任意循环特征下塑性材料持久极限的估算公式

借助塑性材料对称循环持久极限σ－１和静载强度极限σｂ，导出任意循环特征下材料持久极限σｒ的估算公式．     

材料持久极限曲线的逼近和疲劳与屈服强度校核的判据

本文根据材料的对称循环持久极限σ－１和静载强度极限σｂ，提出脉动循环持久极限σ０的估算公式．在此基础上，结合材料持久极限曲线，建立了塑性材料构件在应力循环特性ｒ＞０情况下对其进行疲劳或屈服强度校核的判据．     

持久强度的外推精度

1. 引言持久强度系与时间有关表征材料高温性能的力学量,定义为一定温度下材料至规定时间破断的应力值。通常,时间大多定为十万或十万小时以上。就工业试验而言,不论时间或经费上的考虑,都不能苛求试验室进行这么长的试验,原始数据的外推处理往往不可避免,而最长试验时间和试样总数也应给出合理限度。这样,持久强度的精度,不仅存在材质分散性和试验误差的影响,还必须顾及数据处理过程的可靠性。2. 考核途径持久强度试验获得的一组原始数据,是由试验温度T_i、试样应力σ_i和破断时间τ_i三要素组成。它们间建立的各种关系式,经适当变量变换均可转化为多元线性方程组。其数学模型为y_i=b_0+b_lx_(Ji)+b_2x_(2i)+…+b_px_(pi)+ε_i ε_i是N个相互独立且服从同一正态分布N(0,S)的随机变量。因此,只要估计出b_0、     

涡轮叶片的持久寿命预测模型

镍基高温合金用于制造发动机的高压涡轮叶片.为了提高涡轮叶片持久寿命设计参数选取和设计方法的可靠性,从涡轮叶片代表性部位取材并设计、加工试验试件,进行持久寿命试验.试验过程中记录试件的变形量,进而推算出其蟠变应变,然后利用修正θ-Project Concept法来建立其持久寿命预测方程,并对其进行验证.     

吊耳的极限分析

将吊耳的外形及耳环的横截面作了简化,假定吊钩对耳环的横向压力均匀分布。选择了所有可能的机动模型,推导了相应的上限载荷公式,最后作了数值计算。     

管线上卷极限弯矩分析

利用弹塑性理论对卷管施加弯矩载荷时的截面屈服过程进行了理论分析,得出了卷管上卷时卷管所能承受的极限弯矩;通过实例分析得出X65钢在发生塑性应变为2.5%时的卷管两端截面夹角为1.98rad、滚筒直径为5.05m、极限弯矩为99.2kN m。根据DNV-OS-F101和API-RP-1111的相关规定,可将99.2kN m界定为对卷管施加弯矩的上限值;卷管弯矩上限值对应卷管上卷时的滚筒直径上限值,表明卷管上卷时可通过滚筒直径来限制卷管弯矩值。本文结果可为深水管线卷轴铺设的应用提供理论依据。     

偏心圆管的极限扭矩

本文用Wilson给出的横截面上最大剪应力的解析表达式,求得弹性极限扭矩,并用沙堆比拟法求多连通截面轴极限扭矩的实验近似值,并给出了偏心圆管塑性极限扭矩的简单解析式。     

不均匀及正交异性结构的极限分析与极限设计

1.1 许多应用于承載結构的材料有这样的特性:其塑性性貭随不同点而改变,并且同时取决于所考虑的方向,这表示我們涉及塑性不均匀性及塑性各向异性。 例如,我們試考虑一鋼結构处于一梯度不为零的温度場中。随点而异的温度变化导致材料塑性性貭的不均匀分布;这些塑性性貭(以及相应的塑性模量)随所考虑的点的位置而改变。鋼筋混凝土构件及結构給出了另一个塑性不均匀及各向异性的結构的例子。在此情形下,由于鋼筋分布在不同的方向,我們得到一各向异性的結构。如果除此而外鋼筋的百分比随点而改变,我們得到一个既不均匀而又各向异性的結构的例子。在刚-塑性     

金属塑性变形极限判据

研究出金属材料塑性变形极限判据，它为断裂力学和失效分析提供理论依据。利用这判据，裂纹尖端的临界应变和相应的临界应力可以确定。在板料成形中，拉应力下颈缩失稳的临界应变值可以定出。节省大量试验工作。     

关于壳体的极限承载能力

一、前言 薄壳结构在近代各种工程技术部门中到处得到广泛的应用和重视。普通的实用弹性理论只能给出材料在弹性限度以内的应力和变形状态,这时壳体在强度方面的储备,还远没有竭尽,因此壳体极限承载能力的研究是近代塑性力学中最有实际意义的课题之一。在这方面,伊留兴()、然尼村()、拉包脱诺夫(     

Switching-induced stable limit cycles

Physical limits place bounds on the divergent behaviour of dynamical systems. The paper explores this situation, providing an example where generator field-voltage limits capture behaviour, giving rise to a stable, though non-smooth, limit cycle. It is shown that shooting methods can be adapted to solve for such non-smooth switching-induced limit cycles. By continuing branches of switching-induced and smooth limit cycles, the paper established the co-existence of equilibria, smooth and non-smooth limit cycles. Furthermore, it is shown that when branches of switching-induced and smooth limit cycles merge, the limit cycles are annihilated at a grazing bifurcation.     

On the dynamic elastic limit

The fact that the elastic limit of some solids increases with increasing stress rate has been qualitatively and semiquantitatively established for many decades. Well known experimental difficulties have impeded reliable quantitative measurements of the magnitude or, in some solids, even the existence of such an increase of the elastic limit with stress rate. The present paper describes a simple method for accurately measuring the dynamic elastic limit in any solid which has a linear-elastic domain at small strain, including high-strength structural metal alloys. This method has the advantages of laboratory simplicity, a minimum of complex assumptions, and a close parallel with the manner in which the quasistatic elastic limit generally is determined. Although it is subsidiary to the main focus of this paper, evidence is presented here that a knowledge of the dynamic elastic limit firmly established by experiment, can be of considerable value for subsequent research in the continuum mechanics of solids, particularly with respect to the existence and properties of two distinct yield surfaces during impact loading.     

Analytical methods in limit analysis

Summary The paper presents some analytical kinematic solutions of limit analysis problems in geotechnics and related fields that have been recently obtained at the University of Minnesota. Emphasis is placed on steady and pseudo-steady flow problems that may model certain processes, and on the implications of yield condition nonlinearity. The examples pertain to plane-strain, axisymmetric and general three-dimensional problems. Also, the possible extension of the kinematic method of limit analysis on hardening incompressible and contracting/dilating materials is discussed. Finally, a simple periodic shear band formation process is analyzed.

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Analytische Methoden bei Traglastverfahren

übersicht Der Beitrag stellt einige analytische kinematische Lösungen von Traglastproblemen in der Geotechnik und verwandten Gebieten vor, die jüngst an der UniversitÄt von Minnesota erarbeitet wurden. Der Akzent liegt bei Problemen mit stetigem und pseudo-stetigem plastischem Flie\en und den Folgen einer NichtlinearitÄt der Flie\bedingung. Die Beispiele betreffen ebene FormÄnderung sowie axialsymmetrische und allgemein dreidimensionale Aufgabenstellungen. Darüber hinaus wird die mögliche Erweiterung der kinematischen Methode der Traglastverfahren auf verfestigende Werkstoffe mit oder ohne VolumenÄnderung erörtert. Zum Abschlu\ wird eine einfache periodische Bildung von ScherbÄndern untersucht.

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Presented at the workshop on Limit Analysis and Bifurcation Theory, held at the University of Karlsruhe (FRG), February 22–25, 1988     

Where is the elastic limit?

The elastic limit provides a convenient concept for the design of mechanical and structural parts which should exhibit no permanent deformation after loading and subsequent unloading. The offset tensile yield strength of a material for small offsets, such as the 0.01-percent offset, is considered to be a good approximation of the elastic limit. TheWT-bend tester provides an alternative method to the tension test of determining the offset yield strength of materials. The specimens are subjected to cyclic bending and energy dissipation is used as a yield criterion. The stress as a function of the offset has been determined for a number of alloys. For some of the materials investigated cyclic stresses at levels considerably below the 0.01-percent offset yield strength caused significant changes in mechanical properties. Furthermore, for some highly cold-worked materials substantial cyclic softening could be observed. This raises the question: where is the elastic limit? It is hypothesized that no true elastic limit exists and that it would be possible only to determine an anelastic limit.     

Evaporative instability at the superheat limit

The explosive vaporization of a single bubble inside a droplet of butane heated to the limit of superheat has been investigated experimentally using short-exposure photographs and fast-response pressure measurements. An interfacial instability driven by rapid evaporation has been observed on the surface of the bubbles. It is proposed that the Landau mechanism of instability, originally described in connection with the instability of laminar flames, also applies to rapid evaporation at the superheat limit. Calculations suggest that other technically important fluids may be even more unstable when boiling at the superheat limit. The rate of evaporation after the onset of instability is estimated from the experimental measurements to be two orders of magnitude greater than would be predicted by conventional bubble-growth theories that do not account for the effects of instability. An estimate of the mean density within the bubbles during the evaporative stage indicates that it is nearly equal to the critical density of butane.     

Rigid finite element and limit analysis

According to the lower bound theorem of limit analysis the Rigid Finite Element Method (RFEM) is applied to structural limit analysis and the linear programmings for limit analysis are deduced in this paper. Moreover, the Thermo-Parameter Method (TPM) and Parametric Variational principles (PVP) are used to reduce the computational effort while maintaining the accuracy of solutions. A better solution is also obtained in this paper. The project supported by National Natural Science Foundation of China     

Periodically forced delay limit cycle oscillator

This paper investigates the dynamics of a delay limit cycle oscillator under periodic external forcing. The system exhibits quasiperiodic motion outside of a resonance region where it has periodic motion at the frequency of the forcer for strong enough forcing. By perturbation methods and bifurcation theory, we show that this resonance region is asymmetric in the frequency detuning, and that there are regions where stable periodic and quasiperiodic motions coexist.     

Fracture as the limit of flow

Summary Careful measurement of the shape of the creep curve for a Ni-Cr alloy shows that it closely follows a modifiedAndrade equation. Departures from this form in the near neighbourhood of rupture are shown to be due to the development of cracks so that the initiation of cracking is associated with no observable departure from the preceding regular curve of flow. Results of the analysis of creep rupture data support the indication that cracking is initiated when one or other of the terms of the modifiedAndrade equation reach limiting values.

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Zusammenfassung Sorgfältige Messungen der Gestalt der Kriechkurve für eine Nickelchromlegierung zeigen, daß sie weitgehend einer modifiziertenAndrade- Gleichung folgt. Abweichungen von dieser Form nahe beim Bruch sind offenbar einer Bildung von Rissen zuzuschreiben, derart, daß die Einleitung der Rißbildung mit keiner beobachtbaren Abweichung vom vorangehenden regulären Fließkurvenverlauf verbunden ist. Die Ergebnisse der Analyse der Kriechbruchdaten stützen die Annahme, daß die Rißbildung initiiert wird, wenn der eine oder andere Term der modifiziertenAndrade- Gleichung gewisse Grenzwerte erreicht.

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Presented at the conference Flow, Fatigue and Failure held in Leeds on January 8th and 9th, 1959, by the British Society of Rheology.     

Classical and sequential limit analysis revisited

Classical limit analysis applies to ideal plastic materials, and within a linearized geometrical framework implying small displacements and strains. Sequential limit analysis was proposed as a heuristic extension to materials exhibiting strain hardening, and within a fully general geometrical framework involving large displacements and strains. The purpose of this paper is to study and clearly state the precise conditions permitting such an extension. This is done by comparing the evolution equations of the full elastic–plastic problem, the equations of classical limit analysis, and those of sequential limit analysis. The main conclusion is that, whereas classical limit analysis applies to materials exhibiting elasticity – in the absence of hardening and within a linearized geometrical framework –, sequential limit analysis, to be applicable, strictly prohibits the presence of elasticity – although it tolerates strain hardening and large displacements and strains. For a given mechanical situation, the relevance of sequential limit analysis therefore essentially depends upon the importance of the elastic–plastic coupling in the specific case considered.