DK178289B1 - Light weight composite armor with structural strength - Google Patents

Abstract

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.

Description

LIGHT WEIGHT COMPOSITE ARMOR WITH STRUCTURAL STRENGTHField of the invention

The present disclosure relates generally to passive ballistic armor for military and civilianuse, and in particular, to lightweight composite armor structure with very strongstructural strength based on a multi-material composite for absorbing and limiting thetransfer of impact energy from most types and sizes of kinetic energy (KE) threats andchemical energy (CE) threats.

The present disclosure relates especially to ballistic protection for use in military vehiclesof different types and other moving and stationary military platforms, or for use inpermanent or temporary protection of different types in buildings or other fixed ormobile installations. The ballistic protection according to the present disclosure can beemployed as protection against different types and sizes of low-velocity or high-velocityarmor ballistic threats, such as armor piercing projectiles or threats based on chemicalenergy.

It is an object of the present disclosure to provide a composite armor that will preventthe penetration of projectiles in a structure while also providing structural support to thesame structure.

Desired armor protection levels can usually be obtained if weight is not a consideration.For over a century, metals have been the material of choice in providing load-bearingcapabilities and ballistic protection for military platforms. Especially military vehicleshave traditionally been manufactured from high strength armor plate steel.

Development of modern fighting technology, directed generally towards decreasing ofmass of vehicles construction, creates simultaneously the necessity of increasing thepenetration resistance of protective layers to all types of impact threats. Vehiclesdesigned for land conflict are often lightly or inadequately protected from heavy machinegun (HMG) projectiles, high velocity kinetic energy-based anti-tank long rod penetrators(LRP), asymmetric threats such as fragments from Improvised Explosive Devices (lED's),Explosively Formed Projectiles (EFP's) and chemical energy threats (CE), which are allencountered with rising frequency by troops on operations.

The present disclosure provides an integral composite armor material for absorbing anddissipating kinetic energy from high-velocity armor piercing projectiles, lED’s, EFP’s aswell as chemical energy threats such as RPG’s. It is an improvement to existing ceramic- based armor.

The present disclosure improves upon existing composite armor designs through the useof Fiber Reinforced Cementitious Composite (FRCC), which is a composite material with acementitious matrix and discontinuous reinforcement (short fibers), which are made ofeither inorganic or organic material, mostly steel fibers, polyvinyl alcohol (PVA) fibers andpolyethylene (PE) fibers (FIG. 1).

More specifically, it relates to fully encapsulation and pre-stress of armor-gradematerials, such as ceramics, with FRCC, providing armor with highly enhanced ballisticefficiency, physical durability, structural strength and environmental resistance.

The present disclosure results in superior ballistic characteristics of an armor system andcan be ballistically qualified in different kind of shapes, sizes and thicknesses, and can becustom produced to meet any demand for ballistic protection. The overall shape of thepanel will be determined by end user requirements. Often panels of the presentdisclosure will be generally flat and with generally uniform thickness. For morespecialized end user requirements, a panel can be shaped in almost any form ofcurvature and varied thickness through the panel. The overall dimensions of the panelsof the present disclosure will be determined by end user requirements, such as theimpact conditions which they are required to resist, and the size and/or area of theobject which the panel or an assembly of the panels is required to protect. Theseindividual panels can then be laid into multiple-panel arrays to obtain broad areacoverage of a contoured structure.

The transition between the different degrees of protection (light and heavier protection)can be implemented quickly and efficiently and upgrading/downgrading of existingprotection can be performed quickly without particularly sophisticated equipment.

The result is a lightweight, composite hybrid structure for ballistic protection particularlysuited to tactical ground vehicles. One advantage of the present disclosure is an increasein the ballistic penetration resistance of especially ceramic-based armor with asimultaneous decrease in the armor system weight. It also provides additional strongstructural 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 forpurposes of illustration only and are not intended to limit the scope of the presentdisclosure.

Background of the invention

Ballistic armor is well known in the art as is herein detailed and described together withexplanation why the prior art could be improved, or in some essential features isdifferent from the present disclosure.

To defeat lethal projectiles, laminate systems composed of a hard frontal platesupported by a metallic or polymer composite backing are employed. In this system, thehigh hardness face plate breaks (shatters, erodes, blunts) the projectile defocusing thekinetic energy to allow the backing to effectively catch the residuals. In addition, thebacking serves as a breadboard for attachment to the vehicle. This is employed in armorsystems including dual hard steel, titanium-aluminum laminates and ceramic-composites.Facial ceramics are most effective in this type of application, but have limited fracturetoughness 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 andmost traditional approach makes use of metals. The second approach uses ceramics.Each material has certain advantages and limitations. Metals are more ductile and aregenerally superior at withstanding multiple hits. However, they have a large weightpenalty and are not as efficient at stopping armor-piercing threats. Ceramics areextraordinarily hard, strong in compression, light weight and brittle, making themefficient at eroding and shattering armor-piercing threats, but not as effective atwithstanding multiple hits.

Light-weight metallic and ceramic armor designs are known. For example, metals such astitanium and aluminum alloys can replace traditional steel to cut weight. Ceramics, suchas aluminum oxide, silicon carbide, and boron carbide, are used in combination with asupporting backing plate to achieve even lighter armor. When ceramics are employed inlaminate constructions and are backed with high tensile strength, high-toughness"momentum trap" composites such as Kevlar or Spectra fibers, mass-efficient armorsystems can be designed. The mass efficiency of such ceramic composite armor systemsis generally two times (or more) higher than that associated with high hardness steel orsimilar high strength metallic armor plate.

State-of-the-art military armor systems for different platform protection frequently makeuse of lightweight, very high compressive strength ceramics such as silicon carbide (SiC),boron carbide (B4C) or alumina as the so-called "strike face" of an armor laminate package. The purpose of the strike face material, as often employed in high performanceceramic composite armor systems, is to blunt and defeat incoming metallic projectiles byovermatching the compressive properties of the incoming projectile during the early(compressive shock) portions of the impact event. High modulus, high strength ceramicscan have four to five times the dynamic compressive strength of projectile materials suchas steel, tungsten or tungsten carbide. Thus, it is possible to shock the incomingprojectile to the extent that compressive fracture is initiated. This decreases the ability ofthe projectile to defeat the armor system.

Armor-grade ceramics can be classified in three distinct material categories, a) Oxideceramics such as alumina, zirconia, silica, aluminum silicate, magnesia, aluminum titanateand other metal oxide based materials, b) Non-oxide ceramics such as carbides, borides,nitrides and silicides, c) Composite ceramics, such as particulate reinforced ceramics,fiber-reinforced ceramics, ceramic-metal ceramics, nano-ceramics and combinations ofoxide ceramics and non-oxide ceramics. Grinding of ceramics is part of the process ofmaking ceramics suitable for high performance ceramic applications, such as armorapplications and where tight tolerances are necessary.

An important ceramic material today for ballistic protection of military vehicles and shipsis Alumina (AI203). Owing to its excellent price-efficiency ratio, alumina is thepreeminent ceramic armor material for vehicular applications. When an extremely lowweight is required (e.g. for personal protection or for helicopters), silicon and boroncarbide materials are often used. Other ceramic materials may also be considered for thepurpose of ballistic protection, Silicon nitride (SN), Titanium boride (TiB2), Aluminiumnitride (AIN), SIALON (Silicon aluminium oxynitride), Fiber-reinforced ceramic (e.g. C-SiC)and Ceramic-metal composite materials (CMC). However, ceramic armor is not withoutserious engineering and practical shortcomings. High hardness, high elastic modulusceramic materials such as SiC and B4C are very brittle and have poor durability andresistance to dropping or even rough handling under field conditions. Furthermore, thelow toughness of high performance ceramics implies that essentially all armor-gradeceramics have poor multiple hit capabilities. Once a ceramic tile is impacted, thesubsequent impact response of the armor is seriously compromised.

Multiple hits are a serious problem with ceramic-based armors. Armor-grade ceramicsare extremely hard, brittle materials, and after one impact of sufficient energy, thepreviously monolithic ceramic will fracture extensively, leaving many smaller pieces and a reduced ability to protect against subsequent hits in the same vicinity.

There are several sources of information which shows that confining the ceramics resultsin an increase in penetration resistance One relatively obvious and popular method toovercome the disintegration of ceramic armor is to encapsulate ceramic armor with alayer of surrounding metal.

In the laboratory, ceramics show much higher performance when their boundaries areheavily encapsulated. The problem is to devise methods to realize some or all of thisencapsulation effect so it can be reduced to practical application in real armor systems. Ifthe ceramic tile is not encapsulated, the fractured pieces can move away easily, andresidual protection is lost.

State-of-the-art integral armor designs work by assembling arrays of armor-gradeceramic tiles/spheres/pellets within an encapsulation of polymer composite plating ormetallic frames. Such an armor system will erode and shatter (armor-piercing)projectiles. Different designs are in current use over a range of applications. Substantialdevelopment efforts are ongoing with this type of armor, as it is known that its fullcapabilities are not being utilized.

There are several deficiencies with the encapsulation of ceramic material in the prior art.Because of the properties of the proposed metals, conventional casting processes cannotbe readily and effectively utilized to encapsulate ceramic material. For example, the veryhigh solidification shrinkage of metals precludes this process as the encapsulating metalexerts undue stresses on the ceramic material, and can result in the fracturing of theceramic. Encapsulation of ceramic armor can also be performed by a number of othermeans, such as shrink-fitting ceramic tiles or bricks into metallic containers, or by otherbonding methods involving the use of welded, bolted, brazed or adhesively bondedmetallic containers. In the past, such layers have for instances been formed on or aroundceramic material or tiles by techniques such as powder metallurgical-forming, diffusionbonding, and vacuum casting of liquid metal layers.

Accordingly, a need exists for an armor component formed of an encapsulated ceramicmaterial that has improved penetration resistance, and for an inexpensive method forforming an armor component from a ceramic material that has improved penetrationresistance.

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 areused as disclosed in U.S. Pat. No. 4, 987, 033, which shows methods for metallicencapsulation of ceramic material with powdered metal layers that are cold isostaticallypressed, vacuum sintered and then hot isostatically pressed to final density.

These methods have severe shape limitations, involve the use of relatively costly coldisostatic press tooling, requires a complicated and costly multiple step processingsequence, and still requires complicated and costly post-machining to produce a metallicencapsulating layer with consistent areal density. U.S. Pat. Nos. 3, 616, 115 and 7, 069,836 respectively, shows methods for metallic encapsulation of ceramic armor based onvacuum hot pressing and/or diffusion bonding of ceramic tiles and metallic stiffeninglayers into machined arrays of lattice-type metallic frameworks. While capable ofproducing well-bonded and geometrically-consistent metallic encapsulation layers, thesemethods are also costly and very limited with regard to their shape-forming capabilityand the related ability to be transitioned to large-scale manufacturing, as they requireexpensive restraint tooling and die sets that essentially limit vacuum hot press or diepressing-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 for encapsulation ofdifferent 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 forencapsulation (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 assilicon carbide, boron carbide or alumina frequently leads to liquid metal casting-basedencapsulation results generating very high stresses around the ceramic material, whichcan easily result in the fracturing of the ceramic being encapsulated. This situation wouldalso be worsened for more complex ceramic armor tile geometries.

U.S. Pat. No. 5, 361, 678 issued to Roopchand reveals coated ceramic bodies incomposite armor where the ceramic bodies are embedded in a metal matrix.

French patent No. 2526535 issued to Pequignot reveals ceramic elements embedded intoa 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. Polysulfideis used as an encapsulation component.

U.S. Pat. No. 7, 117, 780 issued to Cohen reveals composite armor plate using a layer ofpellets held by elastic material.

Object of the invention

The present disclosure is in the field of improved and lighter armor materials for militaryand 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 greatersurvivability, mobility and transportability.

It is increasingly difficult to defend against the destructive forces of projectiles beingproduced and developed for the penetration and destruction of current armor materials.These projectiles are also here referred to as ballistic threats or simply "threats". Ballisticthreats vary in size and type.

To meet these threats, exceptional mechanical and physical properties are required inarmor materials. Toughened metal alloys, fiber reinforced plastics and high technologyceramic materials are some of the major armor materials used today. Other materialsused in armor materials are glass, glass-ceramic and sintered refractory material. Manymodern armors are multicomponent systems, using several plates of different materialsbonded together in order to exploit the individual material properties to best advantage.Ceramics like alumina, silicon carbide, boron carbide, beryllium oxide etc., represent oneof the most important developments in armor materials. Their very high compressivestrength and hardness, coupled with relatively low density, provide excellent armorperformance against a wide range of high energy ballistic threats.

Ceramic armor materials are lighter than conventional metallic solutions, includingtitanium, and are two to three times harder. Use of ceramics lowers the weight of apassive armor system while improving the ability to defeat ballistic threats.

Ceramic materials have seen increasing use in ballistic applications where a combinationof high compressive strength and low density are important. The very high compressivestrength of ceramic materials offers the potential for more efficient destruction ofpenetrators than more conventional monolithic metals. However, the extreme localized loading of the ceramic during a ballistic impact often generates early failure andcomminution of these brittle materials, and subsequent considerable loss of ballisticefficiency.

Ceramic materials are hard and brittle. The high hardness contributes to flatten the nosepart 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 thisdamage propagation affects the subsequent ceramic performance. This damage is causedby the activation of preexisting defects by the shear and tensile forces that are generatedon 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 helps to distribute the impact force over alarger area. Another effect of brittleness of ceramic material is the long cracks usuallyexpand from the point of hit due to bending. The long cracks and resulting small pieces ofceramic material are harmful for the defeat of projectiles, because not much constraintexist in-plane to keep the material in the damage zone and to contribute resistanceforces.

Ceramics are usually employed in an armor system where backing and surround platesare utilized in an attempt to increase efficiency of the comminuted ceramic. Suchattempts at backing or surrounding the ceramic with different materials have been metwith mixed success, partly because the underlying principals who influence successfulsystem design are not clearly understood. Engineering a better backing and/orsurrounding that can enhance the performance of the ceramic, would lower its weightand space burden in a structural armor application.

Different factors (backing plate stiffness, ceramic compressive strength, ceramic/-encapsulation impedance mismatch, etc.) have been shown to be important contributorsto the overall efficiency of the ceramic/ encapsulation system. The resulting behavior ofthe ceramic is a complicated combination of the integrated responses of the damagedand 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 thedamage is generated and how well the damage is encapsulated.

Ceramics are currently the subject of intensive research and improvement. The fieldapplications of ceramic armors, however, have been limited by especially the highmaterial cost. Incorporation of ceramics in hybrid armor systems can result in significant weight reductions, and are an excellent prospect for next-generation energy absorbingsystems for ballistic protection.The present disclosure relates to the encapsulation andpre-stress of armor-grade materials, such as ceramics, with fiber reinforced cementitouscomposites (FRCC). The characteristics of FRCC can be utilized to exert a highly beneficialcompressive stress on a ceramic material. Such beneficial compressive stress makes theencapsulated material to defeat the projectiles much more effectively by delaying theformation of cracks in the ceramics.

The objectives of the present disclosure are as follows: • Enhancement of hydrostatic encapsulation of a material made of armor-gradematerials, such as ceramic, to increase dwell time for a projectile on the frontface of the encapsulated material thus promoting mushrooming and defeat ofanti-armor projectiles of any types and sizes.

• Enhancement of multiple hit penetration resistances for individual tiles, tilearrays or more complex shapes in which the encapsulated material is dividedinto.

• Enhancement of the durability and damage tolerance against physical abuse androutine handling for the inner brittle material by providing a robust container forindividual tiles, tile arrays or more complex shapes.

• Providing a composite armor that will prevent the penetration of projectiles in astructure while also providing structural support to the same structure.

The present disclosure is a major improvement to current ceramic-based integral armor,which results in superior ballistic performance and survivability, multi-hit capabilityincluding reduced damaged area, lower areal density, more flexible design as well asstrong structural strength.

Summary of the invention

It is to be understood that the following detailed description present embodiments ofthe present disclosure and are intended to provide an overview or framework forunderstanding the nature and character of the present disclosure as it is claimed. Theaccompanying drawings are included to provide a further understanding of the presentdisclosure and are incorporated into and constitute a part of this specification. Thedrawings illustrate different embodiments of the present disclosure and, together withthe description, serve to explain the principles and operations of the present disclosure but not to limit the present disclosure to these descriptions only.

The main idea of the present disclosure is accomplished by forming fiber reinforcedcementitious material around the perimeter of an encapsulated armor-grade material,especially ceramic, to encapsulate and pre-stress this encapsulated material to increasedwell time and avoiding the inner core from lateral expansion when impacted by aballistic threat, such as a projectile or a fragment.

Fiber reinforced cementitious composite (FRCC) is a very universal term for allcementitious materials that are reinforced by any kind of fibers. FRCC is concretecontaining fibrous material which increases its structural integrity. It contains shortdiscrete fibers that are uniformly distributed and randomly oriented. Fibers are man¬made, such as steel, titanium, glass, carbon, polymers or synthetic. The character of FRCCchanges with varying concretes, fiber materials, geometries, distribution, orientation anddensities.

Many different so called high performance materials have been developed. Theconsidered materials in this disclosure to encapsulate ceramic materials are allcementitious composites reinforced by randomly oriented short discrete fibers. Addingfibers enhances the compressive, tensile and shear strengths, flexural toughness,durability and resistance to impact and penetration as well as resistance to plasticshrinkage cracking.

Concrete is widely used in structural engineering with its high compressive strength, lowcost and abundant raw material. But normal strength 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. Toovercome these deficiencies, additional materials are added to improve the performanceof normal strength concrete.

Cementitious matrices such as concrete have low tensile strength and low strain capacityand therefore fails in a brittle manner. As a result, the mechanical behavior of theconcrete is critically influenced by crack propagation. Concrete can exhibit failurethrough cracks which are developed due to brittleness. A more ductile material can beachieved by the use of fibers in the concrete.

The use of fibers in concrete to improve pre- and postcracking behavior has gainedpopularity, and several different fiber types and materials have been successfully used inconcrete to improve its mechanical and physical properties.

Reinforcing fibers will stretch more than concrete under loading. Therefore, thecomposite system of fiber reinforced concrete is assumed to work as if it wereunreinforced until it reaches its "first crack strength." It is from this point that the fiberreinforcing takes over and holds the concrete together. For fibers reinforcing, themaximum 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 thetoughness of the concrete product in which they are placed.

Many catastrophic failures of reinforced concrete structures subjected to impact areassociated with the brittleness of concrete material in tension. Although a compressivestress wave is generated on the loading side of the structure by impact, it reflects as atensile stress wave after hitting a free boundary on the back side of the structuralelement. In addition, the tensile strength of concrete is lower (by about an order ofmagnitude) than its compressive strength. Therefore, concrete tensile propertiesgenerally govern concrete failure under impact.

Concrete is considered a brittle material, primarily because of its low tensile straincapacity and poor fracture toughness. Concrete can be modified to perform in a moreductile form by the addition of randomly distributed discrete fibers in the concretematrix.

Ductile concrete would be highly desirable to suppress the brittle failure modes andenhance the efficiency and performance of current design approaches. The mosteffective means of imparting ductility into concrete is by means of fiber reinforcement.

An extremely ductile fiber reinforced brittle matrix composite is of great value toprotective structures that may be subjected to dynamic and/or impact loading.Compared to normal strength concrete and fiber reinforced concrete, a ductile concretebased composite has significant improved tensile strain capacity with strain hardeningbehavior, several hundred times higher than that of normal strength concrete and fiberreinforced concrete, even when subjected to impact loading (FIG. 2).

While the fracture toughness of concrete is significantly improved by fiberreinforcement, most fiber reinforced concrete still shows quasi-brittle post-peak tension¬softening behavior under tensile load where the load decreases with the increase ofcrack opening. The tensile strain capacity therefore remains low, about the same as thatof normal concrete. Significant efforts have been made to convert this quasi-brittlebehavior of fiber reinforced concrete to ductile strain hardening behavior resembling ductile metal. In most instances, the approach is to increase the volume fraction of fiberas much as possible. As the fiber content exceeds a certain value, normally 4-10%depending on fiber type and interfacial properties, the conventional fiber reinforcedconcrete may exhibit moderate strain hardening behavior.

The typical stress-elongation response of FRCC indicate two properties of interest, thestress at cracking and the maximum post-cracking stress (FIG. 3). While the crackingstrength of the composite is primarily influenced by the strength of the matrix, the post¬cracking strength is solely dependent on the fiber reinforcing parameters and the bond atthe fiber-matrix interface. Thus, improving the post-cracking strength is the key to thesuccess of the composite.

Special types of concrete are those with out-of-the-ordinary properties or thoseproduced by unusual techniques. Concrete is by definition a composite materialconsisting essentially of a binding medium and aggregate particles, and it can take manyforms.

The first fiber reinforced cementitious material that was developed in the early 1960s issteel fiber reinforced concrete (SFRC). SFRCs exhibit ductile behavior compared to thebrittle matrix, but their flexural and tensile strengths are not very high, and especially thecompressive strengths of these materials do not practically change with the fiber volumefraction. Although it shows certain improvements compared to normal concrete, it is notconsidered to be a high performance material. SFRC consists of a normal strengthconcrete matrix containing fine and coarse aggregates, which is reinforced by relativelylong straight steel fibers. The primary improvements of traditional SFRC compared tonormal strength concrete are a higher toughness and energy absorption capacity, greaterspalling and delaminating resistance, improved durability characteristics through bettercrack control, as well as a higher ductility on a material level. Compared to normalstrength concrete the tensile strength of SFRC is not improved significantly and SFRC stillexhibits "quasi-brittle" failure (no strain hardening). The reasons for this are that lowamounts of fibers are used, that the used fibers are relatively long and that the matrixcontains too much course aggregate. Flowever, compared to unreinforced concrete thefailure is more ductile exhibiting a smother softening behavior.

Originally fiber reinforced materials, straight steel fibers at relatively low volumecontents were used to improve the mechanical properties of traditional concrete. Theaddition of larger volume contents of fibers was mainly prevented by workability problems. In order to improve workability and restrict "balling" of fibers, the quantity ofcement was increased and the amount of coarse aggregate reduced. Furtherimprovement of workability could be achieved by the introduction of high-range water-reducing admixtures. Hereby the possible volume content of fibers could be increased.Together with the use of improved cementitious matrices with fewer coarse aggregateand carefully adjusted properties, this finally leads to the high performance fiberreinforced materials we know today.

FRCC includes the entire class of fiber reinforced cementitious composites (FIG. 4), andcomprises fiber reinforced concrete (FRC), fiber reinforced mortar (FRM), highperformance fiber reinforced composite (HPFRCC) and ductile fiber reinforcedcementitious composite (DFRCC). DFRCC is a broader class of materials than HPFRCC.Examples of modern high performance materials are engineered cementitiouscomposites (ECC), sometimes also called "Bendable Concrete", hybrid fiber concretes(HFC), multi-scale fiber reinforced cementitious composites (MSFRCC), compactreinforced composites (CRC) and reactive powder concretes (RPC), non-high performancefiber reinforced concrete and steel fiber reinforced concrete (SFRC).

Considering the mechanical properties of FRCC, these composites can be categorized intotwo classes: quasi-brittle and pseudo strain-hardening. Conventional FRCC fall into thefirst category whereas HPFRCC fall into the latter.

Quasi-brittle materials, such as normal strength concrete and conventional FRCC, usuallyfail due to the formation of a single macro-crack, whereas pseudo strain-hardeningcementitious materials such as HPFRCC undergo multiple cracking. For conventionalFRCC, the typical upper limit for fiber volume fraction is 3%. For such relatively low fibercontent, the fibers mainly enhance the crack arresting ability, post cracking ductility,fatigue and impact resistance. The stress at first crack, maximum stress and thecorresponding strain are not significantly improved compared to normal strengthconcrete.

HPFRCC show a large improvement in both strength and toughness compared with thenormal strength matrix. The main feature of these materials is the optimum combinationof strength and toughness which approaches the structural properties of steel.

HPFRCC is a generic term encompassing many different materials ranging from those thatemploy ultra-compact matrices and those that do not. However, the common point of allHPFRCC materials is their hardening tensile behavior that helps control cracking to a much better extent than usual FRCC.

The criterion which separates a HPFRCC from a traditional FRCC, such as SFRC, is itsresponse in tension. If the material is strain hardening in the inelastic regime it isconsidered to be a HPFRCC. The fundamental difference between composites that showstrain hardening and those who don't is that the ductility of the latter is only effective ona material level and does not affect the overall structural ductility. Or in other words thematerial does react in a ductile manner, but because it is softening in tension, the plasticdeformations are restricted to a small area, resulting in damage localization and failure ina small zone with large crack openings. On a structural level this kind of ductility has littleinfluence since the failure occurs locally and the rest of the structure remains elastic.Strain hardening fiber reinforced cementitious composites on the other hand retardlocalization and lead to multiple cracking and structural ductility even without theaddition of structural reinforcement bars. For this reason they are called highperformance fiber reinforced cementitious composites.

Because several specific formulas are included in the HPFRCC class, their physicalcompositions vary considerably. However, most HPFRCCs include at least the followingingredients: fine aggregates, a superplasticizer, polymeric or metallic fibers, cement andwater. Thus the principal difference between HPFRCC and typical concrete compositionlies in HPFRCCs lack of coarse aggregates. Typically, a fine aggregate such as silica sand isused in HPFRCCs.

Ultra high performance fiber reinforced concrete (UHPFRC) is a relatively newcementitious material, which has been developed to give significantly higher materialperformance than normal strength concrete, FRC or ECC (FIG. 5). UHPFRCC denotes asubclass of FRCCs that encompasses a number of ultra high strength concretes that arereinforced with steel fibers.

Many UHPFRCCs are also HPFRCCs and therefore exhibit strain-hardening and multiplecracking in direct tension.JJHPFRCC are a sub group of HPFRCC combining the ductility ofstrain hardening cementitious composites with the high compressive strength of DSPconcrete. UHPFRCC is furthermore distinguished between other FRCCs as a materialexhibiting strain hardening in tension, whereas other FRCCs may exhibit a hardeningbehavior in bending, but are characterized by strain softening in tension. UHPFRC havegood potential for absorbing energy through flexure. Studies of this material underdynamic loading have shown an increase in the ultimate strength with increasing strain rate.

UHPFRC can be mixed and cast like normal strength concrete with no special facilities orhandling. "Ultra high performance" refers principally to improved mechanical strength,fractural toughness and durability. A mix is designed to combine high cement contentwith a very low water/cement ratio. The selection of fine aggregates achievesmaximisation of the particle packing density and minimises any localised nonhomogeneity. Post-set heat treatment at 90 degrees Celsius can be applied to furtherimprove the microstructure. This process results in a very high compressive strengthconcrete, typically between 150-200 MPa. The addition of a high dosage of high tensilesteel fibers, normally 13 mm in length and 0.2 mm in diameter, results in a high flexuraltensile strength, typically between 25-50 MPa. This material also has a very high capacityto absorb damage, with fracture energy in the range 20,000-40,000 J/m2.

Fibers are incorporated in UHPFRC in order to enhance the fracture properties of thecomposite material. The additional role of fibers in UHPFRC, in comparison to the role offibers in ordinary and in high strength fiber reinforced concrete, is to provide sufficientductility of the material in tension without a decrease in stress. This is achieved bychoosing the appropriate type and quantity of fibers. In recommendations for UHPFRC,minimal 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 orordinary concretes. Required fiber geometry can be estimated based on the relationshipbetween pullout force and fiber-breaking force. The resulting high-strength and energyabsorbing 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 showsmultiple cracking and strain hardening in tension, therefore in bending as well. On theother hand, DFRCC encompasses a group of FRCCs that exhibit multiple cracking inbending only, in addition to HPFRCCs. Multiple cracking leads to improvement inproperties such as ductility, toughness, fracture energy, strain hardening, strain capacityand deformation capacity under tension, compression and bending. The advantage ofDFRCCs is the increased toughness under tensile stress condition. Among a variety ofDFRCCs, some DFRCCs achieve pure tension toughness and ductility that are comparableto those of metallic materials, while others show increased toughness only under flexuraltension.

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 composite materials. ECC essentiallyconsists of two components: fibers and a cementitious matrix. Using a micro-mechanicalapproach, fiber and matrix properties are adjusted in order to obtain the desiredmacroscopic material behavior. The most characteristic material property of ECC is itsextremely ductile, strain-hardening-like behavior in the inelastic tensile regime. Thisbehavior has been termed "pseudo-strain hardening" referring to the post-yield strainhardening usually exhibited by metals. Although the macroscopic performance of ECC inthe inelastic regime is very similar to the strain hardening observed for metals, theresponsible micromechanical mechanism is a completely different one, hence the term"pseudo". In metals strain hardening is a consequence of a change in the molecularstructure, whereas in ECC it is produced by multiple cracking and bridging of the cracksby the incorporated fibers.

ECC is a special type of HPFRCC which has been microstructurally tailored based onmicromechanics. The most obvious beneficial mechanical property of ECC is its extremelyductile response in tension. Experiments show, that tensile strains up to 6% can bereached before softening sets in. This means that structural elements made from ECC areable to bear very large imposed deformations proved durability characteristics throughbetter crack control, as well as a higher ductility on a material level which are of interestfor impact loading. Microstructure optimization allows ECC to be made with fiber contentless 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 ofsteel. While conventional reinforced concrete members suffer from steel yielding atlocalized cracks, reinforced ECC members attain deformation compatibility and utilize thedeformation capacity of steel to greater extent.

Depending on the matrix constitution, the fiber geometry and the bond propertiesbetween fibers and matrix, a certain minimal volume content of fibers is necessary toachieve the typical pseudo strain hardening behavior of ECC. If the fiber content is toolow, quasi-brittle failure as it is exhibited by traditional fiber reinforced concrete willoccur. In this case damage localization and softening will set in immediately afterformation of the first crack. Therefore the fiber content is usually chosen just above thecritical volume fraction which allows for the pseudo-strain hardening and ductilebehavior, but without using excessive amounts of fibers. In general synthetic fibers such as UHMWPE (ultra high molecular weight polyethylene) or PVA (polyvinyl alcohol) fibersare used (FIG. 7). The difference between these two fiber types is, that UFIMWPE fibersshow little chemical bond with the cement matrix whereas the chemical bond strengthbetween PVA fibers and matrix is considerable. Therefore untreated PVA fibers are notvery adequate for pullout behavior and preliminary surface treatment of the PVA fibersmight be necessary. The used fiber volume contents are usually somewhere between0.5% and 4%. Typically the synthetic fibers in ECC have a higher aspect ratio (fiber lengthto fiber diameter) than steel fibers used in steel fiber reinforced concrete. Theirinterfacial bond strength on the other hand is usually lower. A high aspect ratio isessential in minimizing the fiber volume content necessary for the pseudo-strainhardening behavior. The cementitious matrix in ECC consists of a Portland cement pasteor 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 multiplecracking of the composite. In turn these two mechanisms are responsible for the pseudo¬strain hardening behavior and large strain capacity of ECC. In order to guarantee multiplecracking, the bridging stress that can be transmitted by the fibers has to be larger thanthe first cracking strength of the intact matrix. This condition is referred to as thestrength criterion for multiple cracking.

In general no or little aggregate with a relatively small grain size is used for thecementitious matrix of ECC. The main reason for this is to keep the fracture toughness ofthe matrix low. Adding large amounts of aggregate would result in longer fracture pathdistances and by consequence a higher fracture toughness. Flaving a low fracturetoughness 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 aformwork, and due to the pre-placement of fibers, its fiber volume fraction can amountto 20% at maximum. The confining effect of numerous fibers yields high compressivestrength reaching over 200 MPa, and the strong fiber bridging leads to tensile strainhardening behavior in some SlFCONs. The fracture energies of SlFCON's are about 1350times that of normal strength concrete. SIMCON uses pre-placed fiber mat instead ofsteel fibers.

RPC's represent a new generation of concretes which utilizes reactive powder, and it isdesigned 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 to150 J/m2 for ordinary concretes. The fracture energies of RPCs are about 300 times thatof normal strength concrete. The RPC microstructure has a more compact particlearrangement and is enhanced by the presence of the strongest cementitous hydrates ascompared to HPC. RPCs are produced by using very fine sand, cement, silica fume,superplasticizer and short cut steel fibers. Their very low porosity gives them importantdurability and transport properties and makes them potentially suitable materials forstorage of industrial wastes. These features are achieved by 1) precise gradation of theparticles in the mixture to yield a matrix with optimum density, 2) reducing the maximumsize of the particles for the homogeneity of the concrete, 3) reducing the amount ofwater in the concrete, 4) extensive use of the pozzolanic properties of highly refinedsilica fume, 5) optimum composition of all components, 6) the use of short cut steelfibers for ductility, 7) hardening under pressure and increased temperature, in order toreach very high strengths.

Ductal is a range of ultra high performance concrete (UHPFRC) with very highcompressive strength and non-brittle tensile behavior offering compressive strength of160 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 propertiesare characterized by high strength, high durability and high flowability. Ductal is a cementbased composite reinforced with steel fibers under the concept of high strength and hightoughness. W/C ratio is in the region of 0.2. The sand used has a fine grading, with thelargest grains not exceeding around 600 um in diameter. The addition of silica fume andoptimized use of admixtures are both absolutely essential. Last but not least, theconcrete is reinforced with metal fibers, which have also been optimized for severalcriteria, involving optimizing not only the behavior of the individual fibers, but also theirinteractions within the matrix. A content of 2% by volume of 13-15 mm long fibers withdiameters of around 0.2 mm emerges as a good compromise. Calculating the meanspacing of these fibers in the matrix gives a result of around 1.6 mm. which is perfectlycompatible 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 largeamounts of fly-ash and silica fume and can reach compressive strengths between 120MPa and 250 MPa.

In FRCCs, fibers can be effective in arresting cracks at both macro and micro levels. Mostof the strain hardening FRCC is limited to single fiber type.

Mono fiber composites containing high stiffness fibers normally show high ultimatestrength, low strain capacity and small crack width properties, while those containing lowstiffness fibers show low ultimate strength, high strain capacity and large crack widthproperties.

Recently hybrid fiber reinforced cementitious composites (HFRCC) exhibiting strainhardening behavior is also developed. In hybrid fiber composites, two or more differenttypes of fibers are suitably combined to exploit their unique properties. The use ofoptimized combinations of two or more types of fibers in the same concrete mixture canproduce a composite with better engineering properties than that of individual fibers. Ahybrid composite, with proper volume ratio of high and low stiffness fibers, showsimultaneous improvement in ultimate strength, strain capacity and crack widthproperties.

The hybridization of fibers in FRCC can be done in different ways, such as by combiningdifferent lengths, diameters, modulus and tensile strengths of fibers. Large macro fibersbridge the big cracks and provide toughness while small micro fibers enhance theresponse prior or just after the cracking. Micro fibers also improve the pull out responseof macro fibers, thus produce composites with high strength and toughness.

Over the last three decades, high compressive strength concretes and high tensileductility concretes have emerged as two distinct classes of concrete. Materials at thefrontiers of both of these classes include very high strength concrete (VHSC) withcompressive strength around 200 MPa, and engineered cementitious composites (ECC)with tensile ductility in the range of 3%>-6%. The development of these two concretes wasbased on two different design philosophies that targeted two different structuralperformances. VHSC and similar high strength concretes (RPC, Ductal, MDF and DSP)were designed to achieve size efficiency in structural members for very large structuresand to provide additional strength safety margin for strategically critical and protectivestructures. ECC and similar high performance fiber reinforced cementitious composites(HPFRCCs) were developed to ensure ductility of structural elements and massive energyabsorption in the face of extreme load displacement events such as earthquakes.However, the decoupled development of VHSC and ECC resulted in mutual exclusion ofeach 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. Acombination of high strength and high ductility in one concrete material is highlydesirable.

Recently, there have been a few notable investigations on combining high compressivestrength and high tensile ductility in one concrete with limited success. Some mechanicaltest results of ultra high performance - strain hardening cementitious composites (UHP-SHCC) shows average compressive strength of 96 MPa and tensile ductility of 3.3% at 14days after casting. The development of another such material, ultra high performance -fiber reinforced composite (UHP-FRC) shows compressive strength of about 200 MPa andtensile ductility of 0.6%. Although both of these materials attempt to combine tensileductility and compressive strength in one concrete, UHP-SHCC has a compressivestrength that is only about half that of VHSC, and UHP-FRC has tensile ductility which is atleast five times smaller than ECC.

Newly development of a new composite material, high strength - high ductility concrete(HSHDC), shows both the desirable properties of high compressive strength (similar toVHSC) and high tensile ductility (similar to ECC) and are integrated into a singlecomposite material. This results in higher specific energy absorption (or compositetoughness) in HSHDC as compared to any other material in the class of high performancecementitious composites. The micromechanics-based principles that guide the design ofECC, combined with a VHSC matrix, led to the development of HSHDC.

Due to controlled cracking enabled by micromechanical tailoring of the compositematerial, 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 thestress-strain curve before attaining ultimate stress capacity. A comparison of othercomposite properties of HSHDC with other similar high performance concrete materialsis shown in FIG. 9. It can here be observed, that the specific energy of HSHDC is thelargest among all the materials presented, which is a result of the combination of hightensile strength and high tensile ductility. Such material behavior leads to high energyabsorption, 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 gainedpopularity. The mechanical properties of fiber reinforced concrete depend on the type and the content of the added fibers. Several different fiber types and materials havebeen successfully used in concrete to improve its mechanical and physical properties. Inoften used fiber reinforced concrete steel and synthetic fibers are mainly used, althougha great variety of fibers made of other materials exists. This is related to the strength andstiffness that is required of the desired fiber contribution.

Unlike continuous fiber composites, the external loads are not directly applied to thefibers in FRCC. The load applied to matrix materials is transferred to the fibers via fiberends and the surfaces of fibers. As a consequence, the properties of FRCC greatly dependon fiber length and the diameter (i.e., fiber aspect ratio) of the fibers. Further, severalfactors such as fiber orientation, volume fraction, fiber spacing, fiber packingarrangement, and curing parameters also significantly influence the properties of FRCC.Using micro and macro fibers of different mechanical, geometrical and physicalproperties reduces the brittleness of cementitious materials. Adding short needle-likefibers to cementitious matrices enhances their mechanical properties, particularly theirtoughness, ductility and energy absorbing capacity under impact. The fibers can andshould be engineered to achieve optimal properties in terms of shape, size andmechanical properties, as well as compatibility with a given matrix. All fibers are made ofeither inorganic or organic material. The inorganic category includes materials such asmetals, minerals, ceramic, carbon and glass. The list of organic fiber materials, on theother hand, seems to be limited only by the creativity of nature and the chemicalindustry. 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 includenylon, polypropylene, polyvinyl alcohol (PVA), polyethylene and aramid, among manyothers.

Materials used in fiber reinforcing can include acrylic, asbestos, cotton, glass, nylon,polyester, polyethylene, polypropylene, rayon, rockwool and steel. Of these, acidresistive glass and steel fibers have received the most attention. Plastic fibers haveshown to be of little value in reinforcing concrete until only recently. Natural fibers aresubject to alkali attack and are also determined to have little value. The premium fibersare graphite reinforced plastic fibers, which are nearly as strong as steel, lighter-weightand corrosion-proof. Some experiments have had promising early results with carbonnanotubes. Fiber (steel or "plastic" fibers) reinforced concrete is less expensive thanhand-tied rebar, while still increasing the tensile strength many times.

Short fibers used in concrete can be characterized in different ways. First, according tothe fiber material: natural organic (such as cellulose, sisal, jute, bamboo, etc.); naturalmineral (such as asbestos, rockwool, etc.); man-made (such as steel, titanium, glass,carbon, polymers or synthetic, etc). Second, according to their physical/chemicalproperties: density, surface roughness, chemical stability, non-reactivity with the cementmatrix, fire resistance or flammability, etc. Third, according to their mechanicalproperties: tensile strength, elastic modulus, stiffness, ductility, elongation to failure,surface adhesion property, etc.

Short fibers are mainly characterized by the material and its mechanical properties andby their geometry. Once a fiber type has been selected, an infinite combination ofgeometric properties related to its cross sectional shape, length, diameter or equivalentdiameter and surface deformation can be selected. The cross section of the fiber can beflat, circular, rectangular, diamond, square, triangular or any other significant polygonalshape.

The effectiveness of fibers on the mechanical properties of brittle matrix varies with thegeometrical, mechanical and physical properties of fibers. Fibers with surface roughnessand large specific surface area develop good bond with the matrix due to, as a results,micro-cracking mechanism before the occurrence of peak load is arrested in the presenceof fibers, and fibered concrete exhibits high value of peak load compared to normalstrength concrete without fibers.

The properties of concrete matrix and of the fibers greatly influence the character andperformance of FRCC. The properties of fibers which are of interest include fiberstiffness, 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 ofmagnitude); 2) a bond strength with the concrete matrix preferably of the same order asor higher than the tensile strength of matrix; and 3) unless self-stressing is used throughfiber reinforcement, an elastic modulus in tension significantly higher than that of theconcrete matrix. The Poisson's ratio and the coefficient of thermal expansion shouldpreferably 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 theelastic 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 offibers 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 animportant role. Increasing the average bond strength leads to a direct increase in thepost cracking strength of the composite and other important properties as well, such astoughness and energy absorption capacity. The different bond components are adhesion,friction, mechanical and interlock. In some fibers the surface is etched or plasma treatedto improve bond at the microscopic level.

To develop better bond between the fiber and the matrix, the fiber can be modifiedalong its length by roughening its surface or by inducing mechanical deformations. Thusfibers can be smooth, indented, deformed, crimped, coiled, twisted, with end hooks,paddles, buttons, or other anchorage.

When micro-cracks are developed, the stress in fiber increases gradually with theincrease of crack openings, and a stage of either pulling out from the matrix before thestress in fiber exceeds its tensile strength capacity will happen or a stage of breakage offiber will happen if the fiber are not pulled out from the matrix before the stress in fiberexceeds its tensile strength capacity. In order to enable the transfer of force with a smallcrack opening and sustain tensile force without breaking, a high modulus of elasticity andhigh strengths are required.

To transfer the stress across the crack edges (bridging action of fibers), length of fibercompatible with maximum aggregate size is important. Interfacial transition zonebetween the aggregate and the cement paste is the weakest phase in the concrete. Inorder to bridge this zone and to get highest effect of fibers, length of the fiber and thediameter of the aggregate must be coherent with each other. To develop an efficientbridging action, fiber must be embedded into the matrix on both ends beyond theaggregate particles. For that, fiber length must be at least greater than 2 times maximumaggregate size. Also to get better efficiency, fiber length should be 2 to 3 times themaximum size of the coarse aggregate.

Ideally the amount of fibers and aspect ratio should be as large as possible to maximizethe improvements in the mechanical properties.

The length and diameter of synthetic fibers vary greatly. Single filament fibers can be aslittle 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 inconcrete applications, the aspect ratio, that is, the ratio of length over diameter orequivalent diameter, of very fine fibers exceeds 100 while that of courser fibers is lessthan 100. Most synthetic fibers (glass, carbon, kevlar) are round in cross section; flatsynthetic fibers cut from plastic sheets and fibrillated are suitable when very low volumecontent is used.

Most common steel fibers are round in cross section, have a diameter ranging from 0.4to 0.8 mm, and a length ranging from 25 to 60 mm. Their aspect ratio is generally lessthan 100, with a common range from 40 to 80.

Different types of steel fibers have been developed (FIG. 10). They differ in size, shapeand surface structure. Such fibers have different mechanical properties such as tensilestrength, grade of mechanical anchorage and capability of stress distribution andabsorption). Hence they have different influence on concrete properties. Some othertypes of closed-loop steel fibers such as ring, annulus, or clip type fibers have also beenused and shown to significantly enhance the toughness of concrete in compression.

SFRC with the ring-type steel fibers (RSFRC), fails by more energy consuming mechanismsother than fiber pullout, whereby significant improvements in flexural toughness isobtained as compared to that of SFRC with conventional straight steel fibers. Fiber-matrix interfacial bond strength is provided by a combination of adhesion, friction andmechanical interlocking. While the mechanical performance of traditional straight steelfibers relies on the fiber-matrix interfacial bond strength, ring-type steel fibers aremainly designed to mobilize fiber yielding rather than fiber pullout. Three different typesof flexural failure mechanisms of RSFRC are involved: fiber rupture after yielding andcone-type concrete fracture and separation between ring-type steel fibers and concretematrix. Toughness indices of RSFRC are affected by fiber contents, ring diameter andfiber diameter.

Due to the formulation of the mechanics of the composite, the fiber content in cementmatrices is specified by volume fraction of the total composite. Because of fiber materialsof different densities, the same volume fraction of fibers of different materials leads todifferent weight fractions of fibers. Fibers are purchased by weight, but mechanicalproperties 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 toabout 80 kg/m3 of concrete; however, a 1% volume fraction of polypropylene fibers amounts to about only 9.2 kg/ m3.

A lightweight composite armor is disclosed wherein one or successive layers of discretearmor-grade objects, such as ceramic blocks, are encapsulated within a fiber reinforcedcementitious composite (FRCC). The FRCC is used to (1) encapsulate the armor-gradematerial, (2) pre-stress the encapsulated armor-grade material.

Studies shows that better confining of armor-grade ceramic results in an increase inpenetration resistance, and that ceramic yield much higher performance when theirboundaries are heavily encapsulated, because if the ceramic material is notencapsulated, the fractured pieces can move away easily, and residual protection is lost.The type of encapsulation can influence the ballistic efficiency of the ceramic basedarmor, and that "dwell" type defeat of penetrators can be achieved on the ceramic frontsurfaces. Two key parameters here are suppression of cracked tile expansion and puttingthe ceramic in an initial state of high compressive stress to delay or stop it from goinginto a state of tensile stress during impact. Tensile stresses are the cause of thepremature failure in ceramic components, since in general ceramic have higher strengthin compression than in tension.

The advantage of such compressive stresses on ceramic component is two-fold. First, theceramic material will have a higher tensile strength and will be more effective indefeating the projectile, as the projectile will spend more time (and more energy) beforeit causes the ceramic component to develop cracks and failure. This can allow thedisclosed composite structure to defeat projectiles with minor damage to the ceramiccomponent and therefore will allow the structure to take multiple hits component can bepreserved due to the relative high elastic strain limit of cementitious composites. Eventhough the effectiveness of the system will be reduced (in the case of formation of cracksin ceramic), the remaining compressive stresses will maintain some effectiveness of theceramic for subsequent hits, and at the minimum will keep the un-cracked portion ofceramic in place to defeat the projectile and dissipate its energy.

Viwang Bao etal (materials letters December 2002) reported substantial enhancement inprojectile penetration resistance in encapsulated and pre-stressed ceramic material, andexperimental results by Holmquist and Johnson (EDP Sciences 2003) also shows that pre¬stressed ceramics does improve performance. Other sources shows that threedimensional stressing of ceramic provides a higher enhancement in the penetration resistance than a two dimensional stressing.There is a clear need to improve the impactand penetration resistance, ballistic efficiency and structural integrity of ceramic armoremployed on a widespread basis in many types of armor systems. Over about the pasttwenty years, it has been discovered that the ballistic performance of ceramic armor iscritically dependent on the specific design attributes and geometrical configuration ofthe entire armor system. In particular, it has been observed that enhanced destructionand fragmentation of an incoming projectile can be obtained by increasing the so-called"dwell" time of the projectile on the front face of the ceramic armor during the very earlystages (the first 5-10 microseconds) of the ballistic impact event.

"Dwell," the duration of projectile erosion without target penetration, is an indicator ofthe ceramic's ballistic efficiency. Strategies to prolonging projectile dwell on ceramicsinclude retarding damage and retaining dynamic toughness in the damaged state.Ceramic failure caused by excessive structural bending from ballistic loading is anotherpossible limiting factor to improving ballistic performance. Improving ceramic armor canbe obtained with improved structural support for the ceramics. Desirable attributes of abacking 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 morecompletely the projectile can be attenuated and fragmented. Enhanced dwell time onthe front face of the ceramic armor leads to a phenomenon that is called interfacedefeat, wherein the projectile face mushrooms radially outward without significantpenetration in the thickness direction; this increases the projectile frontal area and thusdecreases its subsequent ability to penetrate the (ceramic) armor (FIG. 11). If theinterface defeat is not sufficient, there will be an initial dwell and subsequentpenetration into the armor (FIG. 12).

The phenomenon of dwell is used to particular advantage in medium or heavy ceramicarmor systems that are intended to defeat larger caliber high kinetic energy projectiles(12.7mm FIMG and above). It has been found that physical encapsulation of ceramicssuch as B4C, SiC or TiB2 increases the dwell time and delays the lateral and axial spreadingof the comminuted zone ahead of the projectile, thus increasing the ballistic efficiency ofthe ceramic.

There are several advantages to using FRCC as a surrounding material for ceramicmaterial. One is the relative high "yield" strength of FRCC that can be utilized to constrainthe ceramic material very effectively and impede the material's disintegration. When the armor package takes a hit, the ceramic material will tend to fracture and dimensionallyexpand due to opening cracks. In this situation, the surrounding FRCC will be forced tostretch out and the material's resistance to yielding will be an important factor inimpeding the disintegration of the ceramic material. This constrain of the ceramicmaterial 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 initialstage 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 impactzone

Terminology

Brittle A material is called brittle if it loose its tensile strength immediately after firstcracking under uniaxial tension and is no longer able to resist any stress.

Cementitious materials The binding component of fiber reinforced cementitiouscomposites. These are: cement, mortar or concrete.

Composite materials (or composites for short) Engineered materials made from two ormore constituent materials with significantly different physical or chemical propertieswhich remain separate and distinct on a macroscopic level within the finished structure.Concrete A construction material composed of cement (commonly Portland cement) aswell as other materials such as fly ash and slag cement, aggregate (generally a coarseaggregate such as gravel, limestone, or granite, plus a fine aggregate such as sand),water, and chemical admixtures. It is known as normal concrete, but also called normalweight concrete or normal strength concrete. DFRCC Stands for a subclass of FRCCscalled ductile fiber reinforced cementitious composites which are hardening underflexural conditions (deflection-hardening) but not strain-hardening in direct tension (seealso FIPFRCC). The expression ductile emphasizes the fact that these composites exhibitmultiple cracking which can be considered to be a form of ductility.

Ductile A ductile material is a material that does not fail immediately under uniaxialtension after reaching its first cracking strength. It first enters a strain-hardening phase,which is then followed by a crack opening phase and localization of failure.

FRCC Stands for fiber reinforced cementitious composites and describes cementitiousmaterials that are reinforced by randomly oriented short fibers.

Glass An amorphous (non-crystalline) solid material. Glasses are typically brittle andoptically transparent. In science, however, the term glass is usually defined in a muchwider sense, including every solid that possesses a non-crystalline (i.e., amorphous)structure and that exhibits a glass transition when heated towards the liquid state. In thiswider sense, glasses can be made of quite different classes of materials: metallic alloys,ionic melts, aqueous solutions, molecular liquids, and polymers. Polymer glasses (acrylicglass, polycarbonate, polyethylene terephthalate) are a lighter alternative to traditionalsilica glasses.

Glass-ceramic Materials that share many properties with both non-crystalline glass andcrystalline ceramics. They are formed as glass, and then partially crystallized by heattreatment.

HPFRCC Stands for high performance fiber reinforced cementitious composite. It delimitsa subclass of FRCCs which are strain-hardening in direct tension. It is also called pseudostrain-hardening or quasi strain-hardening. Strain-hardening refers to a true materialproperty and should not be confounded with hardening due to a redistribution ofinternal 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 wheredeformations start concentrating. It should be characterized by the crack openingdisplacement rather than a strain since the latter is gauge-dependent.

Multiple Cracking Means that a FRCC is capable of arresting the further opening of cracksby 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 expression quasi-brittle describes a material that starts softening di¬rectly after first cracking under uniaxial tension. However, quasi-brittle materials are stillcapable of transferring some reduced amount of stress which gradually decreases withincreasing crack opening.

Strain hardening / pseudo strain hardening Strain hardening describes a phenomenonthat, under uniaxial tension, transmitted tensile stress increases successively even afterfirst cracking, with continued tensile straining. The term "pseudo strain hardening" issometimes used instead, since the strain hardening mechanism of DFRCC is different from that of metallic materials. During strain hardening/pseudo strain hardening, thestress-strain curve is uniquely defined, and is a true material property.

Strain softening Strain softening describes a phenomenon that, under uniaxial tension,transmitted tensile stress decreases upon first cracking or after strain hardening.Structure A structure generally relates to the way elements are organized in relation toeach other and relative to the whole they are in. This can be physical, spatial orsystematically.

Tension toughness, compression toughness, flexure toughness Toughness describesenergy absorption which is given by the area below stress-strain curve or load-displacement curve either in tension, compression, or flexure. In practice, toughness iscalculated based on the area up to a prescribed strain or displacement.

Description of the figures

The accompanying drawings and photos, which are incorporated in and form a part ofthis specification, illustrates the technical background and embodiments of the presentdisclosure, and together with the description, serve to explain by way of example only,the principles of the present disclosure: FIG 1 Illustrates the different compositions between normal strength concrete and UHPC;FIG 2 A photo of a high strength - high ductility concrete (HSHDC) plate under bending; FIG 3 Illustrates characteristics of cementitious materials: Definition of A: brittle, B: quasibrittle, and C: ductile behavior as well as strain softening and strain hardening underuniaxial tensile loading; FIG 4 Illustrates some different high performance fiber reinforced cementitious materialsand their classification; FIG 5 Illustrates flexural performances of beam specimens made of different fiberreinforced 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 fiberreinforced cementitious materials; FIG 10 A photo of typical profiles of steel fibers commonly used in some fiber reinforced concretes; FIG 11 A photo of the sequence of three flash X-ray radiographs showing the initial dwellof a penetrator and subsequent penetration into thick ceramic target; FIG 12 A photo of the sequence of three flash X-ray radiographs showing complete dwellof a penetrator on a thick ceramic target; FIG 13 Photo collage of different forms and sizes of armor-grade ceramics, which can beused in the present disclosure; FIG 14 A schematic cross-sectional perspective view of a first exemplary embodiment ofthe construction of a lightweight composite armor according to the present disclosure; FIG 15 A schematic cross-sectional perspective view of a second exemplary embodimentof the construction of a lightweight composite armor according to the present disclosure;FIG 16 A schematic cross-sectional perspective view of a third exemplary embodiment ofthe construction of a lightweight composite armor according to the present disclosure.

FIG 17 A schematic cross-sectional perspective view of a fourth exemplary embodimentof the construction of a lightweight composite armor according to the present disclosure;FIG 18 A schematic cross-sectional perspective view of a fifth exemplary embodiment ofthe construction of a lightweight composite armor according to the present disclosure; FIG 19 A schematic cross-sectional perspective front view of a sixth exemplaryembodiment of the construction of a lightweight composite armor according to thepresent disclosure; FIG 20 A schematic cross-sectional perspective back view of a sixth exemplaryembodiment of the construction of a lightweight composite armor according to thepresent disclosure;

With reference now to the figures of certain preferred embodiments in detail, it isstressed that the particulars shown are by way of example and for purposes of illustrativediscussion of the preferred embodiments of the present disclosure only, and arepresented in the cause of providing what is believed to be the most useful and readilyunderstood description of the principles and conceptual aspects of the presentdisclosure. In this regard, no attempt is made to show structural details of the presentdisclosure in more detail than is necessary for a fundamental understanding of thepresent disclosure. Only a few examples of how the composite armor structure can beconfigured in different shapes, sizes and thicknesses, and in different configurations, andhow the encapsulated armor-grade material can be configured and arranged in different spacious patterns are shown here.

The description taken with the drawings makes it apparent to those skilled in the art howthe several forms of the present disclosure can be embodied in practice.

One preferred solution is feasible where "a one-piece entity" of the composite materialencapsulate and somehow pre-stresses the armor-grade material, and another preferredsolution is feasible where a plurality of pieces of the composite material act together topre-stress the armor-grade material (such as by sandwiching it and squeezing it).

The first solution where the fiber reinforced cementitious composite materialencapsulates some other material is feasible by embedding armor-grade materials intoflowing not yet cured fiber reinforced cementious composite materal. Different examplesare shown in figure 14 to figure 16.

The second solution is feasible with previous casted and cured fiber reinforcedcementitious composite material encapsulating, sandwiching and squeezing armor-gradematerial. An example is shown in figure 17 to figure 20.

Referring to FIG 14, a cross-sectional perspective view of an embodiment of a ballisticstructure showing an encapsulant (1) confining an armor-grade material (2) for absorbingand limiting the transfer of impact energy from a ballistic threat, such as a kinetic energyprojectile.

The encapsulant is fabricated from a fiber reinforced cementitious composite having alower tensile strength than the tensile strength of the encapsulated material.

The encapsulated material is preferably comprised of armor-grade ceramic material. Butit must be understood that the principles of the present disclosure are applicable to anyarmor-grade materials such as glass, glass-ceramics, sintered refractory material, otherarmor-grade materials having high hardness or mixtures thereof.

The encapsulating structural layer (1) is configured to encapsulate the armor-gradematerial (2). In one embodiment, the encapsulating structural layer (1) pre-stresses theencapsulated material (2). Without pre-stress, at least simple mechanical contact orbinding is needed.

Referring to FIG 15, in another embodiment of the present disclosure, the encapsulatinglayer of fiber reinforced cementitious composite and the encapsulated ceramic materialcan be layered in a laminated structure, where the alternating layers of ceramic materialand fiber reinforced cementitious composite are composed as shown.

Referring to FIG 16, in another embodiment of the present disclosure, the encapsulated ceramic material are formed as spheres and are all held and fully encased with anencapsulating layer of fiber reinforced cementitious composite. The gaps betweenadjacent ceramic spheres are made to be small enough for avoiding the creation of aweak point and stopping an anticipated projectile between the spheres.

Referring to FIG 17, in another embodiment of the present disclosure, the layer of fiberreinforced cementitious composite (1) encapsulates multiple tiles of armor-gradematerial (2); in this example four tiles are used.

Referring to FIG 18, in another embodiment of the present disclosure, two separatelayers of fiber reinforced cementitious composite (2) and (3) encapsulates tiles of armor-grade material (4). Fastening elements (1) are used to obtain that the encapsulationlayers of fiber reinforced cementitious composite provides pre-stress to the encapsulatedarmor-grade material.

Referring to FIG 19, in another embodiment of the present disclosure, two separatelayers of fiber reinforced cementitious composite (1) and (2) and an additional layer offiber reinforced cementitious composite (3) encapsulates armor-grade material.Fastening elements (4) are used to obtain that the encapsulation layers of fiberreinforced cementitious composite (1) and (2) provides pre-stress to the encapsulatedarmor-grade material. The additional layer of fiber reinforced cementitious composite (3)are configured to provide pre-stress to the encapsulated armor-grade material.

Referring to FIG 20, is the same embodiment of the present disclosure as referred to infigure 19, but is here shown in a cross-sectional perspective back view.

In any of the above embodiments of the present disclosure shown in FIG 14 to FIG 20,the thickness of the encapsulating layer can be varied as well as the dimensions of theencapsulated material can be varied.

Often panels of the present disclosure will be generally flat and with generally uniformthickness. For the purpose of constructing the panel, the front face is that which will facethe direction from which the ballistic impact is expected, and the other is the back face.Likewise, the overall dimensions and the overall shape of the panels of the presentdisclosure will be determined by end user requirements, such as the impact conditionswhich they are required to resist, and the size and/or area of the object which the panelor an assembly of the panels is required to protect. For more specialized end userrequirements, a panel can be shaped in mostly any form of curvature. Whatever itsoverall 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 itsthickness.

The shapes shown in FIG 14 to FIG 20 are by way of example only. Other polygonalshapes can be used, such as cylinders and special shaped pellets. In addition, the shapeof the tiles shown in FIG 14 and FIG 15 need not be a regular geometric shape. The tilecan have any shape needed for a particular application, such as triangles, squares,rectangles, hexagons or combinations of polygons thereof, which nest to give completecoverage in one layer. In another configuration, polygon shaped tiles or combinationsthereof are to be used in a first layer, and any gaps in the first layer are protected by asecond layer to obtain complete coverage. It is most often desired to achieve completecoverage in one layer. Often used tile shapes used for this are square and hexagonal.While the present disclosure has been described with reference to certain preferredembodiments, numerous changes, alterations and modifications to the describedembodiments are possible without departing from the spirit and scope of the presentdisclosure, as defined in the appended claims and equivalents thereof.

Claims (8)

1. A structure of composite armor for military and civilian use to absorb and reduce the impact of impact energy from weapon threats with kinetic energy and from chemical energy weapon threats, said armor structure of composite armor comprising: glass, glass-ceramic and / or sintered refractory. A structure of composite armor according to claim 1, wherein said fiber reinforced cementitious composite material comprises fiber reinforced concretes, very high strength concrete, hybrid fiber reinforced cementitious composites, ductile fiber reinforced cementitious composites, ultra ductile fiber reinforced cementitious composites, high performance strain hardening cementitious composites, ultra high performance-strain hardening cementitious composites, engineered cementitious composites and / or high strength-high ductility concrete. A structure of composite armor according to claim 1, wherein said ceramic armor grade material is selected from one or more of the following three groups: a) ceramic oxides including alumina, zirconia, silica, aluminum silicate, magnesium oxide, aluminum titanate and other metal oxide base materials (b) Non-oxide ceramics, including carbides, borides, nitrides and silicides; and (c) composite ceramics including particle-reinforced ceramics, fiber-reinforced ceramics, ceramic-metal composite materials, and nano-ceramics. A structure of composite armor according to claim 1, wherein said armor quality material is arranged in a single pattern, in various shapes, sizes and thicknesses, and in various configurations and combinations. A composite armor structure according to claim 1, wherein said fiber reinforced cementitious composite material is configured to bias said armor grade material. A structure of composite armor according to claim 1, wherein said armor grade material is ground and / or ground. A composite armor structure according to claim 1, wherein said fiber-reinforced cementitious composite material and said armor grade material are attached to each other. A structure of composite armor according to claim 7, wherein said fiber-reinforced cementitious composite material is attached to said armor grade material by coating, bonding and / or fastening elements.