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.     

Controlled Crack Growth Specimen for Brittle Systems

A pure Mode I fracture specimen and test procedure has been developed which provides extended, stable, through-thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson expansion of a compressively loaded cylindrical pin. Stable cracks have been successfully grown in soda-lime glass and monolithic Al₂O₃ for lengths in excess of 20 mm without uncontrollable catastrophic failure.     

Fracture and electric-field-induced crack growth behavior in NBT-6BT relaxor ferroelectrics

The fracture properties of 0.94(Na_0.5Bi_0.5)TiO₃-0.06BaTiO₃ (NBT-6BT) relaxor ferroelectrics were investigated using the Vickers indentation method and computation of crack tip opening displacement. It was found that an unpoled sample had a fracture toughness of around 1.35 MPa m^1/2. In contrast, an electrically poled sample exhibited anisotropy with a lower fracture toughness perpendicular to the poling direction and a higher value in the parallel direction, as compared to the unpoled sample. Upon cyclic electrical loading (with applied electric field amplitudes between 0.73E_C and 1.4E_C), the indented surface crack was found to propagate. In general, the crack grew rapidly during the initial cycles followed by crack arrest, and the principal driving force for crack growth was proposed to be residual stress around the indentation, as evidenced by the limited field dependence of crack growth. There was also a contribution from the electromechanical strain, which played a role at high cycles (>100 cycles) and high fields (>1.3 E_C). Evidence of a saturation threshold of crack propagation is an advantage for the electromechanical reliability of relaxor ferroelectrics in devices.     

硅砖裂纹产生的机制

硅砖生产过程中由于砖体受环境温度的急剧变化,内部产生较强热应力,当强度达到制品的强度极限值时,即产生裂纹甚至开裂。硅砖裂纹主要产生在成型和烧成阶段,但是粉料配料、泥料混炼以及干燥、装车等工序控制不当也会产生裂纹废品。因此,要降低废品率,控制好成型和烧成工序至关重要,但其它工序的操作也不容忽视。     

Evaluation of the Crack Face Bridging Mechanism in a MgAl2O4 Spinel

The effectiveness of a grain bridging mechanism in the following wake zone of two MgAl₂O₄ spinel microstructures has been demonstrated through renotching experiments with double-cantilevered-beam (DCB) specimens. Measurements of d K_R /dδ a agree with previous crack growth resistance curves of similar materials, obtained from single-edgenotch-beam specimens. However, even the extended test ligaments available from the present DCB specimens did not permit the full development of the following wake zone of these coarse microstructures.     

The correlation of slow crack growth in linear polyethylene by the J-integral

The fracture of a polyethylene (PE) homopolymer by slow crack growth was measured with a three point bending (TPB) specimen and a single edge notch tension (SENT) specimen. The crack growth rate correlated with the stress intensity for each type of specimen. However, for a given K, SENT specimens exhibited a faster crack growth rate than TPB specimens. Since PE is non-linear, the J-integral is more appropriate than K and it was found that J does correlate the SENT and TPB results. In addition, the degree of non-linearity of the PE was increased by quenching. For the quenched state it was also found that J correlated the SENT and TPB results in accordance with the dependence of J on the degree of non-linearity of the material.     

Slow stable crack growth in high density polyethylenes

The phenomenon of slow stable crack growth in polyethylene is investigated using notched specimens subject to constant load and the concepts of fracture mechanics. The effect of specimen geometry and dimension, the loading and the mode of loading on the applied stress intensity factor versus crack speed ($\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$) curves has been studied to demonstrate that K_c is the controlling stress parameter for crack growth under suitable conditions. $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves are obtained for a high density polyethylene homopolymer in distilled water and in a diluted detergent solution at four different temperatures. Results are also obtained for a much tougher medium density polyethylene copolymer whenever possible. Several mechanisms can be identified from the form of the $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves. Two, in particular, have been observed but not explained before: (i) crack growth with a time dependence of 0.25, and (ii) the high $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ slopes for crack growth in a tough copolymer. With the help of scanning electron microscopic studies of the fracture surfaces, (i) is postulated to be due to diffusion controlled void growth process and (ii) is considered to be the result of crack tip blunting effects. From the temperature dependence of crack growth, the activation energy of the diffusion controlled crack growth process is found to coincide with that of the x-relaxation process in polyethylene implying that diffusion controlled crack growth may be related to the motion of main chains in the polymer.     

Crack growth angle prediction of an internal crack under mixed mode load for unfilled elastomer using the strain energy density factor

This article investigates the prediction of the crack growth angle of an existing internal crack under mixed mode loading at the crack tip for an unfilled ethylene propylene diene terpolymer rubber (EPDM). For the realization of mixed mode loading, the cracks of the uniaxial loaded specimens were oriented with different angles to the loading direction. The energy density factor was used as a potential criterion for determining the crack growth angle. The determination of the strain energy density factor was carried out simulatively in Abaqus. The second-order Ogden model was used to describe the rubber-like material behavior. The relative local minimum of the strain energy density factor provides the possible growth angle. The experimental investigations show that the initial cracks grow orthogonally to the loading direction for the different crack orientation angles. For the crack orientation angle parallel to the load direction, the crack growth was observed because the strong stretching of the specimen caused strong necking in the crack region. The crack growth for the remaining crack orientation angles were induced due to shear loading at the crack tip. The predictive angle of different crack orientation angles shows very good accordance to the measured crack growth angles.     

Molecular dynamics investigation of the fracture mechanism of a glass-ceramic containing cleavable crystals

In this study on a novel glass-ceramic containing hexagonal CaAl₂Si₂O₈ crystals embedded in a SiO₂–Al₂O₃–CaO glass, we used molecular dynamics simulations to unravel the toughening mechanism of the partially crystallized composite material. The crystalline phase is composed of alternate layers of SiO₄/AlO₄ tetrahedra and calcium ions. After careful modeling of crystals embedded in the glass matrix, we conducted crack propagation simulations using single-notched models. We found that: (a) when a crack propagates parallel to the cleavable calcium layer, the glass-ceramic breaks in a brittle way since the crack passes through the fragile interlayer promptly, (b) the stiffer SiO₄/AlO₄ oxide layer can inhibit crack propagation, and the crack is thus deflected to the interface between the crystal and the glass matrix; and (c) a calcium layer present between the glass matrix and the edge of the CaAl₂Si₂O₈ crystal is more fragile than those inside the crystal, indicating that cracks prefer to travel along the glass-crystal interface. These theoretical simulations successfully demonstrated that the anisotropy and the fragile feature of the crystals lead to microcrack toughening of the glass-ceramic. In addition, we discuss deformation anisotropy in the microscale by constructing a larger model that includes randomly orientated multiple CaAl₂Si₂O₈ crystals.     

Subcritical Crack Growth of Macrocracks in Alumina with R-Curve Behavior

Subcritical crack growth of macroscopic cracks in two Al₂O₃ ceramics is investigated with single-edge-notched bending specimens under constant load. The resulting v - K _I-curves are in complete contrast to the behavior of natural cracks. In spite of the monotonic increase of the externally applied stress intensity factor due to crack extension, the crack growth rates first decrease. This behavior is caused by crack shielding due to crack border interaction and can be described by a rising crack growth resistance. Two methods are applied to determine the R -curve under subcritical crack growth conditions.     

Subcritical Crack Growth in Soda-Lime Glass in Combined Mode I and Mode II Loading

Subcritical crack growth under mixed-mode loading was studied in soda-lime glass. Pure mode I, combined mode I and mode II, and pure mode II loadings were achieved in precracked disk specimens by loading in diametral compression at selected angles with respect to the symmetric radial crack. Crack growth was monitored by measuring the resistance changes in a microcircuit grid consisting of parallel, electrically conducting grid lines deposited on the surface of the disk specimens by photolithography. Subcritical crack growth rates in pure mode I, pure mode II, and combined mode I and mode II loading could be described by an exponential relationship between crack growth rate and an effective crack driving force derived from a mode I-mode II fracture toughness envelope. The effective crack driving force was based on an empirical representation of the non-coplanar strain energy release rate. Stress intensities for kinked cracks were assessed using the method of caustics and an initial decrease and a subsequent increase in the subcritical crack growth rates of kinked cracks were shown to correlate with the variations of the mode I and the mode II stress intensities.     

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

Fracture and subcritical crack-growth characteristics under combined Modes I and III loading were studied using the modified compact tension (CT) specimens of soda-lime glass. The combined mode load was applied to the specimen in the direction β with respect to the initial crack. By superposition of Mode III, the advancing crack begins to rotate at an angle Ψ to the initial crack plane, which nearly maximizes the Mode I stress intensity factor K _I(Ψ), and the crack continues to propagate in the same direction. In this case, unlike combined Modes I and II, the crack breaks into multiple partial fronts, and ligamentary bridging forms fracture lances when these segmented cracks are held together. The crack velocity d a /d t was plotted versus the maximum Mode I stress intensity factor K _I(Ψ) for combined Modes I and III loading. The d a /d t values are initially high, and the crack growth tends to be discontinuous compared with the result for pure Mode I. The subcritical crack growth seems to occur when the K _I value for the initial crack reaches a certain value. The d a /d t - K _I(Ψ) curves for combined Modes I and III lie roughly on the same curve as that for pure Mode I as the crack growth increases.     

陶瓷基复合材料的强韧化研究进展

增韧补强对于脆性陶瓷材料来说是一个永恒的课题,材料科学工作者对此开展了富有成效的研究,并取得了巨大的成功。本文综述了陶瓷基复合材料的相变增韧、纤维增韧、颗粒增韧、自增韧补强的方法、增韧效果及相关的增韧机理。     

无模板剂二次生长法合成MFI分子筛膜的生长机理

Based on two template-free secondary growth methods, the formation mechanism of single-layer and multi-layer zeolite membranes was investigated. The composition of the synthesis solution did not affect the membrane structure, whereas the position-setting of the seeded support in the synthesis solution and the ageing process of the synthesis solution were two critical factors in the formation of the membrane. The synthesis solution with lower viscosity could easily penetrate into the gap between seeds boundaries, resulting in the formation of multi-layer membranes.     

Fatigue Crack Propagation at Polymer Adhesive Interfaces

Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic vs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the effect of phase angle (13 to 54°) on crack growth rates is not significant. However, for epoxy/PMMA interfaces the applied energy release rate for the initiation of crack growth is considerably greater for a phase angle of 66° than for 5°, indicating that increasing shear at the crack tip makes the initiation of crack growth more difficult. These results are discussed in terms of possible mechanisms of fatigue crack growth at polymer adhesive interfaces.     

Crack Propagation in Polymer Adhesive/Glass Sandwich Specimens

Moisture-assisted crack growth in polymer adhesive/glass interfaces was measured as a function of the applied energy release rate, G, using a four-point flexure test coupled with an inverted microscope. The specimens consisted of two glass plates bonded together with an epoxy or an epoxy-acrylate adhesive. It was found that cracks formed and grew on both interfaces if the glass surfaces were both smooth; however, roughening the surface of one of the glass plates increased the fracture resistance of the interface sufficiently so that crack growth occurred only on the remaining “smooth” interface (top or bottom). Finite element analysis was used to determine the G and ψ (phase angle) appropriate for the different crack geometries. It was found experimentally that crack growth rates for all crack geometries depended on the applied G via a power law relationship and that for a given applied G, crack growth rates were sensitive to the crack geometry. The results indicate that the primary driving force for moisture-assisted crack growth at a polymer/glass interface is the applied G at the crack tip and that the effect of the phase angle for the different crack geometries (13° to 54°) is negligible.     

Crack Propagation in Polymer Adhesive/Glass Sandwich Specimens

Moisture-assisted crack growth in polymer adhesive/glass interfaces was measured as a function of the applied energy release rate, G, using a four-point flexure test coupled with an inverted microscope. The specimens consisted of two glass plates bonded together with an epoxy or an epoxy-acrylate adhesive. It was found that cracks formed and grew on both interfaces if the glass surfaces were both smooth; however, roughening the surface of one of the glass plates increased the fracture resistance of the interface sufficiently so that crack growth occurred only on the remaining “smooth” interface (top or bottom). Finite element analysis was used to determine the G and ψ (phase angle) appropriate for the different crack geometries. It was found experimentally that crack growth rates for all crack geometries depended on the applied G via a power law relationship and that for a given applied G, crack growth rates were sensitive to the crack geometry. The results indicate that the primary driving force for moisture-assisted crack growth at a polymer/glass interface is the applied G at the crack tip and that the effect of the phase angle for the different crack geometries (13° to 54°) is negligible.     

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.     

Origin of the Static Fatigue Limit in Oxide Glasses

Oxide glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. The log of crack velocity decreases linearly with decreasing stress intensity factor in Region I. For some glasses, at a lower stress intensity, K_o, log v asymptotically diminishes where there is no measurable crack growth. The same glasses exhibit static fatigue, or a decreasing strength for increasing static loading times, as cracks grow and stress intensity eventually reaches the fracture toughness. In this case, some glasses exhibit a low stress below which no fatigue/failure is observed. The absence of slow crack growth under a low stress intensity factor is called the fatigue limit. Currently, no satisfactory explanation exists for the origin of the fatigue limit. We show that the surface stress relaxation mechanism, which is promoted by molecular water diffusion near the glass surface, may be the origin of the fatigue limit. First, we hypothesize that the slowing down of slow crack growth takes place due to surface stress relaxation during slow crack growth near the static fatigue limit. The applied stress intensity becomes diminished by a shielding stress intensity due to relaxation of crack tip stresses, thus resulting in a reduced crack velocity. This diminishing stress intensity factor should result in a crack growth rate near the static fatigue limit that decreases in time. By performing Double Cantilever Beam crack growth measurements of a soda‐lime silicate glass, a decreasing crack growth rate was measured. These experimental observations indicate that surface stress relaxation is causing crack velocities to asymptotically become immeasurably small at the static fatigue limit. Since the surface stress relaxation was shown to take place for various oxide glasses, the mechanism for fatigue limit explained here should be applicable to various oxide glasses.     

环氧树脂及其改性树脂的低温性能分析和增韧研究(Ⅱ):增韧改性研究

对环氧树脂及其改性后树脂的低温静态和动态的断裂性能进行了分析 ,并对增韧机制进行了相应的研究。得到环氧树脂及其增韧树脂在低温情况下的断口特征以及在低温情况下裂纹快速扩展的特征。文中还分析了材料在低温时快速扩展的机制 ,得出增韧改性后的环氧树脂在低温下有可能提高材料的断裂韧性的结论