J.Am. Ceram.Soc.,901005-1018(2007) DO:10.l111551-2916.2007.01515.x c 2007 The American Ceramic Society urna Dynamic Fracture of Ceramics in Armor Applications Weinong w. chent AAE and MSE Schools, Purdue University, West Lafayette, Indiana 47907-2023 U.S. Army Research Office, Research Triangle Park, North Carolina 27709-2211 Bo Song and Xu Nie AAE and MSE Schools, Purdue University, West Lafayette, Indiana 47907-2023 Ceramic materials have been extensively used in armor appli of possibilities of defeating the threats. The fracture and pulver cations for both personnel and vehicle protection. As the types of ization of the ceramic material are also effective ways to dissi- threats have diversified recently, e.g., improvised explosive de- pate part of the kinetic energy(ke) generated by the projectile vices and explosively formed projectiles, a proper set of ceramic Further, the flow motion of the hard ceramic fragments around material selection criteria is needed to design and optimize cor- the projectile erodes the tip or even the entire length of the pro- responding mitigating structures. However, the dynamic frac- jectile, which further dissipates energy and spreads the impact ture and failure behavior of engineering ceramics is still not well area. This layered armor concept has been used in many vehicle understood Using examples of thin ceramic plates and confined and personnel armor designs. Because of their critical roles in thick ceramics subjected to kinetic energy projectile impact, this layered armors, ceramics have attracted considerable attention article provides a brief summary on the current understanding of for the determination of their mechanical responses and failure dynamic failure processes of ceramics under dynamic penetra- behavior under impact and thermal loadings. Among different tion loading conditions. Laboratory examination of dynamic structural ceramics, some types of oxide ceramics(mostly alumi- fracture of ceramics is conducted using split Hopkinson bars na ceramics) and non-oxide ceramics(mostly carbides, nitrides, with various loading rates, stress states, and loading histories. borides) are commonly used for ballistic protection. A signifi- cant improvement in the cost/performance ratio of silicon carbide ceramics has increased their popularity in recent armor applica- tions relative to more established materials such as alumin The development of armor systems depends on the threats to hen faster and harder hostile threats are encountere be defeated. The anti-armor threats have become more diversi- adding a ceramic plate on top of a metal plate has been fied recently. For example, improvised explosive devices (Ied known to enhance significantly the ballistic protection over the ing both blast and fragment loading on armor systems from monolithic metal armor since the 1960s and 1970s. A ceramic close range and pose significant challenges in protection con- plate on a woven/roven backing also exhibited similar ballistic cepts. An effective armor design is an optimized solution to enhancement on light armors for aircraft protection. Upon im- op a specific threat within the design constrains such as area pact, the ceramic plate surrounding the impact area may be density, cost, and schedule. To achieve high efficiency in armor ractured, pulverized, and ejected depending on different impact development, it is important to have the capability for predicting onditions. However, the dynamic failure processes of the cer- the armor performance before the product is made. With the aid amics effectively extend the time of impact loading and spread of high-speed computers and design software, the possibility the impact load over a larger area on the backing structures. developing such predictive capabilities has become realisticg- Both the loading time extension and the impact area increase However, realistic numerical simulations require accurate con- reduce the stress on the backing structures, thus enhancing the stitutive and failure models for all the materials involved in the impact event. This in turn requires reliable experimental results D. Green-contributing editor and the understanding of the deformation and failure processes of these materials under such extreme loading conditions. How ever, this desired understanding on the impact response of ma- terials is far from comprehensively developed. Many research december 4. 2006. partially supported by the U.S. Army Research Office under Grant No. efforts have been invested to develop a better understanding of different aspects in ceramic fracture and failure under dynamic Author to whom correspondence should be addressed. e-mail: when Feature
Dynamic Fracture of Ceramics in Armor Applications Weinong W. Chenw AAE and MSE Schools, Purdue University, West Lafayette, Indiana 47907-2023 A. M. Rajendran U.S. Army Research Office, Research Triangle Park, North Carolina 27709-2211 Bo Song and Xu Nie AAE and MSE Schools, Purdue University, West Lafayette, Indiana 47907-2023 Ceramic materials have been extensively used in armor applications for both personnel and vehicle protection. As the types of threats have diversified recently, e.g., improvised explosive devices and explosively formed projectiles, a proper set of ceramic material selection criteria is needed to design and optimize corresponding mitigating structures. However, the dynamic fracture and failure behavior of engineering ceramics is still not well understood. Using examples of thin ceramic plates and confined thick ceramics subjected to kinetic energy projectile impact, this article provides a brief summary on the current understanding of dynamic failure processes of ceramics under dynamic penetration loading conditions. Laboratory examination of dynamic fracture of ceramics is conducted using split Hopkinson bars with various loading rates, stress states, and loading histories. I. Introduction WHEN faster and harder hostile threats are encountered, adding a ceramic plate on top of a metal plate has been known to enhance significantly the ballistic protection over the monolithic metal armor since the 1960s and 1970s.1 A ceramic plate on a woven/roven backing also exhibited similar ballistic enhancement on light armors for aircraft protection.2 Upon impact, the ceramic plate surrounding the impact area may be fractured, pulverized, and ejected depending on different impact conditions. However, the dynamic failure processes of the ceramics effectively extend the time of impact loading and spread the impact load over a larger area on the backing structures. Both the loading time extension and the impact area increase reduce the stress on the backing structures, thus enhancing the possibilities of defeating the threats. The fracture and pulverization of the ceramic material are also effective ways to dissipate part of the kinetic energy (KE) generated by the projectile. Further, the flow motion of the hard ceramic fragments around the projectile erodes the tip or even the entire length of the projectile, which further dissipates energy and spreads the impact area. This layered armor concept has been used in many vehicle and personnel armor designs.3,4 Because of their critical roles in layered armors, ceramics have attracted considerable attention for the determination of their mechanical responses and failure behavior under impact and thermal loadings.4–8 Among different structural ceramics, some types of oxide ceramics (mostly alumina ceramics) and non-oxide ceramics (mostly carbides, nitrides, borides) are commonly used for ballistic protection.8 A signifi- cant improvement in the cost/performance ratio of silicon carbide ceramics has increased their popularity in recent armor applications relative to more established materials such as alumina. The development of armor systems depends on the threats to be defeated. The anti-armor threats have become more diversi- fied recently. For example, improvised explosive devices (IED) bring both blast and fragment loading on armor systems from close range and pose significant challenges in protection concepts.4 An effective armor design is an optimized solution to stop a specific threat within the design constrains such as area density, cost, and schedule. To achieve high efficiency in armor development, it is important to have the capability for predicting the armor performance before the product is made. With the aid of high-speed computers and design software, the possibility of developing such predictive capabilities has become realistic.9–11 However, realistic numerical simulations require accurate constitutive and failure models for all the materials involved in the impact event. This in turn requires reliable experimental results and the understanding of the deformation and failure processes of these materials under such extreme loading conditions. However, this desired understanding on the impact response of materials is far from comprehensively developed. Many research efforts have been invested to develop a better understanding of different aspects in ceramic fracture and failure under dynamic loading conditions. Feature D. Green—contributing editor This work was partially supported by the U.S. Army Research Office under Grant No. W911-05-1-0218 to Purdue University. w Author to whom correspondence should be addressed. e-mail: wchen@purdue.edu Manuscript No. 22256. Received September 17, 2006; approved December 4, 2006. Journal J. Am. Ceram. Soc., 90 [4] 1005–1018 (2007) DOI: 10.1111/j.1551-2916.2007.01515.x r 2007 The American Ceramic Society
In this paper, we use two typical ceramic armor summarize briefly the current understanding on the ceramic fracture and failure processes when subject to impact loading. he first case is the ceramic plate in a thin-layered armor syst Ceramic plate to resist impacts by hard objects such as armor-piercing bullets Partially damaged The other case is a confined thick ceramic target subjected to a long-rod penetration. Then, we present the dynamic fracture behavior of brittle materials, which is an important aspect in the processes of ceramic armor failure, under various conditions in loading rates. stress states, and loading histories. A recent effort in determining the dynamic fracture toughness for ceramics un- der valid testing conditions is also introduced. Most of the re- search on this topic described in this paper comes from various Back plate modified split Hopkinson bar experiments. We intend to present Fig 1. Illustration of a short projectile impacting a thin ceramic armor the main concepts and results on these aspects in an attempt to system. illustrate the physical mechanisms behind the failure phenome na, but not an exhaustive review on this subject. For example details related to failure waves, terminal velocities of dynamic the back surface to the impact surface. When the damaged acks, and spalling are not discussed in this paper Modeling zone reaches the projectile and the damaged area is comparable d simulation aspects are discussed only when they are directly to the projectile cross section, the ceramic tile fails. Before the related to the experimental results presented damaged zone reaches the projectile, the projectile is unable to penetrate. This time of non-penetration is known as dwell. If the projectile is short, this damage development time may be suffi- IL. Failure of Ceramics Under Impact ciently long, such that the projectile has completely deformed Upon impact by a KE projectile, the failure process onditierial impact surface. In this case, no penetration occurs. which is in a ceramic plastically or shattered before the damaged zone reaches the properties, geometry, confinement, and interfacial called interface defeat. However. such dwell or interface defe To discuss the failure processes, we take two common ccurs only when the projectile impacting velocity is below cer here: a thin ceramic plate backed by a ductile substrate ain critical values. Detailed numerical analysis also reveals thick ceramic target confined by metal jackets. Thin-plate armor that the damaged zone is conical in shape, pointing to the pro- jectile. Therefore, even the ceramic tile is damaged; the dam- vehicle protection systems, are primarily for defeating projectiles aged or cracked material still piles in the shape of a spreading from small arms and machine guns. Confined ceramic packets, one, which helps to redistribute the impact load to a larger area seen mostly in heavy vehicle armors, are designed to stop heavy n the surface of the backing plate. However, once the projectile metal, long rod, KE projectiles. Multiple layers of either of these velocity is beyond the critical value, the ceramic tile will be per ystems are also seen in some vehicle-protection forated either by a longer projectile and or a higher striking velocity. The prediction of the penetration process and the crit ical impact velocity clearly depends on the accurate modeling of (1) Failure of Ceramic Tiles the material responses and failure under impact conditions, both a typical thin-layered system consists of a ceramic plate and a for the target and the projectile. From the target side, damage is metal or textile backing plate bound together with a thin adhe most likely formed by a series of dynamic fracture processes sive layer, as shown schematically in Fig. 1. The work reported under multiaxial loading conditions. n a series of documents by wilkins et al. 2-6 and Landringham and Casey is probably the earliest systematic description of the dynamic failure in the ceramic plate when impacted by a KE (2) Failure of Confined Thick Ceramics projectile. Using computer simulations, which became powerful When a ceramic armor is subjected to the impact of a long rod tools in the study of ceramic failure under impact loading in the Ke projectile, which is usually made of a heavy metal, the it has been shown that the failure initiated jectile confers high energy density on the impact area over a from the back side of the ceramic tile. Upon impact, high- much longer duration. The efficiency of these projectiles ceramic tile and in the projectile. In the ceramic tile, the com- ensively deformed or damaged, the remaining undamaged por pressive stresses propagate along the thickness. When the waves tion of the long rod will continue to carry on the penetration reach the back surface, which is in contact with the backing process. In this case, the primary defeat mechanism is errosion, 3 plate, portions of the waves are transmitted into the backing plate. The rest of the waves are reflected back into the ceramic which requires a much thicker ceramic layer than the tiles de- scribed in the previous section. Furthermore, Hauver et al. ile. The mechanical impedance of the backing plate is typically and Malaise et al25 found that confined ceramics were much less than that of the ceramic tile; due to the intrinsically lower more efficient in defeating the penetrator. When the ceramic is wave speeds and the necessary low density, the reflected stresses confined, for example by a heat-shrunk metal cover, it takes become tensile As ceramic materials are typically much weaker higher impact load and a longer time to shatter the cerami under tensile loading as compared with their compressive re- Even after the ceramic is shattered the broken ceramic rubble is sponses, failure initiates where the tensile stress exceeds a critical still contained inside the ductile metal cover and can furthe alue. Further upon the point impact load, the target ma erode the projectile during the later stages of the penetration erial just ahead of the projectile deforms more in the out-of process. Both the extended time to shatter and the ability to lane direction than the material away from the impact zone due rode the penetrator contribute to the possible defeat of the to inertia effects. This causes the ceramic tile to bend which also long-rod generates a high tensile stress on the back surface of the tile just As shown schematically in Fig. 2, when a confined thick piece ahead of the projectile. However, this bending effect is unlikely of ceramic is impacted by a long rod, the ceramic materia to be a dominant factor wilkins et al. o observed that initial xtensively cracked as observed by Shockey and marchand racks in ceramic tiles do not significantly affect the penetration Even though the impact event occurs at very high rates of esistance of the target as long as the projectile does not impact deformation, the crack pattern resembles those observed in y close to the cracks. Numerical simulations reveal that brittle materials when subjected to a concentrated indentation time for the cracked or damaged zone to develop from load. If the long rod penetrates into the ceramic target, the
In this paper, we use two typical ceramic armor cases to summarize briefly the current understanding on the ceramic fracture and failure processes when subject to impact loading. The first case is the ceramic plate in a thin-layered armor system to resist impacts by hard objects such as armor-piercing bullets. The other case is a confined thick ceramic target subjected to a long-rod penetration. Then, we present the dynamic fracture behavior of brittle materials, which is an important aspect in the processes of ceramic armor failure, under various conditions in loading rates, stress states, and loading histories. A recent effort in determining the dynamic fracture toughness for ceramics under valid testing conditions is also introduced. Most of the research on this topic described in this paper comes from various modified split Hopkinson bar experiments. We intend to present the main concepts and results on these aspects in an attempt to illustrate the physical mechanisms behind the failure phenomena, but not an exhaustive review on this subject. For example, details related to failure waves, terminal velocities of dynamic cracks, and spalling are not discussed in this paper. Modeling and simulation aspects are discussed only when they are directly related to the experimental results presented. II. Failure of Ceramics Under Impact Upon impact by a KE projectile, the failure process in a ceramic target depends on many parameters that describe the material properties, geometry, confinement, and interfacial conditions. To discuss the failure processes, we take two common examples here: a thin ceramic plate backed by a ductile substrate and a thick ceramic target confined by metal jackets. Thin-plate armor systems, commonly seen in body armors and aircraft or light vehicle protection systems, are primarily for defeating projectiles from small arms and machine guns. Confined ceramic packets, seen mostly in heavy vehicle armors, are designed to stop heavy metal, long rod, KE projectiles. Multiple layers of either of these systems are also seen in some vehicle-protection systems. (1) Failure of Ceramic Tiles A typical thin-layered system consists of a ceramic plate and a metal or textile backing plate bound together with a thin adhesive layer, as shown schematically in Fig. 1. The work reported in a series of documents by Wilkins et al. 12–16 and Landringham and Casey17 is probably the earliest systematic description of the dynamic failure in the ceramic plate when impacted by a KE projectile. Using computer simulations, which became powerful tools in the study of ceramic failure under impact loading in the years up to now,18–20 it has been shown that the failure initiated from the back side of the ceramic tile. Upon impact, highamplitude compressive stress pulses are generated both in the ceramic tile and in the projectile. In the ceramic tile, the compressive stresses propagate along the thickness. When the waves reach the back surface, which is in contact with the backing plate, portions of the waves are transmitted into the backing plate. The rest of the waves are reflected back into the ceramic tile. The mechanical impedance of the backing plate is typically less than that of the ceramic tile; due to the intrinsically lower wave speeds and the necessary low density, the reflected stresses become tensile. As ceramic materials are typically much weaker under tensile loading as compared with their compressive responses, failure initiates where the tensile stress exceeds a critical value. Furthermore, upon the point impact load, the target material just ahead of the projectile deforms more in the out-ofplane direction than the material away from the impact zone due to inertia effects. This causes the ceramic tile to bend, which also generates a high tensile stress on the back surface of the tile just ahead of the projectile. However, this bending effect is unlikely to be a dominant factor. Wilkins et al. 16 observed that initial cracks in ceramic tiles do not significantly affect the penetration resistance of the target as long as the projectile does not impact on or very close to the cracks. Numerical simulations reveal that it takes time for the cracked or damaged zone to develop from the back surface to the impact surface.20 When the damaged zone reaches the projectile and the damaged area is comparable to the projectile cross section, the ceramic tile fails. Before the damaged zone reaches the projectile, the projectile is unable to penetrate. This time of non-penetration is known as dwell. If the projectile is short, this damage development time may be suffi- ciently long, such that the projectile has completely deformed plastically or shattered before the damaged zone reaches the impact surface. In this case, no penetration occurs, which is called interface defeat. However, such dwell or interface defeat occurs only when the projectile impacting velocity is below certain critical values.20,21 Detailed numerical analysis also reveals that the damaged zone is conical in shape, pointing to the projectile.20 Therefore, even the ceramic tile is damaged; the damaged or cracked material still piles in the shape of a spreading cone, which helps to redistribute the impact load to a larger area on the surface of the backing plate. However, once the projectile velocity is beyond the critical value, the ceramic tile will be perforated either by a longer projectile and/or a higher striking velocity. The prediction of the penetration process and the critical impact velocity clearly depends on the accurate modeling of the material responses and failure under impact conditions, both for the target and the projectile. From the target side, damage is most likely formed by a series of dynamic fracture processes under multiaxial loading conditions. (2) Failure of Confined Thick Ceramics When a ceramic armor is subjected to the impact of a long rod KE projectile, which is usually made of a heavy metal, the projectile confers high energy density on the impact area over a much longer duration. The efficiency of these projectiles comes from the fact that even if the front end of the projectile is extensively deformed or damaged, the remaining undamaged portion of the long rod will continue to carry on the penetration process. In this case, the primary defeat mechanism is errosion,3 which requires a much thicker ceramic layer than the tiles described in the previous section. Furthermore, Hauver et al. 22–24 and Malaise et al. 25 found that confined ceramics were much more efficient in defeating the penetrator. When the ceramic is confined, for example by a heat-shrunk metal cover, it takes a higher impact load and a longer time to shatter the ceramic. Even after the ceramic is shattered, the broken ceramic rubble is still contained inside the ductile metal cover and can further erode the projectile during the later stages of the penetration process. Both the extended time to shatter and the ability to erode the penetrator contribute to the possible defeat of the long-rod projectile. As shown schematically in Fig. 2, when a confined thick piece of ceramic is impacted by a long rod, the ceramic material is extensively cracked as observed by Shockey and Marchand.26 Even though the impact event occurs at very high rates of deformation, the crack pattern resembles those observed in brittle materials when subjected to a concentrated indentation load.27,28 If the long rod penetrates into the ceramic target, the Partially damaged Failed Projectile Ceramic plate Back plate Fig. 1. Illustration of a short projectile impacting a thin ceramic armor system. 1006 Journal of the American Ceramic Society—Chen et al. Vol. 90, No. 4
April 2007 Fracture of Ceramics in Armor racture of Ceramics Under Uniaxial Ceramic Target letal confinement Dynamic compressive stress-strain responses of ceramics have been studied extensively. Split Hopkinson pressure bars(SHPB are commonly used tools to generate families of stress-strain curves at controlled strain rates. SHPB, orginally developed by Kolsky, has been modified to determine the dynamic consti- Comminuted material tutive behaviors of a variety of brittle materials including con crete and ceramics. The details of SHPB and its working inciple are well described. However, the focus of these ex perimental investigations is typically on the dynamic stress- strain response, which is another important aspect of ceramic pact response, rather than on dynamic fracture behavior. Re- cently, with improved high-speed imaging systems, dynami fracture processes are more accessible by diagnostic instrument Paliwal et al. imaged the dynamic fracture initiation and prop- Fig. 2. Illustration of a long rod penetrating a thick and confined ation inside a transparent ceramic specimen loaded using an ceramic armor system. he shPb experiments described below were on a silicon carbide(Sic-N) ceramic with a modified SHPb us- ceramic material must make room for the long rod to enter. Conceptually, the material in front of the tip of the projectile SHPB setup(Fig 3)used in this research consisted of a must be compressed by a very high pressure. 8. g T he hr tress. bars, with a density of 8100 kg/m, Young,s modulus of 200 sure, high shear loading turns the ceramic into an extensively GPa, and an elastic bar wave speed of 4969 m/s The bars had a cracked, but still interlocked state commonly known as a com- common diameter of 19.0 mm. The incident and transmission into the ceramic target, fine ceramic fragments ahead of the from strain gauges mounted on the bars were recorded using a penetrator flow radially around the nose of penetrator and are high-speed digital storage oscilloscope through differential pre- then ejected backwards along the shank of penetrator, and thus erode the penetrator. As the penetration process proceeds To minimize the stress concentrations in the brittle specimen the rapidly flowing ceramic fragments continue to erode the due to grip rigidity and alignment, two modifications were i penetrator until the penetrator disappears or the ceramic is per- troduced to the SHPB's testing section, as shown schematically forated. This penetration process will leave a tunnel with a in Fig 3. To prevent the ceramic specimen from indenting into diameter larger than the projectile diameter. The comminuted bar-end faces and thus causing stress concentrations along spec- zone in the ceramic target is rather concentrated around the imen edges, a pair of laterally confined, high-stiffness tungsten projectile. The resistance to comminution has been identified as carbide platens(0 12.7 mm x 6.35 mm) with mechanical im- significant factor governing the ballistic performance of a amic material.37 pedance matching with the bars were placed between the spec- imen and the bars. Ceramic specimens are very sensitive to he descriptions of these two ceramic armor penetration cas stress concentrations due to misalignment of the loadin es indicate that the ceramic failure processes are very compli To correct any slight misalignment, a simplified universal joint cated. The stress state varies from highly confined just ahead of consisted of a pair of hardened tool steel disks with a spherical the projectile to almost stress free on the surfaces away from the joining surface in between was placed between the tungsten striking point. The front surface near the impact area may spall bide platen and the transmission bar(Fig. 3). The disks have off by the tensile stresses during the wave reflections in the tar- the same diameter as the bars, thus introducing no impedance get, forming impact craters. The stress amplitude just ahead of mismatch to wave propagation through the disks. High-vacuum grease was applied on the curved contacting surfaces to minim- ceramic may be possible. The stress on the surface away from ize friction the impact zone is associated only with elastic waves. The rates The surface fracture and failure process in the ceramic spec- of deformation in the ceramic material corresponding to the imen was recorded by a digital high-speed camera synchronized the penetration process, the portion of the intact piece of ce- of the incident bar to generate incident pulses olli end various loading conditions also cover a wide spectrum. During with the SHPB setup. a pulse shaper was used at the uted rubbles that are subsequently rejected back toward the response at constant strain rate projectile direction. The rest of the ceramic target is extensively The dimensions of Sic-N cylindrical specimen used in this cracked. Thus, dynamic fracture ses are associated with esearch were 6.35 mm in diameter and 6. 35 mm in length. The all aspects of the target behavior during penetration. It SIC-N mens were core drilled from a 100 mm 100 ortant to develop a fundamental understanding of the dynamic mm x 50 mm block supplied by U.S. Army Research Labora- fracture behavior of ceramic materials. However, the research tory and then cut to 6.35 mm in length, with both ends precision accumulation on this aspect is still limited and so is the under- ground to within 0.005 mm perpendicularity from the central anding. For example, the processes of comminution of ceram- axis base. The cylindrical surface from core drilling was then ics under a high pressure at high rates are still not well ground to 6.350 mm diameter within 0.005 mm cylindricity. understood. In the following sections, we summarize some of We show two experiments under identical loading conditions the experimental efforts that were designed to develop a better One experiment was performed on a nearly perfect cylindrical understanding of the dynamic fracture behavior of ceramics and pecimen, and the other on a specimen with a small pre-existing the behavior of damaged ceramics. Under uniaxial tensi urface defect. Figure 4 shows the sequential images of the fra here the stress state is one-dimensional (1-D)stress or I-D ture and failure processes of the first specimen. Figure 5 shows strain, the resultant crack runs perpendicular to the tensile load the corresponding positions on the loading history where the g. which will not be discussed here. The stress states in the mages shown in Fig 4 were taken. The loading was provided by cases described below are more complicated of deformation are mostly limited to those ack aiehabpeh with sabes a 203-mm long striker impacting on the incident bar through a composite pulse shaper consisting of a 6.35 mm x 1.57 mm Hopkinson bars annealed copper disk and a 10.31 mm x 0.51 mm mild steel
ceramic material must make room for the long rod to enter. Conceptually, the material in front of the tip of the projectile must be compressed by a very high pressure. High shear stresses are also present in this high-pressure region.28,29 This high pressure, high shear loading turns the ceramic into an extensively cracked, but still interlocked state, commonly known as a comminuted state or a Mescall zone.26,30–35 As the striker penetrates into the ceramic target, fine ceramic fragments ahead of the penetrator flow radially around the nose of penetrator and are then ejected backwards along the shank of penetrator, and thus erode the penetrator.34,36 As the penetration process proceeds, the rapidly flowing ceramic fragments continue to erode the penetrator until the penetrator disappears or the ceramic is perforated. This penetration process will leave a tunnel with a diameter larger than the projectile diameter. The comminuted zone in the ceramic target is rather concentrated around the projectile. The resistance to comminution has been identified as a significant factor governing the ballistic performance of a ceramic material.37 The descriptions of these two ceramic armor penetration cases indicate that the ceramic failure processes are very complicated. The stress state varies from highly confined just ahead of the projectile to almost stress free on the surfaces away from the striking point. The front surface near the impact area may spall off by the tensile stresses during the wave reflections in the target, forming impact craters. The stress amplitude just ahead of the projectile is so high that a brittle–ductile transition in the ceramic may be possible. The stress on the surface away from the impact zone is associated only with elastic waves. The rates of deformation in the ceramic material corresponding to the various loading conditions also cover a wide spectrum. During the penetration process, the portion of the intact piece of ceramic where the projectile passes through turns into comminuted rubbles that are subsequently rejected back toward the projectile direction. The rest of the ceramic target is extensively cracked. Thus, dynamic fracture processes are associated with all aspects of the target behavior during penetration. It is important to develop a fundamental understanding of the dynamic fracture behavior of ceramic materials. However, the research accumulation on this aspect is still limited and so is the understanding. For example, the processes of comminution of ceramics under a high pressure at high rates are still not well understood. In the following sections, we summarize some of the experimental efforts that were designed to develop a better understanding of the dynamic fracture behavior of ceramics and the behavior of damaged ceramics. Under uniaxial tension, where the stress state is one-dimensional (1-D) stress or 1-D strain, the resultant crack runs perpendicular to the tensile loading, which will not be discussed here. The stress states in the cases described below are more complicated, although the rates of deformation are mostly limited to those achievable with split Hopkinson bars. III. Dynamic Fracture of Ceramics Under Uniaxial Compression Dynamic compressive stress–strain responses of ceramics have been studied extensively. Split Hopkinson pressure bars (SHPB) are commonly used tools to generate families of stress–strain curves at controlled strain rates. SHPB, originally developed by Kolsky,38 has been modified to determine the dynamic constitutive behaviors of a variety of brittle materials including concrete and ceramics.39–50 The details of SHPB and its working principle are well described.51 However, the focus of these experimental investigations is typically on the dynamic stress– strain response, which is another important aspect of ceramic impact response, rather than on dynamic fracture behavior. Recently, with improved high-speed imaging systems, dynamic fracture processes are more accessible by diagnostic instruments. Paliwal et al. 52 imaged the dynamic fracture initiation and propagation inside a transparent ceramic specimen loaded using an SHPB. The SHPB experiments described below were conducted on a silicon carbide (SiC–N) ceramic with a modified SHPB using ramp-loading pulses. The SHPB setup (Fig. 3) used in this research consisted of a C-350 maraging steel elastic striker, incident and transmission bars, with a density of 8100 kg/m3 , Young’s modulus of 200 GPa, and an elastic bar wave speed of 4969 m/s. The bars had a common diameter of 19.0 mm. The incident and transmission bars were 2835 and 1371 mm long, respectively. The signals from strain gauges mounted on the bars were recorded using a high-speed digital storage oscilloscope through differential preamplifiers. To minimize the stress concentrations in the brittle specimen due to grip rigidity and alignment, two modifications were introduced to the SHPB’s testing section, as shown schematically in Fig. 3. To prevent the ceramic specimen from indenting into bar-end faces and thus causing stress concentrations along specimen edges, a pair of laterally confined, high-stiffness tungsten carbide platens (+12.7 mm 6.35 mm) with mechanical impedance matching with the bars were placed between the specimen and the bars.53,54 Ceramic specimens are very sensitive to stress concentrations due to misalignment of the loading axis. To correct any slight misalignment, a simplified universal joint consisted of a pair of hardened tool steel disks with a spherical joining surface in between was placed between the tungsten carbide platen and the transmission bar (Fig. 3).55 The disks have the same diameter as the bars, thus introducing no impedance mismatch to wave propagation through the disks. High-vacuum grease was applied on the curved contacting surfaces to minimize friction. The surface fracture and failure process in the ceramic specimen was recorded by a digital high-speed camera synchronized with the SHPB setup. A pulse shaper was used at the impact end of the incident bar to generate incident pulses of linear ramps, which was necessary to deform the specimen with a nearly linear response at constant strain rates.56,57 The dimensions of SiC–N cylindrical specimen used in this research were 6.35 mm in diameter and 6.35 mm in length. The SiC–N specimens were core drilled from a 100 mm 100 mm 50 mm block supplied by U.S. Army Research Laboratory and then cut to 6.35 mm in length, with both ends precision ground to within 0.005 mm perpendicularity from the central axis base. The cylindrical surface from core drilling was then ground to 6.350 mm diameter within 0.005 mm cylindricity. We show two experiments under identical loading conditions. One experiment was performed on a nearly perfect cylindrical specimen, and the other on a specimen with a small pre-existing surface defect. Figure 4 shows the sequential images of the fracture and failure processes of the first specimen. Figure 5 shows the corresponding positions on the loading history where the images shown in Fig. 4 were taken. The loading was provided by a 203-mm long striker impacting on the incident bar through a composite pulse shaper consisting of a +6.35 mm 1.57 mm annealed copper disk and a +10.31 mm 0.51 mm mild steel Ceramic Target Projectile Ejecta Metal confinement Comminuted material Fig. 2. Illustration of a long rod penetrating a thick and confined ceramic armor system. April 2007 Dynamic Fracture of Ceramics in Armor 1007
Journal of the American Ceramic Society--Chen et al. Vol. 90. No 4 computer tigger signal striker fse shape stran gages incident bar transmission bar 松)— stainless steel s Wheatstone bridge Wheatstone bridge universal joint digital osciloscope pre-amplifier specimen assembly Fig 3. Schematic illustration of the experimental setup for dynamic fracture(one-dimensional stress compression) disk. The loading rate of the resulting ramp pulse in the incident the specimen was loaded very evenly because of the precautions bar was 9.76x 10 MPa/s. The first image shows the SiC-N took in the ent design and setup. However, the ejec- specimen sandwiched between two platens in the testing section ion of material also indicates the possible limitation of using of an SHPB. The second image was taken 40 us after the arrival cylindrical specimens to obtain compressive material properti of the front tip of the ramp loading pulse, when the load was of ceramics. At 75 us into the loading, surface cracks just be- only about 10% of the eventual failure load (position I in came visible as shown in the third image. The overall compre Fig 5). Even at this low loading level, fine powder was noticed sive loading reached about 83% of the eventual peak stress to eject from the edges of both ends of the cylindrical specimen. (Fig. 5). The cracks were initiated from the edges and propa- The ejected powder is believed to be the fine particles created by gated approximately along the specimen axial direction. Such the concentrated stresses at the specimen edges, mixed with cracks were populated very quickly. The next image was taken 5 some lubricant. Even though the overall compressive stress was us later, and many cracks had become visible on the cylindrical very low, the local concentrated and multi-axial stresses were surface. Fine particles were ejected in the radial directions from apparently sufficient enough to pulverize the ceramic materials the middle of the specimen, in addition to those ejected from the at the edges. The even pattern of the ejection also verifies that edges. The specimen diameter increased slightly. Another 5 us Unit: microsecond T=0 T T=80 T=85 T=90 Fig 4. Dynamic fracture and failure process of a SiC-N specimen under uniaxial compression
disk. The loading rate of the resulting ramp pulse in the incident bar was 9.76 106 MPa/s. The first image shows the SiC–N specimen sandwiched between two platens in the testing section of an SHPB. The second image was taken 40 ms after the arrival of the front tip of the ramp loading pulse, when the load was only about 10% of the eventual failure load (position 1 in Fig. 5). Even at this low loading level, fine powder was noticed to eject from the edges of both ends of the cylindrical specimen. The ejected powder is believed to be the fine particles created by the concentrated stresses at the specimen edges, mixed with some lubricant. Even though the overall compressive stress was very low, the local concentrated and multi-axial stresses were apparently sufficient enough to pulverize the ceramic materials at the edges. The even pattern of the ejection also verifies that the specimen was loaded very evenly because of the precautions we took in the experiment design and setup. However, the ejection of material also indicates the possible limitation of using cylindrical specimens to obtain compressive material properties of ceramics. At 75 ms into the loading, surface cracks just became visible as shown in the third image. The overall compressive loading reached about 83% of the eventual peak stress (Fig. 5). The cracks were initiated from the edges and propagated approximately along the specimen axial direction. Such cracks were populated very quickly. The next image was taken 5 ms later, and many cracks had become visible on the cylindrical surface. Fine particles were ejected in the radial directions from the middle of the specimen, in addition to those ejected from the edges. The specimen diameter increased slightly. Another 5 ms Fig. 3. Schematic illustration of the experimental setup for dynamic fracture (one-dimensional stress compression). T = 0 T = 40 T = 75 T = 80 T = 85 T = 90 T = 95 T = 100 Unit: microsecond Fig. 4. Dynamic fracture and failure process of a SiC–N specimen under uniaxial compression. 1008 Journal of the American Ceramic Society—Chen et al. Vol. 90, No. 4
April 2007 Fracture of Ceramics in Arm the corresponding stress history in this spec The first was taken before the arrival of the loading pulse. At 15 the arrival of the loading pulse front, when the load in the imen was very low as marked by position I in Fig. 7, an inclined urface crack was already visible as circled in Fig. 6. As the 苏es specimens and the testing conditions were nominally the same, this inclined crack was considered to initiate from a pre-existing urface defect on the specimen. The inclined crack grew as the 3000 loading was increased, as shown in the next two images At 45 us into the loading, when the specimen stress was less than half of its peak level, the inclined crack tip turned into the axial direc- tion. This crack tip behavior is very similar to the well-docu- mented wing cracks observed under quasi-static loading condition Under the dynamic loading in this study, be fore the wing crack could extend through the specimen, other axial cracks were initiated and propagated in the axial direction as seen in the following images taken at 5-us intervals. The Time(microsecond) loading, when the sample had been extensively cracked, as can be seen in the images. Because of the small surface defect, the Fig. 5. Axial compressive stress history in the specimen corresponding ultimate strength of this specimen was much lower than the to the images in fig. 4 previous case(3.3 vs 5.3 GPa), which is an indication that small urface defects can drastically alter the specimen response, lead ing to a scattered nature of the ceramic failure data later, the specimen had been divided into thin axial columns by The images presented in Figs. 4 and 6 reveal some of the many axial cracks. Some of the columns started to colla fundamental aspects associated with the dynamic crack initi- both ends of the specimen. At this point, the axial cor ation and propagation in brittle materials under uniaxial com- tress reached its peak value(position 5 in Fig. 5). The pression. The cracks initiate from stress-concentrated features in became unstable as the load-bearing columns along the axial the specimen, where the features are the specimen edges in the direction became less and less as the columns further collapsed. case of Fig 4, and a surface defect and then edges in the case of Figure 6 shows the images of the fracture and failure pro- Fig. 6. Once the cracks are initiated, they eventually propagate cesses in a specimen with a surface defect under the identical roughly along the compressive loading axis. Before a dominant loading conditions as the previous experiment. Figure 7 shows crack can run through the specimen, many other cracks form 015 Fig. 6. Dynamic fracture and failure process of another SiC-N specimen under uniaxial compression
later, the specimen had been divided into thin axial columns by many axial cracks. Some of the columns started to collapse near both ends of the specimen. At this point, the axial compressive stress reached its peak value (position 5 in Fig. 5). The specimen became unstable as the load-bearing columns along the axial direction became less and less as the columns further collapsed. Figure 6 shows the images of the fracture and failure processes in a specimen with a surface defect under the identical loading conditions as the previous experiment. Figure 7 shows the corresponding stress history in this specimen. The first image was taken before the arrival of the loading pulse. At 15 ms after the arrival of the loading pulse front, when the load in the specimen was very low as marked by position 1 in Fig. 7, an inclined surface crack was already visible as circled in Fig. 6. As the specimens and the testing conditions were nominally the same, this inclined crack was considered to initiate from a pre-existing surface defect on the specimen. The inclined crack grew as the loading was increased, as shown in the next two images. At 45 ms into the loading, when the specimen stress was less than half of its peak level, the inclined crack tip turned into the axial direction. This crack tip behavior is very similar to the well-documented wing cracks observed under quasi-static loading conditions.58–60 Under the dynamic loading in this study, before the wing crack could extend through the specimen, other axial cracks were initiated and propagated in the axial direction, as seen in the following images taken at 5-ms intervals. The specimen became unstable at 65–70 ms after the beginning of loading, when the sample had been extensively cracked, as can be seen in the images. Because of the small surface defect, the ultimate strength of this specimen was much lower than the previous case (3.3 vs 5.3 GPa), which is an indication that small surface defects can drastically alter the specimen response, leading to a scattered nature of the ceramic failure data. The images presented in Figs. 4 and 6 reveal some of the fundamental aspects associated with the dynamic crack initiation and propagation in brittle materials under uniaxial compression. The cracks initiate from stress-concentrated features in the specimen, where the features are the specimen edges in the case of Fig. 4, and a surface defect and then edges in the case of Fig. 6. Once the cracks are initiated, they eventually propagate roughly along the compressive loading axis. Before a dominant crack can run through the specimen, many other cracks form Fig. 5. Axial compressive stress history in the specimen corresponding to the images in Fig. 4. 0 15 30 40 45 50 55 60 65 70 75 µs Fig. 6. Dynamic fracture and failure process of another SiC–N specimen under uniaxial compression. April 2007 Dynamic Fracture of Ceramics in Armor 1009