Cyclic Fatigue Behavior of an Alumina Ceramic with Crack-Resistance Characteristics

The behavior under cyclic tension—tension loading of an alumina ceramic with pronounced crack-bridging (R-curve) characteristics is studied. Tests on disk specimens with indentation cracks reveal no failures below the static fatigue limit. Theoretical predictions of the stress-lifetime response, based on the premise that environmentally assisted slow crack growth is the sole factor determining lifetime, are consistent (within experimental scatter) with the data. The results indicate that there is no significant cyclic degradation from potential damage to the bridges, at least in the short-crack region pertinent to strength properties.     

Subcritical Crack Growth in Lead Zirconate Titanate

Subcritical crack growth in terms of velocity–stress intensity factor ( v – K ) curves in lead zirconate titanate (PZT) were experimentally characterized on poled and unpoled compact tension specimens. The poled specimens were tested under open- and short-circuit electrical boundary conditions, which resulted in an increase in fracture toughness by 0.2 MPa·m^1/2 for the accessible velocity range ( v = 10⁻⁹ to 10⁻⁴ m/s) in the open-circuit case. Subcritical crack growth of unpoled specimens was obtained under ambient (relative humidity = 35%) and dry (relative humidity ∼ 0.02%) conditions over a regime in stress intensity factor of 0.5 MPa·m^1/2.     

Fracture Resistance and Stable Crack-Growth Behavior of 8-mol%-Yttria-Stabilized Zirconia

The crack-growth behavior of a yttria-stabilized zirconia ceramic (8 mol% of cubic-phase yttria) was studied at room temperature. Double-cantilever-beam specimens were loaded with pure bending moments in a specially designed loading fixture inside an environmental scanning electron microscope. Crack-growth data were obtained from truly sharp (arrested) cracks, bypassing interpretation problems that involve crack initiation from a machined notch. The crack-growth study was conducted over a range of applied energy-release rates that allowed crack arrest on one hand and fast fracture on the other. Three energy-release-rate values were relevant: initiation of crack growth (3.5 J/m²), crack arrest (2.8 J/m²), and fast fracture (8.0 J/m²). At the macroscopic scale, subcritical crack growth occurred as a continuous process. In situ observations revealed that, at the microscopic scale, crack growth occurred in small jumps. The fracture mode for stable crack growth was identified to be transgranular.     

Subcritical Crack Growth in Sintered Silicon Nitride Exhibiting a Rising R-Curve

Subscritical crack growth of sintered silicon nitride was analyzed in terms of the R -curve. Provided that the stress intensity at the crack tip governs the subcritical crack-growth velocity, the K _I– V relationship of sintered silicon nitride exhibiting a rising R -curve is shown to shift to the high- K _I region as the crack advances.     

K R -Curve Behavior of Duplex Ceramics

The fracture toughness behavior during crack growth ( K ^R -curve behavior) of duplex ceramics is investigated. Different types of K ^R -curves can be distinguished depending on the microstructural designs of these materials which are characterized by the volume fraction and size of the dispersed pressure zones, and by their effective volume expansion. According to their K ^R -curve behavior, duplex ceramics can be subdivided into two groups consisting of "short-range" and "long-range" toughened materials. The experimental results are discussed regarding the appearance of different toughening mechanisms which are documented by crack path micrographs. An unusual toughening effect, a "crackbranching chain reaction," is documented by in situ observations. The critical stress to nucleate the observed process zone development is calculated and compared with the internal stress intensity factor K _i which has been previously proposed for these materials and with the material strength.     

Rising Crack-Growth-Resistance (R-Curve) Behavior of Toughened Alumina and Silicon Nitride

R -curves for a sinter/HIPed SiC(whisker)-reinforced alumina and a sintered silicon nitride were assessed by direct measurements of lengths of cracks associated with Vickers indentation flaws. The fracture toughness measurements based on (a) initial (as-indented) crack lengths, (b) equilibrium growth of cracks during increasing far-field loading, and (c) crack lengths corresponding to unstable fracture showed definitive trends of R -curves for both materials. The fracture mechanics analyses employed an indenter-material constant that was independently estimated using a physical model for the residual driving force and a free surface correction factor that accounted for the effects of size and shape of the cracks on stress intensity. It is shown that R -curve estimations based on crack length measurements have the intrinsic advantage that crack length dependence of fracture toughness is not assumed a priori as is done in conventional analysis based on strength. The measured fracture toughness of SiC(whisker)-reinforced alumina was in agreement with the prediction of a toughening model based on crack bridging by partially debonded whiskers.     

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.     

Comparison of R-Curves from Single-Edge V-Notched-Beam (SEVNB) and Surface-Crack-in-Flexure (SCF) Fracture-Toughness Test Methods on Multilayered Alumina–Zirconia Composites

The single-edge V-notched-beam fracture testing method was used to measure the short-crack R -curve behavior of a 90-vol%-alumina–10-vol%-ceria-doped zirconia multilayered composite. The initiation and extension of short cracks from the V-notch tip were observed in situ via optical microscopy. Initial crack lengths of 15–50 μm were obtained repeatedly. The resulting R -curve was similar to the R -curve that was measured using the surface-crack-in-flexure testing method. The ability to control the position of the V-notch tip within a material permits study of the influence of specific microscopic features on the fracture behavior.     

Acoustic Emission During Micro- and Macrocrack Growth in Mg-PSZ

Acoustic emission (AE) of a Mg-PSZ (TS grade) material has been studied during three- and four-point bending and stable macrocrack extension tests. During flexure, AE signals increased significantly upon the onset of nonlinear stress-strain behavior and were associated with phase transformation and microcracking on the tensile surface. Observations of crack extension in notched bars revealed relatively little AE during initial loading and onset of non-linear behavior. However, on subsequent loadings, the number of counts was in reasonable agreement with the extent of crack growth. These observations are discussed in terms of AE from phase transformation and crack growth at different grain size. They are compared with AE behavior of other zirconia-based materials.     

Effect of Fiber Bridging Stress on the Fracture Resistance of Silicon-Carbide-Fiber/Zircon Composites

The room-temperature fracture-resistance behavior ( R -curve) of unidirectional silicon-carbide-fiber-reinforced zircon-matrix composites has been studied experimentally and numerically. The composites showed strong rising R -curve behavior from experimental results that used in situ crack-length measurements taken via optical microscopy as well as the compliance method. A numerical calculation, based on the available models, then was performed to determine the bridging-stress function from the experimental R -curve. In addition, the effect of the residual stress and constituent properties on the bridging-stress function also has been considered in the numerical calculations. These results have indicated that the bridging-stress function, which controls the fracture resistance of ceramic composites, can be obtained from the carefully measured R -curve.     

R-Curve Behavior of In Situ Toughened α-SiAlON Ceramics

R -curves of single-phase Y- and Ca-containing α-SiAlON ceramics have been measured. They range from flat ones for fine-grain ceramics to pronounced rising ones when large elongated grains are present. The highest toughness measured reached 11.5 MPa·m^1/2 over a crack extension of about 1000 μm.     

物理钢化对玻璃表面裂纹扩展和强度的影响

物理钢化对玻璃表面裂纹扩展和强度的影响宋力昕，胡行方，陈显求（中国科学院上海硅酸盐研究所２０００５０）ＴｈｅＥｆｆｅｃｔｓｏｆＴｈｅｒｍａｌＴｅｍｐｅｒｉｎｇｏｎｔｈｅＧｒｏｗｔｈｏｆＳｕｒｆａｃｅＣｒａｃｋｓａｎｄｔｈｅＳｔｒｅｎｇｔｈｏｆＧｌａｓ...     

R-Curve Behavior of Si3N4–BN Composites

R -curve behavior of Si₃N₄–BN composites and monolithic Si₃N₄ for comparison was investigated. Si₃N₄–BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si₃N₄. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si₃N₄ grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m^1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si₃N₄.     

Weight Function Analysis on the R-Curve Behavior of Multilayered Alumina–Zirconia Composites

The single-edge V-notched-beam (SEVNB) testing geometry was used to measure the crack growth resistance ( R -curve) behavior of multilayered alumina–zirconia composites. Fracture mechanics weight function analysis was applied to predict the R -curve behavior of multilayered composites having a stepwise change in composition. These results were then used to differentiate the influence of residual stresses from crack-bridging stresses on the measured R -curve behavior.     

Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three-point bend bar test specimens. R -curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room-temperature " K _IC" fracture toughness values of 4.5 MPa · m^1/2 for the chevron-notch specimen and 3.9 MPa · m^1/2 for the straight-notch configuration were obtained. The critical stress intensity factor of the renotched chevron-notch specimen compared very closely with that of the straight-notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.     

Numerical Evaluation of Toughening by Crack-Face Grain Interlocking in Self-Reinforced Ceramics

Crack bridging associated with the pull-out process of interlocking grains in self-reinforced ceramic materials is studied through a micromechanical simulation. The pullout of a single inclined grain is modeled via the numerical solution of a general contact problem. The bridging-force versus crack-opening-distance curve indicates a nonlinear, springlike response for the pullout of interlocking grains. The sliding friction along the debonded interface, induced by highly localized contact stresses, dominates the total bridging force. The bridging force increases with grain inclination until eventual bridge failure. The pullout of misaligned grains mainly affects short-crack toughening, with a rising R -curve, whereas aligned grains contribute to long-crack toughening. The residual stresses of the thermal expansion anisotropy play a minor role in the pull-out process of grain interlocking and the resultant toughening. The proposed mechanism is operative in both single-phase and composite ceramics in which pullout of elongated grains/reinforcements occurs.     

Toughness and R-curve behaviour of laminated Si3N4/SiCw ceramics

Laminated Si₃N₄/SiC_w ceramics were successfully prepared by tape casting and hot-pressing. Its mechanical properties were measured and the impact resistance was discussed. The toughness of the laminated Si₃N₄/SiC_w ceramics was 13.5 MPa m^1/2, which was almost 1.6 times that of Si₃N₄/SiC_w composite ceramics, namely 8.5 MPa m^1/2. Moreover, the indentation strength of laminated Si₃N₄/SiC_w ceramics was not sensitive to increasing indentation loads and exhibited a rising R-curve behaviour, indicating that the laminated Si₃N₄/SiC_w ceramics had excellent impact resistance. The improved toughness and impact resistance of laminated Si₃N₄/SiC_w ceramics was attributed to the residual stress caused by a thermal expansion coefficient mismatch between the different layers, resulting in crack deflection and bridging of SiC whiskers in the interface layer, thus consuming a large amount of fracture work.     

热压氮化硅材料中小裂纹的稳态扩展

对一种HPSN材料中Vickers压痕裂纹的稳态扩展过程进行了研究,发现裂纹扩展阻力随着裂纹尺寸的增大而增大,由于残余应力的影响,其K_(R)～C关系曲线的形状及位置与压痕压制荷载及压痕裂纹初始尺寸有关。经退火消除残余应力后测得的K_(R)～C曲线可能反映了材料的本征性能。