An upper bound limit load for overmatched scarf-joint specimens

A simple kinematically admissible velocity field involving single jump lines is proposed to find an upper bound limit load for overmatched scarf-joint specimens. It is shown that the solution based on the chosen velocity field depends on two dimensionless parameters, while the number of essential dimensional parameters is four. This is a great advantage for engineering applications. The results, for a particular case, are compared with a similar solution for centre-cracked tensile specimens and one which is obtainable from the new solution as a particular case.     

基于AMCQPSO的测试流程优化方法研究

测试流程的优化是测试性方案优化工作的开始,关系到整个测试性设计的好坏.针对现有优化方法的不足,提出带自适应变异的质心量子粒子群算法(AMCQPSO),为粒子群增加质心粒子、建立收缩扩张系数的自适应调节机制,并且引入变异因子,提高算法收敛速度的同时,增强了全局搜索能力,有效地避免了”早熟”现象.最后通过实例验证了算法的有效性,能够实现测试流程的优化.     

An upper bound solution for closed die sinter forging of hexagonal shapes

The paper reports on an investigation into the various aspects of closed die cold forging of hexagonal powder preforms, which have been compacted and sintered from atomized powder. It is found that for certain dimensional ratios of the preform, the die pressure is minimum. An attempt has been made determine the die pressures developed during the closed die forging of the hexagonal powder preform by using an upper bound approach. The results so obtained are discussed critically to illustrate the interaction of various process parameters involved and are presented graphically     

Numerical shakedown analysis of axisymmetric sandwich shells: An upper bound formulation

Shakedown analysis of axisymmetric elastic–perfectly plastic sandwich shells is performed here using a new upper bound formulation based on a special form of Koiter's theorem concerning piecewise linearized yield surfaces. Starting from finite element techniques and the Tresca sandwich yield condition, shakedown analysis is reduced to a linear programming problem which is solved by a powerful simplex algorithm. Numerical results are given for a number of examples and a comparison is made with a previously computed lower bound formulation.     

An improved branch and bound procedure for scheduling a flow line manufacturing cell

This paper presents a new branch and bound procedure for scheduling a flow-line manufacturing cell. This procedure and an existing procedure are tested on several problem sets with varying numbers of families, jobs and machines, and varying setup time distributions. The results show that the new procedure solves small problems dramatically faster than the existing procedure. Three heuristic procedures, based on the new branch and bound procedure, are developed. These heuristic procedures as well as a tabu search procedure are tested on problem sets with larger problem sizes. The results show that one of the new procedures generates solutions with improved makespans compared to the tabu search procedure.     

An upper bound of failure in viscoelastic materials subjected to multiaxial stress states

The failure behavior of a polyrethane rubber is examined under uniaxial, equibiaxial and equitriaxial tensile stresses. The maximal strains in uniaxial and equibiaxial tension are found to be nearly the same. Using the concept of maximal polymer chain extensibility, it is deduced that polymer chain motion is less dependent on the stress state than is commonly assumed. Failure under equitriaxial tension produces gross failure strains much less than those observed for uniaxial and equibiaxial tension. It is concluded that failure starts from defects. For spherical defects, the consequences of a maximum strain are examined. It is found through arguments that smaller cavites are more detrimental to material integrity than larger ones and that the gross equitriaxial strain at Failure is sensitive to the viscoelastic material properties only if the maximal uniaxial extension is large for typical ratios of void volume to total volume.

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Zusammenfassung Das Bruchverhalten eines Polyurethan Polymers ist in ein-, zwei- und drei-axialen Spannungsfeldern untersucht. Die maximalen Dehnungen unter ein-und zwei-achsiger Beanspruchung sind nahezu gleich. Auf Grund maximaler Polymer Kettenextensibilität kann man daher schliessen, dass die Polymer-Kettenbewegung weniger von der Beanspruchung abhängt, als 'allgemein angenommen wird. Bruch unter hydrostatischem Zug erfolgt bei Dehnungen, die viel kleiner sind als beim ein- oder zwei-achsigen Zug. Man folgert, dass der Bruch von Defekten ausgeht. Für kugelförmige Hohldefekte werden die Folgen der maximalen Dehnung geprüft. Dutch einfache Argumente wirdt gezeigt, dass kleine Defekte kritischer sein können als grosse, und dass makroskopische Bruchdehnungen nur vom viscoelastischen Materialverhalten beeinflusstwerden, wenn die maximale ein-achsige Dehnung sehr gross ist, angenommen das Verhältnis von Hohlvolumen zu Material Volumen ist typisch klein.

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Résumé Le comportement à la rupture d' un polymére de la classe des polyuréthanes est examiné sous états de tensions uniaxées, équibiaxées et équitriaxées. On a constaté que les déformations maximales en traction uniaxiale et en traction équibiaxiale étaient sensiblement égales. En recourant au concept de l'extensibilité maximale des chaines des polymères, on déduit de cette constatation que le mouvement de la chaine d'un polymère dépend moins de l'état de tension qu'on ne le suppose généralement.Per contre,la rupture sous traction equitriaxiale s' accompagne de déformations finales beaucoup moins importantes que dons les cas d'uniaxialité et d'équibiaxialité. On en conclut que la rupture sous traction unitriaxiale prend naissance à partir de défauts. On a examiné les effets de défauts sphériques sous déformation maximale, et trouvé, par des arguments simples, que l'intégrité du matériel était plus affectée par la présence de cavités petites que de cavité plus importantes. Par ailleurs, la déformation globale à rupture sous traction équitriaxiale ne dépend des propriétés viscoélastiques du matériel que dans la mesure où l'allongement maximum uniaxial atteint une valeur importante, et ce pour des rapports typiques du volume des cavités au volume total de matière.

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This work was supported by the National Aeronautics and Space Administration through Research Grant NsG-172-60, GALCIT 120.     

A note on upper bound formulations in limit analysis

In this paper, we study some recent formulations for the computation of upper bounds in limit analysis. We show that a previous formulation presented by the authors does not guarantee the strictness of the upper bound, nor does it provide a velocity field that satisfies the normality rule everywhere. We show that these deficiencies are related to the quadrature employed for the evaluation of the dissipation power. We derive a formulation that furnishes a strict upper bound of the load factor, which in fact coincides with a formulation reported in the literature. From the analysis of these formulations, we propose a post‐process, which consists in computing exactly the dissipation power for the optimum upper bound velocity field. This post‐process may further reduce the strict upper bound of the load factor in particular situations. Finally, we also determine the quadratures that must be used in the elemental and edge gap contributions, so that they are always positive and their addition equals the global bound gap. Copyright © 2012 John Wiley & Sons, Ltd.     

An efficient time-windowing procedure for scheduling projects under multiple resource constraints

This paper describes an efficient procedure for resource-constrained project scheduling problems. It starts with a simulated annealing technique to find a base schedule, and then improves the result by a time-windowing process. Every time-window, which is part of the base schedule, is a basis for a small subproject with two sets of specific constraints. Associated subprojects with time-windows are scheduled to optimality and based on their results the base schedule is updated. The overlapping feature of time-windows makes the displacement of an activity possible within the range of the entire project. The process of creating time-windows, scheduling their associated subprojects to optimality and improving the base schedule is controlled by a feed-back based mechanism that realises a trade-off between computational effort and the improvement made. The computational results indicate that the procedure is promising and yields better solutions than several heuristic algorithms presented in the literature.     

基于随机网络演算的无线网络TCP性能上界分析

为了准确地分析、判断无线网络传输控制协议(TCP)性能上界,以提高无线网络在数据传输上的服务质量(QoS),在综合分析无线网络特性的基础上,运用马尔科夫理论对无线网络TCP数据流的随机延迟上界、随机延迟抖动上界以及随机吞吐量理论上界进行了建模,并提出了一种基于随机网络演算的无线网络TCP性能上界的模型。该模型可以简化无线网络流量复杂性分析,而且能够给出网络数据流到达曲线、服务曲线、延迟上界、延迟抖动上界及吞吐量上界。研究表明,用这一模型,可以准确地预测无线网络TCP性能边界情况,这对进行无线网络TCP性能分析有重要意义。     

The lower bound toughness procedure applied to the Euro fracture toughness dataset

The validity of a statistical method for estimating an engineering lower bound fracture toughness in the ductile-to-brittle transition region is investigated using the Euro fracture toughness dataset generated in the European SM&T Project “Fracture Toughness of Steel in the Ductile-to-Brittle Transition Regime”. The lower bound method is based on the empirical evidence that, in the low probability regime, the cumulative failure probability function tends to be a straight line rather than a curve as is the case for Weibull distributions. The investigation demonstrates that the engineering lower bound toughness values as predicted by the method are related to a cumulative cleavage failure probability lower than 2.5%. Such bound predictions can be achieved on the basis of a small number of cleavage toughness values measured at the temperature of interest. The results confirm the validity of the method.     

An improved branching scheme for the branch and bound procedure of scheduling n jobs on m parallel machines to minimize total weighted flowtime

A branch and bound procedure to solve the n job, m parallel machine problem for the weighted flowtime criterion has been developed by Elmaghraby and Park (1974) and further modified by Barnes and Brennan (1977). This paper proposes a branching scheme different from theirs and shows its superiority. Also, some new and simple results are presented which are easy to implement to obtain an efficient branch-and-bound algorithm. In addition, a new and improved lower bound is developed which is easy to compute.     

A posteriori pointwise upper bound error estimates in the finite element method

An error estimate for the finite element method is presented in this paper. The error is identified as the response to a set of residual forces, and a complementary analysis provides an upper bound estimate of the global energy of the error. The inequality proposed by Babu?ka and Miller¹ is then employed to bound the error in stress and displacement at a point. The formula is derived for two-dimensional elasticity, but the procedure is general; and can be applied to three-dimensional and other problems. Numerical experiments using the procedure are carried out and the results are given for the four-node bilinear compatible element and plane stress.     

Comments on 'Cramer?Rao lower bound for tracking multiple targets'

In a recent paper, Ristic et al., 2004, proposed a simple and exact derivation of the theoretical Cramer-Rao lower bound (CRLB) for tracking multiple targets using intensity maps as measurements. Although the formulation and the theoretical derivations presented in their work are indeed correct, the analysis of the proposed bound and the conclusions that follow are wrong. Here, a correct analysis is presented with the key observation that [as opposed to the claim made in the work of Ristic et al. (2004)], the proposed CRLB reflects the interaction between the targets, even if their motion is independent. The multi-target CRLB is also compared with the error performance of a multi-target state maximum likelihood estimator; a remarkable agreement is observed for high signal-to-noise ratio     

Development of an accelerated abrasion test apparatus with a standardized testing procedure

Depending on the nature of the working environment, the abrasive wear of concrete floor slabs can range from scratching or scuffing to impact and local crushing. To assess abrasion resistance it is, therefore, necessary to have a versatile apparatus. This investigation describes the development of a basic apparatus with three modes of action and a standardized testing procedure, for assessing the abrasion resistance of concrete slabs in the industrial environment. It consisted of a rotating plate carrying a total load of 65 kg so that it was sufficiently portable for both laboratory and site testing. Three different types of abrasion head—(a) revolving pads, (b) rolling wheels, (c) dressing wheels—could be fixed to the plate so that the effects of different modes of wear could be assessed. The abrasion resistance is expressed in terms of the depth of the groove produced by 15 min of abrasion. A laboratory programme was undertaken to assess the apparatus and test method. The factors examined included the methods of assessing the abrasive wear, the duration of the test, the minimum number of tests to reliably assess the abrasion resistance, the relative performance of the three types of test head and the mechanism(s) by which each head abraded the concrete surface. Consequently a standardized procedure was developed which was found to be sensitive to variations in surface conditions. This has the additional advantages of simulating service conditions and of being repeatable, easy to follow and with the cost and time of testing not being excessive.     

基于非关联流动法则的滑移线场及上限法研究

当前岩土材料的滑移线场理论及上限法中都广泛采用经典塑性理论中的关联流动法则,由此得出应力特征线与速度滑移线一致的结论。而试验得知,岩土材料并不服从关联流动法则,因而应力特征线与速度滑移线不可能重合。文章分析了基于关联流动法则的滑移线场及上限法中存在的问题,根据广义塑性理论推导了基于非关联流动法则的滑移线场及上限法,消除了现行滑移线场理论及上限法理论中的种种矛盾。