurs, the phenomenon of elongation under stress occurs, and the creep resistance is insufficient.
Poly-p-phenylene Benzobisoxazole Fiber Composites
Poly-p-phenylene benzobisoxazole (PBO) fibers are a kind of polyamide fiber containing heterocyclic aromatic groups; the molecular chain presents a highly conjugated structure, mainly used in military, aerospace, petroleum, and other industries,32,33 known as the 21st-century super fiber. The tensile strength and elastic modulus of standard PBO fibers can reach 5.8 and 180 GPa, respectively, and the elastic modulus is 280 GPa after treatment at 600°C, 34 while its density is only 1.5–1.7 g/cm3, and its unique molecular chain structure gives it excellent heat resistance and flame retardant properties. 35
Professor Sun Shouheng’s team at Brown University in the United States has prepared a new type of PBO material. 36 The new PBO material has a relatively low molecular weight (Mw = 3.6 kDa), and the preparation process is more gentle and environmentally friendly, and has better corrosion resistance and aging resistance than the commercialized high molecular weight polybenzoxazole material (Zylon Mw = 40 kDa) and can be used to prepare high-performance body armor. By comparing the new PBO material with commercial Zylon, the thermal stability and tensile strength of the new PBO material are obviously better than those of Zylon.
But at the same time, PBO fibers also have obvious shortcomings: first, their chemical inertia is very strong, and the interface bond strength with the resin is low; second, they are extremely sensitive to ultraviolet light and visible light, and the irradiated molecular chain is easily degraded; and third, the resistance to humidity and heat aging performance is weak. Although PBO fibers show great advantages and potential in the field of ballistic protection, their strong sensitivity to light and moisture and heat seriously restricts their application in the field of ballistic resistance. In view of the more complex battlefield environment, it will be difficult to achieve large-scale application of PBO fibers in the field of protection if its fatal defects of resistance to light and moisture and heat aging are not effectively solved. 37
Soft and Hard Bulletproof Composite Materials
The soft and hard composite bulletproof materials use hard bulletproof materials as the panel and reinforcement material, while the lining uses soft bulletproof material, so the softness of the composite bulletproof material is between those of the above two materials. The composite materials not only have the advantages of both soft and hard bulletproof materials but it is also possible to change the protection level by changing the insert plate according to different tasks.
Other Bulletproof Composites
In addition to the bulletproof composite materials mentioned above, in recent years, research teams in various countries have also discovered and prepared several other types of composite materials, and their bulletproof and stabproof effects are also excellent.
STF/High-Performance Fiber Composites
STF is a non-Newtonian fluid, which is generally in a suspension state. When it is subjected to impact load, its apparent viscosity will suddenly rise, showing a solid-like state to resist the damage caused by an impact load. 38 When the external force is removed, the apparent viscosity of STF will decrease again, showing a state of suspension again. Therefore, shear thickening behavior is a reversible non-Newtonian fluid behavior. The application of STF in human protective materials can produce a very good anti-stab and bulletproof effect.39,40
Magnetorheological Fluid/High-Performance Fiber Composites
Another thing that could enhance Kevlar bulletproof materials is magnetorheological fluid (MRF), which is an oily substance filled with iron particles. Normally, the surfactant in the liquid surrounds the particles and keeps them suspended in the liquid. Iron particles are small, only 3–10 microns, and make up only 20–40% of the total liquid capacity. However, they have a great effect on the viscosity of the liquid. In a magnetic field, the particles line up and the liquid thickens quickly.
A magnetic field exists around the bulletproof material owing to a thin circuit inside it. When no current flows through the wire, the bulletproof material is flexible, but when the switch is turned on, an electric current is generated in the wire and a magnetic field is created, which makes the bulletproof material harder and stronger. If the switch is flipped to the opposite position, the current disappears, and the bulletproof material becomes pliable again. At present, there are some drawbacks in the application of MRF technology, which is not ready for combat and needs 5–10 years of research and improvement.
Liquid Crystal Elastomer Composites
A research team at the Extreme Materials Institute at Hopkins University has developed a new lightweight bulletproof material that is atomic-scale complex liquid crystal elastomers (LCEs), 36 which are elastic polymer networks in the liquid crystallization stage, giving them both elasticity and stability. LCEs are commonly used to make actuators and artificial muscles for robots, but in the new study, the researchers found that the material has extremely powerful energy absorption and deformation properties. At its core is “elastic buckling instability,” rebound on an atomic scale. At the time of impact, the energy from the impact can be stored by deformation, and after the impact is over, the stored energy can be released so that the material can “bounce” back to its original form.
The material can withstand repeated impacts from objects weighing between 4 and 15 pounds and traveling at up to 22 miles per hour and is a reusable material that will not fail due to damage. This feature has a very practical significance for improving the battlefield survival ability of soldiers, and the body armor made with it can also well solve the problem of female soldiers’ body armor due to large differences in body size and fit, and reduced weight of female soldiers.
Bulletproof Mechanism of Bulletproof Composites
The bulletproof mechanism of bulletproof composite materials mainly has the following kinds.
Bulletproof Mechanism of Hard Bulletproof Composites
Hard bulletproof composites mainly act as by ricocheting bullets or shrapnel, or causing bullets to splinter to dissipate their energy. Taking the new composite ceramic bulletproof plate as an example, when the bullet impacts a composite ceramic bulletproof plate at high speed, according to Newton’s third law, it bounces out into the interior with the opposite force at high speed, forming an approximately circular bullet hole on the surface. Thus, the destruction of only the surface of the bulletproof plate is realized, and there is no fatal damage to the overall composite bulletproof plate, so achieving the purpose of defense against bullets. 41
Bulletproof Mechanism of Soft Bulletproof Composites
The soft bulletproof composites absorb the energy of the bullet through shear fracture and delamination of the material, tensile fracture and deformation of the fiber, and high-speed friction between the projectile and the target plate material, among which the deformation of the material and the fracture of the fiber are the main energy absorption modes.
When the soft bulletproof composite is subjected to the impact energy of the projectile, the fiber will be stretched and deformed, and the absorbed energy will become the work required by the deformation. The work required by the tensile deformation fracture is the fracture energy, also known as the fracture work. The fracture energy of the fiber is related to the number of fibers involved in the tensile deformation fracture. The parameter to measure the bulletproof property of fiber is the energy absorption rate of fiber. 42
When the fiber bulletproof material is subjected to external impact, the longitudinal stress generated by the impact will rapidly propagate around the fiber material, forming a “shock wave” (sound wave). The propagation mode of a shock wave in the fiber is shown in Figure 2. 43 The speed of sound in fiber bulletproof materials will lead to instantaneous diffusion of energy, which will change the number of fibers involved in energy absorption, thus affecting the bulletproof effect of the material. Therefore, the sound velocity in fiber is another important parameter affecting the bulletproof performance of fibers. 44
Propagation mode of the shock wave in the fiber. 43
The fiber shape of bulletproof material is also different. If the fiber shape of the material is straight, then the energy will travel straight along the fiber axis, and the energy will, therefore, spread far away. If the shape of the fiber is curved, or there are broken ends in the fiber, then the bending point or break point in the fiber will reflect part of the energy and reduce the diffusion range of energy, so that the bulletproof effect of the material is reduced. 45
The transfer of energy is often accompanied by the contact of fibers within or between layers. In the process of impact energy transfer, energy reflection occurs at the interface of all materials, and the situation is complicated and diverse. Therefore, the most effective means of impact energy propagation is axial diffusion along the fiber. 46
Bulletproof Mechanism of Soft and Hard Bulletproof Composite Materials
The mechanism of soft and hard composite bulletproof materials can be said to take into account the characteristics of both. When resisting bullets, the first action is from hard bulletproof materials such as a steel plate or reinforced ceramic materials insert plate, providing a blocking effect. In this process, bullets and hard bulletproof materials may be deformed or fractured, which will consume most of the energy of the bullets and reduce the possibility of penetration. The soft bulletproof material then acts as a buffer, diffusing and absorbing the remaining energy of the bullet, and minimizing non-penetrating damage.
Bulletproof Mechanism of STF/High-Performance Fiber Composites
When the projectile penetrates the fabric, the yarn is pushed aside within the fabric and the fiber does not break significantly under impact load. This phenomenon is known as the window effect. 47 The local structure in the impact zone is obviously deformed, and the main yarn in the impact center will slip or break, but the surrounding yarns (secondary yarns) will hardly be disturbed (as shown in Figure 3). STF/high-performance fiber composites attach nanoparticles to the fibers, which can increase the friction between fibers and limit yarn movement. When the fabric is subjected to high-speed impact, the fiber does not easily slip, the shear resistance is enhanced, and the friction between bullets and particles will also consume the impact energy. 48 When the shear rate of fiber slippage under ballistic impact reaches the critical shear rate, the nanoparticles will agglomerate and produce a thickening phenomenon (Figure 3(a)). The viscosity will increase rapidly, and the fabric will be bound and not easily deform, which together with STF produces a coupling effect to help the stress wave propagate along the main yarn to the whole fabric, 49 thus enhancing the bulletproof effect.
Bulletproof Mechanism of MRF/High-Performance Fiber Composites
MRF shows the characteristics of a liquid in the absence of external stimulation; it can flow and maintain a certain activity, and when a certain shear force or impact, magnetic field, electric field, and other external stimulation is applied, its strength and stiffness will suddenly increase, and the material will become hard. This is because the molecular chains in the fluid are interacted with by magnetic polarization and are arranged in order along the magnetic field direction, so the physical properties of the MRF change. This phenomenon is reversible, so after removing the external magnetic field, the MRF returns to its original liquid state. In the process of dynamic phase transformation, the chain structure in the MRF breaks and reforms, and the degree of change depends on the magnitude of the applied magnetic field. 55
MRF and high-performance fibers are made from composite bulletproof material, by setting a fine circuit inside the composite material, so that there is an electric field around the material, Electromagnetic induction generates the corresponding magnetic field. When there is no current through the wire, the bulletproof material is very flexible and the impact resistance is not much different from the original material, but after opening the switch, the electric current is generated in the wire, generating a magnetic field. This magnetic field makes the bulletproof composite stronger and significantly improves impact resistance. Flip the switch back and the magnetic field disappears, and the bulletproof material becomes flexible again.
Bulletproof Mechanism of LCE Composites
LCE, as a new type of liquid crystal polymer material, has the dual characteristics of a liquid crystal and an elastomer, both the anisotropy of a liquid crystal and the rubber elasticity of cross-linked polymers. The driving principle of LCE is mainly based on its phase transition between order and disorder. Compared with LCEs driven by heat, electricity, or magnetism, light-driven LCEs have the advantages of remote and local control.
At present, light-driven liquid crystal drivers are mainly based on two mechanisms: a photochemical effect and a photothermal effect. The former is mainly based on photoisomerization of azobenzol groups under ultraviolet and visible light, but it is affected by side reactions and photodegradation, and its long-term stability is poor. The latter is mainly based on photothermal conversion, but the general photothermal conversion material (photothermal agent) will appear in the LCE agglomeration phenomenon, thereby reducing the photothermal effect, limiting the response rate of the driver, drive performance and cycle performance. 56
By using LCE as a flexible substrate and combining it with high-performance fibers, the orientation of LCE molecules can be controlled by external action, and the order arrangement of LCE molecules can be realized. The deformation behavior of LCE is very dependent on the molecular orientation. When the temperature of the LCE rises above the phase transition temperature, the curved molecular arrangement generates the stress in the LCE along the thickness direction, which causes the LCE to bend to the parallel orientation side of the molecule. After dynamic rearrangement, the liquid crystal structural units form a more stable cross-linked network. The shear resistance of the composite material is improved. After the removal of light, temperature, mechanical force, and other external effects, the polymer network in LCE can be restored to the previous state, with memory function and excellent controllability and programmability.
Factors Affecting Bulletproof Properties of Bulletproof Composites
The bulletproof properties of bulletproof composites are affected by many factors, including resin matrix, reinforcement, interface, fabric structure, and so on.
Resin Matrix
In bulletproof materials, resin is generally selected thermoplastic polyurethane. Its molecular chain contains flexible functional groups, the molecules are flexible, and it has good impact resistance and bending resistance, and good energy absorption. In addition, the resin has strong chemical stability and bonding properties, and can achieve better bulletproof performance through synergistic action with high-strength fibers. 57 Lodewijk and Marie 58 studied high-performance fiber-reinforced amorphous polyurethane resin and found that polyurethane resin has good impact resistance and can effectively reduce the damage to personnel.
The content of resin also affects the bulletproof property of composites. Too low a resin content will make the interlayer bonding strength too low and the fibers loose. Under the high-speed impact of bullets, the fibers easily slip, leading to an increase in gaps, thus reducing the number of effective fibers involved in bulletproofing, affecting the integrity of laminates and reducing the bulletproof performance of the composites. 59 However, when the resin content is too high, it will lead to the thickening of the resin between the composites, which will produce too much constraint on the fiber, and affect the tensile deformation properties of the fiber when it is impacted, and cannot give full play to the role of the fiber.
In addition, due to the good damping performance of a low modulus resin matrix, which is conducive to energy absorption, the bulletproof effect of laminates manufactured using a low-modulus resin matrix is better than that of laminates manufactured using a high-modulus resin matrix. 60
High-Performance Fiber Reinforcement
For high-performance fiber reinforcement, three kinds of fibers are mainly selected, which are UHMWPE fiber, aramid 1414 fiber, and heterocyclic aramid fiber, and their effects on bulletproof performance are compared. Among them, UHMWPE fibers have the highest specific strength and specific modulus, and the main chain of macromolecule is a flexible molecular chain composed of a carbon chain skeleton, which gives UHMWPE excellent tensile and mechanical properties. When UHMWPE fibers are impacted by a high-speed bullet, the main chain of fiber macromolecules will slip first, and the fiber will undergo tensile deformation, which consumes part of the kinetic energy of the bullet. When the tensile stress exceeds the stress limit of the fiber, the main chain of the macromolecule will stretch and break, again consuming the impact energy of the bullet and playing a protective role.
While the macromolecular structure of aramid 1414 fiber is a kind of crystalline polymer structure similar to rigid extended straight chains, the main chain is arranged with benzene rings and amide bonds spaced apart. There are hydrogen bonds between two adjacent chains, so aramid 1414 fiber has high strength and modulus. When aramid 1414 fiber is impacted by a high-speed bullet, due to the rigid structure of benzene rings, the macromolecular main chain of aramid 1414 fiber is directly broken, so the bulletproof ability of aramid 1414 fiber composite is lower than that of the UHMWPE fiber bulletproof composite. 61
Finally, heterocyclic aramid fiber is a new type of aromatic fiber formed by the introduction of a heterocyclic structure into the molecular main chain of aramid 1414 fibers. 62 After the introduction of a heterocyclic ring, the strength of aramid fiber is increased, which improves the bond between fibers and the resin interface and enhances the bulletproof properties. Therefore, the bulletproof properties of these three fibers from high to low are as follows: UHMWPE fibers, heterocyclic aramid fibers, and aramid 1414 fibers.
Fiber Fabric Construction
Woven fabrics with strong shape retention are usually selected for two-dimensional fabrics. Compared with twill and satin fabrics with the same surface density, plain weave has more interweaving points and can disperse more energy to achieve a bulletproof effect. 63 The reinforcement formed by the overlay of the same fabric, 64 the order of fabric stacking, 65 and the stacking angle 66 will also affect the bulletproof property.
Low-surface density fabrics have better bulletproof properties. The bulletproof properties of composite materials are determined by the braiding lines used by the fibers in the material, fabric structure, the number of layers, and the arrangement of fibers. Under a given weight, the thinner and tighter a woven fabric is, and more layers it has, the better the bulletproof performance of the materials. 67 When the surface density of a bulletproof material is fixed, a fabric with more layers and smaller single-side density should be considered. At the same time, improving the properties of the fiber itself will also improve the bulletproof properties of the materials. 68
Liu et al. 69 prepared bulletproof composite materials by using UHMWPE fiber orthogonal layup, plain weave, twill weave, and weft knitted biaxial fabric structure as reinforcement and low-density polyethylene as the matrix. The results show that the orthogonal layup structure is the best, among which the weakness of the two-dimensional woven fabric structure is that the curved yarn will cause a certain internal stress in the yarn itself, resulting in a decrease in the shear resistance of the yarn when it is impacted by fragments. An et al. 70 studied the impact resistance of warp knitted biaxial fabric and Li et al. 71 studied the impact resistance of weft knitted biaxial fabric, which showed that compared with ordinary woven fabric, biaxial fabric responded faster to load.
Özdemir and Mer 64 indicated that plain fabrics have more cross threads, can transfer more energy, and have a better ballistic-resistant effect compared with twill and satin fabrics of the same surface density. Karthikeyan et al. 72 further studied the bulletproof performance of unidirectional weftless fabric with different layering angles. It is found that [0°/90°] has the best ballistic performance, as shown in Figure 4. 73
0°/90° orthogonal layering structure. 73
Li et al. 74 believe that the bulletproof performance of the composite reinforced by three-dimensional braided structure of UHMWPE fiber is better than that of other fabric forms. The existence of transverse yarn enables the fiber to have a better overall structure, which improves the force between fiber layers and increases the penetration resistance of the final material, as shown in Figure 5. 75
3D orthogonal braid structure diagram. 75
Yang et al. 76 studied the impact of hybrid mode on the overall bulletproof performance of composite materials, and the results showed that the addition of aerogel interlayer in bulletproof fiber provided a larger deformation space for the bulletproof panel, greatly improved the efficiency of absorbing bullet kinetic energy, and reduced the incidence of blunt injury.
At the same time, fabric density affects the friction between yarns and also affects the movement of nanoparticles. When the fabric is less dense, the number of intersections or interleaved points between the warp and weft yarns is also lower. Due to the bending of the fibers, reaction forces are applied to these intersections, requiring less force to pull the fibers out of the fabric. For STF-treated fabrics, nanoparticles adhere to fibers, which increases the friction between fibers and significantly increases the fiber pulling force. 50
Interface
The interface is the transition layer between the reinforcement phase and the matrix, so that the reinforcement fiber and the resin matrix form a whole and the stress is transferred through the interface. 77 In UHMWPE fiber composite, the fiber and matrix can absorb the energy of the bullet by interfacial debonding or delamination.
When the crack extends to the fiber, if the interfacial bonding strength is weak, the crack will shift direction and expand along the fiber, resulting in debonding. In this case, the absorbed energy depends on the interfacial bonding strength, so the impact toughness of the material can be improved by appropriately reducing the interfacial bonding strength. 78 But the interface bonding strength should not be too low, otherwise the yarn will be loose, resulting in the bulletproof performance of the composite material being reduced. Taking UHMWPE fiber as an example, the surface bonding performance of UHMWPE fiber is poor. To improve the bulletproof performance of the fiber-reinforced composite material, it is necessary to treat the fiber surface and improve the interface bonding strength. Usually, the surface treatment methods of UHMWPE fibers include the corona method, plasma treatment, chemical oxidation, and crosslinking through the treatment to form polar groups on the surface, so as to improve their bonding performance with the resin matrix.
STF
The most important effect of STF on the bulletproof properties of STF/high-performance fiber composites is the particle size and content of the dispersed phase. The studies show that65,79 with the increase in particle size, the critical shear rate of STF decreases. 80 Because the specific charge on the surface of small particles is large, the repulsive force between particles is strong, so it does not easily form particle clusters. At the same time, the larger particles have a higher inertia effect, showing a plateau region in the large span of shear rate, while the STF of small particles will show shear thinning immediately after reaching the kurtosis viscosity. If the content of a dispersed phase is too high, the particles will condense and disperse unevenly, resulting in a decrease in viscosity. Nano-silicon particles are modified or treated with plasma to increase the interaction between particles, which can show shear thickening behavior and improve the energy absorption capacity at a lower shear rate.
Other Factors
In addition, the rheological properties of STF are easily affected by temperature. 32 Increasing temperature intensifies the Brownian motion of suspended particles, reduces the viscosity of polyethylene glycol (PEG), and increases the difficulty of forming water masses in suspension. At a higher temperature, a larger shear rate is required to trigger shear thickening. The PEG molecules adsorbed on the surface of particles act as a solvated layer. As the temperature decreases, the thickness of solvated layer decreases, the effective volume fraction of STF increases, and the critical shear rate decreases. At the same time, temperature and humidity affect the fiber fracture strength, tensile modulus, and other mechanical properties, resulting in fiber properties being decreased and the bulletproof properties of composites being weakened. Micro-nano fillers affect the friction between fibers, 81 prevent the “window effect” by increasing the shear force between yarns, block the penetration of bullets, and increase the friction force on bullets to consume kinetic energy. Both the weight and velocity of the bullet affect the shear force. 63 The greater the shear force, the shorter the reaction time for STF to have a thickening effect and the faster the impact response to the bullet.
Future Prospects
When it comes to future research directions and advances in bulletproof composites, there are some key areas to focus on. Some possible research directions and advances are as follows:
Diversification and compounding: One of the development trends of bulletproof fibers and composites is to achieve lighter, thinner, and stronger materials. In addition, diversification and compounding are also the direction of future development. By combining different types of fibers and substrates, better properties and materials adapted to different application requirements can be obtained.
Application of nanotechnology: The application of nanotechnology in bulletproof materials is an active research field. The addition of nanoparticles can improve the mechanical properties, wear resistance, and corrosion resistance of the materials. For example, nano-ceramic particles can enhance the hardness and impact resistance of composite materials. 82
New fiber materials: In addition to traditional bulletproof fibers (such as aromatic polyamide fibers and ultra-high molecular weight polyethylene fibers), some fibers that do not have bulletproof properties, such as carbon fiber, high-strength polyimide fibers, polyarylate fibers, and ultra-high-modulus polypropylene fibers, have also joined the ranks of bulletproof fibers due to technological breakthroughs.
Protective performance prediction technology: Develop new prediction methods to more accurately evaluate the properties of bulletproof composites. This includes the use of numerical simulations, experimental testing, and advanced predictive models of material properties.
In short, research on bulletproof composites will continue to focus on innovative design, performance optimization, and cross-cutting and fusion of multiple fields to meet changing needs and challenges.
Conclusion
With their high-specific strength, specific modulus, and versatility, new bulletproof composites are indispensable in many military applications. They are key technologies for personal protection and advanced weapons equipment, and are of great importance to a country in terms of both social and economic benefits.
The composite material has excellent performance because it combines the advantages of reinforcement material and matrix and makes up for the corresponding shortcomings. The research on composite materials makes bulletproof materials gradually diversify, and there are many new bulletproof materials with high hardness and high toughness to deal with the complex protection problems. This article studies and compares the new bulletproof composites from four aspects: the classification of the new bulletproof composites, the bulletproof mechanism of the composites, the factors affecting the bulletproof properties of the composites, and the different bulletproof standards of different countries in the world. Although the new bulletproof composites have various advantages such as light weight, high modulus and high strength, the existing problems such as interface and cost cannot be ignored. Therefore, optimizing the performance of bulletproof composites is constantly still the key point at present.
Footnotes
Acknowledgements
The authors sincerely appreciate the support from the Top-notch Academic Programs Project (TAPP) of Jiangsu Higher Education Institutions and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by School Start-up Fund Project of Changzhou University (grant no. ZMF21020365), by the 4th Leading Innovative Talents Cultivation Project of Changzhou City (grant no. CQ20210106), and the Research Project of MenBo High-tech Jiangsu Co., Ltd (grant no. 2022K3071).
