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The fatigue tests were carried out on door frame containing single-side design and double-side design to assess the effect of different designs on fatigue life. By contrast, the finite element method (FEM) was also used to analyze the stress distribution of the two frames. At last, the fracture morphologies of the specimens after fatigue test were examined by scanning electron microscopy. The results revealed that the double-side design could elevate about 18∼21 times of fatigue life. The direction of crack propagation is transverse in single-side frame, while it is longitudinal in double-side frame.

The purpose of this article is to develop an effective method to evaluate the reliability of structures with epistemic uncertainty so as to improve the applicability of evidence theory in practical engineering problems. The main contribution of this article is to establish an approximate semianalytic algorithm, which replaces the process of solving the extreme value of performance function and greatly improve the efficiency of solving the belief measure and the plausibility measure. First, the performance function is decomposed as a combination of a series of univariate functions. Second, each univariate function is approximated as a unary quadratic function by the second‐order Taylor expansion. Finally, based on the property of the unary quadratic function, the maximum and minimum values of each univariate function are solved, and then the maximum and minimum values of performance function are obtained according to the monotonic relationship between each univariate function and their combination. As long as the first‐ and second‐order partial derivatives of the performance function with respect to each input variable are obtained, the belief measure and plausibility measure of the structure can be estimated effectively without any additional computational cost. Two numerical examples and one engineering application are investigated to demonstrate the accuracy and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

The sensitivity index plays a critical role in the design of product and is used to quantify the impact degree of the uncertainty of the input variable to the uncertainty of the interest output. This paper presents a new local reliability sensitivity method and a global reliability sensitivity analysis method of time-dependent reliability problems. Firstly, according to the Poisson's assumption-based first-passage method, the local reliability sensitivity index is directly obtained by calculating the partial derivative of the failure probability to the distribution parameters of input random variable. Then, the moment-independent global reliability sensitivity index of the time-dependent problems is derived based on the concept of moment-independent. Finally, the efficiency and accuracy of the proposed method are verified with the reference results of Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

Systems with random variables and random excitations exist widely in various engineering problems. Extending the traditional global reliability sensitivity to this double-stochastic system has important guiding significance for its design optimization. However, because there is a certain coupling between the randomness of variables and the randomness of excitation, this coupling mechanism is difficult to determine in practical projects. Therefore, it is difficult to extend the traditional reliability sensitivity analysis method to this double-stochastic system. In this research, it is assumed that there is no correlation between variables and excitations. Then, combining the first-passage method–based dynamic strength formula and the variance-based sensitivity analysis method, an approximate global reliability sensitivity analysis method for this double-stochastic system is proposed. In order to improve the computational efficiency, a nested loop method based on seven-point estimation is proposed for reliability sensitivity analysis. In order to verify the accuracy and efficiency of the proposed method, a Monte Carlo simulation is given as a reference. Three examples are studied and discussed to illustrate the practicality and feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Based on aerodynamic analysis, an optimization method for the profiles of turbine blade is studied in this paper. This method is capable of addressing multiple objectives and constrains without relying on user input. A quintic polynomial is used to build the three-dimensional blade model and a three-dimensional Navier-Stokes solver was used to solve the flow field around the turbine blade. The objective functions are the turbine aerodynamic efficiency and total pressure ratio. The optimization is completed with the K-S function technique and accelerated by approximation technique. Finally, the proposed method is applied to optimizing a true blade to validate its accuracy and efficiency. The obtained result shows that the approximation method is more efficient and accurate than the conventional method. [ABSTRACT FROM AUTHOR]

This paper presents a low cycle fatigue (LCF) failure analysis on [001], [011], and [111] orientations of a Ni‐based single crystal superalloys at 850 °C. Fracture mechanism and microstructure damage evolution are revealed through crystal plastic theory and fracture morphology analysis. Strain‐controlled LCF tests are conducted with the standard round bar samples; the fracture morphology and microstructure characteristics, such as the fracture mechanism and microstructure damage evolution, are observed via scanning electron microscopy (SEM), optical microscopy (OM). Crystal plastic theory is adopted to establish the crystal slip system framework, considering elastic and plastic strain rate, elastic orientation factors, and the resolved shear stress (Rss). The life prediction model is established based on Levkovitch damage theory, theoretical analysis, and test results has good consistency. The results demonstrate that fatigue life on [001] orientation is the longest, but considering the effects of stress amplitude and Young's modulus, [011] orientation exhibits the best fatigue performance. Under LCF loading, the initial crack starts to occur on surface of specimen if the internal defects are small and negligible; in contrast the initial crack starts to occur in subsurface or internal of specimen if the microstructure defects exist in material. The LCF cross‐section of SC belongs to the quasi‐cleavable type, the crack propagates in the direction of {111} plane and then extends along the vertical stress axis until the specimen fractures. Strain‐controlled low cycle fatigue tests are conducted with the standard round bar samples of single crystal at 850°, the fracture morphology and microstructure characteristics, such as the fracture mechanism and microstructure damage evolution, are observed via scanning electron microscopy and optical microscopy. Crystal plastic theory is adopted to establish the crystal slip system framework, considering elastic and plastic strain rate, elastic orientation factors and the resolved shear stress. Fracture mechanism and microstructure damage evolution are revealed through crystal plastic theory and fracture morphology analysis. [011] orientation exhibits the best fatigue performance considering the effects of stress amplitude and Young's modulus; the defect has a significant effect on the position of crack initiation; the LCF cross‐section of SC belongs to the quasi‐cleavable type, the crack propagates in the direction of {111} plane, and then extends along the vertical stress axis until the specimen fractures. [ABSTRACT FROM AUTHOR]

The governing equation of flow-induced vibration is deduced in terms of Hamilton's principle for a variable diameter pipe conveying axial steady flow. Frobenius method is applied to analyze the governing equation. After the recursion formulas of coefficients are obtained, dynamic stiffness method is proposed for free vibration analysis of the variable diameter pipe conveying fluid. In the example, the natural frequencies of uniform pipes conveying fluid are computed and comparisons are made to validate the dynamic stiffness method. Then, the natural frequencies and modal shapes are obtained for the variable diameter pipe conveying fluid with different section variation coefficients and fluid velocities. [ABSTRACT FROM AUTHOR]