Analysis of crack development during processing of laminated ceramic tubes

Models for the strain energy release rate in crack extension and tunnelling are taken from the literature with minor modification to take account of the compressive stresses in multi-layer ceramic tubes or composites. These models are applied to predict the potential for crack development in co-extruded tubes fabricated from combinations of alumina, zirconia (fully and partially stabilized) and zirconia toughened alumina. Experimental results show that the models predict the failure in the composites closely and thus form useful tools in the design of such structures. The main aspect of the predictive capacity is in the selection of the layer numbers and the layer thicknesses to prevent failure. The significance of crack-free layers is improved mechanical properties, in particular strength, and potential improvement in fracture toughness and device performance.     

Alumina versus Zirconia Comparative Survey of Thermic Influence During Dental Ceramic Core Manufacturing Process

For dental ceramic restorations, the manufacturing process presumes a high temperature sample heating treatment process, more than 1,000°C for applied aesthetic veneer and glazer. The present study presents and analyzes a comparative survey for two dental ceramics regarding the link between the heating treatment and reported cases of fractures and failure (delamination) of ceramic core. Two different dental ceramics samples were investigated: alumina based ceramic, respectively zirconia based ceramic materials. For zirconia based ceramic, changes concerning monoclinic (M) and tetragonal (T) crystalline phases occur and peak intensity variation from x-ray diffraction patterns are observed during heating process. For alumina based ceramic, the crystalline structure is less affected by the heating process but a different grain size and orientation were noticed during/after heating treatment. Both of them are affecting the strength and fracture toughness of the dental ceramic core.     

Fracture loads for ceramic samples with rounded notches

Fracture loads of ceramic components with rounded notches cannot be computed by linear elastic fracture mechanics techniques because no stress singularity exists. We propose a procedure to estimate such fracture loads, which is based on the cohesive zone model and supported by experimental evidence with alumina, zirconia and silicon ceramics. Data from 18 ceramic materials and different notched geometries were used. The only material parameters needed were the tensile strength and the fracture toughness.     

Combination of biological mechanisms for a concept study of a fracture-tolerant bio-inspired ceramic composite material

The biological materials nacre and wood are renowned for their impressive combination of toughness and strength. The key mechanisms of these highly complex structures are crack deflection at weak interfaces, crack bridging, functional gradients and reinforcing elements. These principles were applied to a more fracture-tolerant model material which combined porous stiff ceramic layers, manufactured by freeze casting, infiltrated and bonded by a polymer phase reinforced with fabric layers. In the hybrid composites, crack deflection occurred at the ceramic–fabric interface and the intact fabric layers served as crack-bridging elements. Fabric-reinforced epoxy layers stabilized the fracture behaviour and delayed catastrophic failure of the material. The influence of the different components was analysed by varying the ceramic, fabric and interface properties. More ductile fabrics lead to larger strain to failure and more crack bridging but reduced the composite strength and stiffness after initial cracking. Higher elastic mismatch between the components improved crack deflection and bridging but resulted in deterred load transfer and a lower strength. The stiffness and strength of the ceramic layers influenced the elastic properties of the laminar composite and the initial crack resistance. Flaw tolerance was increased with polymer infiltration. We show with our hybrid ceramic–fabric composite as a bio-inspired concept study how fracture toughness, work of fracture and tolerance for cracking can be tailored when the contributing factors, i.e. the ceramic, the fabric and their interface, are modified.     

正交铺设陶瓷基复合材料单轴拉伸行为

采用细观力学方法对正交铺设陶瓷基复合材料单轴拉伸应力-应变行为进行了研究。采用剪滞模型分析了复合材料出现损伤时的细观应力场。采用断裂力学方法、 临界基体应变能准则、 应变能释放率准则及Curtin统计模型4种单一失效模型确定了90°铺层横向裂纹间距、 0°铺层基体裂纹间距、 纤维/基体界面脱粘长度和纤维失效体积分数。将剪滞模型与4种单一损伤模型结合, 对各损伤阶段应力-应变曲线进行了模拟, 建立了复合材料强韧性预测模型。与室温下正交铺设陶瓷基复合材料单轴拉伸应力-应变曲线进行了对比, 各个损伤阶段的应力-应变、 失效强度及应变与试验数据吻合较好。分析了90°铺层横向断裂能、 0°铺层纤维/基体界面剪应力、 界面脱粘能、 纤维Weibull模量对复合材料损伤及拉伸应力-应变曲线的影响。      

ZrO2增韧Al2O3颗粒增强Fe45复合材料拉伸断裂的有限元-离散元耦合方法仿真ZrO2增韧Al2O3颗粒增强Fe45复合材料拉伸断裂的有限元-离散元耦合方法仿真

采用有限元-离散元耦合方法（FEM-DEM方法）,进行了氧化锆增韧氧化铝颗粒增强Fe45复合材料（ZrO₂-Al₂O₃/Fe45）轴对称代表体元模型的拉伸断裂仿真分析。分析了FEM-DEM模型对单元尺寸的敏感性,结果表明采用,二阶实体单元加双零厚度内聚力单元的FEM-DEM模型降低了计算结果对单元尺寸的敏感性。ZrO₂-Al₂O₃/Fe45复合材料拉伸断裂的模拟结果表明,颗粒形状对裂纹的扩展会产生较大影响,复合材料的开裂首先在垂直于拉力方向的界面处发生,界面裂纹扩展至基体应力集中处之后基体发生开裂,裂纹由开裂的界面和基体裂纹共同组成。      

单晶与多晶氧化铝陶瓷材料断裂性能与陶瓷基复合材料

有关多结晶氧化铝陶瓷的断裂力学性能,在国内外已有较深入的研究,但是关于单结晶透明氧化铝陶瓷的断裂力学研究目前尚未见有任何报导、对其断裂机理也尚在探讨。本课题为了开发单晶氧化铝陶瓷作为人工心脏瓣膜的生物医学工程研究,首先测定了两批共10件氧化铝单晶陶瓷材料的断裂韧性值,提出单晶陶瓷三维微裂纹扩展模式,在观察大量电子显微镜图片的基础上阐明陶瓷材料的微观断裂机理,进而提出增加陶瓷韧性之复合材料途径。     

Face/core interface fracture characterization of mixed mode bending sandwich specimens

Debonding of the core from the face sheets is a critical failure mode in sandwich structures. This paper presents an experimental study on face/core debond fracture of foam core sandwich specimens under a wide range of mixed mode loading conditions. Sandwich beams with E‐glass fibre face sheets and PVC H45, H100 and H250 foam core materials were evaluated. A methodology to perform precracking on fracture specimens in order to achieve a sharp and representative crack front is outlined. The mixed mode loading was controlled in the mixed mode bending (MMB) test rig by changing the loading application point (lever arm distance). Finite element analysis was performed to determine the mode‐mixity at the crack tip. The results showed that the face/core interface fracture toughness increased with increased mode II loading. Post failure analysis of the fractured specimens revealed that the crack path depends on the mode‐mixity at the crack tip, face sheet properties and core density.     

基于扩展有限元的陶瓷复合材料多重增韧机制

为了提高陶瓷材料的断裂韧性和可靠度,改善材料抵御破坏的能力,将优化的多重增韧机制应用到氧化铝基陶瓷材料的开发中。相变增韧机制可以耗散部分能量,降低裂纹尖端处的应力集中程度,阻止或延缓裂纹扩展速率。当增强相分布较为合理、材料的致密度较高时,裂纹偏转与桥接增韧机制可以有效地削弱裂纹扩展动力,提高材料的断裂韧性。利用扩展有限元（X-FEM）手段讨论了裂纹扩展问题,为分析陶瓷复合材料的多重增韧机制提供了新思路。     

Strength recovery of machined Al2O3/SiC composite ceramics by crack healing

Alumina is used in various fields as a machine component. However, it has a low fracture toughness, which is a weakness. Thus, countless cracks may be initiated randomly by machining, and these cracks decrease the component's mechanical properties and reliability. To overcome this problem, a crack‐healing ability could be a very useful technology. In this study, Al₂O₃/SiC composite was sintered. This alumina exhibits excellent crack‐healing ability. Small specimens for a bending test were made from the Al₂O₃/SiC. A semicircular groove was machined using a diamond ball‐drill. The machining reduced the local fracture stress from approximately 820–300 MPa. The machined specimens were crack‐healed under various conditions. The fracture stress of these specimens after crack healing was evaluated systematically from room temperature (RT) to 1573 K. It was found that the local fracture stress of the machined specimen recovered almost completely after crack healing. Therefore, it was concluded that crack healing could be an effective method for improving the structural integrity of machined alumina and reducing machining costs.     

Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

The mechanisms that control the lifetime of thermal barrier coating (TBC) systems have been traced by two particular overlay bondcoats serving as model systems: superalloy pins (IN100, CMSX‐4) with two alternative NiCoCrAlRE (RE: Hf, Y) bond coat compositions (i) NiCoCrAlY without and (ii) with co‐dopants of silicon and hafnium. On top an electron‐beam physical‐vapor deposited (EB‐PVD) yttria partially stabilized zirconia (YPSZ) TBC commonly mixed with 2 wt.% hafnia, or, rarely with 10 wt.%, was applied. The test pins were thermo‐cycled at 1100 and 1150 °C until failure. Identical lifetimes in cyclic tests on YPSZ TBCs with 2 (relatively high sintering rate) and 10 wt.% hafnia (relatively low sintering rate) preclude an effect of diffusion mechanisms of the YPSZ TBC on lifetime. The fit of lifetimes and test temperatures to Arrhenius‐type relationships gives activation energies for failure. These energies agree with the activation energies for anion and cation diffusion in alumina for the respective bondcoat variant: (i) for the NiCoCrAlY/TBC system for O^2‐ diffusion in alumina, (ii) for the NiCoCrAlYSiHf/TBC system for Al³⁺ diffusion in alumina. SEM and EDS investigations of the thermally grown oxides (TGOs) confirm the mechanisms responsible for TBC failure as indicated by activation energies. Two categories of failure can be distinguished: (i) NiCoCrAlY coatings fail by an “adhesive mode of failure” along smooth bond coat/TGO interfaces driven by a critical TGO thickness. (ii) NiCoCrAlYSiHf coatings fail later and more reluctantly by a “cohesive” crack mode via de‐cohesion at the TGO/TBC interface. In the latter case a quasi‐integrity of the crack‐affected TGO is lengthily maintained up to failure by a crack‐pinning mechanism which runs via Al³⁺ supply from the bondcoat.     

Hydrogen effect on fracture toughness of thin film/substrate interfaces

The effect of hydrogen on the interface fracture toughness of two nano-film/substrate structures, Ni/Si and Cu/Si, were evaluated using four-point bend specimens with and without hydrogen charging. Hydrogen typically decreases the fracture toughness of materials. However, we found in this study that the interfacial toughness between the Ni film and the Si substrate increased due to the presence of hydrogen, while that of Cu/Si decreased. Nanoindentation experiments for the Ni and Cu films revealed that local plasticity in the Ni and Cu films is promoted by the charged hydrogen. The critical stress intensity at the Ni/Si interface crack considering the plasticity of Ni, namely the true fracture toughness, is scarcely influenced by the existence of hydrogen. The apparent increase in fracture toughness of the Ni/Si interface is due to the large stress relaxation near the crack tip caused by softening due to the presence of hydrogen. Although the promotion of plastic deformation of Cu relaxes the stress intensity at the Cu/Si interface crack, the apparent interfacial toughness still decreases because of the significant decrease in the true toughness due to the presence of hydrogen.     

Dynamic fatigue and strength characterization of three ceramic materials

Fracture strength and fatigue parameters of three ceramic materials submitted to dynamic fatigue were evaluated. A machinable leucite-reinforced dental ceramic, aluminum oxide, and yttria-stabilized zirconia (YSZ) were tested. The inert strength of the materials was determined in air (25 °C) at stressing rates of 70, 250, 400 MPa/s for Porcelain, Alumina and YSZ respectively. The data was analyzed using a two-parameter Weibull distribution. The Weibull modulus (m) and the characteristic of fracture (σ ₀) parameters were determined for each material. Specimens were also tested in 3-point bending at different stressing rates in distilled/deionized water at 37 °C (dynamic fatigue) in order to calculate the fatigue parameters n and ln B. The strength for each material was characterized using Strength–Probability–Time (SPT) diagrams for 1 day, 1 year and 10 years. YSZ showed a high-fracture strength σ ₀ (1,459 MPa) at a failure probability of 63.2% and high resistance to subcritical crack growth. YSZ and alumina showed better resistance to slow crack growth than porcelain, indicating less susceptibility to strength degradation by stress corrosion. Lifetime predictions after 10 years indicate a reduction of 50%, 36% and 29% in strength for porcelain, alumina and YSZ respectively. YSZ seems to be a very promising material for long-term dental and biomedical applications.     

增强体含量对SiC_W/SiC层状陶瓷复合材料强韧性的影响

采用流延-化学气相渗透(TC-CVI)工艺制备SiC晶须(SiC_W)/SiC层状陶瓷复合材料,研究了SiC_W含量对层状陶瓷复合材料力学性能和微观结构的影响,探讨了SiC_W/SiC层状陶瓷复合材料的强韧化机制。结果表明:TC-CVI工艺能够有效提高复合材料中晶须含量(40vol%),减少制备过程对晶须损伤,所制备的SiC_W/SiC层状陶瓷复合材料具有合适的层内及层间界面结合强度。随着SiC_W含量增加,层状陶瓷复合材料的密度和力学性能均有明显提高。含40vol%晶须的SiC_W/SiC层状陶瓷复合材料的密度、弯曲强度和断裂韧性均比含25vol%晶须的分别提高了8.4%、30.8%和26.7%。断口形貌中能够观察到层间及层内的裂纹偏转,层内的裂纹桥接和晶须拔出等,这些为主要的增韧机制。高含量SiC_W及合适的层间和层内界面结合强度,对提高SiC_W/SiC层状陶瓷复合材料强韧性有明显作用。     

Surface Modification of an Alumina‐Based Bioceramic for Cement Application

Biograde zirconia toughened alumina (ZTA) has found wide application in load bearing endoprosthetic implants due to high strength, fracture toughness, and wear resistance. In order to enhance bonding to acrylic bone cement (BC) for implants, fixation modification of ZTA with a thin layer of porous anodic alumina (PAA) was investigated. An Al‐layer of approximately 500 nm was sputtered on the ZTA substrate which subsequently was electrochemically oxidized by anodic polarization in H₂C₂O₄ or H₃PO₄ solution. PAA layers with a total porosity ranging from 11 to 30%, mean pore spacing of 90–200 nm and pore diameters of 30–110 nm were prepared. Compared to unmodified ZTA/BC interface (≈ 30 MPa), the PAA modified specimens (ZTA/PAA/BC) achieved a significantly higher interface bonding strength (≈ 60 MPa) measured by four point bending on composite beam specimens. While crack propagation in the unmodified ZTA/BC specimen was found to proceed along the interface, fracture analysis on the ZTA/PAA/BC specimens showed a mixed mode fracture with part of the fracture propagation localized along the PAA/BC interface and part through BC. Thus, pore structure controlled mechanical interlocking is expected to offer a high potential for applying PAA surface modification to improve biomaterial to BC bonding.     

Sphere contact fatigue of a coarse-grained Al2O3 ceramic

The opposite sphere test is an appropriate tool to determine crack‐growth exponents for fatigue under repeated contact loading. Lifetime measurements for a coarse‐grained Al₂O₃ are reported. To explain the fatigue exponents that strongly deviated from those obtained in cyclic bending tests, a fracture mechanics analysis was carried out. It was aimed at determining the correct stress intensity factor solution for the tests, including limited dimensions of test specimens deviating from the case of a cone crack in a half space. Cone crack development was observed microscopically and the related stress intensity factors were computed for the observed crack shape. For modelling the fatigue behaviour, it is assumed that the fatigue effect is influenced by a reduction of the shielding term of crack growth resistance due to periodical friction between the grain‐interlock bridges in coarse‐grained alumina. This results in a loss of traction at the junctions, crack tip shielding is reduced, and the effective load at the crack tip is increased.     

Modeling Loading/Unloading Hysteresis Behavior of Unidirectional C/SiC Ceramic Matrix Composites

The loading/unloading tensile behavior of unidirectional C/SiC ceramic matrix composites at room temperature has been investigated. The loading/unloading stress–strain curve exhibits obvious hysteresis behavior. An approach to model the hysteresis loops of ceramic matrix composites including the effect of fiber failure during tensile loading has been developed. By adopting a shear-lag model which includes the matrix shear deformation in the bonded region and friction in the debonded region, the matrix cracking space and interface debonded length are obtained by matrix statistical cracking model and fracture mechanics interface debonded criterion. The two-parameter Weibull model is used to describe the fiber strength distribution. The stress carried by the intact and fracture fibers on the matrix crack plane during unloading and subsequent reloading is determined by the Global Load Sharing criterion. Based on the damage mechanisms of fiber sliding relative to matrix during unloading and subsequent reloading, the unloading interface reverse slip length and reloading interface new slip length are obtained by the fracture mechanics approach. The hysteresis loops of unidirectional C/SiC ceramic matrix composites corresponding to different stress have been predicted.     

Fracture Toughness of Veneering Ceramics for Fused to Metal (PFM) and Zirconia Dental Restorative Materials

Veneering ceramics designed to be used with modern zirconia framework restorations have been reported to fracture occasionally in vivo. The fracture toughness of such veneering ceramics was measured and compared to that of conventional feldspathic porcelain veneering ceramics for metal framework restorations. The fracture toughness of the leucite free veneer was measured to be 0.73 MPa m ± 0.02 MPa m, which is less than that for the porcelain fused to metal (PFM) veneering ceramic: 1.10 MPa ± 0.2 MPa. (Uncertainties are one standard deviation unless otherwise noted.) The surface crack in flexure (SCF) method was suitable for both materials, but precrack identification was difficult for the leucite containing feldspathic porcelain PFM veneer.     

Crack propagation in ceramic composites with layered granular structure

Fracture experiments under conditions of subcritical crack extension were performed with double torsion and single-edge notched bend specimens of different alumina-based ceramic composites having layered granular structure. It is shown that it is possible to increase significantly the work-of-fracture as a result of layered granular structure organization. The pecularities of structure influence on the crack propagation kinetics were investigated, and the possibilities of acceleration and deceleration of subcritical crack growth are reported.     

Fatigue crack propagation along interfaces of selective laser melting steel hybrid parts

Selective laser melting (SLM) is an emerging additive manufacturing technology, capable of producing complex geometry components. The current work studied both the effect of substrate material and mean stress on the fatigue crack growth behaviour along interfaces of bi‐material specimens, substrate, and part by SLM. Fatigue tests were carried out in agreement with ASTM E647 standard, using 6‐mm‐thick compact specimens. The substrate steel has only a negligible effect both on the fatigue crack propagation rate and on the crack path. The failure occurs in the material additively manufactured by SLM, near the interface. The mean stress produced only a reduced influence on the fatigue crack propagation rate in the Paris regime. For larger values of ΔK, where K_max approaches K_Ic, a significant influence of the mean stress was observed. In spite of nondetection of crack closure, the application of overloads promoted significant fatigue crack retardation, quite similar for both substrate materials, probably due to the crack bifurcation during the overload.