HT120点焊机器人柔性臂动力学和强度分析

利用多体动力学分析软件ADAMS,直接从三维CAD模型中获取HT120点焊机器人的几何形状、质量分布和转动惯量等重要信息。在此基础上,适当添加约束、力和运动关系,建立了HT120点焊机器人的数字虚拟样机模型。然后进行了动力学仿真,并将机器人小臂等关键部件设为柔性体,利用有限元软件研究小臂动力学仿真过程中的应力分布情况,对造型设计,结构优化以及疲劳分析提供了参考。     

自适应遗传算法用于机械零件的可靠性设计

目的　探索机械零件可靠性优化设计的一种新方法 .方法　在造价、体积、强度和重量等约束条件下 ,机械零件的可靠性优化设计问题是一个具有多局部极值、非线性及具有整数和实数变量的混合优化问题 .本文以发动机气门弹簧为例 ,应用遗传算法建立了机械零件可靠性优化设计的一种新方法 .结果　应用自适应遗传算法建立的新方法比常规优化方法更适合于并行处理优化计算 .结论　将自适应遗传算法用于机械零件的可靠性设计效果是明显的     

Die shape optimal design in three-dimensional shape metal extrusion by the finite element method

A new approach to die shape optimal design in shape extrusion is presented. In this approach, the design problem is formulated as an optimization problem incorporating the three-dimensional finite element analysis model, and optimization of the die shape is conducted on the basis of the design sensitivities. The approach is applied to the determination of the die shapes for extrusion of parts with various cross sections including polygons and T sections. © 1998 John Wiley & Sons, Ltd.     

SHAPE DESIGN SENSITIVITY ANALYSIS AND OPTIMIZATION FOR STRUCTURAL DURABILITY

In this paper, a design sensitivity analysis (DSA) method for fatigue life of 3-D solid structural components of mechanical systems with respect to shape design parameters is presented. The DSA method uses dynamic stress DSA obtained using an analytical approach to predict dynamic stress increment due to design changes; computes fatigue life of the component, including crack initiation and crack propagation, using the predicted dynamic stress; and uses the difference of the new life and the original life at the same critical point to approximate the sensitivity of fatigue life. A tracked vehicle roadarm is presented in this paper to demonstrate accuracy and efficiency of the DSA method. Also, this method is applied to support design optimization of the tracked vehicle roadarm considering crack initiation lives as design constraints. © 1997 by John Wiley & Sons, Ltd.     

Multiaxial small fatigue crack growth in metals

Observations related to the formation and growth of small cracks ranging from subgrain dimension up to the order of 1 mm are summarized for amplitudes ranging from low cycle fatigue (LCF) to high cycle fatigue (HCF) conditions for polycrystalline metals. Further efforts to improve the accuracy of life estimation which address LCF, HCF and LCF–HCF interactions must consider various factors that are not presently addressed by conventional elastic–plastic fracture mechanics (EPFM) or linear elastic fracture mechanics (LEFM) approaches based on long, self-similar cracks in homogeneous, isotropic materials, nor by conventional HCF design tools such as the ε–N curve, the S–N curve, modified Goodman diagram and fatigue limit.Development of microstructure-sensitive fatigue crack propagation relations relies on deeper understanding of small crack behavior, including (a) interactions with microstructure and lack of constraint for microstructurally small cracks, (b) heterogeneity and anisotropy of cyclic slip processes associated with the orientation distribution of grains, and (c) local mode mixity effects on small crack growth. The basic technology is not yet sufficiently advanced in these areas to implement robust damage tolerant design for HCF. This paper introduces an engineering model which approximates the results of slip transfer calculations related to crack blockage by microstructure barriers; the model is consistent with critical plane concepts for Stage I growth of small cracks, standard cyclic stress–strain and strain–life equations above threshold, and the Kitagawa diagram for HCF threshold behaviors. It is able to correlate the most relevant trends of small crack growth behavior, including crack arrest at the fatigue limit, load sequence effects, and stress state effects.     

The effect of stress transients on the HCF endurance limit

Constant amplitude fatigue of a material at a fixed stress ratio, R, and at some limiting stress level, may produce high cycle fatigue (HCF) lives in excess of some large number, typically 10⁷ or higher, which can be treated as an endurance limit. Under vibratory loading, stress transients can exceed this endurance limit amplitude and cause damage that accumulates with repeated transient loading. These HCF transients normally occur at lower stress amplitudes than those needed to cause low cycle fatigue (LCF) where lives, N, are typically in the range N < 10⁴–10⁵. Therefore, the HCF transient stresses produce cycles to failure beyond the normal LCF regime but correspond to amplitudes that are above the fatigue limit stress. In this investigation, a titanium alloy, Ti-6Al-4V, is subjected to HCF stress transients while being cycled under constant amplitude HCF. The HCF transients correspond to blocks of loading above the fatigue limit stress applied for a specified fraction of their expected life. A step-loading procedure is used to determine the fatigue limit stress at a frequency of 420 Hz. Stress transients applied at stresses up to 40% above the endurance limit for cycle counts up to 25% of expected life are found to have little or no effect on the fatigue limit stress. Simple calculations of the propagation life in a test specimen show that most of the life at these transient stress levels is spent in the nucleation phase. Fractography, aided by heat tinting, was unable to detect any prior cracks due to the HCF stress transients on the fractured specimens.     

3D preform shape optimization in forging using reduced basis techniques

Three-dimensional preform shape optimization of complex forgings with a weighted summation of multiple basis shapes is presented in this article. Currently, 2D preform shape optimization is well developed; however, in cases in which the parts are neither axisymmetric nor plane strain, 2D assumptions do not hold well. The number of design variables required to define the 3D preform shape is high, making most iterative design methods impractical for shape optimization. The goal here is to make design optimization practical and efficient by developing reduced-order modeling techniques for 3D preform shape optimization. The preform shape is treated as a linear combination of various billet shapes, called basis shapes, with the weights for each basis shape used as design variables, thereby reducing the number of design variables. It is very difficult to obtain the necessary gradient information for 3D forging simulations, so a non-gradient method is used to build the surrogate model on which optimization is performed. The optimization problem is formulated to minimize strain variance while placing constraints on underfill. Representative problems are used to demonstrate the effectiveness of the approach.     

Probabilistic high cycle fatigue behaviour of nodular cast iron containing casting defects

Theoretical and experimental investigations were combined to characterize the influence of surface casting defects (shrinkages) on the high cycle fatigue (HCF) reliability. On fracture surfaces of fatigue samples, the defect is located at the surface. The shape used for the calculation is a spherical void with variable radius. Finite-element simulations were then performed to determine stress distribution around defects for different sizes and different loadings. Correlated expressions of the maximum hydrostatic stress and the amplitude of the shear stress were obtained by using the response surface technique. The loading representative point in the HCF criterion was then transformed into a scattering surface, which has been obtained by a random sampling of the defect sizes. The HCF reliability has been computed by using the Monte Carlo simulation method. Tension and torsion fatigue tests were conducted on nodular cast iron with quantification of defect size on the fracture surface. The S – N curves show a large fatigue life scattering; shrinkages are at the origin of the fatal crack leading to the final failure. The comparison of the computed HCF reliability to the experimental results shows a good agreement. The capability of the proposed model to take into account the influence of the range of the defect sizes and the type of its statistical distribution has been demonstrated. It is shown that the stress distribution at the fatigue limit is log-normal, which can be explained by the log-normal defect distribution in the nodular cast iron tested.     

A fast, versatile fatigue post-processor and criteria evaluation

This paper is concerned with the development and use of a general High Cycle Fatigue (HCF) post-processor, FAST, Fatigue AnalysiS Tool. It is used to evaluate and develop HCF criteria. FAST can handle many kinds of criteria and evaluate them with fatigue test results from both standard test specimens and actual components. When a reliable criterion exists, FAST will be used for design purposes. The principles of FAST are outlined and an example is given where seven local multiaxial stress based criteria are evaluated, along with the commercial fatigue post-processor Femfat. The conclusion is that a correct treatment of the volume or stress gradient effect is more important than the choice of criterion.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Evolutionary thickness design with stiffness maximization and stress minimization criteria

A shape or topology design with the stiffness maximized and the maximum stress minimized is usually of practical significance in structural optimization. This paper proposes a thickness based evolutionary procedure for such multicriteria design problems. To make the multicriteria optimization suit to more realistic structural situations, multiple maximum stress locations and multiple load cases are taken into account in this paper. To balance the stiffness and stress criteria, a weighting average scheme is adopted to identify the overall effects on the two components of design objective due to varying an element's thickness. Adopting the proposed optimization procedure, a design with maximized static stiffness and minimized peak stress is achieved by gradually shifting material from the under‐utilized regions onto the over‐utilized ones. The examples show the capabilities of the proposed method for solving multicriteria size and topology designs for both single and multiple load cases. Copyright © 2001 John Wiley & Sons, Ltd.     

基于瞬态动力学分析的波纹管结构优化

目的 优化波纹管结构尺寸,最大程度地减小波纹管的应力集中,提高波纹管的疲劳寿命。方法 利用ANSYS Workbench对真空灭弧室用波纹管进行参数化建模,对其耦合速度压力复杂工况进行瞬态动力学分析,借助DOE(DesignofExperiment)技术对波纹管关键几何参数进行单目标优化设计,对优化结果进行强度校核和疲劳寿命计算。结果 优化结果符合设计要求,波纹管在耦合速度压力复杂工况下满足强度的同时,最大等效应力减小了28.8%,疲劳寿命由3 064次提高到32 260次。结论 优化后的结构有效减小了波纹管危险部位的应力集中,疲劳寿命得到提高。     

Two-dimensional shape optimal design by the finite element method

This paper is concerned with the optimal shape design of plane or axisymmetric structures in order to minimize the stress concentration factor along the boundary. This boundary is described by straight lines and circles. The structure is analysed using the finite element method, and the optimization procedure is based on an extended interior penalty function. Three example problems are reported.     

Multi-Disciplinary Optimization for Multi-Objective Uncertainty Design of Thin Walled Beams

The focus of this paper is concentrated on multi-disciplinary and multi-objective optimization for thin walled beam systems considering safety, normal mode, static loading-bearing and weight, in which the uncertainties of the parameters are described via intervals. The size and shape of the cross-section are treated as design parameters during optimization. Considering the lightweight and safety, the design problem is formulated with two individual objectives to measure structural weight and maximum energy absorption, respectively, constrained by the average force, normal mode and maximum stress. The optimization problem with uncertainties is further transformed into a deterministic optimization based on interval number programming. The approximation models, coupled with the design of experiment (DOE) technique, are employed to construct objective functions and constraints. The uncertain optimization problem characterized with these approximation models is performed and applied to a practical thin walled beam design problems.     

A numerical study on the elements of shape optimum design

This paper discusses the main elements of shape optimization. The material derivative of a stress function using the continuum approach is derived by introducing an adjoint problem, which is then transformed into shape design sensitivity by replacing the velocity field with the change of the design variables. The difficulty related with the appearance of the concentrated adjoint loads is discussed, with two proposals for the modelling of the adjoint problem. A numerical example is used to demonstrate the accuracy of the proposed formulation for different adjoint loads.

Two shape optimization examples are used to investigate the numerical characteristics of the optimization process. Two kinds of design boundary modelling are employed, namely the linear and cubic spline boundary representation. The difference of the final design shapes under different design variables and mesh distributions are also studied.     

On the use of the Goodman diagram for high cycle fatigue design

Materials in rotating machinery are typically subjected to vibratory loading from a number of sources which, in turn, is superimposed on mean stresses which result primarily from steady-state centrifugal loads. In addition, components subjected to vibratory stresses can sustain damage during manufacturing, break-in cycles, or during service such as from foreign objects, fretting, or other types of wear. The combination of vibratory and ‘steady’ stress levels can, for certain load levels, produce low cycle fatigue damage in addition to the damage produced from the high frequency (HCF) vibratory loading since the ‘steady’ stresses are actually low cycle fatigue (LCF) which results in one cycle for every startup and shutdown operation. Design for HCF is generally based on a Goodman diagram which takes into account the vibratory as well as the steady stress amplitudes for fatigue runout or fatigue under a given number of cycles. It does not, however, take into account the combined effects of LCF and HCF. In this investigation, the combined effects are demonstrated analytically by numerical examples which consider both the initiation and propagation phases of fatigue. In addition to the analysis of LCF/HCF interactions, considerations which must be accounted for in design are reviewed in light of a number of failures of components in service in U.S. Air Force fighter engines. A critical assessment of the concepts embedded in the use of the Goodman diagram is presented. Comments on the limitations on the use of a Goodman diagram for design are provided. Some suggestions are offered for the improvement of the design methodology for HCF which involve both damage tolerance considerations and methods for assessing and improving the margin of safety.     

Continuum structural topological optimizations with stress constraints based on an active constraint technique

Stress‐related problems have not been given the same attention as the minimum compliance topological optimization problem in the literature. Continuum structural topological optimization with stress constraints is of wide engineering application prospect, in which there still are many problems to solve, such as the stress concentration, an equivalent approximate optimization model and etc. A new and effective topological optimization method of continuum structures with the stress constraints and the objective function being the structural volume has been presented in this paper. To solve the stress concentration issue, an approximate stress gradient evaluation for any element is introduced, and a total aggregation normalized stress gradient constraint is constructed for the optimized structure under the r?th load case. To obtain stable convergent series solutions and enhance the control on the stress level, two p‐norm global stress constraint functions with different indexes are adopted, and some weighting p‐norm global stress constraint functions are introduced for any load case. And an equivalent topological optimization model with reduced stress constraints is constructed,being incorporated with the rational approximation for material properties, an active constraint technique, a trust region scheme, and an effective local stress approach like the qp approach to resolve the stress singularity phenomenon. Hence, a set of stress quadratic explicit approximations are constructed, based on stress sensitivities and the method of moving asymptotes. A set of algorithm for the one level optimization problem with artificial variables and many possible non‐active design variables is proposed by adopting an inequality constrained nonlinear programming method with simple trust regions, based on the primal‐dual theory, in which the non‐smooth expressions of the design variable solutions are reformulated as smoothing functions of the Lagrange multipliers by using a novel smoothing function. Finally, a two‐level optimization design scheme with active constraint technique, i.e. varied constraint limits, is proposed to deal with the aggregation constraints that always are of loose constraint (non active constraint) features in the conventional structural optimization method. A novel structural topological optimization method with stress constraints and its algorithm are formed, and examples are provided to demonstrate that the proposed method is feasible and very effective. © 2016 The Authors. International Journal for Numerical Methods in Engineering published by John Wiley & Sons Ltd.     

Structural shape and topology optimization of cast parts using level set method

This paper presents a level set‐based shape and topology optimization method for conceptual design of cast parts. In order to be successfully manufactured by the casting process, the geometry of cast parts should satisfy certain moldability conditions, which poses additional constraints in the shape and topology optimization of cast parts. Instead of using the originally point‐wise constraint statement, we propose a casting constraint in the form of domain integration over a narrowband near the material boundaries. This constraint is expressed in terms of the gradient of the level set function defining the structural shape and topology. Its explicit and analytical form facilitates the sensitivity analysis and numerical implementation. As compared with the standard implementation of the level set method based on the steepest descent algorithm, the proposed method uses velocity field design variables and combines the level set method with the gradient‐based mathematical programming algorithm on the basis of the derived sensitivity scheme of the objective function and the constraints. This approach is able to simultaneously account for the casting constraint and the conventional material volume constraint in a convenient way. In this method, the optimization process can be started from an arbitrary initial design, without the need for an initial design satisfying the cast constraint. Numerical examples in both 2D and 3D design domain are given to demonstrate the validity and effectiveness of the proposed method. Copyright © 2017 John Wiley & Sons, Ltd.     

Basic issues in the mechanics of high cycle metal fatigue

Mechanics issues related to the formation and growth of cracks ranging from subgrain dimension to up to the order of one mm are considered under high cycle fatigue (HCF) conditions for metallic materials. Further efforts to improve the accuracy of life estimation in the HCF regime must consider various factors that are not presently addressed by traditional linear elastic fracture mechanics (LEFM) approaches, nor by conventional HCF design tools such as the S-N curve, modified Goodman diagram and fatigue limit. A fundamental consideration is that a threshold level for ΔK for small/short cracks may be considerably lower than that for long cracks, leading to non-conservative life predictions using the traditional LEFM approach. Extension of damage tolerance concepts to lower length scales and small cracks relies critically on deeper understanding of (a) small crack behavior including interactions with microstructure, (b) heterogeneity and anisotropy of cyclic slip processes associated with the orientation distribution of grains, and (c) development of reliable small crack monitoring techniques. The basic technology is not yet sufficiently advanced in any of these areas to implement damage tolerant design for HCF. The lack of consistency of existing crack initiation and fracture mechanics approaches for HCF leads to significant reservations concerning application of existing technology to damage tolerant design of aircraft gas turbine engines, for example. The intent of this paper is to focus on various aspects of the propagation of small cracks which merit further research to enhance the accuracy of HCF life prediction. Predominant concern will rest with polycrystalline metals, and most of the issues pertain to wide classes of alloys.