Papers by Keyword: Microcrack

Abstract: The work deals with the transmission electron microscopy (TEM) study of thin films of chromium-nickel Х18Н10 steel. The films were prepared from bulk samples after low cycle fatigue (LCF) tests. Focus was made on the processes accompanying propagation of small microcracks. Particularly, the microstructure changes near the crack tip were analyzed in terms of accommodation processes taking place during crack propagation, such as formation of slip bands, twins etc. The authors conducted crystallographic analysis of the defects formed during crack propagation in correlation with the reasons of their initiation and homogenous length of the slip bands. Thus, the reasons of microcrack deviation from the initial direction were determined. The research has shown that the most convenient microstructure variables in the austenitic crystals of polycrystalline sample, affecting the microcrack deviation, are microstructure, crystallography and the homogenous length of slip bands.

Abstract: Hard phases such as martensite regions affect micro-crack extension by blocking the plastic zone ahead of the crack tip, but also by changing the crack opening which can be taken as loading quantity for cracks. This paper deals with the measurement of crack opening for microcracks in a ferrite/martensite dual phase steel. The methods used are in-situ testing in the SEM, X-ray tomography, and digital image correlation. It was found that martensite regions affect the relative displacement of the crack phases both at the crack tip and in the crack wake.

Abstract: Development of microcracks and microstructures under uniaxial compressive load were studied by the measurement of axial residual strain and the calculation of specific crack area. Two opposite mechanisms could be distinguished for describing the development of microstructures, i.e. densification and micro-crack formation. Based on the experimental results, consistent tendency was found for the development of inside microcracks and overall deformation. Higher load level induced larger residual strain and residual specific crack area after unloading. Significant difference in the development of cracks was found for concrete specimens of different proportions. Specimen with higher water to binder ratio and lower strength developed earlier corresponds to lower initial stress-strength ratio for the beginning of crack propagation, and lower critical stress level for the beginning of unstable development. Effect of uniaxial compressive load on the durability of concrete specimen depends on the relative relationship between the two mechanisms.

Abstract: Based on extremely complex geological characteristics of metal mines, a mining model was built up, and from the micro - meso - macro point of view, relationship between rock AE and failure characteristics of overlying rock mass was studied and AE is divided into four stages. From the view of AE energy and distribution, the generation location, shape and size of caving of overburden rock was predicted. Through studies on the difference of goaf size, the author obtained the displacement variation and failure characteristics of metal mine overburden rock under tectonic stress, self-weight stress and dynamic mining conditions.

Abstract: Analysis of microcrack and mesocrack formation in austenitic steel thin filmsprepared after low-cycle fatigue (LCF) testsfrom bulk samples is presented using TEM techniques. Location, orientation and interaction of microcracks with microstructure components of the steel were determined. Plastic zone ahead of mesocrack tip and the structure changes in it were analyzed. Crystallography of slip bands and deformation twins and their relation with the microcrack propagation direction were also determined. The impact of grain anisotropy and inhomogeneous distribution of stress relaxation ahead of mesocrack tip in plastic zone were considered. Influence of sizes of mesocracks [ and microcracks and their relation with the trajectory and crystallography of propagation are also discussed.

Abstract: Lattice models allow length scale dependent micro-structural features and damage mechanisms to be incorporated into analyses of mechanical behaviour. They are particularly suitable for modelling the fracture of nuclear graphite, where porosity generates local failures upon mechanical and thermal loading. Our recent 3D site-bond model is extended here by representing bonds with spring groups. Experimentally measured distributions of pore sizes in graphite are used to generate models with pores assigned to the bonds. Microscopic damage is represented by failure of normal and shear springs with different criteria based on force and pore size. Macroscopic damage is analysed for several loading cases. It is shown that, apart from uniaxial loading, the development of micro-failures yields damage-induced anisotropy in the material. This needs to be accounted for in constitutive laws for graphite behaviour in FEA of cracked reactor structures.

Abstract: Austenitic chromium-nickel stainless steel CrNiNb 18-10 was studied using TEM technique. Characterizations of thin films prepared from bulk cylindrical samples after low-cycle fatigue (LCF) tests were conducted. Focus was made on the dislocation clusters, slip bands, defects and microstructure changes taking place in the steel during LCF. It is shown that microcracks occur in slip bands. Stereographic and trace analyses revealed the microcrack propagation directions. Two types of microcracks were observed: wedge-shaped and with parallel sides. The obtained results on possible reasons and mechanisms of microcrack formation in the above places are discussed in line with the theoretical assumptions and the existing literature.

Abstract: Effect of plastic stage curing on long-term properties of high performance concrete (HPC) was studied, thereafter, the mechanism is also analyzed. Results showed that compared to compressive strength, the permeability of surface concrete (especially for silica fume concrete) was more sensitive to plastic stage curing, and deteriorations due to insufficient plastic stage curing cant be eliminated by later longer time of wet curing. Furthermore, the deterioration of pore structure and formation of microcracks were main reasons for insufficient plastic stage curing that affected properties of concrete.

Abstract: Cellular concrete characterizes low mass density and low thermal conductivity is one of the durable and economic wall insulation materials which have good market prospects. The present paper investigated the composition of ternary cementitious materials, mixture design and production process of cellular concrete panel (CCP). Experimental results indicated that a higher compressive strength of CCP may be obtained by blended PⅡ52.5 Portland cement with pulverized low temperature clinker (PLWC) made of water treatment sludge and fly ash in the mass ratio of 0.70: 0.15: 0.15. CCP with dry density of 226 kg/m3, thermal conductivity of 0.056 W/ (m•K), compressive strength of 0.6 MPa and water absorption ratio of 7.6 vol. % was modified by high range water reducer, polypropylene fiber and water resistant agent. Mathematic models for controlling temperature of mixing water and for calculation quantity of gas forming admixture were established. Insulated moulds were specially designed for taking advantage of hydration heat of cement to speed up moulds turnover. Meanwhile, internal microcrack of CCP was avoided. CCP incorporating PLWC and fly ash may be used as external insulation material of walls.

Abstract: Three different flaws were detected in a Q345qD hot rolled steel plate by ultrasonic test. The morphologies, chemical compositions and formation causes of these flaws were studied by using metallographic examination, Electron Probe Micros-analyzer (EPMA) and Energy Dispersive X-ray Spectrometer (EDS) analysis. Besides, some corrective measures were proposed to reduce the incidence rate of these detecting flaws.