Papers by Keyword: Microcracking

Abstract: Microwave sintering has emerged in recent years as a new, fast, cheap and green technology for sintering a variety of materials. The main advantages of microwave heating can be summarized as follow: reduced processing times, energy costs and environmental benefits. Nevertheless, understanding how this specific heating drives to obtain ceramic materials with a combination of unique, structural and functional properties is the big challenge. The present work shows the different and improved properties achieved with β-eucryptite nanocomposite ceramic materials by microwave heating compared with the conventional method. Microcracking evolution in addition to the microstructure of the sintered materials along the several thermal cycles has been studied. Mechanical properties changes observed can be related to this process. Thus, the microwave technique is a promising tool for sintering new materials by controlling the composition of the phases, chemical reactivity and nanostructure, using up to 70% less energy in the whole sintering process than conventional heating. This technique becomes part of the new and innovative technologies "eco-green".

Abstract: High strength concrete for the production of concrete railway sleepers was designed more than 20 years ago. The compressive strength of the concrete was very high from the start, but flexure strengths showed some irregular development - a decrease in time. Later, also a significant decrease of fracture properties was recorded. Microcracking was found to be the reason for this; therefore some modifications were performed to avoid this happening (especially the reduction of the maximum size of aggregates from 22 mm to 16 mm or 11 mm). Some problems concerning frost resistance of the concrete with a slag addition were reduced by applying ternary binders. All of the results are discussed from the point of view of a long-term observation of the strengths and fracture properties ́ development during the time period of 5 years or even more.

Abstract: Drying creep of self-compacting concrete (SCC) has two different sources: microcracking and stress-induced shrinkage. Based on theory for pore water and microcracking theory of concrete, the model for drying creep caused by constant stress is derived from rheological equations. The proposed model is coupled with FEM analysis by recursive calculation, so that the creep caused by changing load can be calculated without considering stress history. Verification of the model is conducted through comparison with experimental result and Bazant’s empirical model. The comparing results validate the model for drying creep of SCC.

Abstract: Based on the previous test, SHCC (Strain Hardening Cementitious Composite) with the best fluidity was got through flow test, the different stress-strain full curves of SHCC were obtained by uniaxial tensile test on three dumbbell-shape specimens of SHCC with each group according to different diameter sand and different ages; and crack development was observed during test. The experimental results show that: The finer the sand sizes, the better the strain-hardening performance and more microcrack of SHCC that can be obtained; obvious strain-hardening characteristics of the specimens at different ages are all occured and the ultimate tensile strain can reach or even more than 3%. The crack spacing of various specimens are different, that is to say, there is difference in cracking characteristics among specimens at different ages. The experimental and analytical conclusion can provide a lot of theoretical bases for the research on performances of SHCC.

Abstract: The companion paper presents experimental results and numerical simulation methods to illustrate the behavior of FRP sandwich panels under synergistic effects of low temperature and cyclic loading. This paper aims at verifying the proposed simulation methods and providing several examples of its application to FRP sandwich panels. In this regard, four categories were simulated including: perfect bonding, microcracks, debonding Case I, and debonding Case II. Each category contained six ABAQUS models with two types of loads, thermal loads and applied mechanical loads. Results showed that microcracking would lower the stiffness of the panel, but not as much as debonding between the core and the face sheets.

Abstract: The paper is concerned with using experiments and numerical simulations to study the mechanical performance of a Honeycomb Fiber-Reinforced Polymer (HFRP) sandwich panel at different temperatures, especially at low temperatures coupled with cyclic loadings. All physical tests were performed in a temperature controlled room and used a three-point bending setup where the applied load gradually increased from zero to 36kN. Experimental results show that the stiffness of the panel becomes softer at some lower temperatures. In order to eliminate the influence of the initial conditions, an incremental method was introduced to process the experimental results. This method treated all displacements and strains as zero when applying a load of 4.5kN. Furthermore, the change in stiffness of the panel was obtained through the use of a special equivalent stiffness which involved measuring the change of the stiffness of the panel. After comparing different methods, the composite shell method was used to build finite element models for numerical simulations in ABAQUS. Reduction of moduli and Random Mesh Size Method (RMSM) were employed to simulate microcracking between fibers and matrix and debonding between the core and face sheets, respectively.

Abstract: This work is focused on the modeling of thermal stresses induced during the fabrication of the metal/ceramic composites. On example of Cr-Al2O3 composite processed by powder metallurgy, thermal stresses after fabrication are determined by FEM model for different contents of metal and ceramic phases. Numerical model of microcracking induced by thermal stresses is then proposed and applied to compute the overall elastic properties of the damaged composite. Comparison of the model predictions with the measured data for Young's modulus is presented.

Abstract: Rock macro mechanic character is decided by rock composite and meso configuration. LuHui granite minerals composite, micro configuration and the rock thermal cracking in differ temperature are observed by micro-photometer in meso. LuHui granite is composited by kinds of minerals, and its minerals’ inhomogeneity is visible, the change of cementation among rock crystal grains, dislocation and micro cracking in crystal is produced in crystal grains under temperature. By means of experiment, the development of granite micro cracks and interior configuration, and change laws of micro cracks were observed. The quantitative analysis of the micro mechanism and laws of granite thermal crack was made, granite cracks number increases acutely in 240-260 degree C after a narrow range fluctuation in 80 degree C. Finally, the thermal cracks threshold of granite was determined primarily.

Abstract: In this study, the mechanical behavior of porous thermally microcracked ceramics has been compared with that of solely porous materials, under compressive applied stress. The different aspects of the micro and macroscopic stress-strain curves have been inserted into a coherent analytical model and compared with finite element modeling calculations. The agreement between experiments and models is very good. It is shown that mechanical microcracking, as opposed to thermal, introduces an irreversible aspect in the deformation mechanisms of porous ceramics. In this concern, mechanical loads differentiate themselves from thermal cycling. This leads for instance to a change of the Young’s modulus as a function of applied load, which qualifies those materials as visco-elastic.

Abstract: This article presents a first approach of hygrothermal microcracking in stitched crossply laminates. The presentation includes the definition of the material mesostructure and the selection of a characterization method adapted to the specificity of the study. The hygrothermal loading selected is discussed and the evolution of the crack density in stitched crossply composites during the loading is presented and commented.