Fracture Strength Of Ceramics Research Articles

Finite element analysis of fracture behavior in ceramics: Competition between artificial notch and internal defects under three-point bending

Evaluation of the nonlinear relationship between the surface defect size and fracture strength of ceramics is important for engineering applications. In this study, we aim to predict the apparent nonlinear relationship between the defect size and fracture strength of single-edge notched beams (SENBs) using the finite element method. Specifically, we applied the methodology for predicting fracture strength from microstructure distribution data (relative density, pore size, aspect ratio, and grain size) to a finite element analysis (FEA) model in which the shape and size of the initial defects are defined at notch locations. By reproducing the apparent nonlinearity caused by the competition between the surface and internal defects within the framework of linear elastic fracture mechanics, the effectiveness of the FEA methodology for the evaluation of strength scatter and allowable crack size in ceramics was demonstrated.

Finite Element Analysis of Fracture Strength of Ceramics Considering Both Surface and Internal Defects

Structural ceramics are being considered for use in a variety of fields due to their excellent properties. However, ceramics are very sensitive to defects and minute surface defects can affect fracture strength. In addition, the relationship between surface defects and fracture strength exhibits non-linearity, and thus, the conventional linear elastic fracture mechanics may not predict fracture strength. Therefore, there is a need for an accurate method to determine the effect of surface defects on the fracture strength of ceramics. In this study, we developed a method that extends the model proposed by the authors to incorporate fracture strength reduction due to surface defects. This method can reproduce the non-linearity of the relationship between surface defects and fracture strength of ceramics. Then, we discuss about the competing relationship between internal and surface defects. It can also estimate the allowable size of surface defects by combing with the Weibull distribution of fracture strength.

Prediction of Fracture Strength of Ceramics under Multi-Axial Stress

Prediction of Fracture Strength of Ceramics under Multi-Axial Stress

セラミックスの破壊強度に及ぼすき裂形状の影響の定量的評価

セラミックスの破壊強度に及ぼすき裂形状の影響の定量的評価

Microstructural evolutions, elastic properties and mechanical behaviors of W/Cr Co-doped Bi4Ti3O12 ceramics

For Aurivillius phase Bi4Ti3−xWxO12+x+0.2wt.% Cr2O3 (BTWx-C, x=0–0.1) ceramics prepared by a conventional ceramic process, plate-like grains with random orientations are formed there. The aspect ratio of grains increases after the doping of W for ceramics, at the same time the bulk density of ceramics decreases. The change of Poisson's ratio may be related to the change of bulk density, whilst the change of bond strength could be responsible for the change of Young's modulus. The large-grained samples tend to possess a lower frequency constant but a higher elastic compliance, as well as a lower hardness. Especially, a larger grain size provides a higher fracture toughness for the sample at x=0.05 due to the mechanism of crack deflection. The stress–strain curves of BTWx-C ceramics under a uniaxial compression load are characterized as three stages: linear elastic deformation, ferroelastic domain switch and microcrack propagation. The microcrack propagation along grain boundaries results in the final axial flerry of ceramics. Additionally, the fracture strength of ceramics may be largely influenced by the internal stress resulted from the paraelectric–ferroelectric phase transformation during sintering.

Damage mode effects on fracture strength of ultra-high temperature ceramics

Improving the fracture strength of ceramics has been one of the most important concerns in the ceramics field, and highly accurate evaluation of the fracture strength of ceramics is the right foundation of this topic. In this paper, we analyze fracture strength by using a previously established temperature-damage-dependent strength model. Sensitivities of fracture strength to relative parameters are analyzed, and the influences of different failure modes and typical sizes of different micro-structural characteristics on the fracture strength of ultra-high temperature ceramics (UHTCs) are discussed. This study can provide a theoretical basis and technical platform for the design, application and reliability assessment of UHTCs in applications.

Evaluation of the Uniaxiality of the Stress State in O-Ring Fracture Strength Specimens

Abstract To investigate the experimental implications of using O-ring specimens for evaluating the fracture strength of ceramics in tubular form, a statistically designed series of finite element analysis (FEA) calculations was performed. These calculations focused on the ranges of specimen dimensions and loading distributions required to maintain a uniaxial stress state within the O-ring specimen under diametral loading. Results of the FEA calculations indicated that the degree of uniaxiality of the O-ring stress state could be described completely by a combination of the depth-to-thickness ratio (b/t) and the inner radius-to-outer radius ratio (ri/rc). It was also found that a relatively wide range of geometries can be accurately used to extract fracture strength design data, provided b/t does not get too large nor ri/ro too close to zero. However, there is cause for concern when b/t→0 or ri/ro→1, as this implies a small sample of material (and flaws) in the critical tensile stress region. Results also indicated that the uniaxiality of the O-ring stress state followed the expected 1/v dependence so that the behaviors of different materials can be easily surmised from the current results. On the other hand, load magnitude did not seem to have any effect on the stress-state despite the Boussinesq-type loading, while the distribution of the load over a finite contact area section could.

Influence of Bridging Characteristics on Fracture Strength of Ceramics

The influence of bridging characteristics on the fracture strength of ceramics was investigated by numerical simulation. Factors influencing the fracture strength of ceramics include the defect size, bridging characteristics and the fracture toughness. The relationship between defect size and fracture strength has been studied by many researchers. However, there are few reports on the relationship between bridging characteristics and fracture strength, though this relationship is important for materials development. The purpose of the present investigation was to reveal the relationship between the bridging characteristics and the fracture strength of ceramics by numerical simulation. In the numerical simulation, the stress intensity factor at crack tip and the fracture toughness were used for the fracture criterion. It is assumed that the crack propagates when the crack-tip stress intensity factor reaches the fracture toughness. The effect of bridging on the toughening is not expected when the defect size as an equivalent crack length is smaller than 20 to 50μm. The reason is that the stable crack extension with bridging before a catastrophic fracture is very short for defects smaller than 20 to 50μm.

Evaluation of Fracture Strength for Ceramics with Defects by Numerical Simulation. (1st Report, Fundamental Theory and Application to Alumina Ceramics).

It is well known that the fracture strength of ceramics with small defects is lower than the strength estimated by linear fracture mechanics, based on the strength of a large defect. This phenomenon can be explained by the increase in the crack resistance with crack propagation. This R-curve behavior is caused mainly by grain bridging in the wake of the crack. Recently the grain-bridging stresses for alumina and silicon nitride have been measured experimentally. In this study, the fracture strength is estimated by numerical simulation, based on grain-bridging stresses. In the simulation, the effective stress intensity factor at the crack tip, Ktip, and the effective fracture toughness at the crack tip, (Ktip)c, are used for the conditions of stable and unstable crack propagation. It is found that the fracture toughness for long cracks, KIc, depends on specimen geometries, and the maximum values of Kc approach (Ktip)c as the initial crack length approaches the width of the specimen. Furthermore a convenient method to evaluate (Ktip)c experimentally is discussed.

Threshold stress intensity for oxidative crack healing in sintered silicon nitride

In applying ceramics to stuctural components, the assurance of minimum strength is an important factor in achieving high reliability. The fracture strength of ceramics is generally determined by the worst processing flaw. Flaws induced by machining have taken on greater importance than ever before since ceramic processing technologies have made progress toward the successful elimination of internal flaws such as inclusions, agglomerates, exaggerated particles and voids. This means that the elimination of surface flaws produced during machining is a key to further improvement in strength. When strength is governed by surface flaws, strength recovery can be expected to occur at high temperatures. As indicated by Evans [1], crack healing in ceramics can actually take place at high temperatures below the threshold stress intensity, and negative crack velocity is described as a function of the applied stress intensity. Crack tip blunging, another phenomenon occurring under hightemperature loads, works to counteract subcritical crack growth. It was reported that the strength of sintered silicon nitride, having a Knoop indentation flaw, increased due to crack tip blunting below a stress intensity of 0.1 MPam 1/2 at 1200 °C in a nitrogen atmosphere [2]. These studies seem to indicate that diffusional flow along the crack interface to the crack tip produces crack tip blunting, and that further diffusional flow to the crack interface results in crack healing. Oxidation forming a dense surface layer can heal a crack. The strengthening of silicon carbide as a result of oxidation at 1400 °C was thought to be attributed to the healing of surface cracks [3]. Similar phenomena can be expected to occur in silicon nitride at high temperatures because a dense silicon oxide layer can be formed on the surface. If an oxide layer is formed at the crack interface, blunting the crack, higher threshold stress intensity values may be obtained than those seen for diffusion mechanisms. A novel technique, using a modified stress rupture test, has been proposed as a way of determining the threshold stress intensity at high temperatures, based on the idea that strength can increase under a stress level below the threshold [2, 4]. This technique was used in the present study to explore the threshold stress intensity for oxidative crack healing at high temperatures, resulting in increased strength. The material used in this study was sintered silicon nitride having a density of 3.11 g cm -3. This material was fabricated by compacting and sintering a powder mixture (SN-COA, Ube Co., Ltd) having a composition of 90 wt% Si3N4, 5 wt% Y203 and 5 wt% A1203 in a nitrogen atmosphere under a pressure of 0.1 MPa at 1650 °C for 4 h. Specimens prepared for the tests were ground to the dimensions of 3 mm x 4 mm x 38 mm. At the centre of the surface where tensile stress was applied, a crack was introduced with a Knoop indenter at a load of 198 N. The surface layer with a thickness of 100/xm was ground to eliminate the residual stress induced by the indentation [2, 5]. Fig. 1 shows the apparatus for applying flexural stress at high temperatures. The applied static load was transmitted through the level arm mechanism. In order to detect the time to failure, a microswitch connected to a timer was positioned just below the lever arm. The compact furnace was made of silicon carbide heating elements. The fixtures and the loading rods were machined from dense silicon carbide. The load was applied at the outset at room temperature to avoid healing the crack before reaching the test temperature. It took about 1.5 h to reach the test temperature of 1100 °C, and the temperature was then kept constant for 3 or 20 h. The electric power was switched off to let the furnace cool down after the test in order to remove the specimen. The strength of the specimens after being subjected to high temperature loading was measured in a four-point speed of 0.5 mm min -1. lengths for both high strength measurement at and 30 mm, respectively. flexural test at a crosshead The inner and outer span temperature loading and room temperature were 10

Laser surface modification of metal-coated ceramics

Pulsed excimer laser radiation has been successfully employed in the improvement (> 50%) of fracture strength of metal-coated ceramics. Thin metallic layers (∼500 Å) of nickel were deposited on silicon nitride and silicon carbide substrates and further irradiated with pulsed excimer (xenon chloride, krypton fluoride) laser pulses. The laser energy density was varied from 0.4 to 2.0 J cm −2 to optimize the formation of mixed interfacial layers. The formation of interfacial layers was studied by transmission electron microscopy and Rutherford backscattering spectrometry techniques. Detailed heat flow calculations using implicit finite difference methods were performed to simulate the effects of intense laser irradiation on metal-coated ceramic structures. Three different mechanisms were found to play an important role in the improvement in the fracture strength of these ceramics. Theoretical calculations showed that the displacement of the crack tip away from the free surface by laser surface modification can lead to a 100% improvement in the fracture strength of the ceramic.

Statistical analysis of the effect of surface grinding on the strength of alumina using Weibull's multi-modal function

Statistical analysis of the effect of surface grinding on the fracture strength of ceramics was performed and a new distribution function based on multi-modal Weibull distribution function has been suggested. The experimental verifications were carried out on alumina specimens ground in various directions using diamond wheels which have different abrasive diameters. The theory agreed well with the experimental results.

セラミックスの強度特性評価と信頼性

Strength properties of Ceramics depend upon the shapes, sizes and location of initial flaws, and so they have the strength of significant different levels and dispersions owing mainly to the difference of testing modes or the shapes and sizes of specimens even though the material was taken from the same lot. Therefore, an estimating method for ceramic strength, based on a modified Weibull analysis, was studied in this paper for finding the common property parameters as material constants independent of testing methods or shapes and sizes of specimens. Common Weibull modulus m0 was estimated by Weibull analysis for the values of fracture strength normalized by a mean value of that by each testing method, and was applied to analyze the effective volume of specimens in each test. Finally the strength property parameter σ0 was derived from scale parameter. Weibull modulus m0 was treated as a determined value and the confidence of measured values was considered only about σ0. Consequently, by using these two parameters m0 and σ0, the fracture strength of ceramics can be estimated with regard to failure probability Pf, confidence level 1-γ and sample size n as follows.σf=σ0m0√2nln(1-Pf)-1/Veffχ2(2n, 1-γ)The estimating method proposed in this paper is very useful for estimating the design allowable stress and determining safety factor in ceramic structural design based on reliability engineering.