Crack Appearance during Drying of an Alumina Gel: Thermo-Hydro-Mechanical Properties

Most heterogeneous catalyst supports used in refineries are composed of porous alumina ceramics. Drying has been identified as a critical process for final product mechanical strength. In the literature, numerous papers deal with drying-induced stresses, which can lead to crack initiation. However, there are few papers devoted to experimental study of drying conditions that promote cracking. The objective of this work is to enhance knowledge of cracking behavior, specifically by studying alumina gel drying. First, the relation between drying conditions and first crack initiation is studied experimentally. Then a complete thermo-hydro-mechanical characterization of the alumina gel is made, including moisture content as a parameter.     

Criterion for crack initiation during drying: Alumina porous ceramic strength improvement

Improvement in the mechanical strength of the supports of porous catalysts is increasingly needed by the refining industry because of the high stresses induced by the exploitation of catalysts. Drying is known to be a critical process for mechanical strength during the preparation of supports, leading to micro-cracks in the case of hard drying conditions.In order to predict failure during drying, a crack initiation criterion, combined with a drying induced elastic stress simulation is proposed. Model parameters are the thermo-hydro-mechanical properties of the gel and the criterion is based on the tensile strength of an alumina gel at various moisture contents. The developed tool gives good results and can be easily adapted to several gel formulations as only effective properties of the gel are required.     

Measurement of ceramics cracking during water quenching by digital image correlation

Measuring the thermal shock crack growth process is crucial for revealing ceramic materials and structures’ thermal shock failure mechanisms and evaluating their reliability. We used a self-made water quenching system to conduct thermal shock tests on alumina and zirconia ceramics. The thermal shock process was recorded by high-speed digital image correlation (DIC) during the test. The process of thermal shock crack initiation and propagation in two kinds of ceramics was determined by analyzing the speckle image change on the sample’s surface. It is found that the crack growth rate of alumina is faster than that of zirconia, which is caused by different material parameters. This paper presents an in-situ measurement method for the initiation and propagation of thermal shock cracking in ceramic materials. It can provide a measurement method to identify and predict the thermal shock damage of ceramic components.     

Temporal Characteristics of Desiccation Cracking and Resistivity of Lateritic Soil in Drying Process

To describe and measure the initiation and development of desiccation cracks in lateritic soil and to discuss the relationship between crack ratio and resistivity, three parallel specimens are prepared and dried under different temperatures and humidity. The geoelectrical resistivity technique combined with image processing method is applied. Water content, surface crack ratio, and electrical resistivity are monitored during the drying path. Results indicate that the calculated surface crack ratios can be identified on the basis of five distinct stages. The occurrence of the first crack in three specimens indicates that the first cracking time occurs earlier with increasing temperature and decreasing relative humidity. The time that crack ratios stabilize indicates that environmental parameters significantly influence crack evolution. The temporal characteristics of resistivity can also be identified on the basis of five distinct stages. The resistivity and crack ratio can be effectively described by a mathematical equation, which is considered as the basis for the use of the geoelectrical technique for the assessment of the temporal variability of soil desiccation cracking.     

Contact damage tolerance of alumina-based layered ceramics with tailored microstructures

This work demonstrates how to enhance contact damage resistance of alumina-based ceramics combining tailored microstructures in a multilayer architecture. The multilayer system designed with textured alumina layers under compressive residual stresses embedded between alumina–zirconia layers was investigated under Hertzian contact loading and compared to the corresponding monolithic reference materials. Critical forces for crack initiation under spherical contact were detected through an acoustic emission system. Damage was assessed by combining cross-section polishing and ion-slicing techniques. It was found that a textured microstructure can accommodate the damage below the surface by shear-driven, quasi-plastic deformation instead of the classical Hertzian cone cracking observed in equiaxed alumina. In the multilayer system, a combination of both mechanisms, namely Hertzian cone cracking on the top (equiaxed) surface layer and quasi-plastic deformation within the embedded textured layer, was identified. Further propagation of cone cracks at higher loads was hindered and/or deflected owed to the combined action of the textured microstructure and compressive residual stresses. These findings demonstrate the potential of embedding textured layers as a strategy to enhance the contact damage tolerance in alumina ceramics.     

Thermally Induced Failure of Copper-Bonded Alumina Substrates for Electronic Packaging

Although copper-bonded alumina substrate has been used for some power electronic devices, its use has been rather limited because of lack of information on the thermally induced failure of the substrate. The phenomenology of thermal cracking was experimentally studied in this work. Interface debonding occurred at the side edge of copper chips and grew inwardly along the interface. Subsequently, the interface crack deviated into the alumina after propagating up to approximately 200 μm. The curving of the interface crack was caused by a mixed Mode I and Mode II loading near the edge. The cracking was widespread, whereby small cracks coalesced with one another to form a long, shallow crack along the perimeter of the copper chips. The cracking sensitivity was influenced by such design factors as edge sharpness, nickel plating, and copper backing. Annealing the substrate above 700 K before testing promoted the cracking considerably. The substrate cracking is discussed in terms of stress conditions near the edge.     

水处理容器缠绕不饱和聚酯体系研究

研究了水处理容器缠绕不饱和聚酯树脂体系 ,重点阐述了不饱和聚酯凝胶期控制、表干及树脂增韧方法和纤维缠绕复合层的力学性能 ,分析了水容器受压树脂环向开裂的原因 ,探讨了提高树脂开裂强度的措施     

The relationship between microstructure,fracture and abrasive wear in Al2O3/SiC nanocomposites and microcomposites containing 5 and 10% SiC

Alumina/SiC nanocomposites are much more resistant to severe wear than monolithic alumina. In order to clarify the mechanisms responsible for these improvements, alumina and alumina/SiC nanocomposites with 5 and 10 vol.% SiC and various alumina grain sizes were fabricated. For comparison, a 10 vol.% SiC “microcomposite” was also fabricated using 3 μm SiC particles. The extent of cracking beneath hardness indentations was examined and the specimens were tested in abrasive wear. Quantitative surface fractography of the worn surfaces was carried out. The wear properties depended strongly on the grain size in pure alumina, but were independent of the alumina grain size in the nanocomposites. This is consistent with the idea that much of the improvement in wear resistance when SiC is added to alumina stems from a reduction in the size of the individual pullouts owing to the accompanying change in fracture mode. In addition, crack initiation by plastic deformation during abrasion and indentation was found to be strongly inhibited when 10 vol.% nanosized SiC was added to alumina. The addition of 3 μm “micro-sized” SiC did not have the same effect. The ability of fine SiC particles to suppress cracking is attributed to the blocking of twins and dislocation pileups by intragranular SiC nanoparticles. This reduces the length of the twins or pileups and hence their ability to nucleate microcracks.     

3D gel printing of alumina ceramics followed by efficient multi-step liquid desiccant drying

Dense alumina ceramics were additively manufactured efficiently through a 3D gel printing process. Hydroxyethyl cellulose (HEC) was applied to ensure the printability and rigid of the gel made from boehmite. A multi-step liquid desiccant drying method was implemented to improve the drying efficiency. The results showed that the solid loading and HEC addition were two useful parameters for adjusting the rheology properties of the gel to make it suitable for 3D printing. With polyethylene glycol(PEG) added as liquid desiccants, the printed bodies with section size of 10 mm could be dried within 26 h during which the deformation and crack formation was avoided despite a high linear shrinkage of 45 % was encountered. The successful preparation of dense monolithic alumna ceramics parts with an average grain size of 1 μm, 99 % of the theoretical density and a flexural strength of 380 ± 45 MPa indicated the potential of this process.     

Cracking behavior of ZrB2-SiC-Graphite sharp leading edges during thermal shock

Crack initiation and propagation of ZrB₂-SiC-Graphite (ZSG) sharp leading edges (SLEs) subjected to thermal shock were systematically evaluated by the water spraying method followed by a crack dyeing treatment. Distinct differences in the crack patterns among different test conditions were observed, and the cracking behavior of ZSG SLEs (including crack initiation time, crack number and critical failure temperature) was revealed to be strongly dependent on both the cooling rate and the microstructure. The crack propagation during thermal shock could be considered as a quasistatic process (crack speed was lower than 1?cm/s) that needed to be driven by continuous cooling.     

Drying-Induced Cracks in Thin Film Fabricated from Colloidal Dispersions

As a colloidal dispersion is coated and dried on a nonporous rigid substrate, the enormous stresses developing during the drying process can fracture the thin film. The drying-induced cracks can produce serous technological consequences and even destroy the efficacy of coatings, which is not desirable in most industrial cases. Therefore, as the first step of controlling cracks in thin film, understanding of crack properties and cracking mechanisms leading to fracture is of vital significance. Although numerous experiments and models have been proposed for cracking during drying of colloidal dispersions, there is little consensus on even the most basic mechanisms, and the effect of heat transfer on cracks as well as optimization of drying process are rarely taken into account. Additional, the broad employments of nanosuspensions bring both opportunities and challenges for this area. This review will give a comprehensive physical picture of thin film fabrication by drying of colloidal dispersions and cracking phenomenon, present current investigations for drying-induced cracks, and point out some prospects for cracking researches especially for industrial R&D, as well as propose combination of thin film preparation with drying technique for exploring crack-free thin film.     

Progressive cracking mastercurves of the transverse ply in a laminate

In this study, progressive cracking of a transverse layer in a cross‐ply composite laminate subjected to tensile loading is considered. Using the results of a probabilistic cracking model, approximate relations for crack density as a function of stress are derived for initiation‐controlled and propagation‐controlled cracking. It is shown that the crack density evolution in the transverse ply can be represented by a mastercurve in suitably normalized coordinates. The mastercurve approach is applied to progressive cracking in glass/epoxy laminates. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

RECENT FACTORY EXPERIENCE AND EXPERIMENTS IN DRYING TERRA COTTA1

An attempt was made to determine by experiment the best conditions with respect to temperature, circulation of air, and its relative humidity, by which to obtain shorter drying time, a minimum of war page, and cracking of the ware generally. Also, a very tender clay was given special study with the object of finding a set of drying conditions by which its extreme tendency to warp and crack would be reduced to a point that would permit practical use of this clay for terra cotta body. The drying time was reduced about 407, by increasing the steam pressure in the heating coils and apparently some showing made in reducing the tendency of the ware to warp and crack. A schedule of drying that gave satisfactory results with the tender clay was developed on an experimental scale, but could not be applied to the factory driers because of limitations in the heating and circulation equipment.     

Indentation Crack Initiation and Propagation in Tempered Glass

Indentation crack initiation and propagation in tempered glass surfaces are examined and the results compared to those in annealed and in ion-exchange-strengthened glasses. The presence of surface compression due to tempering inhibits median crack initiation during the loading cycle of the indentation and depresses the radial crack initiation load during unloading. However, the extent of lateral cracking is enhanced in tempered glass surfaces. In situ crack propagation experiments reveal that the compressive stress tends to weakly stabilize crack extension prior to failure. The degree of crack stabilization is considerably lower than expected from a theoretical analysis.     

Drying Behavior of Colloidal Silica Gels

Observations of the drying behavior of thick-wall colloidal silica gel structures are reported. Various techniques are examined to prevent cracking during the drying of these highstrain viscoelastic materials. Experiments are described which illustrate the effect of relative humidity on the drying rate and on the shrinkage of various samples under isothermal conditions. Surface temperature measurements indicate that evaporation occurs at approximately the wet-bulb temperature of the surrounding atmosphere. Acoustic detection of the internal strain activity during drying leads to the conclusion that control of the sol-gel surface tension in the material preparation is crucial to reducing the interior stresses.     

Nonlinear Compliance Testing Applied to Indentation Cracking in Ceramics

Indentation cracking under sharp-pointed indenters is analyzed using compliance-based, nonlinear, fracture mechanics. The stress intensity factor, K , in linear elastic fracture mechanics is well known to be proportional to the load, P ; in this nonlinear analysis K is proportional to P ^3/4. The observed relation between indenter load-point displacement and crack length is based on similitude of crack lengths with load-point displacements as a strain-controlled fracture. The equations that relate load and a function of the crack length to the crack driving force, J , have been found for Vickers indentations. Analysis of the nonlinear load vs displacement assumes an equilibrium crack in the elastic material surrounding the indent. The hardness that describes the load vs load-point displacement during cracking is derived on a constant J line in load-displacement space. The crack length is shown experimentally to be proportional to the load-point displacement after crack initiation for several different indenters in ZnS. The measured loading curves are nonlinear and display crack initiation during loading. The K _Ic expressions found here are very similar to correlations that have been applied to indentation cracks.     

Peridynamic-based investigation of the cracking behavior of multilayer thermal barrier coatings

Numerical simulations of the cracking behavior of the top layers of multilayer thermal barrier coatings (TBCs) can effectively reveal the failure mechanisms of the TBCs. Current finite element method (FEM)-based simulation means have been applied to solve certain simple cracking problems in TBCs; however, they cannot effectively describe complex cracking problems in TBCs such as coalescence, intersection, and interference among multiple cracks. Peridynamic (PD), a newly developed mechanical theory, has been widely studied to provide analysis for cracking problems in TBCs. In this paper, a numerical model of TBCs is built by the bond-based PD (BB-PD) theory. Complex cracking behaviors, such as spontaneous crack propagation at both interfacial and internal regions, coalescence, and interference among multiple cracks, are simulated under isothermal cooling and gradient cooling conditions. In addition, the effects of interfacial roughness and calcium–magnesium–alumina–silicate (CMAS) inclusions on the cracking behavior are discussed. The results show that the PD model accurately captures complex cracking behaviors observed via scanning electron microscopy (SEM). Given the ability of the model for analyzing discontinuities in TBCs, it can help to further clarify the fracture mechanisms of TBCs.     

凝胶3D打印制备细晶氧化铝陶瓷研究

本文采用溶胶-凝胶法制备了适用于直写打印的拟薄水铝石凝胶,通过改变拟薄水铝石含量以及羟乙基纤维素含量来调控凝胶的流变性,并评价了凝胶的打印性能。针对凝胶坯体干燥所面临的挑战,创新性地提出了多步液相介质干燥方法,采用低分子量聚乙二醇作为干燥介质,乙酸乙酯作为萃取剂,在26 h内实现了厚度为10 mm的凝胶坯体的干燥,并且有效地避免了干燥缺陷的产生,这是传统的低温高湿干燥方法所难以实现的。常压烧结后,成功制备出了平均晶粒尺寸约为1.5 μm、相对密度为99%、弯曲强度为(351±53) MPa的致密氧化铝陶瓷零件。