Grain-Size and R-Curve Effects in the Abrasive Wear of Alumina

Results of sliding wear tests on three alumina ceramics with different grain sizes are discussed in the light of crackresistance (R-curve, or T-curve) characteristics. The degree of wear increases abruptly after a critical sliding period, reflecting a transition from deformation-controlled to fracture-controlled sulface removal. This transition occurs at earlier sliding times for the aluminas with the coarser-grained microstructures, indicative of an inherent size effect in the wear process. A simplistic fracture mechanics model, incorporating the role of internal thermal expansion mismatch stresses in the crack-resistance characteristic, is developed. The results suggest an inverse relation between wear resistance and large-crack toughness for ceramics with pronounced Rcurve behavior.     

The One-Dimensional Car Parking Problem and Its Application to the Distribution of Spacings between Matrix Cracks in Unidirectional Fiber-Reinforced Brittle Materials

Experimental observations of the spacings between cracks formed in the matrix in unidirectional fiber-reinforced brittle materials in a situation where a tensile stress is applied parallel to the lengths of the fibers, and where the fibers have a longer elongation to failure than the matrix, suggest that the distribution of crack spacings does not follow the one expected on the basis of a model where the matrix cracks at a well-defined single-valued stress. A simple model which is able to account for this apparent discrepancy between theory and experiment is developed and discussed.     

Transformation Zone Shape, Size, and Crack-Growth-Resistance [R-Curve] Behavior of Ceria-Partially-Stabilized Zirconia Polycrystals

Transformation zone shape, size, and crack-growth-resistance ( R -curve) behavior were studied in precracked and annealed single-edge notch bend specimens of commercial-grade ceriapartially-stabilized zirconia polycrystals as a function of applied load. Well-defined transformation zones with a characteristic elongated shape in the plane of the crack were observed. It is shown that the observed zone shape is significantly different from the shape predicted by a combined shear/dilatation yield criterion and the stress field of the crack prior to the transformation. The length of the transformation zone directly ahead of the crack tip is in better agreement with the prediction of the Dugdale plastic strip zone model. The fracture toughness increment showed the characteristic square root dependence on the transformation zone width, but the magnitude of the toughness increment was not consistent with the predictions of the theoretical models of transformation toughening.     

Statistics of Flaw Interaction in Brittle Materials

The problem of elastic interaction of an array of microcracks is considered. The solution is based on an efficient surface integral method using distribution of edge dislocations to represent the microcracks in the mathematical model. From the analysis of the interaction between two identical cracks, it is seen that interaction is most likely to produce a slight enhancement in the stress intensity factor. By performing computer experiments on a random array of microcracks, the effect of crack interaction is studied statistically as a function of the density of the microcracks. The significance of the interaction effects for fracture in ceramics is discussed.     

R-Curve Behavior and Thermal Shock Resistance of Ceramics

The influence of R -curve behavior of ceramics on the strength degradation associated with thermal shock is explored. Of particular significance for this interdependence is the observed nonlinear stress-strain behavior of materials that exhibit minimal strength degradation under severe thermal shock conditions. These two features, R -curve behavior and nonlinear behavior, are incorporated into a fracture mechanics analysis to provide a framework with which to understand severe thermal shock of ceramics. This analysis enables the estimation of the crack growth due to thermal shocking and also the anticipated strength degradation. The influence of specimen size is also addressed, and it is shown that greater strength degradation is anticipated with increasing specimen size. Experimental results for an alumina-zirconia composite material are presented to support the simple analysis.     

Subcritical Crack Growth of Macrocracks in Alumina with R-Curve Behavior

Subcritical crack growth of macroscopic cracks in two Al₂O₃ ceramics is investigated with single-edge-notched bending specimens under constant load. The resulting v - K _I-curves are in complete contrast to the behavior of natural cracks. In spite of the monotonic increase of the externally applied stress intensity factor due to crack extension, the crack growth rates first decrease. This behavior is caused by crack shielding due to crack border interaction and can be described by a rising crack growth resistance. Two methods are applied to determine the R -curve under subcritical crack growth conditions.     

Nanoindentation Method for Measuring Residual Stress in Brittle Materials

The lowered threshold load for cracking with the cube-corner indenter has been used in developing a method that can be used to measure residual stresses in small volume brittle materials. By studying a series of orthogonal cracks generated at loads not exceeding 10 mN with the cube-corner indenter, a variation of crack length with position around a large Vickers impression in soda–lime glass was observed. Using an indentation fracture mechanics approach residual stresses were evaluated at the positions where the cube-corner indents had been made. The stress values thus evaluated were generally higher than those reported in the literature where micro-Vickers indents had been used to measure the stresses. Possible reasons for the disparity are discussed.     

Evaluation of the bond strengths of two novel bioceramic cement using a modified thin-slice push-out test model

OrthoMTA (OrthoMTA, BioMTA, Korea) and RetroMTA (OrthoMTA, BioMTA, Korea) have been newly developed bioceramic cements or tricalcium silicate cements (TCSs). The aim of this study was to examine the dislocation resistance of these two novel TCSs from radicular dentin, using a modified thin slice push-out test design that simulated canal spaces of uniform dimensions. Twenty-five single-rooted human canine teeth were used. Longitudinal slabs were obtained from each tooth using a diamond saw. Standardized canal-like holes were created using size #2 Peeso Reamer along the coronal, middle and apical thirds of longitudinal tooth slabs. The cavities were filled with OrthoMTA or RetroMTA. The slabs were kept at 37°C for 14-day in 100% humidity before push-out evaluation (MPa). Failure modes were examined with stereomicroscopy. Data were calculated using unpaired t test (P < 0.05). Location of the TCS-filled cavities did not affect push-out strengths. OrthoMTA (9.809 ± 2.176 MPa) had significantly higher push-out bond strength values than ProRoot MTA (7.103 ± 2.145 MPa) (P < 0.0001). The leaf-like bodies were seen in the fractured specimens of both TCS groups. Failure modes were predominantly adhesive in both TCS groups. It can be concluded 14-day push-out bond strength of OrthoMTA is higher than that of RetroMTA.     

Effect of Tangential Loading on Critical Conditions for Radial Cracking in Brittle Coatings

Results of Hertzian contact tests investigating the effects of superposed tangential loads on the critical conditions for radial cracking at the undersurfaces of brittle coatings on compliant substrates are reported. It is demonstrated that these effects are secondary, so that conventional normal indentation remains an appropriate test procedure for characterizing this highly deleterious mode of coating fracture under a wide range of complex loading conditions.     

R-Curve Behavior of Long Cracks in Alumina

Coarse-grained alumina is among those monolithic ceramics which can exhibit an increase in crack resistance with crack extension. This R -curve behavior is most pronounced for intergranular fracture and does not depend exclusively on material properties. Crack and specimen geometries also influence the shape of the R -curves. The magnitude of the effect increases with increasing crack surface roughness, which is microstructure-dependent, and with crack-opening displacement, which is geometry-dependent. Based on experimental observations, a "dynamic" R -curve model is presented which relates the increasing resistance to an increasing crack tip shielding caused by crack surface bridging. Applying a J -integral approach, R -curves are calculated for two specimen geometries (short double cantilever beam and single-edged notched beam) and different grain sizes. The good agreement between calculation and experiment indicates that the R -curve behavior of long cracks in alumina can be predicted by a simple wake model.     

Microstructural Aspects of the Fracture of Hardened Cement Paste

For microstructural studies, samples of hardened cement paste were fractured, impregnated with epoxy using a procedure that does not produce drying cracks, and examined using a scanning electron microscope. Cracks generally appear to pass around unhydrated cement grains and along calcium hydroxide cleavage planes. Cracks are often branched or forked at their tips, and a few microcracks are sometimes observed just ahead of the tips. Perhaps more importantly, the crack path is often offset, with the interjacent material between these offset crack segments forming a bridge across the crack. Even though the crack plane is most likely continuous in three dimensions, these bridging materials may provide local regions of ligamentary tractions across the crack, thereby providing a plausible explanation for the rising fracture resistance ( R -curve behavior) of hardened cement paste.     

Factors Controlling the Fracture Resistance of Brittle Cellular Materials

Previous work on brittle cellular solids has indicated that the fracture toughness may be independent of cell size. The results showed agreement with two theoretical approaches proposed by Maiti et al. to predict the toughness of porous cellular solids. One approach indicates a strut strength and cell size dependence of the bulk material toughness, whereas the other depends only on the toughness of the solid portion. The present work attempted to experimentally discern between the two approaches and identify the controlling parameters governing the mechanical behavior. Reticulated vitreous carbon samples were oxidized to introduce surface flaws into the struts. The toughness and compressive strength of the bulk material were measured before and after oxidation. Fractographic analysis of the fracture surface of individual struts was essential in being able to estimate the strut strength. It was concluded that it is the strut strength which controls the strength and toughness of the bulk cellular solid.     

脆性材料强度评价

本文介绍以先进结构陶瓷为主要对象的脆性材料抗张强度的各种评价方法。包括直接单轴拉伸法和各种间接测定法。     

脆性材料的亚临界裂纹扩展和双向应力的影响

分析了陶瓷,玻璃等脆性材料的断裂机理和表征.指出了脆性材料的失效过程的三种不同的裂纹发展模式.实验研究了玻璃在静载下的亚临界裂纹扩展特征及其受双向应力的影响.从而为脆性材料的可靠性和寿命评价提供理论基础和手段.     

牙科修复粘结性材料

牙科治疗和修复应用的粘结性材料包括粘固剂、合成粘结剂和复合树脂三大类。本文对上述三类材料的组成和性能,在牙科临床的应用情况,研究进展,改性方向等进行了介绍。     

Crack Stability and T-Curves Due to Macroscopic Residual Compressive Stress Profiles

Stabilization of the fracture process and resistance to strength degradation have been observed for materials with increasing T -curves. In this study, the possibility of using residual compressive stresses to induce crack stabilization is examined theoretically. Nonmonotonic forms for the residual compressive stress profiles are assumed. The stress intensity factors for linear through-the-thickness cracks subjected to these profiles are derived. The stress intensity factors are then used to construct the T -curves for the stress profiles considered. It is demonstrated that the presence of these T -curves leads to crack stability under the action of applied tensile stresses, and to strength insensitivity to the initial flaw size. The effects of additional localized stress fields (similar to those produced by indentation) on crack growth in these materials are also considered. In this case, the strength is found to be relatively insensitive to the magnitude of the localized loading. It is therefore concluded that residual stresses can be used to improve mechanical reliability in ways which are usually associated with microstructural toughening mechanisms.     

Long‐term ceramic reliability analysis including the crack‐velocity threshold and the “bathtub” curve

A thermally activated crack‐velocity formulation that includes a threshold at thermodynamic equilibrium is used in the prediction of long‐term time‐to‐failure for brittle materials. A new closed‐form time‐to‐failure solution is derived for straight cracks propagating under the influence of constant stress. Explicit connections are made between the macroscopic crack‐velocity parameters and the underlying bond‐rupture parameters. A feature of the solution is the divergence of time‐to‐failure for applied loading approaching the thermodynamic threshold. A new reliability framework is developed and long‐term reliability and hazard predictions made using the time‐to‐failure solution. A bathtub hazard curve is shown to be generated by a single crack‐velocity failure mechanism.     

Crack-Growth-Velocity-Dependent R-Curve Behavior in Lead Zirconate Titanate

Crack-velocity ( v – K ) curves and crack-resistance ( R ) curves for unpoled ferroelectric and ferroelastic lead zirconate titanate (PZT) were determined for long cracks in compact-tension (CT) geometry using an in situ fracture device on the stage of an optical microscope. The steady-state crack length and the plateau value of R -curves measured at controlled constant velocities increased with increased velocity. The plateau value for 10⁻⁶ m/s was 1.2 MPa·m^1/2 after 1.3 mm of crack extension and for 10⁻⁴ m/s was 1.4 MPa·m^1/2 after 2.2 mm.     

Strength of brittle materials in moderately corrosive environments

The strengths of four brittle materials―cordierite glass ceramic, fused silica, silicon, and polycrystalline alumina were measured after exposure to weakly corrosive water and moderately corrosive buffered HF (BHF) solution. Exposure to water did not alter the strengths in subsequent inert strength tests and decreased the strengths in reactive strength tests. Exposure to BHF increased the strengths in both tests, but only after an incubation exposure time. Prior to the incubation time, the BHF had the same effect as water, suggesting that the bond rupture kinetics were unaffected. Examination of strength‐controlling indentation flaws after the incubation time showed clear corrosive effects on the flaw geometry indicative of reductions in the indentation residual stress fields. The implication is that moderately corrosive environments increase the strength or lifetime of a brittle component by reducing the crack driving force via flaw alteration and do not, as perhaps expected, decrease the strength or lifetime through enhanced chemical reactivity.