Double Torsion testing of thin porous zirconia supports for energy applications: Toughness and slow crack growth assessment

Thin, porous zirconia-based ceramic components are of high interest in energy application devices where they are used as structural ceramics. Mechanical reliability of such devices is not only dependent on the fracture toughness of the ceramic components, but also on their sensitivity to slow crack growth (SCG). In this work, the fracture toughness and SCG behavior of porous (4.5–45.5%) and thin (∼ 0.25 mm) 3Y-TZP ceramics are investigated using the Double Torsion method. The analysis of the double torsion data, previously developed for dense materials, was here assessed and adapted. The compliance of the samples was observed to change linearly with crack length and the measured stress intensity factor was dependent on crack length, as for dense materials. This dependency decreased by increasing the sample porosity. For all materials, the ratio of the SCG threshold to fracture toughness was of 0.56 ± 0.06.     

Evaluation of the Microtensile Bond Strength between Resin Composite and Hydrofluoric Acid Etched Ceramic in Different Storage Media

The aim of this study was to evaluate the effects of storage media on the bond strength between resin composite and ceramic that was etched with hydrofluoric acid and silanized. Two types of ceramics were used: lithium disilicate and leucite-reinforced. The ceramic surface was etched with 4.7% hydrofluoric acid and bonded to the resin composite using a silane coupling agent. Specimens were divided into 10 groups and each group of specimen was subjected to different types of storage conditions for 7 days: de-ionized water (control), 15% ethanol, lemon soda, and cola. The microtensile test was used to measure the bond strength. The results showed that storage in food beverages significantly reduced bond strength compared to controls because of the acidity of beverages (ANOVA; p < 0.05). Lithium disilicate ceramics stored for 7 days in ethanol produced bond strengths significantly lower than those stored in 7 days in de-ionized water (p < 0.05). Leucite-reinforced ceramics stored for 7 days in lemon soda generated mean bond strengths significantly lower than those stored for 7 days in de-ionized water (p < 0.05). The significance of this in vitro study is that patients who have just repaired fractured crowns or inserted laminates should refrain from drinking acidic substances because it may weaken the resin-to-ceramic bond.     

Room temperature R-curve and stable crack growth behaviour of ZrB2–SiC ceramic composites

ABSTRACT

R-curve and controlled stable crack growth behaviour of ZrB₂–17vol.-%SiC and ZrB₂–45vol.-%SiC ceramic composites was studied on V-notched samples using four-point bending at room temperature. The rising K_1R behaviour was determined as a function of the crack extension Δa with a crack bridging mechanism being dominant in such behaviour. Significant differences in crack growth rates were found within the same composition of ceramics simply as the crack length varied during crack growth processes. These differences are indicative of the significant influence of microstructural parameters of the ceramics on crack propagation. The peculiarities of stress intensity factor K₁ and the crack growth-specific behaviour in ZrB₂–SiC particulate ceramic composites are discussed.     

Fracture mechanics properties of fused silicas used for international space station windowpanes

The fracture toughness and slow crack growth (SCG) parameters of a quartz-based silica and a high-purity fused silica were measured as part of a program to review the reliability of the International Space Station windows. The materials exhibit the same fracture toughness (.75 MPa m^1/2 in N₂) and very similar SCG parameters. The literature on fused silica indicates excellent agreement of fracture toughness, but a very wide range of SCG parameters, even from the same institution, with strength-based methods usually yielding a lower power law exponent than direct crack velocity measurements. Use of the exponential function is shown to provide better agreement between test methods, with velocity curves derived from strength tests of bare fiber and polished or ground test specimens paralleling those from wide-range, direct crack velocity observations, implying that constant stress rate tests can predict long lifetime via the exponential function. However, much variation still exists. SCG parameters for soda–lime silicate are much less sensitive to the test method than fused silica. Static load tests and stress intensity measurements resulted in a fatigue threshold of .3 MPa m^1/2 for fused silica.     

Time-Dependent Strength Degradation of a Siliconized Silicon Carbide Determined by Dynamic Fatigue

Both fast-fracture strength and strength as a function of stressing rate at room temperature, 1100°, and 1400°C were measured for a siliconized SiC. The fast-fracture strength increased slightly from 386 MPa at room temperature to 424 MPa at 1100°C and then dropped to 308 MPa at 1400°C. The Weibull moduli at room temperature and 1100°C were 10.8 and 7.8, respectively, whereas, at 1400°C, the Weibull modulus was 2.8. The very low Weibull modulus at 1400°C was due to the existence of two exclusive flaw populations with very different characteristic strengths. The data were reanalyzed using two exclusive flaw populations. The ceramic showed no slow crack growth (SCG), as measured by dynamic fatigue at 1100°C, but, at 1400°C, an SCG parameter, n , of 15.5 was measured. Fractography showed SCG zones consisting of cracks grown out from silicon-rich areas. Time-to-failure predictions at given levels of failure probabilities were performed.     

聚乙烯管材热熔对接接头抵抗慢速裂纹扩展性能评价

采用应变硬化试验(SH)对不同焊接工艺下的聚乙烯管材热熔对接接头抵抗慢速裂纹扩展（SCG）性能进行评价。通过建立焊接温度梯度（190~250 ℃）、焊接压力梯度（0.6~1.4 MPa）和吸热时间梯度（40~140 s）试验,分析在不同焊接工艺参数条件下,不同聚乙烯管材热熔对接接头耐SCG性能的变化规律,探索冷焊及过焊2种典型缺陷对管材接头耐SCG性能的影响。结果表明,焊接温度、焊接压力和吸热时间都是影响管材热熔对接接头耐SCG性能的重要工艺参数,试验测得PE100, d_n110, SDR11型管材的最佳焊接参数为焊接温度230 ℃,焊接压力1 MPa及吸热时间100 s,当焊接参数选取过高或过低时,会造成管材接头出现过焊或冷焊缺陷,降低管材接头的耐SCG性能。     

Effects of Er,Cr:YSGG laser on repair bond strength of 5-year water-aged and non-aged CAD/CAM ceramics

The aim of this study is to examine the repair bond strength of three different 5-year water-aged and non-aged computer-aided design/computer-aided manufacturing (CAD/CAM) ceramics (leucite-reinforced, lithium disilicate, and feldspathic ceramic) on which four different surface treatments (bur-grinding, sandblasting, acid-etching, and laser irradiation) have been applied with composite resin. Note that 360 ea. samples have been attained from CAD/CAM blocks. Each CAD/CAM ceramic has been randomly separated into two sub-groups depending on aging procedure. The designed 5-year water-aged and non-aged samples have been separated into four sub-groups. Ceramic surfaces were repaired then the samples have been placed into shear test device. Three-way variance analysis has been used in the comparison of the repair bond strengths depending on the ceramic type, surface treatment, and aging. Results have revealed that the repair bond strength values show differences depending on CAD/CAM ceramic type, surface treatment, and the aging of the surface (p < .001). While the aged and laser-irradiated feldspathic CAD/CAM ceramics showing the highest shear bond strength, the lowest shear bond strength values were in aged and bur-grinded feldspathic CAD/CAM ceramics. Irradiation with erbium chromium: yttrium scandium gallium garnet (Er,Cr:YSGG) laser has significantly increased the repair bond strength in leucite-reinforced and feldspathic CAD/CAM ceramics, acid-etching is suggested surface treatment for the lithium disilicate CAD/CAM ceramics.     

Postindentation slow growth of cracks in structural ceramics

Different forms of representing data on subcritical crack growth are discussed, which makes it possible to compare materials and (or) interpret the physical meaning of the parameters of slow crack growth (SCG). The SCG parameters are determined for hot-pressed OTM-914 materials and an Si₃N₄ — SiC (NK) composite. The scattering of the results can be explained by errors of the method, variation of the properties over the bulk of the material, surface oxidation, and other causes.     

Crack Growth along Interfaces in Porous Ceramic Layers

Crack growth along porous ceramic layers was studied experimentally. Double cantilever beam sandwich specimens were loaded with pure bending moments to obtain stable crack growth. The experiments were conducted in an environmental scanning electron microscope enabling in situ observations of various mechanisms associated with crack growth. The macroscopic fracture energy of the interface between dense lanthanum strontium chromite and a porous lanthanum strontium manganite was measured to lie in the range of 1.4–3.8 J/m². Several micromechanisms were observed ahead of, or in the wake of, the crack tip. The measured variation of the fracture energy along the crack length may be attributed to such phenomena.     

Subcritical crack growth models for static fatigue of Hi-NicalonTM-S SiC fiber in air and steam

Three subcritical crack growth (SCG) laws were used to model strain-rates and failure times for static fatigue of Hi-Nicalon^TM-S SiC fiber tows in air and Si(OH)₄(g)-saturated steam. Models were fit to tow failure times ( t_f ) and steady-state strain rates ( ἑ ) for brittle creep measured at 700 to 1100°C under initial applied stresses ( σ_A ) of 260 to 1260 MPa. A power law, a reaction-rate law, and a bond-energy law were used to describe SCG that caused sequential filament failure, and ultimately tow failure. Two versions of each model were developed. One allowed access of chemisorbed species to flaws throughout the fiber (mode 1) and another only allowed access to flaws at the SiC-SiO₂ interface (mode 2). The stress increase on intact filaments as others fractured and as filaments oxidized, and the increase in stress intensity geometric factors ( Y ) as crack size increased were incorporated in the models. The fits to data were compared for the different models by using both simple regression analysis and orthogonal distance regression (ODR) analysis. Faster convergence and more consistent results were achieved using ODR analysis. Regression analyses found parameters for all models with similar error in data fits, so validity of a model could not be distinguished by regression analysis alone. For all models, the stress dependence of SCG rates was much stronger in steam than in air, and for most models activation energies were between 300 and 420 kJ/mol, regardless of environment. For the steam environment, the bond-length parameter ( δ ) for the bond-energy model was very close to the lattice parameter of β-SiC (.436 nm), but in air it was significantly lower at 0.25-0.26 nm, but still larger than the Si-C bond length of 0.189 nm. Other factors suggest that either a bond-energy based law or a modified version of a reaction-rate law are the best choices for a SCG law. Filament strength distributions initially described by Weibull distributions could not be described by such distributions after application of the models. SCG mechanisms are discussed.     

Fracture Toughness of a Silicon Nitride/Silicon Carbide Nanocomposite at 1350°C

The fracture toughness of a hot-pressed silicon nitride/silicon carbide (Si₃N₄/SiC) nanocomposite and reference monolithic Si₃N₄ has been investigated in four-point bending at 1350°C in air, using different loading rates (0.01-1 mm/min). Single-edge V-notched bend specimens that were prepared by polishing the notch tip to a radius of <10 µm, using 1 µm diamond paste, were used for the fracture toughness measurement. Slow crack growth (SCG) prior to catastrophic failure was detected at all applied loading rates at 1350°C. The fracture toughness at 1350°C, as calculated using the actual crack size measured on the fracture surface after the bend test, increased in both ceramics with decreasing loading rate and increasing area of the SCG region.     

Evaluation of Slow Crack Growth Resistance in Ceramics for High-Temperature Applications

The slow (subcritical) crack growth (SCG) resistance of Si₃N₄ and SiC ceramics has been evaluated by a stepwise loading test on bending bars precracked by Vickers indentation. Three highly refractory materials were selected for the evaluation: i.e., (1) high-purity Si₃N₄ sintered by hot isostatic pressing (HIP) without additives and (2,3) α - and β - SiC pressureless sintered with B and C addition. Under the hypothesis of linear elastic behavior at high temperature, which was found satisfied in the present materials, the SCG resistance was expressed in terms of initial stress intensity factor critical for SCG failure within a predetermined lifetime. The present method was found useful in shortening the testing time and consistent with other traditional fatigue tests (e.g., static-fatigue test): It is recommended as a screening test for materials under research and development. Among the materials tested in the present study, the highest SCG resistance up to 1440°C was found in the high-purity Si₃N₄ without additives.     

Dynamic fatigue behavior of lithium hydride at elevated temperatures

The fatigue properties of lithium hydride (LiH) are crucial to its application as neutron shielding and moderating at elevated temperatures. The dynamic fatigue tests of LiH were investigated with the notched 3-point bend (3 PB) specimens over ranges of loading rates at RT up to 400 °C. At RT, the results showed that slow crack growth (SCG) occurred prior to failure as the minor deviation from linearity to nonlinearity in the load-deflection curves. In addition, the fracture strength of LiH decreased with decreasing stress rate and the SCG zone gradually became smaller with higher stress rates, indicating evident dynamic fatigue phenomenon. However, the trends were quite different at 200, 300 and 400 °C due to accumulative creep damage for low stress rates at elevated temperatures. With increasing temperature and decreasing stress rate, there existed a transition of the dominated failure mechanism, from SCG to creep rupture. Evidence of very small SCG zone could also be detected at the notch for the failure dominated by creep rupture.     

Three-parameter (3P) weibull distribution for characterization of strength of ceramics showing R-Curve behavior

Strength distribution of advanced ceramics is commonly characterized by two-Parameter (2P) Weibull distribution. However, deviation of strength distribution from 2P-Weibull distribution may occur in ceramics due to various mechanisms. R-curve behavior is one of these mechanisms where increase of fracture resistance with the extension of crack occurs. In such cases, 2P-Weibull distribution may not be the best fitting distribution function based on the goodness-of-fitness tests. This article examines the effectiveness of three-parameter (3P) Weibull distribution function for fitting the strength variation due to R-curve effect by using experimental and virtual strength data. The effect of Weibull parameters, degree of increase in crack resistance and number of samples on effectiveness of fitting via 3P-Weibull distribution is investigated. It is reported that 3P-Weibull distribution function fits the strength distribution better than 2P-Weibull distribution function for materials showing R-curve behavior when the crack resistance curve is steep and Weibull modulus is high. Furthermore, it is shown that at least 100 samples should be used for a reliable estimate when the material exhibits R-curve behavior.     

Slow crack growth resistance of electrically conductive zirconia-based composites with non-oxide reinforcements

Slow crack growth (SCG) behavior of four zirconia-based composites reinforced with 40 vol% WC, TiC, NbC or TiCN were studied by means of double-torsion testing. Compared to monolithic zirconia, the composites had a higher resistance to fast fracture, i.e., higher fracture toughness. The extent of toughening depended on the reinforcement type, shifting the V-K_I (crack velocity versus stress intensity factor) curve parallel to higher K_I values. More importantly, these composites were less sensitive to SCG. Identical V-K_I/K_IC curves with steeper slopes compared to monolithic zirconia were observed for the investigated composites, independent on the reinforcement type. No rising R-curve was measured, at least in the crack-size domain investigated by SCG. Therefore, the higher SCG resistance of the composites was due to the intrinsic stress-assisted corrosion resistance of the covalent non-oxide secondary phase.     

聚乙烯管材应变硬化模量与裂纹扩展速率相关性研究

采用应变硬化试验和锥体试验分别得到不同聚乙烯100（PE100）管材试样的应变硬化模量和裂纹扩展速率,并对它们之间的相关性进行研究,以期验证应变硬化试验法评价PE管材耐慢速裂纹扩展性能的正确性。结果表明,锥体试验中PE管材试样的裂纹扩展程度随着试验时间的增加而增大;应变硬化试验和锥体试验对不同PE管材试样的耐慢速裂纹扩展性能评价结果完全相同,应变硬化试验法评价PE管材耐慢速裂纹扩展性能的正确性得到验证;应变硬化试验不仅误差较小,而且可以区分不同牌号PE100管材耐慢速裂纹扩展性能的细微差异,评价结果相对于锥体试验要更加可靠。     

Oxidation-induced crack-healing behavior of SiC-Al2O3-TiB2 composites at 600°C–800°C

It is necessary to give self-healing function to ceramic materials because of their notch sensitivity. In the past, studies on self-healing ceramics have mainly focused on the high-temperature stage, and less research has been done below 1000°C. In this study, SiC/Al₂O₃/TiB₂ ceramic composites were prepared by spark plasma discharge sintering, and cracks were introduced on the surface of the polished specimens. Crack healing at 600°C–800°C was investigated, and the recovery of macroscopic bending strength and the change of microscopic crack morphology after heat treatment were used to evaluate the crack-healing effect. It was found that the surface cracks of the material were completely filled and healed by oxidation products after heat treatment at 700°C for 60 min, and the highest healing efficiency exceeded 95% for both specimens with different crack lengths, and the main mechanism of crack by Si-Al-B-Na-Ca-O type glass produced by the reaction of TiB2 and a small amount of SiC with oxygen to produce oxides and glass powder. Good healing effect and fast healing speed effectively improve the service life and reliability of ceramic materials, which has very far-reaching significance for the practical application with ceramic materials.     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

Micromechanics of Creep-Crack Growth in a Glass-Ceramic

The near-tip strain and opening behavior of creep cracks in a glass-ceramic were studied at temperatures for which grain-boundary sliding is the dominant deformation mechanism. Using the stereoimaging displacement measurement technique, near-tip creep strain and crack-opening displacement (COD) increments were obtained as functions of distance from the crack tip and time of creep. Both the time and radial dependence of the strain and COD were observed to be consistent with the Riedel and Rice (RR) field when the time of creep and crack extension were small. Increasing creep time and crack extension led to the formation of a localized shear zone located directly ahead of the crack tip. Further crack extension tended to follow this shear zone within which creep damage accumulated. Neither the rate of COD nor the strain-rate distribution within this shear zone could be described by the RR field or the Hui and Riedel (HR) field for growing cracks. Steady-state, stable crack growth exhibiting both Mode I and II components was observed in the glass-ceramic despite a creep exponent of ≅2. Varying the test temperature and applied stress intensity influenced both the development of the crack-tip shear zone and the crack growth process. These observations are discussed on the basis of the interrelationships between the near-tip field, creep damage accumulation, and grain-boundary sliding.