Yue Zhu-feng

Support vector machine for structural reliability analysis

Support vector machine (SVM) was introduced to analyze the reliability of the implicit performance function, which is difficult to implement by the classical methods such as the first order reliability method (FORM) and the Monte Carlo simulation (MCS). As a classification method where the underlying structural risk minimization inference rule is employed, SVM possesses excellent learning capacity with a small amount of information and good capability of generalization over the complete data. Hence, two approaches, i.e., SVM-based FORM and SVM-based MCS, were presented for the structural reliability analysis of the implicit limit state function. Compared to the conventional response surface method (RSM) and the artificial neural network (ANN), which are widely used to replace the implicit state function for alleviating the computation cost, the more important advantages of SVM are that it can approximate the implicit function with higher precision and better generalization under the small amount of information and avoid the “curse of dimensionality”. The SVM-based reliability approaches can approximate the actual performance function over the complete sampling data with the decreased number of the implicit performance function analysis (usually finite element analysis), and the computational precision can satisfy the engineering requirement, which are demonstrated by illustrations.

Dynamic Response Analysis of Bird Strike on Aircraft Windshield Based on Damage-modified Nonlinear Viscoelastic Constitutive Relation

Abstract Damage-modified nonlinear viscoelastic constitutive equation and failure criterion are introduced and the three-dimensional incremental forms are deduced based on the updated Lagrangian approach. A simple tensile test model and a split Hopkinson pressure bar model are built to verify the accuracy of the subroutine implemented within the non-linear finite element program LS-DYNA. A numerical model of bird strike on windshield is established to study the responses of windshield under three different bird velocities at three sites. The bird is represented by a cylinder with a hemisphere at each end and the contact-impact coupling algorithm is used in this study. It is found that the implemented subroutine can properly describe the mechanical behavior of polymethyl methacrylate under low and high strain rates and large deformation, and can be used validly.

Advanced response surface method for mechanical reliability analysis

Based on the classical response surface method (RSM), a novel RSM using improved experimental points (EPs) is presented for reliability analysis. Two novel points are included in the presented method. One is the use of linear interpolation, from which the total EPs for determining the RS are selected to be closer to the actual failure surface; the other is the application of sequential linear interpolation to control the distance between the surrounding EPs and the center EP, by which the presented method can ensure that the RS fits the actual failure surface in the region of maximum likelihood as the center EPs converge to the actual most probable point (MPP). Since the fitting precision of the RS to the actual failure surface in the vicinity of the MPP, which has significant contribution to the probability of the failure surface being exceeded, is increased by the presented method, the precision of the failure probability calculated by RS is increased as well. Numerical examples illustrate the accuracy and efficiency of the presented method.

STATISTIC MODELING OF THE CREEP BEHAVIOR OF METAL MATRIX COMPOSITES BASED ON FINITE ELEMENT ANALYSIS

The aim of the paper is to discover the general creep mechanisms for the short fiber reinforcement matrix composites (MMCs) under uniaxial stress states and to build a relationship between the macroscopic steady creep behavior and the material micro geometric parameters. The unit cell models were used to calculate the macroscopic creep behavior with different micro geometric parameters of fibers on different loading directions. The influence of the geometric parameters of the fibers and loading directions on the macroscopic creep behavior had been obtained, and described quantitatively. The matrix/fiber interface had been considered by a third layer, matrix/fiber interlayer, in the unit cells with different creep properties and thickness. Based on the numerical results of the unit cell models, a statistic model had been presented for the plane randomly-distributed-fiber MMCs. The fiber breakage had been taken into account in the statistic model for it starts experimentally early in the creep life. With the distribution of the geometric parameters of the fibers, the results of the statistic model agree well with the experiments. With the statistic model, the influence of the geometric parameters and the breakage of the fibers as well as the properties and thickness of the interlayer on the macroscopic steady creep rate have been discussed.

Transient response analysis of multi-span pipe conveying fluid

The method of reverberation-ray matrix proposed by Pao et al. is applied to analyze the transient response of a multi-span pipe conveying fluid. The pipe under transverse vibration is considered as Timoshenko beam conveying axial flow. In the process of analysis, fast Fourier transform is used to convert the partial differential equation into ordinary differential equation. The support points of the pipe are chosen as the elements’ nodes. The transient wave motions at each node are divided into incident and reflected wave. By virtue of continuity and balance condition, the scattering relation is established between the above wave motions. The transient response is obtained by inverse fast Fourier transform. In the examples, the natural frequencies are calculated for single- and multi-span pipe conveying fluid with different fluid velocity, and great agreement is shown with the published literature. And then the transient responses, such as deformation, velocity, shear force and bending moment are obtained for a two-span pipe conveying fluid under different fluid velocity.

Stress Distribution Near Grain Boundary in Anisotropic Bicrystals and Tricrystals

The rate dependent crystallographic finite element program was implemented in ABAQUS as a UMAT for the analysis of the stress distributions near grain boundary in anisotropic bicrystals and tricrystals, taking the different crystallographic orientations into consideration. The numerical results of bicrystals model with the different crystallographic orientations shows that there is a high stress gradient near the grain boundaries. The characteristics of stress structures are dependent on the crystallographic orientations of the two grains. The existing of triple junctions in the tricrystals may result in the stress concentrations, or may not, depending on the crystallographic orientations of the three grains. The conclusion shows that grain boundary with different crystallographic orientations can have different deformation, damage, and failure behaviors. So it is only on the detail study of the stress distribution can the metal fracture be understood deeply.

A crystallographic model for the orientation dependence of low cyclic fatigue property of a nickel-base single crystal superalloy

Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [1 1 2], [0 1 1] and [1 1 4] oriented single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950°C. The cyclic strain rates were chosen as 1.0×10−2, 1.33×10−3 and 0.33×10−3s−1. The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the scanning electron microscopy (SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950°C, and an orientation and strain rate modified Lall-Chin-Pope (LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied.

Study on Fatigue Properties of Turbine Disk Groove Modeling Specimens of GH4720 Alloy

Abstract Based on the structural features and the work status of the aero-engine turbine disk groove, the experimental and theoretical analysis of the fatigue failure mechanism and the crack growth life of the groove modeling specimen of GH4720 alloy were carried out. The fatigue failure of the groove modeling specimen of GH4720 alloy can be divided into three stages: (i) due to the high stress concentration, slip systems are activated and then cracks initiate; (ii) with the increase of loading cycles, adjacent intergranular dislocations and slip systems are activated, cracks transfer between grains; (iii) with the increase of the stress intensity factor range and the cooperation of shear stress and principal stress, the dislocations motion in different slip systems and the crack propagation are accelerated. A fatigue crack growth life model of GH4720 alloy has been established. The crack growth lives obtained from the finite element analysis and the life model have a good agreement with the results of the experiment. It is demonstrated that the life model can be used in engineering to predict the remaining life of turbine disk.

Numerical Simulation on the Creep Damage Evolution of Nickel-Based Single Crystal Specimens with Slant-Angled Film Cooling Holes

Abstract Numerical calculations have been performed to study the creep damage evolution of nickel-based single crystal specimens with slant-angled film cooling holes. The emphasis is the effect of different slant angles on creep properties. The slant angles are 0°, 15°, 30° and 45°. The results show that the distributions of the resolved shear stress around film holes are different under different slant angles. The directions of crack propagation are θ =±54° when α =0°, 15°, 30°; however when α =45° the crack propagation directions are along θ =±46°. The specimen with α =30° has the longest failure life. In the higher stress region near the cooling hole, the phenomenon of stress relaxation will appear, such as resolved shear stress, Mises equivalent stress and stress σ 11 . And with the time increasing, they reach stable values. Different specimens have different damage distributions. The maximum damage points appear at locations 0°, 0°, 13° and 26° from horizontal direction when the slant angles are 0°, 15°, 30° and 45°, respectively. And thickness has influence on the distribution of resolved shear stress along the holes.

Dynamical strength and design optimization of pipe-joint system under pressure impact load

This article is focused on the dynamical strength analysis and design optimization of a pipe-joint system. A three-dimensional model of a fuel pipe-joint system is established and the strength and dynamic analysis of the system under the pressure impact load is analysed. In order to avoid dangerous forced vibrations or resonance, and to lower vibration level, the clamps’ numbers and locations are optimized. The results show that: (a) under the pressure impact load, near the T-branch pipe, clamps and elbows, the pipe system’s failure can easily occur; and (2) the numbers and locations of clamps of pipe-joint system can be optimized and the optimized pipe-joint system has better anti-vibration ability.