Structure of Crack in Thermally Dried Sludge Cake

We examined in this work the drying characteristics of wastewater sludge, considering the weight loss using an electronic balance and the morphology change of drying cake by an X-ray micro-computerized tomography scanner (micro-CT). The drying rates for sludge cake was 50-60% of the pure water test. The corresponding micro-CT images were scanned to explore the change in cake volume and development of internal crack over drying. The Otsu's method was applied to bilevel the scanned images, based on which the three-dimensional model for crack's internal structure was reconstructed. The cake porosity and the geometric factors of cracks, such as the compactness, the boundary fractal dimension, and the Sierpinski carpet fractal dimension, were evaluated. The shrinkage of cake volume and the development of internal crack occurred simultaneously. The skin layer formed by the former mechanism retards further drying. On the other hand, the internal cracks were shown to exhibit zigzagging with a noncircular cross section and fractal-like boundary, which should be able to enhance local drying. A possible role of change in crack structure on the rate of drying was discussed.     

水工压力隧洞衬砌裂缝计算及抗裂设计

给出了有防渗要求的水工压力隧洞衬砌环向应力与纵向应力计算式,分析了隧洞衬砌产生纵向裂缝与环向裂缝的原因,提出了相应抗裂设计原则与伸缩缝间距计算式.     

Mechanical properties and failure characteristics of fractured sandstone with grouting and anchorage简

Based on uniaxial compression experimental results on fractured sandstone with grouting and anchorage, we studied the strength and deformation properties, the failure model, crack formation and evolution laws of fractured sandstone under different conditions of anchorage. The experimental results show that the strength and elastic modulus of fractured sandstone with different fracture angles are significantly lower than those of intact sandstone. Compared with the fractured samples without anchorage, the peak strength, residual strength, peak and ultimate axial strain of fractured sandstone under different anchorage increase by 64.5–320.0%, 62.8–493.0%, and 31.6–181.4%, respectively. The number of bolts and degree of pre-stress has certain effects on the peak strength and failure model of fractured sandstone. The peak strength of fractured sandstone under different anchorage increases to some extent, and the failure model of fractured sandstone also transforms from tensile failure to tensile–shear mixed failure with the number of bolts. The pre-stress can restrain the formation and evolution process of tensile cracks, delay the failure process of fractured sandstone under anchorage and impel the transformation of failure model from brittle failure to plastic failure.     

Comments on Simulations of Fatigue Crack Propagation by Blunting and Re-sharpening: The Mesh Sensitivity

A selection of results of extensive analysis of mesh sensitivity of largedeformation elastoplastic finite element (FE) simulations of a crack under cyclic loading is presented. Notorious mesh sensitivity, which commences at spontaneous shear localization, is evidenced. This is argued to be not a mere numerical artefact, but a consequence of the inherent bifurcating behaviour of the boundary value problem solutions, where different mesh layouts and element technologies could trigger a variety of deformation patterns near the crack tip.     

Crack and flaw identification in elastodynamics using Kalman filter techniques

Filter-driven optimization based on the extended Kalman filter concept is used here for the numerical solution of crack and flaw identification problems in elastodynamics. The mechanical modeling of the studied two-dimensional problem, which includes the effect of unilateral contact along the sides of the crack, is done with the help of the boundary element method. The effect of various dynamical test loads and the applicability of this method for crack and defect identification in disks are investigated. The work has been supported by the German Research Foundation (DFG). Partial support has been provided by a Greek-German Research Cooperation Grant (IKYDA2001). This support is greatfully acknowledged.     

Multi-region Trefftz boundary element method for fracture analysis in plane piezoelectricity

This paper presents a multi-region Trefftz boundary element method for fracture analysis in plane piezoelectricity. To model the sub-region that contains the crack, a special set of Trefftz functions that satisfy the traction-free and charge-free conditions along the crack faces are constructed. To model the remaining sub-regions, the basic set of Trefftz functions co-derived previously by the authors are employed. With the two sets of Trefftz functions, the multi-region Trefftz boundary element method is formulated by point collocation. The special set of Trefftz functions exempts all the boundary treatment of the crack faces and enables the direct determination of the electromechanical intensity factors. Numerical examples are presented to illustrate the efficacy of the formulation.     

Fatigue cracks nucleation on steel, acoustic emission and fractal analysis

In this paper, a new acoustic emission (AE) diagnostic technique, for the study of fatigue cracks nucleation and propagation on steel, was investigated. Using the fractal analysis, and the box-counting method (BCM) in particular, it is possible to characterize the spatial distribution of the prime AE sources through the fractal dimension (D) that evolve with the number of fatigue cycles (N) of the specimen. D–N curves were found useful to identify the condition of incipient collapse due to the nucleation and propagation of fatigue cracks on steel. It is possible to use the fractal dimension as a damage parameter. In all tested specimens, the crisis occurs within the same range of values of fractal dimension. The results suggest that it is possible to anticipate the detection of crack beginning relating to the other theoretical or experimental techniques.     

Prediction of the crack extension under fretting wear loading conditions

Fretting is associated with small amplitude oscillatory movements between two surfaces in contact. One possible consequence of fretting is the formation and subsequent growth of cracks at the edges of the contact. This paper presents an experimental investigation of the cracking behaviour under fretting loading of two different aluminium alloys: 2024-T351 and 7075-T651. Systematic and controlled experiments with a cylinder-flat contact under partial slip fretting conditions were carried out. A model which combines both crack nucleation and propagation processes is used to predict the crack extension throughout the life of the component. The direction of crack propagation experimentally observed was taken into account by the model. Furthermore, an analytical prediction of crack nucleation based on the process volume approach is made. The predictions of both crack extension and nucleation are compared with the experimental results, and show good agreement.     

Analysis of superalloy turbine blade tip cracking during service

High Pressure Turbine (HPT) first stage blade is the most important rotational components of aero-engine. It operates at high temperature and under conditions of extreme environmental attack such as oxidation and corrosion, is especially subjected to degradation by oxidation, corrosion and wear. During the service of aero-engine, the HPT first stage blades made of c 6 y nickel-based superalloy suffer from increasing blade tip cracks after hundreds of hours service. This significantly affects the whole engine function and, of course, safety of the aircraft. In this paper, the premature tip cracking of the HPT first stage blade is investigated. The research result shows that turbine blade tip is initially damaged by rubbing and corrosion over a period time, premature cracking of blade tip is caused by a combination mechanism of environment and thermal stress. During turbine blade maintenance and refurbishment, coating of both oxidation-resistance and abrasion-resistance should be applied in blade squealer tip to counteract the problem.