Effect of Lateral Cracks on Fracture Toughness Determined by the Surface-Crack-in-Flexure Method

The surface-crack-in-flexure (SCF) method uses a Knoop indenter to create small, semielliptical surface precracks in beam specimens. Lateral cracks may interfere with the primary median crack and cause errors of up to 10% in determination of fracture toughness, particularly for materials for which the fracture toughness is ∼3 MPa·m^1/2 or less. Although the residual-stress-damage zone is ground or polished away by hand by removing 4.5–5 times the indentation depth, this amount may not be sufficient to completely remove the lateral cracks in low-fracture-toughness materials. A series of tests were conducted on sintered alpha silicon carbide with different amounts of material removed after indentation. Once the lateral cracks were fully removed, the SCF results concurred with single-edged-precracked-beam and chevron-notched-beam data collected in accordance with ASTM Designation C1421. A simple remedy for the SCF method is to examine the outer ground surface for remnants of lateral cracks before fracture and to remove more material if necessary.     

Effect of Micmcracking on the Fracture Toughness and Fracture Surface Fractal Dimension of Lithia-Based Glass-Ceramics

The effect of thermally induced microcracks on the fracture toughness and fractal dimension of fully crystalline lithia disilicate glass-ceramics was studied. The fracture toughness, K _IC, for the nonmicrocracked lithia disilicate, 3.02 ± 0.12 MPa·m^1/2, was significantly greater than the value of 1.31 ± 0.05 MPa·m^1/2 for the microcracked specimens. The fractal dimensional increment, D *, was 0.24 ± 0.01 for nonmicrocracked lithia disilicate specimens compared with a value of 0.18 ± 0.01 for the microcracked specimens. The relationship between K _IC and D * implies that the two materials exhibit dissimilar fracture behavior because of microstructural differences. Estimates of the characteristic length involved in the fracture process, a ₀, indicate that the materials have an identical fracture process at the atomic level. This apparent contradiction may be explained by the scale on which the measurements were taken. It is suggested that fractal analysis at the atomic level would yield equivalent D * values for the two different microstructures.     

Flexure Strength, Fracture Toughness, and Slow Crack Growth of YSZ/Alumina Composites at High Temperatures

Flexure strength and fracture toughness of zirconia–alumina composites, fabricated by hot pressing 10 mol% yttria-stabilized zirconia (10-YSZ) reinforced with 0–30 mol% alumina particulates or platelets, were determined as a function of alumina content at 1000°C in air. Both strength and fracture toughness of the two composite systems increased with increasing alumina content. For a given alumina content, flexure strength of the particulate composites was greater than that of the platelet composites at higher alumina contents (≥20 mol%); whereas, fracture toughness of the platelet composites was greater than that of the particulate counterparts, regardless of the alumina content. The susceptibility to slow crack growth (SCG), determined at 1000°C via constant stress-rate testing, was greatest for 30 mol% particulate composite with SCG parameter n =5–8 and was least for 30 mol% platelet composite with n =33. Elastic modulus of both composite systems decreased below 400°C and then remained almost unchanged up to 1000°C, forming a unique transition around 400°C, irrespective of alumina content.     

Crack Stability and Its Effect on Fracture Toughness of Hot-Pressed Silicon Nitride Beam Specimens

The effect of stable crack extension on fracture toughness test results was determined using single-edge precracked beam specimens. Crack growth stability was examined theoretically for bars loaded in three-point bending under displacement control. The calculations took into account the stiffness of both the specimen and the loading system. The results indicated that the stiffness of the testing system played a major role in crack growth stability. Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions. Hot-pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension. The fracture toughness values from the unstable tests were 10% higher than those from the stable tests. This difference, albeit small, is systematic and is not considered to be due to material or specimen-to-specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughness values.     

Effect of Subcritical Crack Growth on Fracture Toughness and Work-of-Fracture Tests Using Chevron-Notched Specimens

A correlation between the plane strain stress intensity factor K_I , load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron-notched, four-point-bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity v_t at K_I equal to a threshold value K_t . Maximum crack velocity is reached at K_I= K_IC , the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with K_Ic . An error in K_IC calculation is estimated if different displacement rates are applied. Repeated loading-unloading work-of-fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, K_t , only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, K_IC are not generated in WOF tests.     

Fracture Toughness Predictions for Crack Bowing in Brittle Particulate Composites

The bowing of cracks between obstacles has been proposed as a mechanism for increasing the .fracture toughness of brittle materials. An analytical expression is presented based on previous theoretical studies that can be used to predict the increase in fracture toughness. The approach incorporates the effect of obstacle failure and the assumptions needed to compare the predictions with experimental data.     

Indentation Fracture Toughness of Sintered Silicon Carbide in the Palmqvist Crack Regime

The fracture toughness of a sintered dense α-SiC was estimated by the Vickers indentation microfracture method in the low-load Palmqvist crack regime. It was observed that the use of simultaneously obtained Vickers hardnesses does not yield reliable fracture toughness values, nor does application of the median-crack-derived equations. It is necessary to utilize a load-independent, crack-free hardness value with this toughness estimation method. Although several of the curvefitting equations yield similar toughnesses, it is concluded for the Palmqvist crack system in this α-SiC that the Niihara-Morena-Hasselman equation is the only one which yields fracture toughness values in agreement with conventional measurement techniques.     

Influence of Surface Structure on the Fracture Toughness of Tempered Glass

A nondestructive measurement of the surface stress and an indentation technique were used to determine the relationship between the fracture toughness and the surface compressive stress of a tempered glass plate. Ignoring the effect of surface compression, one can determine the fracture toughness concerned only with a change in the glass structure. It is found that the fracture toughness is lower for tempered glass than for annealed glass. It is indicated that the decrease is due to structural changes in the surface layer during the heat treatment in tempering; this is confirmed by infrared reflection spectroscopy.     

Effect of Indenter Geometry on Controlled-Surface-Flaw Fracture Toughness

Fracture toughness values obtained using both Knoop and Vickers-indentation-produced controlled surface flaws were compared as a function of indentation load for a well-characterized glass-ceramic material. At the same indentation load, Knoop cracks were larger than Vickers. As-indented K_c values calculated from fracture mechanics expressions for surface flaws were higher for Knoop flaws than Vickers, but both types gave low K_c values due to indentation residual stress effects. Analysis suggested that theoretical formalisms for indentation residual stress effects based on fracture mechanics solutions for a center-loaded penny crack in an infinite medium should apply to both indentation types. K_c values calculated using the residual stress approach were identical for Knoop and Vickers controlled surface flaws when a "calibration" value for a constant term in the expression for K_c was used for both indentation types.     

Determination of Fracture Toughness at High Temperatures after Subcritical Crack Extension

As a consequence of R -curve behavior, ceramic materials may exhibit increased fracture toughness ( K _Ic) following slow crack extention. In this investigation, the effect of crack propagation on fracture toughness is studied in static bending tests. For the calculation of stress intensity factors ( K _I) the stress distribution must be known at the moment of fracture. As a consequence of creep, this stress distribution must deviate from the linear distribution. The corresponding stress intensity factors are computed using the fracture mechanical weight function. Experimental results for fracture toughness are communicated for a 2.5%-MgO-doped hot-pressed Si₃N₄ at 1300°.     

Effect of Grain-Boundary Oxidation on Fracture Toughness of SiC

Hot-pressed SiC was oxidized at temperatures from 900° to 1300°C, and the fracture toughness was determined by the indentation method. The apparent fracture toughness of the surface layers increased with oxidation temperature and then decreased. The observed variations in apparent fracture toughness were consistent with earlier observations of strength variations in oxidized SiC. Some specimens were fractured, and the fracture surfaces were characterized. Based on these observations, the variations in fracture toughness were attributed to variations in residual compressive stresses induced by oxidation.     

Effect of Surface Tension on the Toughness of Glass

Mechanical stresses (i.e., surface tension) inherently exist on a general free surface because its atomic structure differs from its bulk counterpart. The effect of surface tension is amplified at a crack tip because of curvature enhancement. An integral equation describing this effect on the toughness of glass is derived and first-order approximations using a weight function technique were made. The qualitative results indicate that the geometry-induced toughening is linearly proportional to surface tension and crack tip curvature and to the square root of crack tip zone size. An illustrative example of a recently observed crack tip in SiO₂ glass is given which shows that toughness is enhanced by ∼3 times the intrinsic K _1C value.     

Silicon Carbide Whisker/Alumina Matrix Composites: Effect of Whisker Surface Treatment on Fracture Toughness

The fracture toughness of a 30 vol% SiC whisker/Al₂O₃ matrix composite was evaluated as a function of whisker surface chemistry. Two types of SiC whiskers (Silar-SC-9 and Tateho-SCW-1-S) were investigated. Modification of the whisker surface chemistry was achieved by subjecting the whiskers to thermal treatments under controlled atmospheres. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy, was correlated to the fracture toughness of the composites.     

Evaluation of Fracture Toughness for Ceramic Materials by a Single-Edge-Precracked-Beam Method

As a substitute for the fatigue-cracked-beam method prescribed in ASTM E399 A2, a recently devised precracking method was applied to the evaluation of fracture toughness of ceramic materials. Straight-through precracks proved to be easily introduced into rectangular beams of several ceramic materials. This method gives K_ic values almost identical with those of the fatigue-cracked-beam method. The K_ic values are almost constant over wide ranges of the pop-in precrack length and the loading rate of the three-point bend test. The test can be easily performed even at elevated temperatures although its validity should be further examined.     

Effect of a Microheterogeneous Structure on the Fracture Toughness of Glasses

A model is presented that describes the variation of the critical stress intensity factor, K _k., of glasses as a function of composition. It is based on a microheterogeneous structure that produces nonannealable stresses in the material. The effect of these internal stresses on the measured values of K _IC is expressed as a function of the mechanical and thermal parameters of the material. Application to several borosilicate glasses is presented and discussed.     

不同泊松比复合材料的层板断裂韧性和裂纹尖端应力场

比较了两组具有不同平面泊松比的玻璃纤维／环氧树脂复合材料层板的断裂韧性和缺口断裂强度，发现低泊松比复合材料层板具有较高的断裂韧性和缺口断裂强度；用复变函数一变分方法计算了它们的裂纹尖端应力场；研究了主应力方向与纤维夹角的关系；结果表明：低泊松比材料独特的裂纹尖端应力场有利于提高缺口断裂强度。     

压力容器材料断裂韧性值的估算方法

断裂韧性值KIC是评定材料安全性能的主要参数,寻找KIC的估算方法就显得非常有必要。利用估算方法,既能减少断裂韧性试验的费用,又能取得评价所需要的参数值。本文简述了压力容器材料在几种不同环境下的断裂韧性值的估算方法,使得一些缺陷材料在不能通过实验获得其断裂韧性值的情况下利用估算方法间接获取,为压力容器设备的安全评定提供了方便。     

Effect of Notch-Root Radius on the Fracture Toughness of a Fine-Grained Alumina

The dependence of K _Ic on the notch-root radius has been examined for a notch radius as small as a few micrometers in a dense, fine-grained, polycrystalline alumina ceramic. The notch radius can be systematically varied by using a semimanual procedure in a special jig which polishes out rather than cuts the specimen. K _Ic is independent of the notch sharpness for notch-root radii < 10 μm. The results are critically compared with those obtained by other standard techniques and discussed in terms of residual compressive stresses introduced during the notching procedure.