Toughness Of Ceramics Research Articles

An Experimental Investigation of the Failure Modes in Hybrid Ceramic Matrix Composites

A material system has been developed that employs the benefits of both fiber and whisker reinforcement in the strengthening and toughening of ceramics. Testing was performed on this hybrid material system in three-point flexure using a combination of acoustic emissions analysis and microscopy techniques for damage analysis. It is shown that the hybrid material system will withstand a higher stress before microcracking occurs in the matrix and has improved transverse strength. In the longitudinal direction, microcracking occurred at 121 MPa for the hybrid composite, compared to 47 MPa for composites reinforced solely with fibers. The transverse flexure strength was 62 MPa for the hybrid, compared with 24 MPa for fiber-reinforced composites. The hybrid composite also had higher stiffness and better damage tolerance in both the longitudinal and transverse directions. Material properties are presented for three material systems: a whisker-reinforced cordierite composite, a fiber-reinforced cordierite composite, and a fiber/whisker hybrid-reinforced cordierite composite.

Relationship between fracture surface pattern, crack geometry and fracture toughness in brittle materials

Mecholsky et al. [1] experimentally investigated the relationship between the fractal dimension of the fracture surface profile, D' (1 _-< D ' N 2) and the fracture toughness, Klo, in alumina and glass ceramics. They found that the increment of fracture toughness (Ko = Klc K0) was in proportion to the increase in the square root of the fractional part of the fractal dimension (D* = D ' 1 ) , such that K~ = A(D*) 1/2, where K0 is the toughness of the materials for a smooth (Euclidean) fracture surface and A is a constant. The author [2] analysed the relationship between the fracture toughness and the fractal dimension of the crack (D", 1 =< D " N 2) in brittle materials on the basis of a fractal geometry model, and obtained an equation: In Kj~ = 1/ 2{ln(2FE/(1 v2)) (D" 1)lnrL} (E = Young's modulus: v = Poisson's ratio; F = the work done in creating a unit crack surface; tL: t 'min/rmax, the ratio of the lower limit (rmin) and the upper limit (rm~x) of the scale length (r), between which a crack exhibits a fractal nature). A similar functional dependence can be expected between the fracture toughness, KI¢, and the fractal dimension of the fracture surface, D (2 _-< D =< 3), in brittle materials, although the generalized analysis based on the fractal geometry has not been made on the relationship between them. Let us consider a fracture surface in brittle materials (Fig. 1). The fracture surface area, S, can be measured by covering with squares of side length, r [3]. The number of squares covering the fracture surface, N, is generally correlated through the fractal dimension of the fracture surface, D (2 ~ D ~< 3), with the side length r [4]:

酸化物超伝導体の合成と特性 ＳｉＣ／Ａｌ２Ｏ３複合体の機械的特性におけるＳｉＣ粒子サイズの影響

Recent approaches to toughening of ceramics are mainly carried out by incorporation of fillers such as particles, whiskers and fillerments into ceramics matrix. In this study, three types of SiC fillers, namely, thin or thick whiskers, or rod like particle are used to fabricate composites. The diameters of fillers were 0.3, 0.7 and 2.5μm, respectively. The toughening behavior and micro-structures of these composites are examined in relation to the size effect of these fillers. Since fracture strength and fracture toughness were both measured for specimens, fracture mechanical analysis was used to determine size of critical flaws.

High‐Temperature Fracture Toughness of Duplex Microstructures

The temperature dependence of the fracture toughness of ceramics exhibiting duplex microstructures was studied relative to their single‐phase constituents using two test methods: bend testing of chevron‐notched beams, and the indentation‐crack‐length technique. The two materials systems studied were Al2O3:c‐ZrO2(Y) and A12O3:Y3A1SO12 (YAG), and the testing temperature ranged from room temperature to 1200°C. The study showed that in both systems the duplex materials showed higher toughness values than their single‐phase constituents above 800°C. This result was attributed to the contribution of low‐energy interphase boundaries to the overall composite toughness. Indentation crack length measurements gave toughness values and trends comparable to those determined by the chevronnotched beam method. By comparing the results of the two test methods it was possible to demonstrate that the indentation calibration constant (ξ) shows no significant temperature or material dependence. For the zirconia‐containing materials, however, indentation at elevated temperatures is accompanied by significant localized plasticity, which suppressed the radial cracking. Under such conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the fracture toughness.

Ferroelectric ceramic toughening by fracture: Cokputer models

Abstract Toughening mechanisms by the ferroelectric ceramic (FC) fracture using computer simulations are studied. Within bounds the joint consideration of the sintering and fracture the processes of the microcracking, crack branching and bridging, crack interactions with damaged material and morphotropic transitions regions (MTR) effects are investigated.

Toughening of ceramics by short aligned fibers

Abstract The tensile strength of a ceramic composite reinforced by short, aligned fibers is studied. The composite contains an initial, through-the-fibers Mode-I crack of arbitrary size. It is assumed that the parallel fibers have equal lengths and are randomly arrayed. The fiber length is chosen to be “optimal” in the sense that it has the largest value consistent with the occurrence of pull-out before fiber fracture, no matter where a propagating matrix crack intersects a fiber. The crack-bridging fibers are modeled by an array of non-linear bridging springs, and the consequent nominal fracture toughness associated with a long crack is calculated. The tensile strength of the composite containing an initial crack of finite size is determined by monitoring the load history as a matrix crack emanating from the tips of the original through-the-fibers crack extends. It is found that the long-crack fracture toughness produced via optimal short-fiber reinforcement is much larger than that provided by continuous fibers of equal deterministic strength. However, for sufficiently small initial cracks, the continuous fibers provide more strength, approaching double that of the short-fiber composite as the initial crack size decreases. For realistic values of physical parameters, the cross-over initial crack length for equal strengths of composites reinforced by short or long fibers is in the millimeter range, comparable to the optimal fiber length.

Effect of Grain Size on Scratch Interactions and Material Removal in Alumina

Dramatic effects of scratch interactions on material removal are observed in alumina. A series of parallel scratches are made in aluminas with different grain sizes to investigate the influence of scratch interactions on the material removal process in abrasive machining. The separation distance between the two scratches and the normal load are varied and subsurface microfracture and damage modes are examined to assess the mechanisms of material removal. A very small amount of material is removed when the separation distance between the two parallel scratches is large or when the two scratches completely overlap. However, at intermediate distances the volume of material removed increases dramatically as a result of the interactions between the two scratches. The maximum amount of material removed and the corresponding distance between the two scratches are found to depend strongly on the grain size and the load. Observations of surface and subsurface damage reveal that grain dislodgement is the predominant mechanism of material removal, irrespective of the grain size. The relation between grain size, scratch interactions, and the material removal process in grinding and abrasive machining of ceramics is discussed in terms of the short‐crack toughness of ceramics.

Toughening of sic ceramics by a bimodal grain size distribution produced by hiding ultrafine and coarse grained SiC powders

Toughening of ceramic materials by using a bimodal grain size distribution has already been mentioned early in literature. This kind of toughening can be explained by an interlocking of the large grains during crack propagation. This so called bridging can operate very effectively for large grain size ratios and large aspect ratios of the coarse grained phase. In this approach ultrafine particles have been used to produce the bimodal grain size distribution. The main advantages over conventional powders is that large coarse grained powders can be densified. Then the established microstructure reveals a large grain size ratio. Furthermore the defect size which is correlated to the size of the largest grains is still quite low and hence high strength values can be expected. As starting powders mixtures of ultrafine SiC powder (particle size below 20 nm) and 10 or 20% coarse grained α-SiC powder with a particle size of 10 μm were used. These mixtures and pure ultrafine powders were Hot Isostatically Pressed (HIPed) at low temperatures (below 1700°C) and high pressures (350 MPa). At 1500°C densities above 90%TD and at 1600°C above 95%TD could be achieved. A microstructural evaluation shows that the ultrafine powder particles exhibit large grain growth but still remain in the submicron range. The large particles show only a negligible coarsening. The resultant large grain size ratio larger than 10 leads to a significant increase in fracture toughness. While the fine grained SiC without addition of coarse grained SiC shows fracture toughness values of about 4.5 MPam 1 2 , the addition of 10% coarse grained powders leads to increase of fracture toughness of about 10%. SiC samples with 20% of coarse grained powders even reach K 1c values of 6 MPam 1 2 .

拘束したスラブの一面加熱によるセラミックスの熱破壊じん性の測定

A new technique to evaluate the thermal fracture toughness of ceramics was proposed by using the precracked slab specimens. Three kinds of mullite ceramics were used in the experiment. An initial precrack was introduced by the water quenching method, and was propagated by the three-point loading. The precracked slab specimen was held firmly by theree points and was heated from one side by a ruthenium thin film heater. The repulsive load at the restricted point was continuously measured by a load cell to monitor the thermal stress. The numerical analysis of stress intensity factor of the single edge precracked slab was conducted to determine the thermal fracture toughness from the generated repulsive load at fracture. A slight difference in thermal fracture toughness among three kinds of mullite could be detected by this technique. This experimental method is applicable to determine the thermal fracture toughness of ceramics under actual thermal shock conditions.

Shear induced transformation toughening in ceramics

There is experimental evidence that the tetragonal to monoclinic transformation in zirconia (ZrO 2) can be triggered not only by hydrostatic pressure but also by shear stresses. In this paper we consider the implications of a transformation criterion which depends on both mean stress and shear stress on transformation toughening in ceramics reinforced with zirconia inclusions. We treat zirconia toughened ceramics (ZTC) as dilute composites of randomly oriented inclusions in an elastic matrix and develop the constitutive model for the composite from the behaviour of single inclusions by orientation averaging. We thenstudy quasistatic crack propagation and predict the transformation zone, distribution of transformed inclusions and toughness enhancement.

High temperature stability of the matrix-fibre interface in a SiC/SiC composite

The intrinsic brittleness, or low resistance to crack propagation, characteristic of most ceramic materials severely restricts their use and applications. There is great interest, and much research effort is being invested, in developing methods to improve the fracture toughness of ceramics. Amongst these methods the reinforcement of ceramic materials with high strength fibers is believed to offer great potential. The increase in toughness which can be achieved by means of fiber reinforcement arises from both crack deflection at the fiber-matrix interface and from bridging of the crack faces by unbroken fibers. The effectiveness of these processes depends strongly on the strength of the fiber-matrix interface. An efficient way of tailoring the strength of fiber-matrix interfaces in ceramic matrix composites is to form a thin layer of adequate shear strength around the fibers. The principal challenge is to identify a coating material, which has the necessary mechanical properties and is also stable in the working environment and at elevated temperatures. The thermal stability of fiber, matrix and interface should therefore be fully investigated. The purpose of this paper is to investigate the high temperature stability of the carbon coating interface, and its effect on mechanical properties, in a SiC fiber-SiC matrixmore » composite.« less

Crack‐Shape–Wake‐Area Effects on Ceramic Fracture Toughness and Strength

It is argued that, where crack wake‐area effects are a significant factor in the toughness of ceramics, the rate of change of wake area with crack growth normalized by the instantaneous wake area should be a factor in their mechanical behavior. Simple geometrical considerations show that differences in such wake area changes can vary by a factor of two or more for different test/crack geometries. This raises key questions regarding the (1) applicability of certain toughness tests for predicting strengths, (2) possible favoring of certain initial crack shapes or shape changes during failure, and (3) interaction with fracture mechanics, e.g., how stress intensity and such geometrical wake effects balance to control crack shape and failure.

Progress in Transformation Toughening of Ceramics

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical ...Read More

Whisker Toughening of Ceramics: Toughening Mechanisms, Fabrication, and Composite Properties

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical ...Read More

The stiffness and strength of transformation toughening ceramics with misoriented microcracks

Theoretical studies on misoriented transformation particles and microcracks in transformation toughening ceramics are presented using the Eshelby equivalent inclusion method. The stress field, stiffness and strength were calculated. Experiments were done by the three-point bend method using Al2O3/ZrO2 ceramics and the stiffness and strength were also measured. Comparison between theoretical and test results confirmed the important role of microcracks.

Cohesive Crack Model for Geomaterials: Stability Analysis and Rate Effect

The cohesive (or ficticious) crack model, characterized by softening stress-displacement relations, provides a good description of fracture of quasibrittle materials such as conrete, rock, or tough ceramics. The cohesive crack model is formulated in terms of compliance influence functions and the failure is analyzed as a stability problem. The size effect is determined by means of an eigenvalue problem. In this problem, the structure size for which a given relative crack length yields the maximum load is the eigenvalue. The model is further generalized to time dependence. The opening displacement is considered as a function of the cohesive stress and the opening rate of the crack. Finally, applications to rock and concrete are discussed.

Eigenvalue analysis of size effect for cohesive crack model

The paper analyses the effect of structure size on the nominal strength of the structure that is implied by the cohesive (or fictitious) crack model proposed for concrete by Hillerborg et al. A new method to calculate the maximum load of geometrically similar structures of different sizes without calculating the entire load-deflection curves is presented. The problem is reduced to a matrix eigenvalue problem, in which the structure size for which the maximum load occurs at the given (relative) length of the cohesive crack is obtained as the smallest eigenvalue. Subsequently, the maximum load, nominal strength and load-point displacement are calculated from the matrix equilibrium equation. The nonlinearity of the softening stress-displacement law is handled by iteration. For a linear softening law, the eigenvalue problem is linear and independent of the matrix equilibrium equation, and the peak load can then be obtained without solving the equilibrium equation. The effect of the shape of the softening law is studied, and it is found that the size effect curve is not very sensitive to it. The generalized size effect law proposed earlier by Bažant, which describes a transition between the horizontal and inclined asymptotes of strength theory and linear elastic fracture mechanics, is found to fit the numerical results very well. Finally some implications for the determination of fracture energy from the size effect tests are discussed. The results are of interest for quasibrittle materials such as concrete, rocks, sea ice and modern tough ceramics.

セラミックス基高機能材料 粒子分散強化

セラミックス基高機能材料 粒子分散強化

Whisker Toughening of Ceramics: Toughening Mechanisms, Fabrication, and Composite Properties

Gallium is a metal that literally melts in your hand. It has low toxicity, near-zero vapor pressure, and a viscosity similar to water. Despite possessing a surface tension larger than any other liquid (near room temperature), gallium can form nonspherical ...Read More

Small specimen compact tension testing of ceramics

The small size compact tensile specimen is useful for measurement of the fracture toughness of ceramics. The tabular data available for the compliance factor are accurately fitted by a polynominal expression. The elimination of residual stress is important for accurate measurement of the fracture toughness. [on SciFinder (R)]