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.     

Controlled Surface Flaws in Hot-Pressed Si3N4

Surface flaws of controlled size and shape were produced in high-strength hot-pressed Si₃N₄ with a Knoop microhardness indenter. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a 4-point-bend specimen, with attendant reduction in the measured magnitude and scatter of the fracture strength. The stress required to propagate the controlled flaw was used to calculate the critical stress-intensity factor, K _IC, from standard fracture-mechanics formulas for semielliptical surface flaws in bending. After the bend specimen had been annealed, the room-temperature K _IC values for HS-130 Si₃N₄ increased to a level consistent with values obtained from conventional fracture-mechanics tests. It was postulated that annealing reduces the residual stresses produced by the microhardness indentation. The presence of residual stresses may account for the low K _IC, values. Elevated-temperature K_IC values for HS-130 Si₃N₄ were consistent with double-torsion data. Controlled flaws in HS-130 Si₃N₄ exhibited slow crack growth at high temperatures.     

Mixed-Mode Fracture of Ceramics in Asymmetric Four-Point Bending: Effect of Crack-Face Grain Interlocking/Bridging

The mixed-mode fracture of a large-grain-size alumina ceramic and a soda-lime glass is investigated. These ceramics are tested using straight-through precracked or notched specimens. The straight-through precrack is introduced by the single-edge-precracked beam method. Precracked or notched specimens are subjected to combined mode I/II or pure mode II fracture, under asymmetric four-point bending, and pure mode I fracture, under symmetric four-point bending. A pure mode II fracture is never achieved in the precracked polycrystalline alumina by the crack-face friction inevitably induced by grain interlocking/bridging. The crack-face friction in sliding mode reduces the local mode II stress intensity factor in the crack-tip region and produces a sizable amount of mode I deformation. Accounting for the contribution of the crack-face friction to the crack-tip local stress intensity factors, K _I and K _II, in mixed-mode fracture tests, the experimental results of the K _I/ K _{I c} versus K _II/ K _{I c} envelope and the initial angle of noncoplanar crack extension are in good agreement with the theoretical predictions of the maximum hoop-stress theory.     

Controlled Surface Flaw-Initiated Fracture in Reaction-Densified Silicon Carbide

Controlled surface flaws were produced in commercial reaction-densified SiC by Knoop microhardness indentation. The flaws themselves could not be observed easily, thus an etching technique was used to delineate their semielliptical shape, thereby enabling calculation of the critical stress- intensity factor K _IC at room temperature. Room-temperature fracture was insensitive to annealing environment (air or vacuum), flaw "healing" being observed at ≫1000°C. The variation in fracture stress of indented specimens with temperature showed 3 distinct regions of behavior which were interpreted in terms of residual stress relief, flaw healing, and Si-SiC bond weakening.     

Effect of Surface Flaw Size on Fracture Strength of Alumina Ceramics

Semielliptical surface flaws of different sizes were introduced into Al₂O₃ by Knoop microhardness indentation. The specimens were fractured by four-point bending and the profiles of the indentation flaws were determined by observing the fracture surfaces with a scanning electron microscope. The relation between the indentation flaw size and the fracture strength could be well explained by applying the fracture-mechanics analysis for semielliptical surface flaw in bending. The calculated values of the as-indented critical stress intensity factor, K_IC, were lower than previously reported presumably because of the influence of the residual stresses produced by the indenter.     

Fracture and Subcritical Crack Growth in Soda-Lime Glass under Combined Modes I and II Loading

Fracture and subcritical crack growth characteristics under combined modes I and II loading were studied using the compact tension shear specimens of soda–lime glass. The maximum normal stress criterion gives a good agreement with the experimental mode I–mode II fracture toughness envelope for initially straight cracks and kinked cracks. Subcritical crack growth characteristics were determined under sustained modes I and II loading in water. The values of K _I and K _II were calculated approximately by replacing the subcritical kinked crack with an assumed straight crack ā, and the K _Imax value based on the maximum normal stress criterion was used to describe this subcritical kinked crack growth. The experimental results show that subcritical crack growth under pure mode I, pure mode II, and various combined modes I and II loading can be well described by the K _Imax value based on the approximate maximum normal stress criterion.     

Controlled Surface Flaws in Hot-Pressed SiC

Controlled semi-elliptical surface flaws were produced in hot-pressed SiC by Knoop microhardness indentation. Flawed specimens were placed in 4-point bending in order to determine their critical stress intensity factor, K_IC, at both room and high temperatures. Room-temperature fracture and K_IC values after annealing were sensitive to the annealing environment; this behavior correlated with the active/passive nature of the oxidation process. Flaw healing was observed for annealing exposures in air. Room-temperature K_IC values increased with increasing annealing temperature. High-temperature K_ICvalues decreased with increasing temperature as a result of a decrease in the fracture surface energy.     

Mixed-Mode Fracture of Hot-Pressed Si3N4

The mixed-mode fracture of hot-pressed Si₃N₄ was investigated using inclined indentation surface flaws in bending and large crack geometries in combined tension/torsion. Non-coplanar fracture was observed in all cases. Values of K_Ic, K_IIc, and K_IIIc stress intensity factors were obtained, with ratios K_IIc/K_Ic= 0.79 and K_IIIc/K_Ic= 1.55 observed. For large cracks, mode II conditions had more of an effect on mode I fracture than mode III conditions. The mixed-mode I-II fracture of surface flaws was significantly different from that for large cracks, suggesting surface flaw shear resistance effects. A model describing these effects was derived, based on the ratio of the crack-opening displacement to the crack surface asperity height.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

Effect of Precrack "Halos" on Fracture Toughness Determined by the Surface Crack in Flexure Method

The surface crack in flexure method, which is used to determine the fracture toughness of dense ceramics, necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient, room-temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this article, fractographic techniques are used to characterize evidence of stable crack extension, a "halo," around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high-purity Al₂O₃, an AlN, a glass-ceramic, and a MgF₂ reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF₂ is due to indentation-induced residual stresses initiating crack growth. However, for the Al₂O₃ and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.     

Fracture Toughness of Ceramics Measured by a Chevron-Notch Diametral-Compression Test

A diametral-compression test that features disk specimens with chevron notches was developed and applied to measure fracture toughness (K_lc) of three ceramics. Fracture–toughness values calculated from the fracture loads and a compliance analysis were in good agreement with values measured by other fracture–mechanics techniques. Advantages of the proposed test include simple specimen geometry, minimal requirements for fixtures, and consequent potential for application at elevated temperatures.     

Mixed-Mode Fracture from Controlled Surface Flaws in Hot-Pressed Si3N4

The room-temperature mixed-mode fracture of commercial hot-pressed Si₃N₄ was examined using controlled surface flaws in 4-point bending, oriented at various angles 6 with respect to the outer fiber tensile stress direction. Catastrophic fracture paths were non-coplanar with the initial flaw plane, and the stress intensity factor ratio K_I/K_IC was < 1 for fracture in modes II and III. A non-coplanar maximum strain-energy release rate fracture criterion best described mixed-mode fracture.     

Mixed-Mode Fracture in Soda-Lime Glass

Soda-lime-silica glass was fractured under combined mode I and mode II loading from flaws produced by hardness indentations. Critical stress intensities calculated from K₁ and K₁₁ combined using four analyses, are compared to K_IC measured by a fracture mechanics technique and to values of K_IC determined by measuring fracture-mirror size. The comparison showed that K_IC calculated using a noncoplanar strain-energy release-rate analysis gave the best agreement with values obtained by fracture mechanics techniques over the widest range of crack orientations. K_ICcalculated from mirror sizes was constant regardless of the orientation of the original flaw.     

Fracture Toughness of a Toughened Silicon Nitride by ASTM C 1421

The fracture toughness of a sintered reaction-bonded silicon nitride was measured by the single-edge precracked beam and surface crack in flexure methods, which are two of the three complementary test methods in ASTM C 1421. Results were compared with chevron-notched beam results that were available from another source. Precracks ranged from tiny artificial flaws introduced by Knoop indentation to millimeter-long precracks in single-edge precracked beams. The fracture toughness values from the three methods were in good agreement at 5.6 MPa·m^1/2.     

Advances in the Grinding Efficiency of Sintered Alumina Abrasives

The study relates the grinding power of different grades of sintered alumina abrasives to their microstructures and to basic mechanical properties in comparison with conventionally fused electrocorundum and with an electrofused alumina/ zirconia eutectic. Contrary to the traditional approach of the Battelle test, the fracture toughness K _{I c} of individual grains is measured by a quantitative indentation analysis. Compared with fused corundum, sintered alumina grits exhibit an increased toughness and grinding efficiency, but the further increase of K _{I c} in the eutectic does not improve the grinding performance. The key parameter for grinding is the inherent hardness of the abrasive. The elimination of flaws by a new approach results in a strong increase in the grinding power of sintered alumina abrasives.     

Effects of an Electric Field on the Fracture Toughness of Poled Lead Zirconate Titanate Ceramics

Compact tension tests and indentation-fracture tests have been conducted to study the effects of an applied electric field on the fracture toughness ( K _{I C} ) of poled commercial lead zirconate titanate (PZT) ceramics. The experimental results show that an applied electric field, either parallel or antiparallel to the poling direction, considerably reduces the K _{I C} value of the PZT ceramics. The reduction in K _{I C} for a negative field is larger than that for a positive field of the same strength. The failure mode in the PZT ceramics is basically transgranular, insensitive to the applied electric field.     

Mechanical Behavior of a Borosilicate Glass Under Aqueous Corrosion

In France, fission products are being vitrified for a possible final geological disposal. Under disposal conditions, corrosion of the glass by groundwater as well as stress corrosion because of stresses occurring at surface flaws cannot be excluded. Within this framework, the mechanical behavior of the French simulated nuclear waste glass SON68 was studied by Vickers indentation and fracture experiments in air and in a corrosive solution. The glass was corroded at 90°C in a solution enriched with Si, B, and Na. The results showed that the glass corrosion enhances the cracks propagation relative to experiments in air. The indentation fracture toughness ( K _{I C} ) obtained using a four-point bending test showed that the K _{I C} of the glass decreased with increasing corrosion time.     

Influence of Microstructure on Crack Propagation in ZnSe

The rate of crack propagation as a function of the stress intensity factor, K ₁, for chemically vapor-deposited (CVD) ZnSe was significantly larger in H₂O than in air. Lower flexural strengths were measured in H₂O (29 MN/m²) than in air (40 MN/m²). Fracture surface analysis of these flexural specimens showed that failure usually resulted from flaws contained within 1 or 2 large grains. These flaws were shown to propagate at a stress-intensity factor of 0.33 MN/m^3/2 compared to the measured value of 0.70 MN/m^3/2 for polycrystalline ZnSe. Fracture mechanics parameters obtained in the present study were used to show that failure time predictions using values of K ^1C for propagation through a single grain are 25 times shorter, and proof-to-service stress ratios are 1.1 times greater, than those obtained for polycrystalline ZnSe.     

Foreign Object Damage Resistance of Silicon Nitride and Silicon Carbide

To clarify the foreign object damage (FOD) resistance of ceramics, chipping fracture mode and flexural fracture mode were investigated using several types of Si₃N₄ and Sic. The critical velocity which is the threshold impact velocity of the projectile for chipping fracture and flexural fracture was determined. The critical velocity of the chipping fracture mode is explained as a function of K ^5/2_IC a ^–5/4, and depends on the hardness and the shape of the projectile. The critical velocity of the flexural fracture mode is explained as a function of σ^5/6_C t ^5/3. The mechanisms of impact damage are discussed.