Light weight composite armor with structural strength

专利摘要:
Future fighting vehicles will require lighter, stronger and more space efficient armor for better protection, survivability and greater mobility. The invented lightweight armor component consists of armor-grade materials (2L such as ceramic, enclosed in fiber reinforced cementitious composite (FRCC) (1). The FRCC is used to substantially confine and pre-stress the armor-grade materials. The resulting armor component of the present invention can provide excellent ballistic protection against all types and sizes of Kinetic Energy (KE) threats and Chemical Energy (CE) threats. The armor component has low areal density, reduced damage area, improved multi-hit capability, flexible design and also provides high structural strength. lt furthermore has the advantage that it can be formed in virtually any shape. This invention results in superior ballistic characteristics of a (passive) armor system. An object of the present invention is to increase penetration resistance of especially ceramic based armor, while lowering system weight.
公开号:DK201200491A
申请号:DK201200491
申请日:2012-08-07
公开日:2014-02-08
发明作者:Nielsen Frank
申请人:Nielsen Frank；
IPC主号:

专利说明:

LIGHT WEIGHT COMPOSITE ARMOR WITH STRUCTURAL STRENGTH The scope of the invention
The present invention generally relates to passive ballistic armor for military and civilian use, and in particular, to lightweight composite armor with very strong structural strength based on a multi-material composite for absorbing and limiting the transfer of impact energy from all types and sizes of Kinetic Energy (KE) threats and Chemical Energy (CE) threats.
This invention relates particularly to ballistic protection for use in military vehicles of various types and other moving and stationary military platforms, or for use in permanent or temporary protection of various types in buildings or other fixed or mobile installations. The ballistic protection according to the present invention can be employed as protection against various types and sizes of low-velocity or high-velocity armor piercing projectiles.
It is an object of the present invention to provide a composite armor which will prevent the penetration of projectiles into a structure while also providing structural support to the same structure.
Desired armor protection levels can usually be achieved if weight is not a consideration. For over a century, metals have been the material of choice in providing load-bearing capabilities and ballistic protection for military platforms. Especially military vehicles have traditionally been manufactured from high strength armor plate steel.
Development of modern fighting technology, directed generally towards decreasing mass of vehicle construction, simultaneously creates the necessity of increasing the resistance of protective layers to all types of impact threats. Vehicles designed for land conflict are often lightly or inadequately protected from heavy machine gun (HMG) projectiles, high velocity kinetic energy-based anti-tank long rod penetrators (LRPs), asymmetric threats such as fragments from Improvised Explosive Devices (lED's), Explosively Formed Projectiles (EFPs) and Chemical Energy Threats (CE), which are all encountered with rising frequency by troops on operations.
This invention provides an integral composite armor material for absorbing and dissipating kinetic energy from high-velocity armor piercing projectiles, LEDs, EFPs as well as Chemical Energy threats such as RPGs. It is an improvement to existing ceramic-based armor.
This invention improves upon existing composite armor designs through the use of Fiber Reinforced Cementitious Composite (FRCC), which is a composite material with a cementitious matrix and discontinuous reinforcement (short fibers) made of either inorganic or organic material, mostly steel fibers. , polyvinyl alcohol (PVA) fibers and polyethylene (PE) fibers (FIG. 1).
More specifically, it relates to fully encapsulation and pre-stress of armor-grade materials, such as ceramics, with FRCC, providing armor with highly enhanced ballistic efficiency, physical durability, structural strength and environmental resistance.
This invention results in superior ballistic characteristics of an armor system and can be ballistically qualified in all kinds of sizes and shapes, and can be custom produced to meet any demand for ballistic protection. The overall shape of the panel will be determined by end user requirements. Often panels of the invention will be generally flat and with generally uniform thickness. For more specialized end user requirements, a panel may be shaped in any form of curvature and varied thickness through the panel. The overall dimensions of the panels of the invention will be determined by end-use requirements, such as the impact conditions they are required to resist, and the size and / or area of the object on which the panel or assembly of the panels is required to protect. These individual panels can then be laid into multiple-panel arrays to obtain broad area coverage of a contoured structure.
The transition between the various degrees of protection (light and heavier protection) can be implemented quickly and efficiently and upgrading / downgrading of existing protection can be performed quickly without particularly sophisticated equipment.
The result is a lightweight, composite hybrid structure for ballistic protection particularly suited to tactical ground vehicles. One advantage of this invention is an increase in the ballistic penetration resistance of especially ceramic-based armor with a simultaneous decrease in the armor system weight. It also provides additional strong structural reinforcement.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Prior art - prior art
Ballistic armor is well known in the art as is herein detailed and described together with explanation why the prior art could be improved, or in some essential features is different from the present invention.
To defeat lethal projectiles, laminate systems composed of a hard frontal plate supported by a metallic or polymer composite backing are employed. In this system, the high hardness face plate breaks (shatters, erodes, blunts) the projectile defocus the kinetic energy to allow the backing to effectively catch the residuals. In addition, the backing serves as a breadboard for attachment to the vehicle. This is employed in armor systems including dual hard steel, titanium-aluminum laminates and ceramic composites. Facial ceramics are most effective in this type of application but have limited fracture toughness and damage tolerance; thus the ceramic is parasitic to the vehicle structure. There are two prevalent hard passive armor technologies in general use. The first and most traditional approach uses metals. The second approach uses ceramics. Each material has certain advantages and limitations. Metals are more ductile and are generally superior to withstanding multiple hits. However, they typically have a large weight penalty and are not as efficient at stopping armor-piercing threats. Ceramics are extraordinarily hard, strong in compression, light weight and brittle, making them efficient at eroding and shattering armor-piercing threats, but not as effective at withstanding multiple hits.
Light-weight metallic and ceramic armor designs are known. For example, metals such as titanium and aluminum alloys can replace traditional steel to cut weight. Ceramics, such as aluminum oxide, silicon carbide, and boron carbide, are used in combination with a supporting backing plate to achieve even lighter armor. When ceramics are employed in laminate constructions and are backed with high tensile strength, high-toughness "momentum trap" composites such as Kevlar or Spectra fibers, mass-efficient armor systems can be designed. The mass efficiency of such ceramic composite armor systems is generally two times (or more) higher than that associated with high hardness steel or similar high strength metallic armor plate.
State-of-the-art military armor systems for various platform protection frequently make use of lightweight, very high compressive strength ceramics such as silicon carbide (SiC), boron carbide (B4C) or alumina as the so-called "strike face" of and armor laminate package. The purpose of the strike face material, typically employed in high performance ceramic composite armor systems, is to blunt and defeat incoming metallic projectiles by overmatching the compressive properties of the incoming projectile during the early (compressive shock) portions of the impact event. High modulus, high strength ceramics can have four to five times the dynamic compressive strength of projectile materials such as steel, tungsten or tungsten carbide. Thus, it is possible to shock the incoming projectile to the extent that compressive fracture is initiated. This decreases the ability of the projectile to defeat the armor system.
The most important ceramic materials today for ballistic protection of military vehicles and ships are Alumina (AI203). Owing to its excellent price-efficiency ratio, alumina is the preeminent ceramic armor material for vehicular applications. Only when an extremely low weight is required (e.g. for personal protection or for helicopters), silicon and boron carbide materials may be used. Other ceramic materials may also be considered for the purpose of ballistic protection, such as Silicon nitride (SN), Titanium boride (TiB2), Aluminum nitride (AIN), SIALON (Silicon aluminum oxynitride), Fiber-reinforced ceramic (eg C-SiC ) and Ceramic-metal composite materials (CMC).
However, ceramic armor is not without serious engineering and practical shortcomings. High hardness, high elastic modulus ceramic materials such as SiC and B4C are very brittle and have poor durability and resistance to dropping or even rough handling under typical field conditions. Furthermore, the low toughness of high performance ceramics implies that essentially all armor-grade ceramics have poor multiple hit capabilities. Once a ceramic tile is impacted, the subsequent impact response of the armor is seriously compromised.
Multiple hits are a serious problem with ceramic-based armors. Armor-grade ceramics are extremely hard, brittle materials, and after one impact of sufficient energy, the previously monolithic ceramic will fracture extensively, leaving many smaller pieces and a reduced ability to protect against subsequent hits in the same area.
There are several sources of information which shows that confining the ceramics results in an increase in penetration resistance One relatively obvious and popular method to overcome the disintegration of ceramic armor is to encapsulate ceramic armor with a layer of surrounding metal.
In the laboratory, ceramics show much higher performance when their boundaries are heavily confined. The two key parameters are suppression of cracked tile expansion and putting the ceramic in an initial state of high compressive stress to delay or stop it from going into a state of tensile stress during impact. The problem is to devise methods to realize some or all of this confinement effect so it can be reduced to practical application in real armor systems. If the ceramic tile is not encased, the fractured pieces can move away easily, and residual protection is lost.
State-of-the-art integral armor designs work by assembling arrays of armor-grade ceramic tiles / spheres / pellets within an encasement of polymer composite plating or metallic frames. Such an armor system will erode and shatter (armor-piercing) projectiles. Various designs are in current use over a range of applications. Substantial development efforts are ongoing with this type of armor, as it is known that its full capabilities are not being utilized.
There are several deficiencies with the encapsulation of ceramic cores in the prior art. Because of the properties of the proposed metals, conventional casting processes cannot be readily and effectively utilized to encapsulate ceramic cores. For example, the very high solidification shrinkage of metals precludes this process as the encapsulating metal exerts undue stresses on the ceramic core, and can result in the fracturing of the ceramic. Confinement of ceramic armor can also be performed by a number of other means, such as shrink-fitting ceramic tiles or bricks into metallic containers, or by other bonding methods involving the use of welded, bolted, brazed or adhesively bonded metallic containers. In the past, such layers have for instance been formed on or around ceramic cores or tiles by techniques such as powder metallurgical forming, diffusion bonding, and vacuum casting of liquid metal layers.
Accordingly, a need exists for an armor component formed from an encapsulated ceramic material that has improved impact resistance, and for an inexpensive method for forming an armor component from a ceramic material that has improved impact resistance. Snedeker, et al. used a hybrid metal / ceramic approach in U.S. Pat. No. 5, 686, 689. Ceramic tiles were placed into individual cells of a metallic frame consisting of a backing plate and thin surrounding walls. A metallic cover was then welded over each cell, encasing the ceramic tiles.
More expensive encapsulation processes, such as, powder metallurgy techniques are used as disclosed in U.S. Pat. No. 4, 987, 033, which shows methods for metallic encapsulation of ceramic cores with powdered metal layers that are cold isostatically pressed, vacuum sintered and then hot isostatically pressed to final density.
These methods have severe shape limitations, involving the use of relatively costly cold isostatic press tooling, require a complicated and costly multiple step processing sequence, and still require complicated and costly post-machining to produce a metallic encapsulating layer with consistent areal density (which is required for armor system design). U.S. Pat. Nos. 3, 616, 115 and 7, 069, 836, respectively, show methods for metallic encapsulation of ceramic armor based on vacuum hot pressing and / or diffusion bonding of ceramic tiles and metallic stiffening layers into machined arrays of lattice-type metallic frameworks. While capable of producing well-bonded and geometrically consistent metallic encapsulation layers, these methods are also costly and very limited with regard to their shape-forming capability and the related ability to be transitioned to large-scale manufacturing, as they require expensive restraint tooling. and those sets that essentially limit vacuum hot press or die-based diffusion bonding to flat plate geometries. Modifications of conventional liquid metal casting processes have been used as in U.S. Pat. No. 7, 157, 158. These methods, while capable of providing encapsulation of different ceramic materials as well as complex shapes, require complex and costly molds, and the casting process itself presents many challenges since most metals of interest for encapsulation (Al, Mg , Ti etc.) shrink anywhere from about 3 to 12% upon solidification. The high coefficient of thermal expansion relative to armor-grade ceramics such as silicon carbide, boron carbide or alumina frequently leads to liquid metal casting-based encapsulation results generating very high stresses around the ceramic core, which can easily result in the fracturing of the ceramic being encapsulated. This situation would also be worsened for more complex ceramic armor tile geometries. U.S. Pat. No. 5, 361, 678 issued to Roopchand reveals coated ceramic bodies in composite armor where the ceramic bodies are embedded in a metal matrix.
French patent no. 2526535 issued to Pequignot reveals ceramic elements embedded in a metallic plate and thermally stressed. U.S. Pat. No. 6, 532, 857 issued to Shih reveals a ceramic array armor confined with shock isolated ceramic tiles with rubber between the tiles and over the top of them. Polysulfide is used as an encapsulation component. U.S. Pat. No. 7, 117, 780 issued to Cohen reveals composite armor plate using a layer of pellets held by elastic material.
The particular one achieved by the invention
This invention is in the field of improved and lighter armor materials for military and civilian purposes. "Smaller and lighter" is today's paradigm for future combat vehicles. Passive armor is, and will be for many years to come, the last line of defense for vehicular survivability. Future fighting vehicles will require lighter and more efficient armor materials for greater survivability, mobility and transportability.
It is increasingly difficult to defend against the destructive forces of projectiles being produced and developed for the penetration and destruction of current armor materials. These projectiles are referred to as ballistic threats or simply "threats". Modern threats vary in size and type.
To meet these threats, exceptional mechanical and physical properties are required in armor materials. Toughened metal alloys, fiber reinforced plastics and high technology ceramic materials are some of the major armor materials used today. Many modern armors are multicomponent systems, using several plates of different materials bonded together in order to exploit the individual material properties to the best advantage.
Ceramics like alumina, silicon carbide, boron carbide, beryllium oxide etc., represent one of the most important developments in armor materials. Their very high compressive strength and hardness, coupled with relatively low density, provide exceptional armor performance against a wide range of high energy ballistic threats.
Ceramic armor materials are lighter than conventional metallic solutions, including titanium, and are two to three times harder. Use of ceramics lowers the weight of a passive armor system while improving the ability to defeat ballistic threats.
Ceramic materials have seen increasing use in ballistic applications where a combination of high compressive strength and low density are important. The very high compressive strength of ceramic materials offers the potential for more efficient destruction of penetrators than more conventional monolithic metals. However, the extreme localized loading of the ceramic during a ballistic impact typically generates early failure and comminution of these brittle materials, and subsequent considerable loss of ballistic efficiency.
Ceramic materials are hard and brittle. The high hardness contributes to flatten the nose portion of the incoming projectiles, which increases the forces to stop the projectiles. Regardless of the ceramic material used, ceramic armor is damaged on impact, and this damage propagation affects the subsequent ceramic performance. This damage is caused by the activation of preexisting defects by the shear and tensile forces generated on impact on the ceramic.
The brittle properties of ceramics are not good for sustained defeating of projectiles, however, the damage zone forms due to this help to distribute the impact force over a larger area. Another effect of brittleness of ceramic material is the long cracks usually expand from the point of hit due to bending. The long cracks and resulting small pieces of ceramic material are detrimental to the defeat of projectiles because not much constraint exists in-plane to keep the material in the damage zone and to contribute resistance forces.
Ceramics are usually employed in an armor system where backing and surround plates are utilized in an attempt to increase the efficiency of the comminuted ceramic. Such attempts at backing or surrounding the ceramic with various materials have been met with mixed success, partly because the underlying principles that influence successful system design are not clearly understood. Engineering a better backing and / or surrounding that can enhance the performance of the ceramic would lower its weight and space burden in a structural armor application.
Various factors (backing plate stiffness, ceramic compressive strength, ceramic / encapsulation impedance mismatch, etc.) have been shown to be important contributors to the overall efficiency of the ceramic / confinement system. The resulting behavior of the ceramic is a complicated combination of the integrated responses of the damaged and undamaged regions. Since ceramics is rarely, if ever, used as a stand-alone armor, the mechanical response of the entire system determines the degree to which the damage is generated and how well the damage is confined.
Ceramics are currently the subject of intensive research and improvement. The field applications of ceramic armors, however, have been limited by especially the high material cost. Incorporation of ceramics into hybrid armor systems can result in significant weight reductions, and are an excellent prospect for next-generation energy absorbing systems for ballistic protection.
The present invention relates to the encapsulation and pre-stress of armor-grade materials, such as ceramics, with fiber reinforced cementitous composites (FRCC). The characteristics of FRCC can be utilized to exert a highly beneficial compressive stress on a ceramic core. Such beneficial compressive stress makes the encapsulated inner core defeat the projectiles much more effective by delaying the formation of cracks in the ceramics.
The objectives of this invention are as follows: Enhancement of hydrostatic confinement of an inner core made of armor-grade materials, such as ceramic, to increase dwell time for a projectile on the front face of the inner core thus promoting mushrooming and defeat of anti-armor projectiles of all types and sizes. • Enhancement of multiple hit resistances for individual tiles, tile arrays or more complex shapes into which the inner core is divided. • Enhancement of the durability and damage tolerance against physical abuse and routine handling for the inner brittle core by providing a robust container for individual tiles, tile arrays or more complex shapes. • Providing a composite armor that will prevent the penetration of projectiles into a structure while also providing structural support to the same structure.
The present invention is a major improvement to current ceramic-based integral armor, which results in superior ballistic performance and survivability, multi-hit capability including reduced damaged area, lower area density, more flexible design as well as strong structural strength.
The new technical means
It is to be understood that the following detailed description represents embodiments of the invention and is intended to provide an overview or framework for understanding the nature and character of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated into and constitute a portion of this specification. The drawings illustrate various embodiments of the invention and, together with the description, serve to explain the principles and operations of the invention but not to limit the invention to these descriptions only.
The main idea of this invention is accomplished by wrapping fiber reinforced cementitious material around the perimeter of an inner armor-grade material, typically ceramic, to confine and pre-stress this inner core to increase dwell time and avoid the inner core from lateral expansion when impacted by a projectile.
Fiber reinforced cementitious composite (FRCC) is a very universal term for all cementitious materials that are reinforced by any kind of fiber. FRCC is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers are manmade, such as steel, titanium, glass, carbon, polymers or synthetic. The character of FRCC changes with varying concretes, fiber materials, geometries, distribution, orientation and densities.
Many different so called high performance materials have been developed. The considered materials in this disclosure to confine ceramic materials are all cementitious composites reinforced by randomly oriented short discrete fibers. Adding fibers enhances the compressive, tensile and shear strengths, flexural toughness, durability and resistance to impact as well as resistance to plastic shrinkage cracking.
Concrete is widely used in structural engineering with its high compressive strength, low cost and abundant raw material. But common concrete has some shortcomings, for example, shrinkage and cracking, low tensile and flexural strength, poor toughness, high brittleness, low shock resistance and so on, that restrict its applications. To overcome these deficiencies, additional materials are added to improve the performance of concrete.
Cementitious matrices such as concrete have low tensile strength and low strain capacity and therefore fail in a brittle manner. As a result, the mechanical behavior of the concrete is critically influenced by crack propagation. Concrete may exhibit failure through cracks which are developed due to brittleness. A more ductile material can be achieved by the use of fibers in the concrete.
The use of fibers in concrete to improve pre- and postcracking behavior has gained popularity, and several different fiber types and materials have been successfully used in concrete to improve its mechanical and physical properties.
Reinforcing fibers will stretch more than concrete during loading. Therefore, the composite fiber reinforced concrete system is assumed to work as if it were unreinforced until it reaches its "first crack strength." It is from this point that fiber reinforcement takes over and holds the concrete together. For fiber reinforcing, the maximum load carrying capacity is controlled by fibers pulling out of the composite. Some fibers are more "slippery" than others when used as reinforcing and will affect the toughness of the concrete product in which they are placed.
Many catastrophic failures of reinforced concrete structures subject to impact are associated with the brittleness of concrete material in tension. Although a compressive stress wave is generated on the loading side of the structure by impact, it reflects as a tensile stress wave after hitting a free boundary on the back side of the structural element. In addition, the tensile strength of concrete is typically much lower (by about an order of magnitude) than its compressive strength. Therefore, concrete tensile properties generally govern concrete failure under impact.
Concrete is considered a brittle material, primarily because of its low tensile strain capacity and poor fracture toughness. Concrete can be modified to perform in a more ductile form by the addition of randomly distributed discrete fibers in the concrete matrix.
Ductile concrete would be highly desirable to suppress the brittle failure modes and enhance the efficiency and performance of current design approaches. The most effective means of imparting ductility into concrete is by means of fiber reinforcement.
An extremely ductile fiber reinforced brittle matrix composite is of great value to protective structures that may be subjected to dynamic and / or impact loading. Compared to normal concrete and fiber reinforced concrete, a ductile concrete based composite has substantially improved tensile strain capacity with strain hardening behavior, several hundred times higher than that of conventional concrete and fiber reinforced concrete, even when subjected to impact loading (FIG. 2). .
While the fracture toughness of concrete is significantly improved by fiber reinforcement, most fiber reinforced concrete still shows quasi-brittle post-peak tensile reduction behavior under tensile load where the load decreases with the increase of crack opening. The tensile strain capacity therefore remains low, about the same as that of normal concrete. Significant efforts have been made to convert this quasi-brittle behavior of fiber reinforced concrete to ductile strain hardening behavior resembling ductile metal. In most instances, the approach is to increase the volume fraction of fiber as much as possible. If the fiber content exceeds a certain value, typically 4-10% depending on fiber type and interfacial properties, the conventional fiber reinforced concrete may exhibit moderate strain hardening behavior.
The typical stress-elongation response of FRCC indicates two properties of interest, the stress at cracking and the maximum post-cracking stress (FIG. 3). While the cracking strength of the composite is primarily influenced by the strength of the matrix, the postcracking strength is solely dependent on the fiber reinforcing parameters and the bond at the fiber-matrix interface. Thus, improving the post-cracking strength is key to the success of the composite.
Special types of concrete are those with old ^ of-the-ordinary properties or those produced by unusual techniques. Concrete is by definition a composite material consisting essentially of a bonding medium and aggregate particles, and it can take many forms.
The first fiber reinforced cementitious material that was developed in the early 1960s is steel fiber reinforced concrete (SFRC). SFRCs exhibit ductile behavior compared to the brittle matrix, but their flexural and tensile strengths are not very high, and especially the compressive strengths of these materials do not practically change with the fiber volume fraction. Although it shows some improvements compared to normal concrete, it is not considered to be a high performance material. SFRC consists of a normal strength concrete matrix containing fine and coarse aggregates, which is reinforced by relatively long straight steel fibers. The primary improvements of traditional SFRC compared to normal concrete are higher toughness and energy absorption capacity, greater spalling and delaminating resistance, improved durability characteristics through better crack control, as well as higher ductility on a material level. Compared to normal concrete the tensile strength of SFRC is not significantly improved and SFRC still exhibits "quasi-brittle" failure (no strain hardening). The reasons for this are that low amounts of fibers are used, that the fibers used are relatively long and that the matrix contains too much course aggregate. However, compared to unreinforced concrete the failure is more ductile exhibiting a smother softening behavior.
Originally fiber reinforced materials, straight steel fibers at relatively low volume contents were used to improve the mechanical properties of traditional concrete. The addition of larger volume content of fibers was mainly prevented by workability problems. In order to improve workability and restrict "balling" of fibers, the quantity of cement was increased and the amount of coarse aggregate reduced. Further improvement of workability could be achieved by the introduction of high-range water-reducing admixtures. Hereby the possible volume of fiber content could be increased. Together with the use of improved cementitious matrices with fewer coarse aggregate and carefully adjusted properties, this finally leads to the high performance fiber reinforced materials we know today. FRCC includes the entire class of fiber reinforced cementitious composites (FIG. 4), and includes fiber reinforced concrete (FRC), fiber reinforced mortar (FRM) and high performance fiber reinforced composite (HPFRCC) and ductile fiber reinforced cementitious composite (DFRCC). DFRCC is a broader class of materials than HPFRCC. Examples of modern high performance materials are engineered cementitious composites (ECC), sometimes also called "Bendable Concrete", hybrid fiber concretes (HFC), multi-scale fiber reinforced cementitious composites (MSFRCC), compact reinforced composites (CRC) and reactive powder concretes (RPC), non-high performance fiber reinforced concrete and steel fiber reinforced concrete (SFRC).
Considering the mechanical properties of FRCC, these composites may be categorized into two classes: quasi-brittle and pseudo strain-hardening. Conventional FRCC falls into the first category whereas HPFRCC falls into the laughter.
Quasi-brittle materials, such as plain concrete and conventional FRCC, usually fail due to the formation of a single macro-crack, whereas pseudo strain-hardening cementitious materials such as HPFRCC undergo multiple cracking. For conventional FRCC, the typical upper limit for fiber volume fraction is 3%. For such relatively low fiber content, the fibers mainly enhance the crack arresting ability, post cracking ductility, fatigue and impact resistance. The stress at first crack, maximum stress and the corresponding strain are not significantly improved compared to plain concrete. HPFRCC shows a great improvement in both strength and toughness compared to the plain matrix. The main feature of these materials is the optimum combination of strength and toughness which approaches the structural properties of steel. HPFRCC is a generic term encompassing many different materials ranging from those that employ ultra-compact matrices and those that do not. However, the common point of all HPFRCC materials is their hardening tensile behavior which helps control cracking to a much better extent than usual FRCC.
The criterion that separates an HPFRCC from a traditional FRCC, such as SFRC, is its response in tension. If the material is strain hardening in the inelastic regime it is considered to be an HPFRCC. The fundamental difference between composites that show strain hardening and those that do not is that the ductility of the laughter is only effective on a material level and does not affect the overall structural ductility. Or in other words, the material reacts in a ductile manner, but because it is softening in tension, the plastic deformations are restricted to a small area, resulting in damage localization and failure in a small zone with large crack openings. At a structural level this kind of ductility has little influence since the failure occurs locally and the rest of the structure remains elastic. Strain hardening fiber reinforced cementitious composites on the other hand retard localization and lead to multiple cracking and structural ductility even without the addition of structural reinforcement bars. For this reason they are called high performance fiber reinforced cementitious composites.
Because several specific formulas are included in the HPFRCC class, their physical compositions vary considerably. However, most HPFRCCs include at least the following ingredients: fine aggregates, a superplasticizer, polymeric or metallic fibers, cement and water. Thus the principal difference between HPFRCC and typical concrete composition lies in HPFRCC's lack of coarse aggregates. Typically, a fine aggregate such as silica sand is used in HPFRCCs.
Ultra high performance fiber reinforced concrete (UHPFRC) is a relatively new cementitious material, which has been developed to give significantly higher material performance than conventional concrete, FRC or ECC (FIG. 5). UHPFRCC denotes a subclass of FRCCs that encompasses a number of ultra high strength concretes that are reinforced with steel fibers.
Many UHPFRCCs are also HPFRCCs and therefore exhibit strain-hardening and multiple cracking in direct tension JJHPFRCC are a subgroup of HPFRCC combining the ductility of strain hardening cementitious composites with the high compressive strength of DSP concrete. UHPFRCC is furthermore distinguished between other FRCCs as a material exhibiting strain hardening in tension, whereas other FRCCs may exhibit a hardening behavior in bending, but are characterized by strain softening in tension. UHPFRC should have good potential for absorbing energy through flexure. Studies of this material under dynamic loading have shown an increase in ultimate strength with increasing strain rate. UHPFRC can be mixed and cast as normal concrete with no special facilities or handling, ultra high performance "basically refers to improved mechanical strength, fractural toughness and durability. A mix is designed to combine high cement content with a very low water / cement ratio. The selection of fine aggregates achieves maximization of the particle packing density and minimizes any localized non homogeneity. Post-set heat treatment at 90 degrees Celsius can be applied to further improve the microstructure. This process results in a very high compressive strength concrete, typically between The addition of a high dosage of high tensile steel fibers, normally 13 mm in length and 0.2 mm in diameter, results in a high flexural tensile strength, typically between 25-50 MPa. This material also has a very high capacity to absorb damage, with fracture energy in the range 20,000-40,000 J / m2.
Fibers are incorporated into UHPFRC in order to enhance the fracture properties of the composite material. The additional role of fibers in UHPFRC, compared to the role of fibers in ordinary and in high strength fiber reinforced concrete, is to provide sufficient ductility of the material in tension without a decrease in stress. This is achieved by choosing the appropriate type and quantity of fibers. In recommendations for UHPFRC, minimum fiber strength is limited to 2000 MPa. Fibers used in UHPFRC are typically short, smooth and straight, while hooked fibers are more often used in high-strength or ordinary concrete. Required fiber geometry can be estimated based on the relationship between pullout force and fiber-breaking force. The resulting high-strength spirit energy absorbing properties of UHPFRC are far superior to those of normal concrete. DFRCC is a broader class of materials than HPFRCC. HPFRCC is an FRCC that shows multiple cracking and strain hardening in tension, therefore in bending as well. On the other hand, DFRCC encompasses a group of FRCCs that exhibited multiple cracking in bending only, in addition to HPFRCCs. Multiple cracking leads to improvement in properties such as ductility, toughness, fracture energy, strain hardening, strain capacity and deformation capacity under tension, compression and bending. The advantage of DFRCCs is the increased toughness under tensile stress condition. Among a variety of DFRCCs, some DFRCCs achieve pure tension toughness and ductility comparable to those of metallic materials, while others show increased toughness only under flexural tension. ECCs make up a particular type of HPFRCC. HPFRCC also includes Slurry-infiltrated Fibrous Concrete (SIFCON) and Slurry-Infiltrated Mat Concrete (SIMCON). ECC are ultra ductile fiber reinforced cementitious materials. ECC essentially consists of two components: fibers and a cementitious matrix. Using a micro-mechanical approach, fiber and matrix properties are adjusted in order to obtain the desired macroscopic material behavior. The most characteristic material property of ECC is its extremely ductile, strain-hardening-like behavior in the inelastic tensile regime. This behavior has been termed "pseudo-strain hardening" referring to the post-yield strain hardening usually exhibited by metals. Although the macroscopic performance of ECC in the inelastic regime is very similar to the strain hardening observed for metals, the responsible micromechanical mechanism is a completely different one, hence the term "pseudo". In metal strain hardening is a consequence of a change in the molecular structure, whereas in ECC it is produced by multiple cracking and bridging of the cracks by the incorporated fibers. ECC is a special type of HPFRCC which has been microstructurally tailored based on micromechanics. The most obvious beneficial mechanical property of ECC is its extremely ductile response in tension. Experiments show that tensile strains up to 6% can be reached before softening sets. This means that structural elements made from ECC are able to bear very large imposed deformations, proven durability characteristics through better crack control, as well as higher ductility at a material level which are of interest for impact loading. Microstructure optimization allows ECC to be made with fiber content less than 2-3%. ECC deforms pseudo uniformly due to dense and fine multiple cracks (FIG. 6); therefore it shows deformation capacity comparable and compatible to that of steel. While conventional reinforced concrete members suffer from steel yielding at localized cracks, ECC members reinforce deformation compatibility and utilize the deformation capacity of steel to a greater extent.
Depending on the matrix constitution, the fiber geometry and the bond properties between fibers and matrix, a certain minimum volume content of fibers is necessary to achieve the typical pseudo strain hardening behavior of ECC. If the fiber content is too low, quasi-brittle failure as it is exhibited by traditional fiber reinforced concrete will occur. In this case damage localization and softening will set in immediately after formation of the first crack. Therefore, the fiber content is usually chosen just above the critical volume fraction which allows for the pseudo-strain hardening and ductile behavior but without using excessive amounts of fibers. In general synthetic fibers such as UHMWPE (ultra high molecular weight polyethylene) or PVA (polyvinyl alcohol) fibers are used (FIG. 7). The difference between these two fiber types is that UHMWPE fibers show little chemical bond with the cement matrix whereas the chemical bond strength between PVA fibers and matrix is considerable. Therefore untreated PVA fibers are not very adequate for pullout behavior and preliminary surface treatment of the PVA fibers may be necessary. The used fiber volume contents are usually somewhere between 0.5% and 4%. Typically, the synthetic fibers in ECC have a higher aspect ratio (fiber length to fiber diameter) than steel fibers used in steel fiber reinforced concrete. Their interfacial bond strength on the other hand is usually lower. A high aspect ratio is essential in minimizing the fiber volume content necessary for the pseudo-strain hardening behavior. The cementitious matrix in ECC consists of a Portland cement paste or mortar. The addition of fly ash and microsilica is possible and often used (FIG. 8).
The function of the synthetic fibers in ECC is to lead to steady state cracking and multiple cracking of the composite. In turn, these two mechanisms are responsible for the pseudostrain hardening behavior and large strain capacity of ECC. In order to guarantee multiple cracking, the bridging stress that can be transmitted by the fibers has to be greater than the first cracking strength of the intact matrix. This condition is referred to as the strength criterion for multiple cracking.
A general no or little aggregate with a relatively small grain size is used for the cementitious matrix of ECC. The main reason for this is to keep the fracture toughness of the matrix low. Adding large amounts of aggregate would result in longer fracture path distances and as a result a higher fracture toughness. Having a low fracture toughness is a further condition to obtain multiple cracking with moderate fiber volumes. SIFCON, SIMCON: SIFCON is produced by infiltrating slurry into pre-placed steel fibers in a formwork, and due to the pre-placement of fibers, its fiber volume fraction can amount to 20% at maximum. The confining effect of numerous fibers yields high compressive strength reaching over 200 MPa, and the strong fiber bridging leads to tensile strain hardening behavior in some SlFCONs. The fracture energies of SlFCON's are about 1350 times that of normal strength concrete. SIMCON uses pre-placed fiber instead of steel fiber. RPCs represent a new generation of concrete that utilizes reactive powder, and it is designed with optimal packing theory. It's cube strengths between 200 and 800 MPa, tensile strengths between 25 and 150 MPa, and unit weights of 2500 - 3000 kg / m3. The fracture energy of these materials can reach up to 40000 J / m2, as compared to 100 to 150 J / m2 for ordinary concrete. The fracture energies of RPCs are about 300 times that of normal strength concrete. The RPC microstructure has a more compact particle arrangement and is enhanced by the presence of the strongest cementitous hydrates as compared to HPC. RPCs are produced using very fine sand, cement, silica fume, superplasticizer and short cut steel fibers. Their very low porosity gives them important durability and transport properties and makes them potentially suitable materials for storage of industrial wastes. These features are achieved by 1) precise gradation of all particles in the mixture to yield a matrix with optimum density, 2) reducing the maximum size of the particles for the homogeneity of the concrete, 3) reducing the amount of water in the concrete, 4) extensive use of the pozzolanic properties of highly refined silica fume, 5) optimum composition of all components, 6) the use of short cut steel fibers for ductility, 7) hardening under pressure and increased temperature, in order to reach very high strengths .
Ductal is a range of ultra-high performance concrete (UHPFRC) with very high compressive strength and non-brittle tensile behavior offering compressive strength of 160 to 240 MPa and tensile strength of over 10 MPa and with true ductile behavior. Ductal is an inorganic composite material based on the concept of RPC. The properties are characterized by high strength, high durability and high flowability. Ductal is a cement based composite reinforced with steel fibers under the concept of high strength and high toughness. W / C ratio is in the region of 0.2. The sand used has a fine grading, with the largest grains not exceeding around 600 um in diameter. The addition of silica fume and optimized use of admixtures are both absolutely essential. Last but not least, the concrete is reinforced with metal fibers, which have also been optimized for several criteria, involving optimizing not only the behavior of the individual fibers, but also their interactions within the matrix. A content of 2% by volume of 13-15 mm long fibers with diameters of around 0.2 mm emerges as a good compromise. Calculating the mean spacing of these fibers in the matrix gives a result of around 1.6 mm. which is perfectly compatible with the sand grading used. UHSCs or Ultra High Strength Concretes are concretes with a very densely packed matrix, which causes them to withstand high compressive loads. They usually contain large amounts of fly-ash and silica fume and can reach compressive strengths between 120 MPa and 250 MPa.
In FRCCs, fibers can be effective in arresting cracks at both macro and micro levels. Most of the strain hardening FRCC is limited to single fiber type.
Mono fiber composites containing high stiffness fibers normally show high ultimate strength, low strain capacity and small crack width properties, while those containing low stiffness fibers show low ultimate strength, high strain capacity and large crack width properties.
Recently hybrid fiber reinforced cementitious composites (HFRCC) exhibiting strain hardening behavior has also been developed. In hybrid fiber composites, two or more different types of fibers are suitably combined to exploit their unique properties. The use of optimized combinations of two or more types of fibers in the same concrete mixture can produce a composite with better engineering properties than that of individual fibers. A hybrid composite, with proper volume ratio of high and low stiffness fibers, shows simultaneous improvement in ultimate strength, strain capacity and crack width properties.
The hybridization of fibers in FRCC can be done in different ways, such as by combining different lengths, diameters, modulus and tensile strengths of fibers. Large macro fibers bridge the big cracks and provide toughness while small micro fibers enhance the response before or just after the cracking. Micro fibers also improve the pull out response of macro fibers, thus producing composites with high strength and toughness.
Over the last three decades, high compressive strength concretes and high tensile ductility concretes have emerged as two distinct classes of concrete. Materials at the frontiers of both of these classes include very high strength concrete (VHSC) with compressive strength around 200 MPa, and engineered cementitious composites (ECC) with tensile ductility in the range of 3% -6%. The development of these two concretes was based on two different design philosophies that targeted two different structural performances. VHSC and similar high strength concrete (RPC, Ductal, MDF and DSP) were designed to achieve size efficiency in structural members for very large structures and to provide additional strength safety margin for strategically critical and protective structures. ECC and similar high performance fiber reinforced cementitious composites (HPFRCCs) were developed to ensure ductility of structural elements and massive energy absorption in the face of extreme load displacement events such as earthquakes. However, the decoupled development of VHSC and ECC resulted in mutual exclusion of each other's desirable properties. VHSC is an order of magnitude less ductile than ECC, whereas the compressive strength of ECC is three to four times less than VHSC. A combination of high strength and high ductility in one concrete material is highly desirable.
Recently, there have been a few notable investigations on combining high compressive strength and high tensile ductility into one concrete with limited success. Some mechanical test results of ultra high performance - strain hardening cementitious composites (UHP-SHCC) show average compressive strength of 96 MPa and tensile ductility of 3.3% at 14 days after casting. The development of another such material, ultra high performance fiber reinforced composite (UHP-FRC) shows compressive strength of about 200 MPa and tensile ductility of 0.6%. Although both of these materials attempt to combine tensile ductility and compressive strength into one concrete, UHP-SHCC has a compressive strength that is only about half that of VHSC, and UHP-FRC has tensile ductility which is at least five times smaller than ECC.
Newly developed a new composite material, high strength - high ductility concrete (HSHDC), shows both the desirable properties of high compressive strength (similar to VHSC) and high tensile ductility (similar to ECC) and are integrated into a single material. This results in higher specific energy absorption (or composite toughness) in HSHDC as compared to any other material in the class of high performance cementitious composites. The micromechanics-based principles that guide the design of ECC, combined with a VHSC matrix, led to the development of HSHDC.
Due to controlled cracking enabled by micromechanical tailoring of the composite material, HSHDC exhibits high tensile ductility in spite of a high strength brittle matrix. Specific energy (or composite toughness) of HSHDC, is calculated as the area under the stress-strain curve before attaining ultimate stress capacity. A comparison of other composite properties of HSHDC with other similar high performance concrete materials is shown in FIG. 9. It can be observed here that the specific energy of HSHDC is the largest among all the materials presented, which results from the combination of high tensile strength and high tensile ductility. Such material behavior leads to high energy absorption, which is critical for structures to withstand extreme loading conditions.
Fibers used in FRCCs
The use of short fibers in concrete to improve pre- and post cracking behavior has gained popularity. The mechanical properties of fiber reinforced concrete depend on the type and content of the added fibers. Several different fiber types and materials have been used successfully to improve its mechanical and physical properties. Often used fiber reinforced concrete steel and synthetic fibers are mainly used, although a large variety of fibers made from other materials exists. This is related to the strength and stiffness required of the desired fiber contribution.
Unlike continuous fiber composites, the external loads are not directly applied to the fibers in FRCC. The load applied to matrix materials is transferred to the fibers via fiber ends and the surfaces of fibers. As a consequence, the properties of FRCC greatly depend on fiber length and the diameter (i.e., fiber aspect ratio) of the fibers. Further, several factors such as fiber orientation, volume fraction, fiber spacing, fiber packing arrangement, and curing parameters also significantly influence the properties of FRCC. Using micro and macro fibers of different mechanical, geometric and physical properties reduces the brittleness of cementitious materials. Adding short needle-like fibers to cementitious matrices enhances their mechanical properties, particularly their toughness, ductility and energy absorbing capacity under impact. The fibers can and must be engineered to achieve optimal properties in terms of shape, size and mechanical properties, as well as compatibility with a given matrix. Once a fiber has been selected, an infinite combination of geometric properties related to its cross sectional shape, length, diameter or equivalent diameter and surface deformation can be selected. The cross section of the fiber can be circular, rectangular, diamond, square, triangular, flat, polygonal, or any substantially polygonal shape.
All fibers are made of either inorganic or organic material. The inorganic category includes materials such as metals, minerals, ceramic, carbon and glass. The list of organic fiber materials, on the other hand, seems to be limited only by the creativity of nature and the chemical industry. Nature still holds the record for the strongest fibers, which are spun by spiders. Other natural fibers include cellulose, silk and cotton. Manmade organic fibers include nylon, polypropylene, polyvinyl alcohol (PVA), polyethylene and aramid, among many others.
Materials used in fiber reinforcing can include acrylic, asbestos, cotton, glass, nylon, polyester, polyethylene, polypropylene, rayon, rockwool and steel. Of these, acid-resistant glass and steel fibers have received the most attention. Plastic fibers have been shown to be of little value in reinforcing concrete until only recently. Natural fibers are subject to alkali attack and are also determined to have little value. The premium fibers are graphite reinforced plastic fibers, which are nearly as strong as steel, lighter-weight and corrosion-proof. Some experiments have had promising early results with carbon nanotubes. Fiber (steel or "plastic" fibers) reinforced concrete is less expensive than hand-tied rebar, while still increasing tensile strength many times.
Short fibers used in concrete can be characterized in different ways. First, according to the fiber material: natural organic (such as cellulose, sisal, jute, bamboo, etc.); natural mineral (such as asbestos, rock wool, etc.); man-made (such as steel, titanium, glass, carbon, polymers or synthetic, etc). Second, according to their physical / chemical properties: density, surface roughness, chemical stability, non-reactivity with the cement matrix, fire resistance or flammability, etc. Third, according to their mechanical properties: tensile strength, elastic modulus, stiffness, ductility , elongation to failure, surface adhesion property, etc.
J
Short fibers are mainly characterized by the material and its mechanical properties and by their geometry. Once a fiber type has been selected, an infinite combination of geometric properties related to its cross sectional shape, length, diameter or equivalent diameter and surface deformation can be selected. The cross section of the fiber can be circular, rectangular, diamond, square, triangular, flat, polygonal, or any substantially polygonal shape.
The effectiveness of fibers on the mechanical properties of brittle matrix varies with the geometric, mechanical and physical properties of fibers. Fibers with surface roughness and large specific surface area develop good bond with the matrix due to, as a result, micro-cracking mechanism before the occurrence of peak load is arrested in the presence of fibers, and fibered concrete exhibits high value of peak load compared to normal concrete without fibers.
The properties of concrete matrix and of the fibers greatly influence the character and performance of FRCC. The properties of fibers that are of interest include fiber stiffness, bond between fiber and concrete matrix, fiber concentration, fiber geometry, fiber orientation, fiber distribution and fiber aspect ratio.
In order to be effective in concrete matrices, fibers must have the following properties: 1) a tensile strength significantly higher than that of the concrete (two to three orders of magnitude); 2) a bond strength with the concrete matrix preferably of the same order as or higher than the tensile strength of matrix; and 3) unless self-stress is used through fiber reinforcement, an elastic modulus in tension is significantly higher than that of the concrete matrix. The Poisson's ratio and the coefficient of thermal expansion should preferably be of the same order for both the fiber and the matrix.
In relation to the elastic modulus, fibers are divided into two types, those where the elastic modulus of fibers is less than the elastic modulus of the matrix: i.e. cellulose fiber, polypropylene fiber, polyacrylonitrile fiber, etc .; and those where the elastic modulus of fibers is greater than the elastic modulus of the matrix: i.e. asbestos fibers, glass fiber, steel fiber, carbon fiber, aramid fiber, etc.
To develop bond with matrix, specific surface area and surface conditions of fiber play an important role. Increasing the average bond strength leads to a direct increase in the post cracking strength of the composite and other important properties as well, such as toughness and energy absorption capacity. The various bond components are adhesion, friction, mechanical and interlock. In some fibers the surface is etched or plasma treated to improve bond at the microscopic level.
To develop a better bond between the fiber and the matrix, the fiber can be modified along its length by roughening its surface or by inducing mechanical deformations. Thus fibers can be smooth, indented, deformed, crimped, coiled, twisted, with end hooks, paddles, buttons, or other anchorage.
As micro-cracks are developed, the stress in fiber gradually increases with the increase of crack openings, and a stage of either pulling out of the matrix before the stress in fiber exceeds its tensile strength capacity will occur or a stage of breakage of fiber will happen if the fiber is not pulled out of the matrix before the stress in fiber exceeds its tensile strength capacity. In order to enable the transfer of force with a small crack opening and sustain tensile force without breaking, a high modulus of elasticity and high strengths are required.
To transfer the stress across the crack edges (bridging action of fibers), length of fiber compatible with maximum aggregate size is important. Interfacial transition zone between the aggregate and the cement paste is the weakest phase in the concrete. In order to bridge this zone and to get highest effect of fibers, length of fiber and diameter of aggregate must be coherent with each other. To develop efficient bridging action, fiber must be embedded into the matrix on both ends beyond the aggregate particles. For that, fiber length must be at least greater than 2 times maximum aggregate size. So to get better efficiency, fiber length should be 2 to 3 times the maximum size of the coarse aggregate.
Ideally, the amount of fiber and aspect ratio should be as large as possible to maximize the improvements in the mechanical properties.
The length and diameter of synthetic fibers vary greatly. Single filament fibers can be as little as 10 micrometers in diameter such as for Kevlar or carbon fibers, and as large as 0.8 mm such as with some polypropylene or poly-vinyl-alcohol (PVA) fibers. Generally in concrete applications, the aspect ratio, that is, the ratio of length over diameter or equivalent diameter, of very fine fibers exceeds 100 while that of courser fibers is less than 100. Most synthetic fibers (glass, carbon, kevlar) are round in cross section; Flat synthetic fibers cut from plastic sheets and fibrillated are suitable when very low volume content is used.
Most common steel fibers are round in cross section, having a diameter ranging from 0.4 to 0.8 mm, and a length ranging from 25 to 60 mm. Their aspect ratio is generally less than 100, with a common range from 40 to 80.
Various types of steel fibers have been developed (FIG. 10). They differ in size, shape and surface structure. Such fibers have different mechanical properties such as tensile strength, grade of mechanical anchorage and capability of stress distribution and absorption). Hence they have different influence on concrete properties. Some other types of closed-loop steel fibers such as ring, annulus, or clip type fibers have also been used and shown to significantly enhance the toughness of concrete in compression. SFRC with the ring-type steel fibers (RSFRC), fails by more energy consuming mechanisms other than fiber pullout, whereby significant improvements in flexural toughness are obtained as compared to that of SFRC with conventional straight steel fibers. Fiber-matrix interfacial bond strength is provided by a combination of adhesion, friction and mechanical interlocking. While the mechanical performance of traditional straight steel fibers relies on the fiber-matrix interfacial bond strength, ring-type steel fibers are mainly designed to mobilize fiber yielding rather than fiber pullout. Three different types of RSFRC flexural failure mechanisms are involved: fiber rupture after yielding and cone-type concrete fracture and separation between ring-type steel fibers and concrete matrix. Toughness indices of RSFRC are affected by fiber contents, ring diameter and fiber diameter.
Due to the formulation of the mechanics of the composite, the fiber content in cement matrices is specified by volume fraction of the total composite. Because of fiber materials of different densities, the same volume fraction of fibers of different materials leads to different weight fractions of fibers. Fibers are purchased by weight, but mechanical properties of composites are based on volume fraction, not weight fraction of fibers. Typically a 1% volume fraction of steel fibers in normal-weight concrete amounts to about 80 kg / m3 or concrete; however, a 1% volume fraction of polypropylene fibers amounts to about only 9.2 kg / m3.
The Technical Impact A lightweight composite armor is disclosed as one or successive layers of discrete armor-grade objects, such as ceramic blocks, encapsulated within a fiber reinforced cementitious composite (FRCC). The FRCC is used to (1) substantially confine the segmented armor-grade core, (2) pre-stress the confined segmented armor-grade core. Studies show that better confining of armor-grade ceramic results in an increase in penetration resistance, and that ceramic yields much higher performance when their boundaries are heavily confined, because if the ceramic core is not encased, the fractured pieces can move away easily, and residual protection is lost. The type of confinement or encapsulation can influence the ballistic efficiency of the ceramic based armor, and that "dwell" type defeat of penetrators can be achieved on the ceramic front surfaces. Two key parameters here are suppression of cracked tile expansion and putting the ceramic in an initial state of high compressive stress to delay or stop it from going into a state of tensile stress during impact. Tensile stresses are the cause of the premature failure of ceramic components, since in general ceramic have higher strength in compression than in tension.
The advantage of such compressive stresses on ceramic component is two-fold. First, the ceramic core will have a higher tensile strength and will be more effective in defeating the projectile, as the projectile will spend more time (and more energy) before causing the ceramic component to develop cracks and failure. This can allow the disclosed composite structure to defeat projectiles with minor da ge to the ceramic component and therefore will allow the structure to take multiple hits component can be preserved due to the relatively high elastic strain limit of cementitious composites. Even though the effectiveness of the system will be reduced (in the case of formation of cracks in ceramic), the remaining compressive stresses will maintain some effectiveness of the ceramic for subsequent hits, and at a minimum will keep the un-cracked portion of ceramic in place to defeat the projectile and dissipate its energy.
Yiwang Bao etal (materials letters December 2002) reported substantial enhancement in projectile penetration resistance in confined and pre-stressed ceramic cores, and experimental results by Holmquist and Johnson (EDP Sciences 2003) also show that prestressed ceramics does improve performance.
Other sources show that three-dimensional stressing of ceramic provides a higher enhancement in the impact resistance than a two-dimensional stressing.
There is a clear need to improve the impact resistance, ballistic efficiency and structural integrity of ceramic armor employed on a widespread basis in many types of armor systems. Over the past twenty years, it has been discovered that the ballistic performance of ceramic armor is critically dependent on the specific design attributes and geometric configuration of the entire armor system. In particular, it has been observed that enhanced destruction and fragmentation of an incoming projectile can be achieved by increasing the so-called "dwell" time of the projectile on the front face of the ceramic armor during the very early stages (the first 5- 10 microseconds) of the impact event. "Dwell," the duration of projectile erosion without target penetration, is an indicator of the ceramic's ballistic efficiency. Strategies for prolonging projectile dwell on ceramics include retarding damage and retaining dynamic toughness in the damaged state. Ceramic failure caused by excessive structural bending from ballistic loading is another possible limiting factor to improve ballistic performance. Improving ceramic armor can be obtained with improved structural support for the ceramics. Desirable attributes of a backing material include shock mitigation and high stiffness to resist bending.
In general, the longer the dwell time on the front face of the ceramic armor, the more completely the projectile can be attenuated and fragmented. Enhanced dwell time on the front face of the ceramic armor leads to a phenomenon called interface defeat, radiating the projectile face mushrooms radially outward without significant penetration in the thickness direction; this increases the projectile frontal area and thus decreases its subsequent ability to penetrate the (ceramic) armor (FIG. 11). If the interface defeat is not sufficient, there will be an initial dwell and subsequent penetration into the armor (FIG. 12).
The phenomenon of dwell is used to particular advantage in medium or heavy ceramic armor systems that are intended to defeat larger caliber high kinetic energy projectiles (12.7mm HMG and above). It has been found that physical confinement of ceramics such as B4C, SiC or TiB2 increases the dwell time and delays the lateral and axial spreading of the comminuted zone ahead of the projectile, thus increasing the ballistic efficiency of the ceramic.
There are several advantages to using FRCC as a surrounding material in ceramic cores. One is the relative high "yield" strength of FRCC that can be utilized to constrain the ceramic core very effectively and impede the material's disintegration. When the armor package takes a hit, the ceramic core will tend to fracture and dimensionally expand due to opening cracks. In this situation, the surrounding FRCC will be forced to stretch out and the material's resistance to yielding will be an important factor in impeding the disintegration of the ceramic core. This constrain of the ceramic core when hit by an incoming projectile results in: • Constraints of material to prevent material "flee" from the impact zone • Improved hardness of ceramic to flatten the tip of projectile at the initial stage of impact • Transference of impact force to surrounding and supporting materials • Small damage zone • Other aspects to defeat projectile by involving more materials in the impact zone
Terminology A material is called brittle if it loses its tensile strength immediately after first cracking under uniaxial tension and is no longer able to resist any stress.
Cementitious materials The bonding component of fiber reinforced cementitious composites. These are: cement, mortar or concrete.
Composite materials (or composites for short) Engineered materials made from two or more constituent materials with significantly different physical or chemical properties that remain separate and distinct at a macroscopic level within the finished structure. DFRCC Stands for a subclass of FRCCs called ductile fiber reinforced cementitious composites which are hardening under flexural conditions (deflection-hardening) but not strain-hardening in direct tension (see also HPFRCC). The expression ductile emphasizes the fact that these composites exhibit multiple cracking which can be considered to be a form of ductility. A ductile material is a material that does not fail immediately under uniaxial tension after reaching its first cracking strength. It first enters a strain-hardening phase, which is only then followed by a crack opening phase and localization of failure. FRCC Stands for fiber reinforced cementitious composites and describes cementitious materials that are reinforced by randomly oriented short fibers. flpFRCC Stands for high performance fiber reinforced cementitious composite. It delimits a subclass of FRCCs which are strain-hardening in direct tension. It is also called pseudo strain-hardening or quasi-strain-hardening. Strain hardening refers to a true material property and should not be confounded with hardening due to a redistribution of internal stresses such as within the cross-section of a beam (referred to as deflection hardening).
Localized Crack A localized crack is a crack at which the damage accumulates and where deformations start concentrating. It should be characterized by the crack opening displacement rather than a strain since the laughter is gauge-dependent.
Multiple Cracking Means that a FRCC is capable of arresting the further opening of cracks by fiber bridging action and by consequence new cracks tend to form in the close vicinity. This is a fundamental property of HPFRCCs. .Quasi-Brittle The quasi-brittle expression describes a material that starts softening directly after first cracking under uniaxial tension. However, quasi-brittle materials are still capable of transferring some reduced amount of stress which gradually decreases with increasing crack opening.
Shrain hardening / pseudo strain hardening Strain hardening describes a phenomenon that, under uniaxial tension, transmitted tensile stress increases successively even after first cracking, with continued tensile strain. The term "pseudo strain hardening" is sometimes used instead, since the strain hardening mechanism of DFRCC is different from that of metallic materials. During strain hardening / pseudo strain hardening, the stress-strain curve is uniquely defined, and is a true material property.
Strain softening: Strain softening describes a phenomenon that, under uniaxial tension, transmits tensile stress decreases upon first cracking or after strain hardening.
Tension toughness, compression toughness, flexure touehnpss Toughness describes energy absorption which is given by the area below stress-strain curve or load-displacement curve either in tension, compression, or flexure. In practice, toughness is calculated based on the area up to a prescribed strain or displacement.
Figure Description
The accompanying drawings and photos, which are incorporated into and form part of this specification, illustrate the technical background and embodiments of the invention, and together with the description, serve to explain by way of example only, the principles of the invention: FIG. 1 Illustrates the different compositions between conventional concrete and UHPC; FIG 2 A photo of a high strength - high ductility concrete (HSHDC) plate during bending; FIG 3 Illustrates characteristics of cementitious materials: Definition of A: brittle, B: quasi brittle, and C. ductile behavior as well as strain softening and strain hardening under uniaxial tensile loading; FIG 4 Illustrates some different high performance fiber reinforced cementitious materials and their classification; FIG. 5 Illustrates flexural performances of beam specimens made of different fiber reinforced cementitious materials; FIG. 6 Illustrates multiple cracking pattern of PVA-ECC under uniaxial tension; FIG 7 Photos of polyethylene (PE) fibers and polyvinyl alcohol (PVA) fibers used in ECC; FIG 8 illustrates the composition of a typical ECC formulation and a concrete formulation, showing weight percent; FIG 9 Illustrates and compares mechanical properties of HSHDC with other fiber reinforced cementitious materials; FIG. 10 A photo of typical profiles of steel fibers commonly used in some fiber reinforced concrete; FIG. 11 A photo of the sequence of three flash X-ray radiographs showing the initial dwell of a penetrator and subsequent penetration into thick ceramic target; FIG. 12 A photo of the sequence of three flash X-ray radiographs showing complete dwell of a penetrator on a thick ceramic target; FIG. 13 Photo collage of different shapes and sizes of armor-grade ceramics, which can be used in the present invention; FIG. 14 A schematic cross-sectional perspective view of a first exemplary embodiment of the construction of a lightweight composite armor according to the present invention; FIG. 15 A schematic cross-sectional perspective view of a second exemplary embodiment of the construction of a lightweight composite armor according to the present invention; FIG. 16 A schematic cross-sectional perspective view of a third exemplary embodiment of the construction of a lightweight composite armor according to the present invention.
With reference now to the figures of certain preferred embodiments in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings makes it apparent to those skilled in the art how the various forms of the invention may be embodied in practice.
Referring to FIG. 14, a cross-sectional perspective view of an embodiment of a ballistic structure showing an encapsulant (1) substantially encasing and confining an armor-grade core material (2) for limiting the transfer of impact energy from a projectile.
The encapsulant is fabricated from a fiber reinforced cementitious composite having a greater tensile strength than the tensile strength of the encapsulated core material.
The core preferably is comprised of armor-grade ceramic material. But it must be understood that the principles of the invention are applicable to any similar armor-grade materials such as glass, glass-ceramics, sintered refractory materials, mixtures thereof or other armor-grade materials having high hardness.
The encapsulating structural layer (1) is configured to substantially enclose the armor-grade core material (2). In one embodiment, the encapsulating structural layer (1) presses the encapsulated core material (2). Without pre-stress, at least simple mechanical contact is needed.
Referring to FIG. 15, in another embodiment of the invention, the encapsulating layer of fiber reinforced cementitious composite and the encapsulated ceramic material can be layered in a laminated structure, where the alternating layers of ceramic material and fiber reinforced cementitious composite are composed as shown.
Referring to FIG. 16, in another embodiment of the invention, the encapsulated ceramic material is formed as spheres and are all held and fully enclosed with an encapsulating layer of fiber reinforced cementitious composite. The gaps between adjacent ceramic spheres are made to be small enough to avoid the creation of a weak point and stop an anticipated projectile between the spheres.
In any of the above embodiments of the invention shown in FIG. 14, FIG. 15 and FIG. 16, the thickness of the encapsulating layer may be varied as well as the dimensions of the encapsulated core may be varied.
Often panels of the invention will be generally flat and with generally uniform thickness. For the purpose of constructing the panel, the front face is that which will face the direction from which the ballistic impact is expected, and the other is the back face. Likewise, the overall dimensions and overall shape of the panels of the invention will be determined by end-use requirements, such as the impact conditions they are required to resist, and the size and / or area of the object which the panel or an assembly of the panels is required to protect. For more specialized end user requirements, a panel may be shaped in any form of curvature. Whatever its overall shape, the fact that it is a panel implies that its thickness will be smaller than its other dimensions, e.g. its length and width, and it will have two faces separated by its thickness.
The shapes shown in FIG 14, FIG 15 and FIG 16 are by way of example only. Other polygonal shapes may be used, such as cylinders and specially shaped pellets. In addition, the shape of the tiles shown in FIG 14 and FIG 15 need not be a regular geometric shape. The tile may have any shape needed for a particular application, such as triangles, squares, rectangles, hexagons or combinations of polygons thereof, which provide complete coverage in one layer. In another configuration, polygon shaped tiles or combinations thereof are used in a first layer, and any gaps in the first layer are protected by a second layer to obtain complete coverage. It is most often desired to achieve complete coverage in one layer. Typical tile shapes used for this are square and hexagonal.
While the invention has been described with reference to certain preferred embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims and equivalents thereof.

权利要求:
Claims (6)
[1] 2. A composite armor structure according to claim 1 wherein said encapsulated core material is selected from the group consisting of armor-grade ceramics as classified in three distinct material categories, a) Oxide ceramics such as alumina, zirconia, silica, aluminum silicate, magnesia, aluminum titanate and other metal oxide based materials, b) Non-oxide ceramics such as carbides, borides, nitrides and silicides, c) Composite ceramics, such as particulate reinforced, fiber-reinforced, ceramic-metal composite materials, nano-ceramics, combinations of oxides and non-oxides and mixtures thereof.
[2] 3. A composite armor structure according to claim 1 wherein said encapsulated core material is selected from the group consisting of glass.
[3] 4. A composite armor structure according to claim 1 wherein said encapsulated core material is selected from the group consisting of glass-ceramic.
[4] 5. A composite armor structure according to claim 1 wherein said encapsulated core materia! is selected from the group consisting of sintered refractory material.
[5] 6. A composite armor structure according to claim 1 wherein said encapsulant is selected from the entire group of fiber reinforced cementitous composites (FRCC) consisting of a) fiber reinforced concretes, b) fiber reinforced mortars, c) very high strength cementitious composites, d) hybrid fiber reinforced cementitious composites, e) ductile fiber reinforced cementitious composites, f) ultra ductile fiber reinforced cementitious composites, g) high performance strain hardening cementitious composites, h) ultra high performance strain hardening cementitious composites, i) engineered cementitious composits and j) high strength high ductility concrete.
[6] 7. A composite armor structure according to claim 1 wherein said encapsulated core material may be grinded, ungrinded, plasmasiret and/or coated with a primer adapted to facilitate the binding with the encapsulant.

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公开号 | 公开日

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CA2881271A1|2014-02-13|

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US20150369568A1|2015-12-24|

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DK178289B1|2015-11-09|

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EP2883015A4|2015-11-25|

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EP2883015A1|2015-06-17|

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WO2014023309A1|2014-02-13|

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法律状态:
2017-03-20| PBP| Patent lapsed|Effective date: 20160831 |

优先权:

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申请号 | 申请日 | 专利标题

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DK201200491|2012-08-07|

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DKPA201200491A|DK178289B1|2012-08-07|2012-08-07|Light weight composite armor with structural strength|DKPA201200491A| DK178289B1|2012-08-07|2012-08-07|Light weight composite armor with structural strength|

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CA2881271A| CA2881271A1|2012-08-07|2013-08-05|Light weight composite armor with structural strength|

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PCT/DK2013/000048| WO2014023309A1|2012-08-07|2013-08-05|Light weight composite armor with structural strength|

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US14/419,663| US20150369568A1|2012-08-07|2013-08-05|Light weight composite armor with structural strength|

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EP13827556.5A| EP2883015A4|2012-08-07|2013-08-05|Light weight composite armor with structural strength|