Key Engineering Materials Vols. 348-349

Abstract: A special specimen was created cutting a rectangular notched area from the surrounding of the upper left corner of a wide body aircraft door. Then a fatigue traction load was applied in order to induce an MSD crack initiation and propagation. An innovative DBEM (Dual Boundary Element Method) modelling approach was devised, capable of explicitly modelling the different test article layers with their rivet connections even in a 2d approach. The rivets that are close to the propagating crack are coupled with the corresponding holes by non linear contact conditions, and the accuracy improvements are assessed in comparison with a previous linear analysis, in which traction and displacements continuity conditions on the hole-rivet interface had been imposed. The importance of such influence on the simulation precision need to be assessed due to the strong impact that a non linear analysis produces on computational times. For such a complex problem (three different panels, made of different materials, each one with a variable thickness and connected by numerous rivets), experimental crack propagation data were available for the numerical-experimental comparison. With such non linear approach, a significant improvement on the growth rate correlation is obtained, that justify the increased computational effort.

Abstract: In order to overcome the weakness in traditional high arch dam safety evaluation without considering the randomness, failure mode and risk analysis, it is proposed to apply three indexes namely failure probability, economic loss and life loss to high arch dam safety evaluation. On the basis of the analysis of accidents and on-site measured data, expert discussion as well as analytical analogy and by means of Fault Tree Analysis, a probe is made into the four major failure modes and the causes of high arch dam failure, namely destabilization collapse, shear-slipping collapse, excess cracking and man-caused destruction. Based on the calculating of the probability of high arch dam shearing-slipping failure, the computational method and procedures are established by means of Second Moment Method for the calculation of the occurrence probability of the major failure modes of high arch dam. A study is made of the degree of correlation between the major failure modes and of the method for calculating the high arch dam failure probability under multi-failure-modes.

Abstract: It is proposed to make use of dam risk rate, economic loss risk value and life loss risk value in evaluating the risk of high arch dam. The acceptable standards of risk for high arch dam in the developing country should not be set too high,it should reflect the level of economic and technology development, the policy for energy sources,the life value as well as the degree of the importance attached. Based on study of the characteristics of high arch dam and by means of the comparison of the levels of economic development, the life values and the degrees of the importance attached between developing and developed countries, as well as the analogy analysis of rare risk events occurring in developing countries, the reference value of the single high arch dam acceptable risk rate, economic loss risk and life loss risk in developing countries are found to be 10 /(yr dam) 6 ⋅ − , 10000$/(yr ⋅ dam) and 0.001person/(yr ⋅ dam) respectively;a method is worked out for determining the acceptable risk rate,the acceptable economic loss risk value and life loss risk value.

Abstract: Composite materials are used in aerospace structures due to their considerable bending stiffness and strength-to-weight ratio. A composite sandwich is composed of a face-sheet and an aluminum core. The face-sheet of the sandwich takes the bending stress and core of sandwich takes the shear stress. A compression test and FEM analysis accomplished about composite sandwich panels that have curvature. The FEM analysis was performed using a commercial code, ANSYS and the compression test was performed until failure occurred in the sandwich panel. A strain gauge and a displacement gauge were used to acquire the data. In this paper, the failure strength and failure mode was checked. Additionally, the results of the test and analysis were compared.

Abstract: Forced vibration and liquid corrosion are important reasons for last stage blade failure in steam turbine. In order to improve reliability of blades in wet steam stages, the dynamic stress and the liquid-solid impact stress of blade were analyzed firstly, then Neuber’s rule, rainflow counting method and improved cumulative damage rule were introduced to develop a new model to evaluate service life of last stage blade. Many factors such as manufacturing quality, metal quality and operation status were considered, so it is more reasonable for service life assessment of last stage blade. At last, the service life of a last stage blade of 680mm was analyzed and the result approximates the practical service life. It is shown the new fatigue model can give some significant data and is usable for engineering application.

Abstract: The distribution of stress in a nonrectangular compressed area, such as a section under a biaxial moment, is different than that of a rectangular compressed area. The properties of the compressive stress distribution of concrete have only a minor effect on the pure bending moment capacity of reinforced concrete beams, but they are important influencing factors for columns subjected to a combined axial load and a biaxial bending moment. The variation of ultimate strain of concrete according to the angle and depth of a neutral axis was investigated. Thus, the modified rectangular stress block (MRSB) model for nonrectangular compressed areas is formulated and compared to the experimental results and the existing RSB model. The MRSB method is able to provide more accurate predictions of test results for the biaxial bending moment strength than the RSB method.

Abstract: Spalling is defined as damages to concrete exposed to high temperature during fire, causing cracks and localized bursting of small pieces of concrete. As the concrete strength increases, the degree of damage caused by spalling becomes more serious due to impaired permeability. It has been reported that polypropylene(PP) fiber has an important role in protecting concrete from spalling, and the optimum dosage of PP fiber is 0.2%. However, this result was based on the fire test of non-reinforced concrete specimens. The high-temperature behavior of highstrength reinforced concrete columns with various concrete strength and various quantity of PP fibers is investigated in this study. The results revealed that the ratio of unstressed residual strength of columns increased as the concrete strength increased and as the quantity of PP fiber increased from 0% to 0.2% the residual strength of columns increase. However, the effect of PP fiber quantity on residual strength of column was barely above 0.2%.

Abstract: The present paper deals with the research on the crack growth in a bolt connection of a lug for crane counter weight bars. Counter weight bars are structural elements that are subjected to very heavy loads and therefore special attention must be paid. The main purpose of this research is to determine the number of the load cycles required for a crack to propagate from initial to critical crack length, when the final failure can be expected to occur. All required material parameters and the experimental results were determined in our previous research. The influence of the initial crack size upon the remaining life of the lug is researched numerically by means of finite element analysis and analytically by use of the corrected analytical model.

Abstract: A review is given about FEM-techniques to compute the coupled electromechanical boundary value problem of cracks in piezoelectric structures under static and dynamic loads. To calculate the relevant fracture parameters very precisely and efficient, the following numerical techniques are presented: i) Special singular crack tip elements, ii) Modified crack closure integral, iii) Computation of electromechanical J-integral and iv) Usage of interaction integrals. Special emphasis is devoted to different electric crack face boundary conditions. The accuracy, efficiency and applicability of these techniques are examined by various example problems and discussed with respect to their advantages and drawbacks for practical applications.