步降应力加速寿命试验的可靠性仿真

研究武器装备寿命实验准确评估可靠性问题,为进一步完善步降试验的统计分析方法,建立依赖于加速模型的数据折算公式,再对步降试验下的三步分析法做出改进,改进后的方法在处理数据时更具灵活性,且计算复杂度有所降低.进而,为比较步进试验与步降试验下三步分析法的精确性与稳定性,首次利用Monte - Carlo仿真法对两试验下的三步分析法进行仿真.仿真结果显示,相比步进试验,步降试验能显著提高三步分析法的精度和稳定性,说明了步降应力试验具有优越性为可靠性评估提供了依据.     

恒定应力加速寿命试验及其数据统计分析软件

加速寿命试验是为了缩短试验时间,在不改变产品失效机理的条件下,用加大应力（如温度应力、电压应力等）的方法对产品进行的寿命试验。通过加大应力水平,加快产品失效,获得失效数据,运用加速寿命模型外推产品在正常应力水平下的可靠性特性。本文主要介绍恒定应力加速寿命试验的组织实施方法,以及其试验数据统计分析软件。     

双应力交叉步降加速寿命试验的数值模拟

双应力交叉步降加速寿命试验是可靠性工程中不同于传统增应力加速寿命试验的一种新思路,投入实践前,针对其是否能提高可靠性试验效率进行计算机模拟。仿真相似条件下双应力交叉步降加速寿命试验和原有双应力交叉步进加速寿命试验的步骤,应用于产品寿命服从Weibull分布的实际算例,统计得出二者的试验平均耗费时间和效率比。由效率比曲线,证实双应力交叉步降加速寿命试验在应用于高可靠性、长寿命产品可靠性分析时多数情况下能较大程度提高试验效率。     

压力变送器加速工作寿命试验

压力变送器的加速工作寿命试验,是通过施加反复工作压力,检验应力累积效应而引起应力关系不稳定,造成变送器性能变差和失效。本文介绍一种简单可行的方法,实现各种量程的压力变送器加速工作寿命试验。     

加速寿命试验方法及评估

加速寿命试验是为缩短试验时间,在不改变故障模式和故障机理的条件下,用加大应力的方法进行的试验。随着元器件水平的迅速提高,高可靠、长寿命的产品越来越多,在正常应力水平下进行寿命试验来评定产品的可靠性已不能满足实际需要,代价很高,不现实,目前广泛采用了加速寿命试验。本文介绍了加速寿命试验方法及评估,并给出了案例。     

恒定应力加速寿命试验算法探讨

恒定应力加速寿命试验具有试验方法简单、理论成熟、得到信息多、试验结果较为准确等优点。本文利用三种被广泛运用的统计推断方法对同一恒定应力加速寿命试验结果进行推断,分析比较算法的优劣：在样本量较小的情况下极大似然估计优于线性无偏估计,但是当各应力水平下的样品失效数较大时,极大似然估计与线性无偏估计几乎等效,而线性回归的方法计算较为简单,但是在应力水平较少的情况下误差较大。     

恒定应力加速寿命试验模型及应用——威布尔分布

加速寿命试验是可靠性试验技术的基本方法之一。检验加速寿命试验中的数据是否服从威布尔分布,进而得出加速系数和加速寿命方程,用在不同应力水平下试验得到的数据,运用威布尔分布模型,并对其中的待估参数进行点估计和区间估计,最终得出产品在正常应力水平下的各项可靠性特征量的估计。     

基于Monte-Carlo仿真的双应力步降加速退化试验优化设计统计分析研究

针对高可靠性长寿命产品的性能往往受到多个应力的影响,且在有限试验时间内难以获得大量性能退化信息的问题,采用Monte-Carlo对试验过程进行仿真,运用退化失效环境因子对Monte-Carlo仿真产生数据进行折算,利用最小二乘对双应力步降加速退化试验统计分析,建立基于Monte-Carlo仿真的双应力步降加速退化试验优化设计统计分析模型。通过仿真实例,验证该方法的有效性、可行性。     

基于湿热试验的光伏组件功率衰减与使用寿命研究

光伏组件功率衰减与使用寿命对于光伏电厂安全、经济运行具有重要意义。针对光伏组件功率衰减和使用寿命问题,在分析加速湿热条件下光伏组件归一化功率倒S形衰减规律基础上,提出以指数时间尺度变换函数对光伏组件非线性归一化功率进行变换,并运用Gamma过程对加速湿热条件下光伏组件归一化功率衰减数据进行建模,进而根据光伏组件使用寿命定义估计光伏组件使用寿命。通过加速湿热试验下光伏组件实测使用寿命与估计的使用寿命,验证了所提方法的正确性与合理性。方法表明,在50℃、45%相对湿度条件下光伏组件平均寿命近似为20~25年。与此同时,还对现有的加速退化试验进行了分析改进,探讨了运用步降加速退化试验代替传统的恒定应力加速退化试验的可能性。     

随机振动环境中双端四梁结构微加速度计的寿命预测方法

微加速度计可完成运动载体加速度信息的获取,主要应用于航天、航空等伴随振动干扰的惯导领域。为分析微加速度计在振动环境下的寿命指标,本文提出一种随机振动加速恒定应力下的微加速度计可靠性试验与寿命预测方法。针对双端四梁结构微加速度计在振动应力下的典型失效模式,采用加速恒定应力试验方法并对试验数据统计分析,由各加速应力下的形状参数估计值满足Weibull分布参数恒等的约束条件,利用逆幂律模型建立了微加速度计的可靠度预测模型,并合理预测出微加速度计在正常振动应力下的工作状态。预测结果表明：某型号双端四梁结构微加速度计在正常振动应力下可靠度优于0.9时,寿命可达895 h。     

桁架结构优化设计的满应力法仿真研究

研究应用计算机进行桁架结构优化设计中满应力齿行法的应用问题。目前满应力齿行法在同时考虑应力、位移以及压杆稳定性约束时,计算结果往往存在难以收敛的缺陷,导致方法在实际应用中效果不佳。为解决以上问题,提出从算法中的松弛系数设置入手,将满应力步和满位移步的松弛系数分开设置,通过合理设置该参数使得计算能较好地收敛,经实验证明:当考虑压杆稳定时,传统的松弛系数设置方法难以得到理想效果,而本文提出的设置方法能避免计算过程中的振荡现象,能得到较好效果。     

Guidelines for selecting finite element grids based on an optimization study

The requirements for optimum finite element grids are presented and discussed. The work focuses on the introduction of nodal co-ordinates as independant unknowns, in addition to nodal displacements which have characterized the method in the past. A study is made of optimum grids in two dimensional plane stress problems from which specific guidelines are suggested such that near optimum grids can be selected by the analyst. These grids lead to improved stresses and displacements.     

Concurrent material and structural optimization of hollow plate with truss-like material

In this paper, optimum stress distribution for hollow plates composed of linear cellular materials (LCMs), a kind of truss-like material, is investigated. To reduce the computational cost, we model the material as micropolar continua representation. Two classes of design variables, relative density, and cell-size distribution of truss-like materials are to be determined by optimization under given total material volume constraint. And the concurrent designs of material and structure are obtained for three different optimization formulations. For the first formulation, we aim at the minimization of the maximum stress that appears at the initial uniform design; for the second formulation, we minimize the highest stress within the specified point set. As the yield strength of truss-like material is dependent on the relative material density, we minimize the ratio of stress over the corresponding yield strength along the hole boundary in our third formulation, which maximizes the strength reserve and seems more rational. The numerical results for the three objectives validate the concurrent optimization method proposed in this paper. And the influence of ply angle (angle between the principle direction of material and the axes of the system’s coordinates) on the optimum result is discussed. The dependence of optimum design on finite element meshes is also investigated. An approximate discrete model is established to verify the method proposed in this paper, and the stress concentration near a hole is reduced significantly.     

Optimum spacing design of grillage systems using a genetic algorithm

In this study a genetic algorithm based method is developed for the optimum design of grillage systems. The algorithm not only selects the optimum sections for the grillage elements from a set of standard universal beam sections, but also finds the optimum spacing required for the grillage system. Deflection limitations and allowable stress constraints are considered in the formulation of the design problem. Due to the fact that grillage elements are thin walled sections, warping plays an important role in their design, particularly, when they are subjected to significant torsional loading. The algorithm developed has the flexibility of including or excluding the effect of warping in the design process. Grillage structures are designed for uniformly distributed loading. The optimum spacings are determined both considering and not taking into account the effect of warping in the design. The comparison of the results shows that inclusion of warping in the design process has a significant effect on the optimum spacing.     

Adaptive stress limit approach for SLP method of structural optimization

A convergence aid based on adaptive stress limits and for use with the sequential linear programming (SLP) method of structural optimization is presented. The structural optimization problem is formulated using linearized behavioral constraints and is solved by the SLP method with a sequence of stress limits. The stress limits, for each linear programming stage, are modified using information derived from the previous iterations. This approach, by suitably modifying the feasible region, prevents the oscillation of solution due to linearization for nonvertex optimum and also permits disjoint global optimum to be attained. The effectiveness of the approach is demonstrated by applying it to truss structures with vertex, nonvertex, and disjoint global optima.     

Strength-based topology optimisation of plastic isotropic von Mises materials

Conventionally, topology optimisation is formulated as a non-linear optimisation problem, where the material is distributed in a manner which maximises the stiffness of the structure. Due to the nature of non-linear, non-convex optimisation problems, a multitude of local optima will exist and the solution will depend on the starting point. Moreover, while stress is an essential consideration in topology optimisation, accounting for the stress locally requires a large number of constraints to be considered in the optimisation problem; therefore, global methods are often deployed to alleviate this with less control of the stress field as a consequence. In the present work, a strength-based formulation with stress-based elements is introduced for plastic isotropic von Mises materials. The formulation results in a convex optimisation problem which ensures that any local optimum is the global optimum, and the problems can be solved efficiently using interior point methods. Four plane stress elements are introduced and several examples illustrate the strength of the convex stress-based formulation including mesh independence, rapid convergence and near-linear time complexity.

     

Optimum design of steel frames with stability constraints

Optimum design algorithms based on the optimality criteria approach are proven to be efficient and general. They have the flexibility of accomodating variety of design constraints such as displacement, stress, stability and frequency in the design problem. The design methods developed recently, although considering one or more of these constraints, lack the necessity of referring to any relevant design code. The algorithm presented for the optimum design of street frames implements the displacement and combined stress limitations according to AISC. The recursive relationship for design variables in the case of dominant displacement constraints is obtained by the optimality criteria approach. The combined stress inequalities which include in-plane and lateral buckling of members are reduced into nonlinear equations of design variables. The solution of these equations gives the values of bounds for the variables in the case where the stress constraints are dominant in the design problem. The use of effective length in the combined stress constraints makes it possible to study the effect of the end rigidities on the final designs. The design procedure is simple and easy to program which makes it particularly suitable for microcomputers. A number of design examples are considered to demonstrate the practical applicability of the method. It is also shown that the design procedure can be employed in selecting the optimum topology of steel frames.     

Stress-based topology optimization

Previous research on topology optimization focussed primarily on global structural behaviour such as stiffness and frequencies. However, to obtain a true optimum design of a vehicle structure, stresses must be considered. The major difficulties in stress based topology optimization problems are two-fold. First, a large number of constraints must be considered, since unlike stiffness, stress is a local quantity. This problem increases the computational complexity of both the optimization and sensitivity analysis associated with the conventional topology optimization problem. The other difficulty is that since stress is highly nonlinear with respect to design variables, the move limit is essential for convergence in the optimization process. In this research, global stress functions are used to approximate local stresses. The density method is employed for solving the topology optimization problems. Three numerical examples are used for this investigation. The results show that a minimum stress design can be achieved and that a maximum stiffness design is not necessarily equivalent to a minimum stress design.     

An optimised silicon piezoresistive microcantilever sensor for surface stress studies

Surface stress is a versatile and efficient means to study various physical, chemical, biochemical and biological processes. This work focuses on developing high sensitive piezoresistive microcantilever designs to study surface stress. The cantilevers are made of silicon with rectangular holes at their base that also circumscribe a piezoresistor sensing element. To find the optimum design, the effects of change in cantilever width, rectangular hole length and type of dopant on mechanical properties like deflection, frequency and maximum stress are characterised using finite element analysis software. The surface stress sensitivity characteristics of the different cantilever designs is ascertained by applying a surface stress on their top surfaces. Results show that the sensitivity is increased by increasing the cantilever width as well as the length of the hole and the sensitivity of p-type designs is more than two times the n-type.