Fully-developed FPZ length in quasi-brittle materials

This paper studies the development of fracture processes in quasi-brittle materials. We propose to use the length of the fracture process zone (FPZ) once it is fully developed as a material parameter. This assumption allows us to build an analytical formulation that reproduces the mechanical behavior of any specimen as a cohesive crack advances. Extensive comparisons with experimental results lead us to define a new characteristic length that commensurates with the fully-developed FPZ and that together with the new analytical model, is used to provide a complete and consistent study on the fracture process. In particular, the size-effect deriving from our formulation coincides with the statistical size-effect law of Ba?ant for small and medium sizes, whereas it smoothly converges to size-independent results as size increases. The analytical cohesive formulation developed here is validated against experimental results on various types of normal and high-strength concretes as well as construction ceramics for several experimental set-ups and test scales. Because of its simplicity as compared with numerical models for fracture, this analytical formulation constitutes a powerful tool for studying fracture processes in a wide variety of mechanical configurations. Meanwhile, analytical expression for a fully-developed FPZ length is given for a general type of cohesive law.     

Fracture behavior of shear key structures with a softening process zone

The objective of this paper is to investigate the fracture behavior of short fiber reinforced ceramic structures by means of the fracture mechanics approach. In this paper, structural stability in relation to crack growth in shear key structures with a softening process zone under effectively bending loads is studied through residual strength diagrams and load deflection curves. In addition, the behavior of process zone size preceding traction free crack in the shear key structure is investigated.Shear key structures with a softening process zone can behave stably under loading in the presence of a crack. Results of this analytical study potentially indicate that short fiber reinforcement for ceramics could eliminate catastrophic failure or unstable fracture behavior of ceramic structures, and also indicate the plausible applicability of ceramics as construction materials.     

Numerical simulation of the damage behavior of bidirectionally reinforced ceramic cross-ply laminates

The behavior of bidirectionally reinforced SiC/SiC cross-ply laminates is studied with the help of numerical simulations based on the finite element method (FEM). Within the presented model the composite is regarded on the layer scale considering each layer as homogeneous with `layer properties'. Brittle cracking as well as damage effects can appear within each layer, which is why both a damage and a fracture model for the plies is derived. The damage model is based on damage variables depending on the strain state. Fracture is checked using a fracture criterion, a crack and a post-failure model. In this way fracture can be considered for multiaxial stress states and the statistical distribution of strength values as well as load transfer effects after crack initiation can be taken into account. By subjecting the structure to a cooling down process before mechanical loading in one of the fiber directions the residual thermal stresses within the layers can also be regarded. The purpose of the simulations is to indicate the influence of important parameters on the composite behavior.     

Minimum solid area models applied to the prediction of Young's modulus for cancellous bone

In developing models for the mechanical behavior of cancellous bone, accurate prediction of Young's modulus as a function of the pore fraction and morphology is a requirement. Previous workers have suggested models which provide good statistical fits, but most of these models are highly idealized, with no treatment of the actual morphology of the porosity. In the field of engineering ceramics, simple minimum solid area models have been developed over the past four decades to describe the mechanical properties of porous structural ceramics. This paper applies these models to data for cancellous bone, and it is shown that one, developed specifically for high porosity materials, gives realistic predictions of tissue modulus and a good statistical fit to well-established data. This model should prove to be useful in biomechanical analyses involving cancellous bone tissue.     

Effects of designed tubular porosity on compressive strengths of honeycomb ceramics

Three-dimensional honeycomb porous ceramics were fabricated using the fused deposition process. The structures were designed to have a controlled interconnected porosity. Elastic interactions between pores of honeycomb ceramic materials were then evaluated for different pore parameters using the finite element modeling (FEM) approach. The FEM results were compared experimentally with the compressive failure strengths of the honeycomb ceramics. The effect of relevant pore parameters on the compressive strength was studied. An analytical fracture mechanics based approach is presented for comparison with the FEM to reflect on the relative importance of pore parameters for different volume fractions of porosity. A detailed insight is also provided into the interrelation between various porosity parameters, mechanical behavior and design of these structures.     

Effect of the manufacturing process on the interfacial properties and structural performance of multi-functional composite structures

A composite integral armor (CIA) structure consists of various layers such as ceramics, rubber and polymer composites assembled in a precise sequence to provide superior ballistic and structural performance at low areal density. CIA structures were originally manufactured in a labor-intensive multi-step process. In recent years, vacuum-assisted resin transfer molding (VARTM) has emerged as an affordable manufacturing method for CIA structures. In this paper, the relationship between the manufacturing processes (i.e. VARTM and multi-step) and the mechanical performance of CIA beams is investigated by four-point bend tests. The behavior of the CIA is found to be highly dependent on the mechanism of stress transfer between the layers and the structures are found to fail progressively and provide significant ductility and capacity. The VARTM process is found to produce structures with superior mechanical performance. Moreover, the level of interface adhesion achieved during processing is shown to control the structural behavior of the CIA. Consequently, the Mode I fracture testing of VARTM and multi-step manufactured double-cantilever beams, representative of one interface of the CIA, is characterized. The resistance to crack growth of the specimens is also related to the manufacturing process, with the VARTM specimens achieving the highest fracture toughness.     

自韧化氮化硅陶瓷的研究与进展

本文阐述了近年来自韧化Ｓｉ３Ｎ４陶瓷技术的研究情况，对自韧化Ｓｉ３Ｎ４的生长机理、影响柱状Ｓｉ３Ｎ４生长的各种因素，以及自韧化Ｓｉ３Ｎ４的断裂韧性、强度、韦伯模数、Ｒ曲线行为、疲劳行为、蠕变行为、氧化行为、抗热震性和热导做了全面的分析和说明，提出了增韧的技术关键是控制柱状Ｓｉ３Ｎ４的尺寸与玻璃相的合理运用。     

Development and cytotoxicity evaluation of SiAlONs ceramics

SiAlONs are ceramics with high potential as biomaterials due to their chemical stability, associated with suitable mechanical properties, such as high fracture toughness and fracture resistance. The objective of this work was to investigate the mechanical properties and the cytotoxicity of these ceramic materials. Three different compositions were prepared, using silicon nitride, aluminum nitride and a rare earth oxide mixture as starting powders, yielding Si₃N₄–SiAlON composites or pure SiAlON ceramics, after hot-pressing at 1750 °C, for 30 min. The sintered samples were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Furthermore, hardness and fracture toughness were determined using the Vicker's indentation method. The biological compatibility was evaluated by in vitro cytotoxicity tests. Ceramic with elevated hardness, ranging between 17 and 21 GPa, and high fracture toughness of 5 to 6 MPa m^1/2 were obtained. Since a nontoxic behavior was observed in the cytotoxicity tests, it may be assumed that SiAlON-based ceramics are viable materials for clinical applications.     

A physical model of the oxygen subsystem evolution in PLZT ceramics under neutron irradiation and annealing

A physical model describing the evolution of defects in the oxygen subsystem of ferroelectric PLZT ceramics under neutron irradiation and isochronous annealing conditions is proposed. The model takes into account the dependence of the material properties on the lanthanum content. The oxygen vacancy concentration variations calculated using this model agree with the experimental data on the polarization behavior in annealed ceramics.     

Stereolithographic 3D Printing of Ceramics: Challenges and Opportunities for Structural Integrity

This article reviews the current activities at the Montanuniversität Leoben on the design, processing, and characterization of 3D printed advanced ceramics using the lithography-based manufacturing technology. An overview of the challenges and the opportunities offered to improve the mechanical properties of printing ceramics is given. Their brittle failure is analyzed within the framework of linear elastic fracture mechanics, considering specific aspects of additive manufacturing. Several issues associated with the printing process are addressed, such as surface quality, geometry control, influence of printing directions, as well as the need to establish testing protocols for 3D printed parts. Based on the layer-by-layer capabilities of the stereolithographic process, bio-inspired material design concepts are discussed aiming to enhance the mechanical resistance of 3D-printed ceramics. By tailoring the layer architecture and microstructure of the parts, high strength and fracture resistance may be achieved.     

Microwave sintering of zirconia ceramics

The potential of a microwave heating technique for the sintering of 3Y-TZP ceramics is demonstrated. High density samples were obtained by short duration firing in a domestic microwave oven. The ultrafine and monomodal size distribution of grains resulting from the process has important implications in terms of mechanical properties. The hardness and fracture toughness values compare very well with long duration, conventionally fired 3Y-TZP ceramics.     

Numerical analysis of ferroelectric/ferroelastic domain switching in ferroelectric ceramics

A numerical approach predicting the behavior of ferroelectric ceramics under electric field and mechanical loading is proposed in this paper. In the model, macroscopic properties of ferroelectric ceramics are determined by microscopic structures. Ferroelectric ceramics are seen to be composed of many domains with different orientations, and domain switching is the source of the nonlinear constitutive behavior of the ferroelectric ceramics. Numerical calculations based on the model were carried out, and the computational results are compared with the experimental results, which shows the two sets of results consist with each other. The calculation approach can provide a guidance for the ceramics component design.     

rGO/SiC复合材料的制备与性能研究

碳化硅(SiC)陶瓷具有优异的力学性能, 但是其断裂韧性相对较低。石墨烯的引入有望解决碳化硅陶瓷的断裂韧性较低的问题。本研究采用热压烧结工艺, 制备了具有不同还原-氧化石墨烯(rGO)掺入量的SiC复合材料。经过2050℃保温、40 MPa保压1 h后, 所制备的复合材料均烧结致密。对复合材料中rGO的掺入量、微观结构和力学性能的相互关系进行分析和讨论。加入4wt%的rGO后, 复合材料的三点抗弯强度达到564 MPa, 比热压SiC陶瓷提高了6%; 断裂韧性达到4.02 MPa•m^1/2, 比热压SiC陶瓷提高了54%。加入6wt%的rGO后, 复合材料的三点抗弯强度达到420 MPa, 略低于热压SiC陶瓷, 但其断裂韧性达到4.56 MPa•m^1/2, 比热压SiC陶瓷提高了75%。裂纹扩展微观结果显示, 主要增韧机理有裂纹偏转、裂纹桥连和rGO片的拔出。     

Fracture strength distribution of porous ceramics under quasi-static load

The purpose of this paper is to estimate the fracture strength distribution of porous ceramics under quasi-static load. Four-point bending test was performed for SiC-porous ceramics at room temperature under quasi-static load. Fracture strength distributions obtained in the above test were estimated with the aid of a conventional probabilistic time-dependent fracture model on the basis of the slow crack growth concept in conjunction with two-parameter Weibull distribution. The results showed that the estimated fracture strength distribution curves were not in good agreement with the experimental data at stress rates. Porous ceramics have damage-tolerable property due to failure of a lot of grain boundaries. Therefore, this is because the dispersion of applied stress was not considered in the conventional model. A new probabilistic time-dependent fracture model considered the dispersion of applied stress was proposed based on Markov process in conjunction with local load sharing rule. The fracture strength distribution curves estimated the aid of the new model were in reasonably good agreement.     

微机模拟法研究纤维增强金属基复合材料拉伸断裂过程

本研究在统计分析及材料断裂力学分析的基础上,建立了长纤维增强金属基复合材料板材拉伸断裂过程的微计算机模拟方法。用该方法得到的模拟结果与已有实测结果吻合良好,从而初步证实了分析模型及其模拟程序系统的有效性。     

Statistical assessment of notch toughness against cleavage fracture of ferritic steels

The work is an initial effort on adopting a statistical approach to correlate the fracture behavior between a notched and a fracture mechanics specimen. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness, and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch brittleness of ferritic steels according to a recently proposed local approach model of cleavage fracture. The temperature independence of the 2 Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the 2 Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with 2 different microstructures.     

陶瓷基复合材料界面结合强影响断裂过程的有限元研究

提出了纤维增强复合材料断裂有限元模型,该模型既用弹簧单元考虑了基体与纤维之间的分离,又用接触单元考虑了基体与纤维之间的摩擦,较真实地模拟了纤维增强复合材料的断裂过程。通过有限元计算,预测了基体与纤维之间的界面结合强度对整个复合材料断裂模式的影响。还对强弱两种不同基体弹性模量的材料进行进一步的探讨。对比其他文献 , 本文中预测结果与真实情况较为吻合。结果表明,对于纤维增强复合材料,不论是强基体还是弱基体,适中的界面结合强度有助于提高其韧性及整体抗拉强度。       

Fracture criterion for conductive cracks in soda-lime glass under combined mechanical and electrical loading

Fracture tests of electrically conductive cracks on pre-notched four-point bending soda-lime glass samples were conducted under combined mechanical and electrical loading. The experimental results show that the critical stress intensity factor at fracture is reduced if an electric field is applied, thereby indicating that the electric field makes contributions to the fracture of conductive cracks. Base on the charge-free zone (CFZ) model, the total local J-integral including the local mechanical and electrical J-integrals serves as a fracture criterion for conductive cracks in dielectric ceramics under combined mechanical and electrical loading. The experimental results confirm the fracture criterion deduced from the CFZ model.     

Computational modeling of debonding behavior at the bone/cement interface with experimental validation

Both clinical examinations and in vitro physical experiments have shown that the fixation interfaces of cemented components are actually critical sites affecting the long-term stability and survival of prosthetic implants after implantation. This study aims to investigate the interfacial debonding behavior of bone/cement composite structures and attempts to establish an analysis model for clinical applications involving cemented prosthetic components. The mechanical properties of the bonded interface were characterized by interfacial strength, interfacial stiffness, and fracture toughness; the measured values of tensile strength, shear strength, and fracture toughness were 4.94 MPa, 5.94 MPa, and 0.34 MN/m^3/2, respectively. The measured strengths of the different configurations from this study are in good agreement with the experimental results available in the literature. In addition, we generated a finite element model with the same geometry as that of the experimental specimen used in the fracture test. The extent of interfacial debonding was further determined by means of the surface damage criteria and the fracture characteristics of the interface crack. The finite element model with an elastic interface predicted that the stress intensity factor (SIF) at the bone/cement interface crack varies nonlinearly with the applied load, which shows that the interface disintegrates at the load level, as was measured in the fracture experiments. It was possible to verify that the proposed simulation model was capable of describing the interfacial mechanical behavior of cemented components.     

Microplane damage model for fatigue of quasibrittle materials: Sub-critical crack growth,lifetime and residual strength

In contrast to metals and fine grained ceramics, fatigue in concrete and other quasibrittle materials occurs in a large fracture process zone that is not negligible compared to the structure size. This causes the fatigue to be combined with triaxial softening damage whose localization is governed by a finite material characteristic length. A realistic model applicable to both has apparently not yet been developed and is the goal of this paper. Microplane model M7, shown previously to capture well the nonlinear triaxial behavior of concrete under a great variety of loadings paths, is extended by incorporating a new law for hysteresis and fatigue degradation, which is formulated as a function of the length of the path of the inelastic volumetric strain in the strain space. The crack band model, whose band width represents a material characteristic length preventing spurious localization, is used to simulate propagation of the fatigue fracture process zone. Thus the fatigue crack with its wide and long process zone is simulated as a damage band of a finite width. For constant amplitude cycles, the model is shown to reproduce well, up to several thousands of cycles, the Paris law behavior with a high exponent previously identified for concrete and ceramics, but with a crack growth rate depending on the structure size. Good agreement with the crack growth histories and lifetimes previously measured on three-point bend beams of normal and high strength concretes is demonstrated. The calculated compliance evolution of the specimens also matches the previous experiments. The model can be applied to load cycles of varying amplitude, to residual strength under sudden overload and damage under nonproportional strain tensor variation. Application to size effect in fatigue is relegated to a follow-up paper, while a cycle-jump algorithm for extrapolation high-cycle fatigue with millions of cycles remains to be researched.