THE USE OF ULTRASONICS IN STRESS ANALYSIS

A bstract : Some methods that have been proposed for ultrasonic stress analysis are described. These depend on the measurement of stress-induced velocity changes, analogous to the light-wave velocity changes in photoelasticity. The theory of the nonlinear stress-strain relationships giving rise to the velocity changes is introduced, and the practical and fundamental difficulties yet to be resolved are pointed out. The results of some measurements of stress-induced changes are quoted.     

THE USE OF SIMULATED FRACTALS IN THE ANALYSIS OF AGGREGATES

A procedure is described for the simulation of clusters and aggregates which are constrained to have a definite fractal dimension. The fractal (Hausdorff-Besicovitch) dimension of the simulated clusters was determined for two dimensional projections of clusters using methods based on the radius of gyration and ¹Voss. The two approaches are compared and used to develop a correlation between the fractal dimension of the cluster and its two dimensional projection.     

THE USE OF SPARK MACHINING IN THE DETERMINATION OF LOCAL RESIDUAL STRESSES

A method of measuring residual stress by a stress relief process in flat bars and cylindrical specimens is developed and correction factors discussed.     

梁的剪力动力系数的确定

分别应用振型叠加法和非线性有限元方法,研究了线性和非线性条件下梁的剪力响应,给出了抗爆结构设计计算所需要的剪力动力系数。研究结果表明:(1)在突加三角形荷载作用下,正向剪力动力系数随荷载作用时间与基本周期比(td/T1)的增大而增大,负向剪力动力系数先随td/T1的增大而增大,在保持常值前随td/T1的增大而略有减小;而在升压平台型荷载作用下,正向剪力动力系数随荷载升压时间与基本周期比(tr/T1)的增大而减小。(2)正向和负向剪力动力系数都随厚跨比(h/l)的增大而增大。(3)当梁进入塑性响应阶段后,随着延性比的增大,与弹性响应相比支座剪力逐渐减小;剪力的降低幅度随荷载作用时间(td)或升压时间(tr)的增大而减小。     

渡桥电厂冷却塔倒塌的塔型因素分析

针对渡桥电厂冷却塔倒塌原因中的塔型因素,建立与之同壁厚、同高度、同筒底直径和相同人字柱的双曲线型冷却塔,分析两者动力特性的差别,比较自重、规范平均风荷载和风洞试验脉动风荷载作用下响应的差异。研究表明：渡桥冷却塔比双曲线型冷却塔的基频低30%;风荷载作用下渡桥冷却塔的径向位移比双曲线型冷却塔大很多,壳体扭曲程度较大;渡桥冷却塔迎风面的子午向薄膜力随塔高减小而迅速增大,最大拉力比双曲线型冷却塔大40%以上。渡桥冷却塔的塔型不如双曲线型合理,塔型不合理是构成渡桥电厂冷却塔倒塌的一个因素。     

结构连续变更定理在可靠性分析中的应用

以平面桁架为例将结构连续变更定理用于结构的可靠性分析,以分枝限界法为基础编制了电算程序.避免了由于结构拓朴不断变更引起的总刚度阵多次组装及多次求逆运算,引入了结构退化为机构的新判据.实现了不断变更结构的连续分析,还连续生成极限状态函数.从而边进行结构分析,边进行失效链失效概率计算.在搜索中,有效地筛选出可能失效链并得到可能失效模式的集合.因此提出的理论和方法可以用来解决某些工程实际问题.算例表明结果令人满意.     

ON THE USE OF VIBRATION AND ELECTRIC FIELDS TO FLUIDIZE AND DISPERSE PARTICULATE MATTER

ABSTRACT

Particulate matter is a statistical ensemble of interacting, non-identical particles. The fluidization and dispersal of dust is considered here from kinetic arguments and experiment. It is concluded that fluidization and dispersal of dust are independent physical processes arising from the action of vibration and steady stresses, respectively. Although the transition or correlation function for particulate systems is not as sharply defined as for the ideal-gas case, its general character is analogous with that for a condensing phase-transition with its critical condition broadened by the distribution of binding energies between the particles. The ratio of the net steady stress on the particles to the vibrational energy is the controlling factor in particle capture and entrainment.     

INVERSION OF THE STRAIN-LIFE AND STRAIN-STRESS RELATIONSHIPS FOR USE IN METAL FATIGUE ANALYSIS

Abstract— Two methods are described for inverting the strainrange/life and strainrange/stressrange equations commonly used in fatigue analysis in order to obtain closed-form expressions for life and stressrange in terms of strainrange. In the Collocation approach the form used is N _f= A (Δε—Δε₀)γ or N _f= A (Δε)Ψ(Δε—Δε₀)γ. In the Spline-Function approach the curve is divided into two regions. At strainranges above where the elastic and plastic lines intersect the equation is N _f= N _T R ^1/c exp δ R α; at lower strainranges it is N _f= N _T R ^1/b exp δ R β, where N _T is transition life, R is strainrange normalised to transition strainrange, and b, c , α, β, δ are constants determinable from the constants of the equation to be inverted. Similar expressions are derived for the cyclic stress/strain curve in terms of the same constants. The methods are illustrated by an example, and found to have close conformity to the basic equations to be inverted.     

LIMITATIONS ON THE USE OF THE STRESS INTENSITY FACTOR, K, AS A FRACTURE PARAMETER IN THE FATIGUE PROPAGATION OF SHORT CRACKS

Abstract— It is well known that for very short cracks the stress intensity factor K is not a suitable parameter to estimate the stress level over the small but finite Stage II process zone activation region of size r_s near the crack tip, within which crack growth events take place. A critical appreciation of the reasons for the limitations on the applicability of ΔK as a fatigue crack propagation (FCP) parameter, when the crack length a is of the same order of magnitude or smaller than the size of the ‘fatigue-fracture activation region’, r_s is presented. As an alternative to ΔK the range Δσ_s of the cyclic normal stress at a point situated at the fixed distance s=r_s/2, ahead of the crack tip, inside the fatigue-fracture activation region, is proposed. It is observed that the limitation on the use of ΔK when the crack is short, is mathematical (and not physical) but this inconvenience is easily circumvented if the stress Δσ_s at the prescribed distance is used instead of ΔK since nowadays Δσ_s can be obtained numerically by using finite element methods (FEM). It follows that the parameter Δσ_s is not restricted by the mathematical limitations on ΔK and so it would seem that there is, a priori, no reason why the validity of the parameter Δσ_s cannot be extended to short cracks. It is shown that if the Paris law is expressed in terms of Δσ_s (πrr_s)^½ instead of ΔK the validity of the modified Paris law can be extended to short cracks. A coherent estimate of the value of the fatigue-fracture activation region r_s is derived in terms of the fatigue limit Δσ_FL obtained from S-N tests and of the threshold value ΔK_th obtained from tests on long cracks where both relate to Stage II crack growth that ends in failure, namely, r_s= (ΔK_th/Δσ_FL)²/π. An overall, threshold diagram is presented based on the simple criterion that, for sustained Stage II FCP, Δσ_s must be greater than Δσ_FL. The study is based on a simple continuum mechanics approach and its purpose is the investigation of the suitability of both ΔK and Δσ_s to characterise the crack driving force that activates complex fracture processes at the microstructure's scale. The investigation pertains to conditions that lead to the ultimate failure of the component at values of Δσ_s > Δσ_FL.     

MEASUREMENTS AND ANALYSIS OF THE PARTICULATE EMISSION FROM A DIRECT INJECTION DIESEL

For five different operating conditions of varying speed and load, measurements have been made of the particulate emissions using a mini-dilution tunnel operated at different dilution ratios and air temperatures. Particles were sized by electron microscopy and employment of interactive computer graphics.

Analysis of results leads to conclusions that the organic fraction of the particulates can be explained by either an adsorption or condensation interaction with the gas phase. Account must be taken of the multi-component nature of the organic species and that adsorption is multilayer in modeling these processes.     

MEASUREMENTS AND ANALYSIS OF THE PARTICULATE EMISSION FROM A DIRECT INJECTION DIESEL

For five different operating conditions of varying speed and load, measurements have been made of the particulate emissions using a mini-dilution tunnel operated at different dilution ratios and air temperatures. Particles were sized by electron microscopy and employment of interactive computer graphics.

Analysis of results leads to conclusions that the organic fraction of the particulates can be explained by either an adsorption or condensation interaction with the gas phase. Account must be taken of the multi-component nature of the organic species and that adsorption is multilayer in modeling these processes.     

MEASUREMENTS AND ANALYSIS OF THE PARTICULATE EMISSION FROM A DIRECT INJECTION DIESEL

For five different operating conditions of varying speed and load, measurements have been made of the particulace emissions using a mini-dilution tunnel operated at different dilution ratios and air temperatures.Particles were sized by electron microscopy and employment of interactive computer graphics.

Analysis of the results led to conclusions that the organic fraction of the particulate can be explained by either an adsorption or condensation interaction with the gas phase.In modeling these processes one has to account for the multi-component nature of the organic species and that adsorption is multilayer.     

大角度倾转法在碳化物研究中的应用

蠕变过程中碳化物形貌，类型，数量和分布状态的变化直接影响着材料的高温性能，因而在材料的蠕变得到了广泛的重视，介绍了一种在碳膜萃取复型样品上利用Ｈ－８００透射电子显微镜的双倾样品台进行了大角度倾转，来观察碳化物形貌并鉴定其类型的试验方法。     

关于瞬态应力强度因子和裂纹扩展加速度之间的关系探讨

将动态焦散线法与高速摄影技术相结合,本文记录了高分子材料悬臂梁内裂纹扩展加速、减速过程。一方面,利用动态焦散线可确定扩展裂尖的瞬态应力强度因子;另一方面,利用最小二乘法将裂纹扩展的水平、垂直位移分量拟合成时间(t)的四次多项式,描述了裂纹扩展方向、裂纹扩展加速度以及裂纹扩展加速度与动态应力强度因子的关系。结果表明:裂纹扩展加速度与动态应力强度因子之间存在一定的依赖关系;利用最小二乘法将裂纹扩展的水平、垂直分量拟合成时间(t)的四次多项式,可以对裂纹的扩展方向、加速度进行定量分析。     

离子推力器束流下降原因分析

离子推力器的推力与其引出的束流成正比,束流的大小直接确定了推力.离子推力器在点火启动后,在工作条件不变的条件下,其引出束流随工作时间而下降.为找出束流下降的原因,以离子推力器为研究对象,通过分析引起离子推力器束流下降的各种因素,并对这些因素进行分析与验证.经过对因素的分析定位,找出引起束流下降的主要原因.分析与验证表明：影响离子推力器束流下降的决定因素为栅极组件固有特性、磁场固有特性和阴极固有特性,其中阴极固有特性是导致束流下降的主要原因.     

A NOTCH INTENSITY FACTOR APPROACH TO THE STRESS ANALYSIS OF WELDS

In the context of linear elastic stress gradients that are present in welded joints, a stress field approach based on notch stress intensity factors is presented with the aim of describing stress distributions in the neighbourhood of weld toes, since fatigue strength is dependent on such distributions. This paper summarizes the analytical fundamentals and gives an appropriate definition of the parameters for stress components under opening and sliding modes. Then, by comparing the expected results with those obtained by numerical analysis, the contributions of the symmetric and skew-symmetric loading modes are quantified for different geometries, and summarized into concise expressions which also take into account the influence of the main geometrical parameters of the welded joint. The range of validity and the application limits of this field approach in the presence of weld toe radii are discussed. Finally, a synthesis of experimental fatigue strength data based on the new field parameters is reported.     

A PRACTICAL USE OF DUALITY IN SHAKEDOWN AND LIMIT ANALYSIS OF STRUCTURES

The paper examines the use of duality theory in the plastic limit analysis and shakedown analysis of structures. Classical results are reviewed and both primal and dual formulations of the above problems are given. The use of formulations and the information which can be derived from them are studied with the aid of an example.