Mechanisms of crack propagation in dry plaster

Controlled crack propagation tests were performed on single edge notched bend samples to investigate the crack growth behaviour of dry plaster. The influence of the relative notch depth on the crack resistance curve has been studied and appears to be very important. The results are discussed considering a qualitative model based on the specific microstructure of plaster and in situ observations of the crack propagation. Two mechanisms acting at different scales and undergoing complex interactions are involved: crack bridging by small gypsum crystals acting locally behind the crack tip and secondary cracking in a macroscopic frontal process zone. Interaction of the main crack with secondary ones undergoes substantial branching and crack accelerations leading to bridging destruction due to sudden crack opening.     

Crack propagation speed in ceramic during quenching

The effects of water quenching temperature and specimen size on the propagation speed of thermal shock crack are investigated in real time by water quenching of translucent ceramic and high-speed imaging. The results show that the crack growth rate increases with the increase of quenching temperature difference or specimen size. Within 100?ms, average crack speed is 20.3?mm/s at a temperature difference of 400?°C in 20?mm wide ceramic and is 11.9?mm/s at a temperature difference of 220?°C in 5?mm wide ceramic, respectively. Compare with specimen size, the influence of quenching temperature difference on the crack propagation speed is larger. The calculations based on meso-damage mechanics have similar results to those of experiments. This paper quantitatively studies the thermal-shock crack growth of ceramic in real time and expands the scientific understanding of thermal shock cracking phenomenon of ceramic.     

Thermal-shock resistance and nature of crack resistance of alumina material with a layer-granular structure

Conclusions Comparative investigation of the crack resistance and thermal-shock resistance of aluminum oxide materials with a granular and with a layered-granular structure shows the advantages of using the latter in conditions of high-velocity heating and cooling. The breakdown of such a material is characterized by stages of acceleration and retardation of the dissemination of cracks, as a result of a change in its orientation in relation to the direction of the action of the maximum tensile stresses.Translated from Ogneupory, No. 3, pp. 9–12, March, 1986.     

Vertical crack distribution effect on the TBC delamination induced by crack growth from the ceramic surface and near the interface

The propagation of vertical crack on the surface of thermal barrier coatings (TBCs) may affect the interface cracking and local spallation. This research aims to establish a TBC model incorporating multiple cracks to comprehensively understand the effects of vertical crack distribution on the coating failure. The continuous TGO growth and ceramic sintering are together introduced in this model. The influence of the vertical crack spacing and non-uniform distribution on the stress state, crack driving force, and dynamic propagation is examined. Moreover, the influence of coating thickness on the crack growth driving is also explored. The results show that large spacing will lead to early crack propagation. The uniform distribution of vertical cracks can delay the spallation. When the spacing is less than 4 times ceramic coat thickness, the cracking driving force will come in a steady-state stage with the increase of vertical crack length. Prefabrication of vertical cracks with spacing less than 0.72 mm on the coating surface can greatly decrease the strain energy. The results in this study will contribute to the construction of an advanced TBC system with long lifetime.     

Thermal-shock resistance of composite materials based on periclase and chromium oxide

Conclusions The possibility of controlling the structure, thus making it possible to obtain thermal-shock resistant materials in the MgO-Cr₂O₃ system, by combined synthesis and sintering in the same firing, has been studied. The fracture diagrams in load-bend coordinates and the criteria of thermal-shock resistance used to evaluate the tendency of the material to resist the movement of a crack have been studied. An effective composition for a thermal-shock resistant material based on pure magnesium chromite has been obtained.Translated from Ogneupory, No. 8, pp. 44–47, August, 1981.     

Mechanical behavior of SiC ceramics with single flaw under three-point bending

As a common defect in ceramics, the flaw-like defect will affect the fracture damage and mechanical properties of ceramics. For the current experimental equipment, it is difficult to accurately prefabricate defects in ceramics and observe the internal crack propagation when the specimen is damaged under loading, however, numerical simulation can solve this problem well. In order to explore the effect of various parameters of a single flaw on the crack evolution and bending strength of SiC ceramics under three-point bending, a SiC ceramic discrete element model with a prefabricated single-flaw defect is established and calibrated in this study, and the effects of the flaw length, inclination angle, and position on the bending strength of SiC ceramic specimens are investigated. The results show that the SiC ceramic specimen has the highest sensitivity to the existence of single-flaw defect when the flaw height ratio is 0 and inclination angle is 90°. As the height ratio of the single-flaw defect increases, the weakening effect of the flaw on the bending strength is reduced. Moreover, the influence of the flaw inclination angle on the bending strength of the specimen is weaker than that of the height ratio, but a change in the flaw inclination angle may induce the generation of secondary cracks in the specimen. In addition, the effect of the horizontal offset of the flaw on the bending strength is not obvious, but secondary cracks may also be generated in the specimen.     

Thermal shock damage assessment of a BNW/SiO2 ceramic: Insight from temperature-dependent material properties

The high-temperature service performance of nearly fully dense 20 wt% BN_W/SiO₂ ceramic was systematically investigated. The oxidation damage and strength degradation of the whiskers combined with the surface microstructures of the samples predominantly influence the flexural strength from RT to 1000 °C. In previous work, the temperature dependence of the material properties is invariably ignored when evaluating thermal stress crack initiation and propagation behaviour. In this work, modified thermal shock models that include temperature-dependent material properties were established based on thermal-shock fracture (TSF) theory and thermal-shock damage (TSD) theory. Then, the thermal shock resistance (TSR) of the BN_W/SiO₂ ceramic was evaluated by preforming a water quenching test. The modified models could better explain the TSR behaviour of the ceramic, indicating that considering the temperature-dependent material properties will reveal the thermal shock damage mechanism more precisely.     

Electrical boundary condition at the crack surface in ferroelectrics

Electrical boundary condition (EBC) at the crack surface has significant influence on the fracture behavior of ferroelectrics. Here, we characterized the electrical displacement (ED) response of ferroelectrics specimens with through cracks under cyclic electric field. The physical processes occurred between the crack surfaces are in situ observed. It is found that when the distance between the crack surfaces was smaller than a critical value, significant ED was measured, which cannot be rationalized by the previous EBCs. The appearance of bubbles and arcing between the crack surfaces demonstrates that dielectric breakdown occurs during electric field loading. These results imply that the EBC of the crack surface is permeable, and the dielectric breakdown process should be taken into consideration.     

Fatigue crack retardation in polycarbonate

Results are presented from a recent study of the influence of tensile overloads on fatigue crack growth in polycarbonate. Fatigue cracks were grown under conditions of constant range in stress intensity factor in four-point bend specimens. The data presented here indicate that tensile overloads may significantly retard subsequent fatigue crack growth in polycarbonate. The period of delay in crack growth was shown to increase with the magnitude of the overload. Recovery of stable crack extension following the overload appeared to involve reinitiation of separate crack growth sites at the tip of the blunted crack tip, similar to the original crack initiation at sharp V-notches.     

Unique crack behaviors of glass BK7 occurred in successive double scratch under critical load of median crack initiation

Successive double scratch tests under the critical load of median crack initiation were conducted to investigate the crack behaviors under the influence of interaction between the arranged grits during grit-arranged grinding BK7 glass. After the inspection of the subsurface crack morphology by the advanced focused ion beam, it is found that with the separation distance increasing, the crack behaviors under the 2nd scratch experience three stages: a new type of crack—premature lateral crack initiation, no crack initiation and median crack reappearing. The crack behaviors that occurred in the former two stages can alleviate the subsurface damage. The initiation mechanisms of the unique crack behaviors were analyzed by a stress field analytical model. Also, the benefit of these unique crack behaviors on the grinding process was discussed.     

Microscopic approach to energy dissipation during viscoelastic crack propagation

The microscopic factors relating to the energy dissipation increase in the viscoelastic dynamic crack propagation such as in PMMA [poly(methyl methacrylate)] were investigated through the observation on the microfracture process ahead of the crack tip up to the microcrack formation. It is found that the energy dissipation remains almost constant in a certain low velocity range but increases sharply in the higher velocity range for the PMMA case. A cleavagelike fracture mode works in the former, and both the cleavagelike and a ductile-fracture mode work in the latter. The increase in energy dissipation is attributed to the secondary cracks which remarkably increase in number depending on the crack velocity. The energy dissipation is proved to be independent of the crack velocity in cases where the crack is propagated by the cleavagelike fracture mode.     

Hertzian crack analysis in alumina–chromium composites

Ceramic metal composites are of interest for their good resistance to crack propagation. We have prepared different kinds of alumina chromium composites, observed their microstructures and made an analysis of Hertzian cracks in order to identify the principle parameters of crack propagation in relation with the metallic phase size and distribution in the matrix. The crack is analysed at two scales, a macroscopic one to estimate the fracture toughness from the overall crack and a microscopic one to study, at the local level, the influence of the metallic phase on crack propagation. Using macroscopic models the fracture toughness estimation highlights the benefit of the presence of chromium particles. Observations and measurements made on the crack path and metallic phase, from the microstructure analysis, combined with the knowledge of the residual stress state, provide the principal parameters governing crack propagation in these composites.     

The effect of flexoelectricity on domain switching in the vicinity of a crack in ferroelectrics

Ferroelectrics are widely used in the manufacture of transducers, actuators, and memory devices, due to their attractive electromechanical properties. However, the reliability and failure of devices is greatly dependent on their brittleness. In view of the fact that both the polarization distribution and elastic field are at nanoscale and vary greatly in the vicinity of the crack tip, flexoelectricity is expected to strongly affect the domain configuration. In this work, Ginzburg-Landau (TDGL) theory and the phase field method (PFM) are employed to analyze the influence of flexoelectric effect on the domain switching process in the vicinity of the crack tip of ferroelectric materials. The results obtained show that, the domain configuration would become asymmetric with increasing flexoelectric coefficients, and the flexoelectric effect has a larger influence on the polarization field than on the elastic field in the vicinity of the crack tip of ferroelectric materials.     

针刺参数对玄武岩纤维预制体力学和热导率影响的有限元分析

针刺密度、针刺深度等针刺工艺会对纤维针刺预制体的性能造成显著影响。本文通过有限元分析方法,利用Python语言对ABAQUS软件进行二次开发,建立了玄武岩纤维针刺预制体的代表性体积单元(RVE)。研究了针刺深度和针刺密度对玄武岩纤维针刺预制体层间力学和热导率的影响,并结合实验结果分析了其影响机制。结果表明,不同纤维网胎层间主要依靠针刺纤维进行结合,增加针刺密度和针刺深度均可提高针刺预制体的剪切性能,但提高针刺密度的效果比提高针刺深度的效果更加明显。当沿针刺预制体z轴方向施加温度梯度时,热流矢量主要集中于针刺纤维中,且当增加针刺深度时上述现象更加明显。因此,增加针刺深度不利于针刺预制体的防热性能。     

Atomic picture of crack propagation in Li2O-2SiO2 glass-ceramics revealed by molecular dynamics simulations

Fundamental understanding of the interaction between glass and ceramic phases with a running crack and the mechanisms by which propagation is hindered or altered as compared to the precursor parent glasses remains elusive for the lithium disilicate glass-ceramics. We herein conduct extensive molecular dynamics simulations to reveal some atomic details that are otherwise extremely challenging to be probed by experiments. The results show that the crack propagation pathway in glass-ceramics is dramatically different as compared to the precursor parent glass. In glassy sample, clean crack branching seems to be triggered by multiple cavitation events ahead of crack tip. In glass-ceramic samples, however, branching is characterized by microcracks at multiple glass-nanocrystal interface sites, clean nanocrystal cleavage along certain crystalline plane, and even secondary cracks due to the percolation of multiple microcracks. Additionally, the nanocrystal distribution, total volume fraction, and aspect ratio also have pronounced effects on the propagation of a primary crack and can lead to very diverse crack patterns.     

Influence of crystal structure on crack propagation under cyclic electric loading in lead–zirconate–titanate

Crack propagation under cyclic electric loading was studied in two non-commercial compositions of lead–zirconate–titanate and compared to earlier results from a commercial composition. These materials were chosen to provide a well-defined variation in crystal structure, ranging from rhombohedral to tetragonal, including a composition from the morphotropic phase boundary. The results are presented in terms of crack propagation as a function of various electric load amplitudes. While the crack propagation rates were of the same order of magnitude in all three compositions, fracture occurred in an either trans- or intergranular manner with crack extension either in the form of a singular crack, a microcrack zone or with extensive secondary cracking. These differences in crack propagation are discussed in the context of different piezoelectric material properties.     

STANDARD THERMAL-SHOCK TEST FOR PORCELAIN ENAMELED UTENSILS*

The equipment and method used in testing porcelain enameled utensils for thermal-shock resistance is described. Test results are given which show that increased enamel thickness and decreased metal thickness decrease the thermal-shock resistance of enameled ware. The mechanics of thermal-shock failures is discussed. This study is one of a series sponsored by the Enameled Utensil Manufacturers' Council.     

Analysis of interfacial crack initiation mechanism of thermal barrier coatings in isothermal oxidation process based on interfacial stress state

In this paper, the interfacial stress state is used to analyze the interfacial crack initiation mechanism of the thermal barrier coatings (TBCs) during isothermal oxidation. The influence of thermal growth stress, initial residual stress, and creep behavior on the stress distribution is considered to have an accurate simulation result. A parameter that integrates the effects of interfacial normal and tangential stress is modified for evaluating interfacial crack initiation. It is found that, in the cooling stage, the interfacial cracks sprout at the top coat (TC)/thermally grown oxide (TGO) interface valley region and the TGO/bond coat (BC) interface peak region, which agrees with the experimental results. Furthermore, the influence of interfacial roughness on crack initiation is investigated. The result shows that different interfacial roughness affects the sprouting region of interfacial cracks and cracks within the TC layer.     

Influence of traversing crack on chloride diffusion into concrete

This study examined the effects of traversing cracks of concrete on chloride diffusion. Three different concretes were tested: one ordinary concrete (OC) and two high performance concretes with two different mix designs (HPC and HPCSF, with silica fume) to show the influence of the water/cement ratio and silica fume addition. Cracks with average widths ranging from 30 to 250 μm, were induced using a splitting tensile test. Chloride diffusion coefficients of concrete were evaluated using a steady-state migration test. The results showed that the diffusion coefficient of uncracked HPCSF was less than HPC and OC, but the cracking changed the material behavior in terms of chloride diffusion. The diffusion coefficient increased with the increasing crack width, and this trend was present for all three concretes. The diffusion coefficient through the crack D_cr was not dependent of material parameters and becomes constant when the crack width is higher than  80 μm, where the value obtained was the diffusion coefficient in free solution.     

Statistical methods of determining the thermal-shock resistance of refractories

Conclusions A statistical analysis of the test results for the thermal-shock resistance of a large batch of specimens of zirconium dioxide established that there is satisfactory correspondence between the experimental data and the exponential distribution of Weibull. The appreciable spread in the thermal-shock resistance criteria R for brittle refractory materials should be considered as regular, and therefore to obtain reliable calculation characteristics for thermal-shock resistance it is necessary to treat the test results statistically.It is proposed that one of the causes of variation in the thermal-shock resistance criterion R is the slight (unavoidable even within the limits of a single batch of specimens) change in the density of the material.Translated from Ogneupory, No. 2, pp. 53–56, February, 1971.