Impact of characteristic length and loading rate upon dynamic constitutive behavior and fracture process in alumina ceramics

Understanding the impact performance of ceramic materials requires accurate corresponding relationship between mechanical response and fracture behavior. In this study, constitutive behaviors of alumina ceramics were successfully determined via split-Hopkinson pressure bar (SHPB) system coupled with high-speed camera to track the deformation and failure process. Failure strength of alumina demonstrated a strong dependency on strain rate beyond a critical value (namely transition strain rate). Inelastic deformation in the dynamic stress-strain curves implied that degradation of modulus does occur. The incorporating such degradation (damage evolution) in modulus enabled a more accurate evaluation of transition strain rate as a function of characteristic length of specimen. On-line observation revealed that longitudinal cracks dominated the failure process of alumina with negligible interfacial friction. However, interfacial friction became significant with the decreased characteristic length, thus the inclined cracks dominated fracture in alumina. It was found that the effect of interfacial friction can be minimized by lowering the impact velocity to maintain the uniaxial loading status in SHPB loads. Finally, it is suggested that an aspect ratio of 1.0 for the specimen should be suitable for alumina due to its insensitivity to interfacial friction within the achievable strain rate.     

Strain-rate-dependent compressive and compression-shear response of an alumina ceramic

This paper assessed the microstructure and properties of CeramTec ALOTEC 98 SB alumina ceramic through microscopic characterization and mechanical experiments. The rate-dependent strength and failure response of an alumina ceramic were studied under both uniaxial compression and compression-shear loading. Under quasi-static uniaxial compression at rates of 10^?5 to 10³ s^?1, the strength had an average of 3393 ± 306 MPa, and at dynamic strain rates of 10² to 10³ s^?1, the strength ranged from 3763 to 4645 MPa. The CeramTec ALOTEC 98 SB alumina ceramic was found to have greater mechanical properties than other commercial alumina ceramics from the literature (i.e., AD-995). To monitor the strain field and the failure process of the alumina ceramic during testing, an ultra-high-speed camera coupled with digital image correlation (DIC) was used to visualize crack initiation and propagation processes, and obtain quantitative stress-strain information. A new data processing method was then proposed in this study to calculate the shear components for the compression-shear tests. Validation of the proposed method was confirmed by the shear strain obtained from the DIC analysis with the ultra-high-speed camera. Using the results obtained by the proposed model and the DIC analysis, new observations and understandings of failure mechanisms are obtained. (1) In compression-shear tests, the shear failure happens before complete failure, and shear behavior plays an important role during the failure process. (2) The equivalent peak stress (strength) of compression-shear test is smaller than the uniaxial compression one. (3) The directional cracks have weak influence on the compressive stiffness, but have a strong influence on the shear response.     

Influence of panel/back thickness on impact damage behavior of alumina/aluminum armors

Using modified SHPB device, damage behaviors of alumina/aluminum armors under impact load were studied. The influences of panel/back thickness on the target damage characteristics were investigated. The transmitted stress wave increased and the reflected stress wave decreased distinctly with the increase of back thickness, while the panel thickness variation had little influence on the stress wave propagation features. The vertex angle of ceramic inverted cone increased with the increase of back thickness and decrease of panel thickness, but the number of radial cracks reduced with the increase of back thickness and the decrease of panel thickness. Furthermore, the failure mechanism of the ceramic panels, including the cone and radial cracks formation mechanism was analyzed. A “composite beam” model has been established to estimate the local bending stress. The model calculation showed that the local bending stress is related to the panel thickness, back thickness and the panel/back moduli ratio.     

Dynamic response of ceramic shell for titanium investment casting under high strain-rate SHPB compression load

The mechanical performances of ceramic mold are crucial for the quality of casts in investment casting. However, most of the previous researches were focused on the quasi-static performance which is not sufficient for the accurate failure analysis of shell mold under complex stress state. In this investigation, dynamic mechanical behaviors of Al₂O₃-SiO₂ ceramic shell for investment casting have been studied using split Hopkinson pressure bar (SHPB) at high strain rates. Sand pack samples and pure slurry samples were considered for the testing in order to further understand the mechanism of fracture. Weibull approach was then applied to describe the strength distribution of ceramic shells. The dynamic increase factor (DIF) of compressive strength increased from 1.23 (863?s^?1) to 2.03 (1959?s^?1) indicating the high dependency of mechanical property to strain-rate. The cross-section and fracture surface were analyzed through scanning electron microscopy (SEM). The microstructural investigations showed that the crack propagation in the ceramic shell is mainly through the weak interface between sand particles and slurry region under quasi-static load. At high strain-rate, the crack propagation path is different which extends through the well sintered slurry region and even runs through the sand particles. The mechanism of crack propagation path is analyzed based on Griffith criterion. In addition, the feature of stress-strain curves indicates the layered structure which plays an important role in the process of fracture.     

Static and Dynamic Compressive Behavior of Aluminum Nitride under Moderate Confinement

An experimental technique for imposing lateral confinement on specimens subjected to dynamic or quasi-static uniaxial compression has been developed. Lateral confinement is provided by a shrink-fit metal sleeve installed on the lateral surface of a cylindrical ceramic specimen. Experiments using this technique were performed on sintered aluminum nitride (A1N). The results show that the failure mode changes from fragmentation by axial splitting under conditions of uniaxial stress to localized faulting under moderate lateral confinement. The compressive failure strength of the A1N increases with the increase of confinement pressure under both static and dynamic loading conditions. The effect of strain rate on the failure strength appears to be independent of the confinement pressure.     

Dynamic Compressive Response of Intact and Damaged AD995 Alumina

The dynamic compressive responses of AD995 alumina have been determined as a function of loading rates and damage levels. To determine the dynamic properties under loading conditions simulating those encountered in ceramic armors subjected to impact, a novel dynamic compressive experimental technique modified from a split Hopkinson pressure bar (SHPB) was employed to load the ceramic specimen by two consecutive stress pulses. The first pulse determines the dynamic response of the intact ceramic material and then crushes the specimen. Before the specimen disintegrates, the second pulse determines the dynamic compressive constitutive behavior of the ceramic rubble that is still interlocked. The results show that the compressive strengths of damaged ceramics are insensitive to strain rates once the damage level exceeds a critical value.     

Critical Appraisal of Limiting Strain Rates for Compression Testing of Ceramics in a Split Hopkinson Pressure Bar

The split Hopkinson pressure bar (SHPB) is being widely used to determine the dynamic compressive strength of ceramics and ceramic composites. However, extreme caution needs to be exercised while testing these highstrength ceramics at high strain rates. The highest strain rate at which ceramics can be tested using an SHPB without violating the underlying assumptions is found to be in the range of 2500–3000/s. It is also shown that at very high loading rates, dispersion in the transmitted pulse can lead to discrepancies in measuring the dynamic failure strength of ceramics.     

High strain rate compressive response of the Cf/SiC composite

Carbon fiber reinforced ceramic owns the properties of lightweight, high fracture toughness, excellent shock resistance, and thus overcomes ceramic's brittleness. The researches on the advanced structure of astronautics, marine have exclusively evaluated the quasi-static mechanical response of carbon fiber reinforced ceramics, while few investigations are available in the open literature regarding elastodynamics. This paper reports the dynamic compressive responses of a carbon fiber reinforced silicon carbide (C_f/SiC) composite (CFCMC) tested by the material test system 801 machine (MTS) and the split Hopkinson pressure bar (SHPB). These tests were to determine the rate dependent compression response and high strain rate failure mechanism of the C_f/SiC composite in in-plane and out-plane directions. The in-plane compressive strain rates are from 0.001 to 2200?s^?1, and that of the out-plane direction are from 0.001 to 2400?s^?1. The compressive stress-strain curves show the C_f/SiC composite has a property of strain rate sensitivity in both directions while under high strain rate loadings. Its compressive stiffness, compressive stress, and corresponding strain are also strain rate sensitive. The compressive damage morphologies after high strain rate impacting show different failure modes for each loading direction. This study provides knowledge about elastodynamics of fiber-reinforced ceramics and extends their design criterion with a reliable evaluation while applying in the scenario of loading high strain rate.     

Strength Evaluation of Piezoelectric Ceramics under Transient Thermal Environments

The strength of piezoelectric ceramics is analyzed for a plate suddenly exposed to an environmental medium of different temperatures. The admissible temperature jump the material can sustain is studied using the stress- and fracture-toughness-based failure criteria. The critical parameters governing the level of the transient thermal stress in piezoelectric ceramics are identified. Solutions are obtained for the maximum thermal shock that the plate can sustain without failure, under the conditions that (i) maximum local tensile stress equals the tensile strength of the ceramic, and (ii) maximum stress intensity factor for representative pre-existing cracks equals the fracture toughness of the ceramic.     

The microstructure,coefficient of thermal expansion and flexural strength of cordierite ceramics prepared from alumina with different particle sizes

Cordierite ceramics were produced from alumina with 5 and 0.65 μm particle sizes or AlOOH and talc, clays and feldspar, to determine the influence of the alumina particle size on the microstructure, coefficient of thermal expansion (CTE) and flexural strength (FS) of the ceramics. After sintering at 1300 °C the ceramics made from 5-μm-sized alumina consisted of cordierite, glass, quartz, mullite and alumina, and had the highest density, FS and CTE. The alumina grains act as inclusions, from which the trajectories of the cracks were deflected or terminated, which increases the FS and CTE. The ceramics from sub-micrometre-sized alumina or AlOOH contained a negligable amount and no alumina, respectively, together with other phases. This is reflected in the low CTE and FS. The cordierite ceramic with the lowest CTE of ∼2.0 × 10⁻⁶ K⁻¹ and a high FS of 100 MPa was prepared from the 0.65-μm-sized alumina particles.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Determination of the multiaxial failure criteria for alumina ceramics under tension–torsion test

Failure probability of ceramic components in multiaxial stress state can be predicted using the uniaxial test results (e.g. tension test, 4-point-bending test) when a suitable multiaxial criterion, which introduces the triaxiality of stress state, is known. In this article, tension–torsion tests were performed with alumina (Alsint 99.7) specimens from a standard manufacturer under two different load cases. Next experimental results were compared with the numerically calculated effective volume and effective surface values according to different multiaxial failure criteria. It was concluded that the specimens failed due to surface flaws and the normal stress criterion is the most appropriate criterion for the strength prediction of alumina ceramics under multiaxial stress state. Furthermore, it was shown that the Weibull modulus does not play a big role for the prediction of strength of alumina ceramics.     

Compressive properties of pristine and SiC-Te-added MgB2 powders,green compacts and spark-plasma-sintered bulks

Pristine and (SiC+Te)-added MgB₂ powders, green and spark plasma sintered (SPS) compacts were investigated from the viewpoint of quasi-static and dynamic (Split-Hopkinson Pressure Bar, SHPB) compressive mechanical properties The amount of the additive (SiC+Te) was selected to be the optimum one for maximization of the superconducting functional parameters. Pristine and added MgB₂ show very similar compressive parameters (tan δ, fracture strength, Vickers hardness, others) and fragment size in the SHPB test. However, for the bulk SPSed samples the ratio of intergranular to transgranular fracturing changes, the first one being stronger in the added sample. This is reflected in the quasi-static K_IC that is higher for the added sample. Despite this result, sintered samples are brittle and have roughly similar fragmentation behavior as for brittle engineering ceramics. In the fragmentation process, the composite nature of our samples should be considered with a special focus on MgB₂ blocks (colonies) that show the major contribution to fracturing. The Glenn-Chudnovsky model of fracturing under dynamic load provides the closest values to our experimental fragment size data.     

Fabrication and mechanical properties of nacre-like alumina with addition of silicon nitride

How to deal with the brittleness of ceramic materials is always one of the hot topics in the field of materials science. Design of layered ceramics with textured structure is one of the effective methods to improve their fracture toughness. The introduction of additives as interlayer phases can balance fracture toughness and flexural strength. However, the research about addition of interlayer phases and their mechanisms in the layered ceramics is still limited. In this work, nacre-like alumina ceramics were successfully fabricated by freeze casting followed by hot pressing. Silicon nitride was incorporated as the interlayer phase, and the effect on the mechanical properties of the nacre-like alumina was investigated. The addition of silicon nitride was beneficial to improvement of interlayer bonding between the alumina layers due to formation of sialon phase, leading to increase of flexural strength but decease of fracture toughness. When the content of silicon nitride was 3.3 wt%, flexural strength and fracture toughness of the sample was 468 MPa and 6.2 MPa m^1/2, respectively. Compared with the sample without silicon nitride, the flexural strength was enhanced significantly. Additionally, both flexural strength and fracture toughness were improved as compared the sample without any additives. This work can provide available references for design and fabrication of high-strength and high-toughness ceramics by properly tuning the layer structure and interlayer phase composition.     

Cyclic contact fatigue behavior of baria-silicate glass-ceramics as a function of crystal aspect ratio

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, evidence of contact fatigue damage in ceramics raises considerable concern regarding its effect on the survival probability predicted for dental prostheses. To simulate intraoral conditions, Hertzian indentation loading with steel indenters was applied in this study to characterize the fatigue failure mechanisms of ceramic materials. Baria silicate glasses and glass-ceramics with different aspect ratios of crystals were selected because the glass and crystal phases have similar density, elastic modulus, and thermal expansion coefficients. Therefore, this system is a model ceramic for studying the effect of crystal geometry on contact cyclic fatigue failure. The subsequent flexural strength results show that the failure of materials with a low fracture toughness such as baria-silicate glass (0.7 MPa m^1/2) and glass-ceramic with an aspect ratio of 3.6/1 (1.3 MPa m^1/2) initiated from cone cracks developed during cyclic loading for 10³ to 10⁵ cycles. The mean strengths of baria-silicate glass and glass-ceramics with an aspect ratio of 3.6/1 decreased significantly as a result of the presence of a cone crack. Failures of baria-silicate glass-ceramics with an aspect ratio of 8.1/1 (K_c = 2.1 MPa m^1/2) were initiated from surface flaws caused by either grinding or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8.1/1 after loading in air for 10³ to 10⁵ cycles. A reduction of approximately 50 % in fracture stress levels was found for specimens with an aspect ratio of 8.1/1 after loading for 10⁵ cycles in deionized water. Thus, even though this glass-ceramic with an 8.1/1 crystal aspect ratio material is tougher than that with a 3.6/1 crystal aspect ratio, the fatigue damage induced by a large number of cycles is comparable. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3.6/1. For an aspect ratio of 8.1/1, a transgranular fracture mode was dominant.     

Mechanical behavior of ceramic-metal joint under quasi-static and dynamic four point bending: Microstructures,damage and mechanisms

This paper studies the mechanical behavior of Alumina ceramic-Kovar joint under quasi-static and dynamic four-point bending (FPB). The joint is fabricated by molybdenum-manganese (Mo-Mn) metallization method with extra additions. The bend strength of the joint is improved by glass phase migration. Electronic universal testing machine and modified split Hopkinson pressure bar (SHPB) are employed to realize the loading process. The microstructure of the joint is investigated by scanning electron microscope (SEM) and the chemical composition is determined by energy dispersive spectrometer (EDS). Digital image correlation (DIC) technique is used to determine the displacement contours and ultra-high speed camera is used to monitor the deformation and crack evolution around the joint. It is found that the specimen will slip due to the different Young's modulus of base material. The dynamic bend strength of the joint is lower than the quasi-static bend strength. The failure mechanism of the ceramic-metal joint is mainly intergranular failure for the dynamic bend but mixed transgranular/intergranular failure for the quasi-static case. The crack starts from the inherent voids inside the ceramic and then expands along the metallization band between solder and ceramic.     

Notched Fracture Behavior of an Oxide/Oxide Ceramic-Matrix Composite

The fracture behavior of an oxide/oxide ceramic-matrix composite, alumina/alumina-silica (Nextel610/AS), was investigated at 23° and 950°C using a single edge notched specimen geometry with clamped ends. Crack growth and damage progression were monitored during the tests using optical microscopy, ultrasonic C-scans, and crack mouth opening displacement. The net section strength of Nextel610/AS was less than the unnotched ultimate tensile strength. The failure mode was nonbrittle with considerable nonlinear deformation prior to and after the peak load at 23° and 950°C. The effect of temperature on the notched strength was significant. Net section failure stress decreased 50% when temperature was increased from 23° to 950°C. Observations of damage progression indicated that the reduction in notch strength with temperature was associated with self-similar crack growth at 950°C. Ultrasonic C-scans were found to be an effective method of monitoring damage progression. Ultrasonic attenuation ahead of the notch tip was correlated with surface matrix cracks and exposed fiber lengths on the fracture surface.     

More on penetration of ceramic based targets by non-deforming projectiles

Abstract

In this work, we examine the calculation of the mean resisting stress offered to a penetrating projectile by several ceramic targets that are intact or that have been predamaged through explosive loading. When comparing the results of intact and predamaged samples, it is shown that the mean resisting stress offered by alumina targets does not fall much, whereas for silicon carbide ceramics, there is a much larger dropoff in resistance. The mean resisting stress offered by predamaged ceramic, which is analogous to the strength of the damaged material, has been estimated for alumina and silicon carbide and was found to be ～4·3 GPa.     

部分烧结陶瓷材料力学特性的DEM模拟

通过开展三维离散单元法数值模拟,考察了部分烧结陶瓷在单轴拉伸和压缩加载条件下的力学响应行为.模拟结果表明,拉伸加载下试样的破坏表现为裂纹"成核"效应,而压缩加载下则呈现为裂纹"聚并"效应;通过追踪固体键的断裂顺序和断裂模式发现,拉伸加载下固体键的破坏主要源于拉伸作用,而压缩加载下则为剪切作用.试样的宏观断裂强度与固体键...     

Foreign Object Damage by Spherical Steel Projectiles in an N720/Alumina Oxide/Oxide Ceramic Matrix Composite

Foreign object‐damage (FOD) phenomena of an N720/alumina oxide/oxide ceramic matrix composite (CMC), impacted by 1.59‐mm spherical chrome steel projectiles up to Mach 1, were assessed at ambient temperature at a normal incidence angle in both partial and full supports. The impact damage was in the form of craters, matrix/fiber tow breakage, compaction of the material, delamination and cone cracks, and their occurrence and degree depended on both impact velocity and type of target supports. The partial support resulted in significant damage with increasing impact velocity, accompanying substantial strength degradation. The presence of tensile stress and presumably stress wave interaction at the backside of a target could have been responsible for greater impact damage in partial support. Although the CMC targets impacted at 340 m/s were on the verge of being penetrated, the targets still survived catastrophic failure retaining about 68% of the as‐received strength, indicative of relatively superior FOD resistance as compared to monolithic ceramic counterparts. A quasi‐static analysis of impact force prediction was made based on the energy balance principle and was validated indirectly using the experimental data on frontal impact damage size.