Local Minima and Gradients of Stiffness and Conductivity as Indicators of Strength Reduction of Brittle-Elastic Materials

It is often suggested in literature to use change in the effective elastic properties (stiffness loss) as an indicator of reduction of strength due to defects such as cracks and pores. We argue that the key parameter is not the reduction of the average (over the specimen) stiffness but its local minimal values caused by formation of defect clusters. These defect clusters can be identified by the emergence of spatial gradients of elastic stiffness on a smaller scale. A convenient tool of detecting these gradients is provided by the elasticity-conductivity connection: the electric conductance gradient is usually easier to measure than the stiffness gradient. This concept is supported by computational and experimental results reported in two accompanying papers published in the present issue (by Sevostianov et al and Caiulo and Kachanov).     

UL94与聚合物三种燃烧实验的相关性

包括UL94实验在内的一些聚合物燃烧实验被广泛用于评价火灾危害和阻燃性能,但同一产品的UL94等级与其他燃烧实验得出的阻燃级别有时相差较大。文中综述了UL94等级与锥形量热仪、微型量热仪和氧指数实验结果的关联关系,论述其间存在的关联性的理论基础及影响关联程度高低的因素,并建议对UL94实验条件、不稳定燃烧过程和滴落行为作深入细致的研究,以便更好地理解UL94实验与其它实验之间的异同。     

Commentary on “Quantitative Acoustic Emission Source Characterization of Microcracking in Steel”

Abstract

Results published recently by Takatsubo and Kishi concerning acoustic emission during ductile fracture of A533B steel indicated that high-amplitude emissions observed were associated with tension/shear mixed-mode cracking. These results appear to be consistent with (and complementary to) the conclusions reached by the present author in work on pressure vessel steels, including A533B, in which the onset of slow stable cracking was associated with the occurrence of high-amplitude emissions, and in which these emissions were shown to be associated with a fracture mechanism involving shear linkage of voids. The present paper examines the conclusions which can be drawn from these complementary programs, and discusses the link between ductile fracture and acoustic emission.     

水泥基胶凝材料氧化物含量与氯离子结合量的定量关系

为揭示氯盐溶液及存在硫酸盐复合盐溶液中氯离子对水泥基材料的侵蚀特性,合理选用氯盐环境下水泥基材料组分,减小海洋环境和盐湖卤水对混凝土造成的氯离子侵蚀问题,本工作通过添加矿物掺合料(矿渣和粉煤灰)改变主要氧化物含量,研究水泥基材料氧化物含量与氯离子结合量的关系.采用等温吸附法测试样品的氯离子结合量,采用X射线衍射仪(XRD)和同步热分析仪(TG-DSC)表征侵蚀产物.实验结果表明,在氯盐溶液侵蚀下,水泥基材料的氯离子结合量与其Al2 O3、SiO2含量成正比,与CaO含量成反比.在硫酸盐存在的复合盐溶液侵蚀下,硫酸盐浓度较低(3.5％NaCl+0.5％Na2 SO4)时,氯离子结合量略有下降,测试结果表明Friedel盐的生成量不会因硫酸根离子的引入而降低,此时两种离子没有明显的竞争关系;当硫酸盐浓度提高到5％Na2 SO4时,氯离子结合量下降显著,实验结果表明Friedel盐的生成量降低,两种离子间竞争关系显著,但不影响氧化物含量与氯离子结合量之间的相关性.     

Connection Between Strength Reduction,Electric Resistance and Electro-Mechanical Impedance in Materials with Fatigue Damage

In this paper we report experimental results on the connection between the strength reduction due to accumulated damage in elastic electrically conductive material, resistance variation across the damaged specimen and changes in electro-mechanical impedance (EMI) response. We compare behavior of a cyclically loaded specimen (44,000 loading cycles with the stress amplitude near 0.5 of the yield stress) and intact one (nonloaded). Electro-mechanical impedance testing of the specimens was carried out and the electrical resistances of different parts of the specimens were measured. Finally, both the specimens were tested for ultimate strength under uniaxial tension. We observed qualitative correlation between changes in EMI response, electrical resistance and strength reduction in damaged specimen.     

Strength of Materials and Structures

Book reviewAll engineering structures designed according to modern principles have to be strong and sufficiently rigid. Scientists and engineers have long recognized the importance of the strength ofmaterials and structures, and dedicated much their efforts to bothfundamental and industrial research into the theory for vast engineering materials and various structures. A lot of engineers needto be familiar with the fundamental principles of strength in materials and structures in order to desi…     

三维编织复合材料刚度和强度性能研究进展

综述了近年来国内外对三维编织复合材料的刚度和强度性能预测方法的研究进展.三维编织复合材料刚度性能的预测分析方法主要有两大类:解析法和有限元法,而强度特性的预测大多采用有限元方法来进行.最后,本文对有待于进一步研究的问题进行了展望.     

Strength of Structural Carbon Materials

The effect of microstructure on the strength of various carbon materials is analyzed. The strength of structural carbon materials is shown to decrease upon oxidation and to rise during neutron irradiation.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 523–530.Original Russian Text Copyright © 2005 by Virgilev.     

Asymptotic Correlations Between Rotated Solutions in Factor Analysis

The asymptotic correlations between differently rotated solutions in exploratory factor analysis are derived. The solutions are orthogonally or obliquely rotated for unstandardized or standardized manifest variables. To obtain the asymptotic correlations between different solutions, the covariance models for manifest variables have been constructed so that two sets of solutions are involved in a single covariance structure. The asymptotic correlations can be used for the statistical test of the differences of rotated solutions. The correlation matrix between the rotated factors of the first solution and those of the second is also introduced in the models with appropriate restrictions to identify the models. The asymptotic standard errors of the estimates of the correlations between the factors in different solutions are simultaneously provided. A numerical example is presented with simulated values.

     

Absence of a Competition Between Magnetism and Superconductivity in Layered Nickel Borocarbides: Common Correlations of Tc with Crystal Chemical Parameters for Magnetic and Nonmagnetic Compounds

In the present work, an analysis of the crystal chemical parameters of nickel borocarbides RNi₂B₂C (R = rare earth) is given. The reasons for the dependence of superconducting transition temperature (T _c) on crystal chemical parameters by two separate curves for magnetic and nonmagnetic R are considered. For all R, a common pattern of dependence of T _c on crystal chemical parameters similar to that existing in layered quasi two-dimensional systems (HTSC cuprates and diborides) is established. The absence of the influence on the T _c of borocarbides of magnetic properties R is also established. On the basis of the correlations found, the radii of a number of rare earths are more precisely defined, and T _c of compounds at various substitutions R are calculated.     

Correlations Between Indentation Hardness and Yield Stress in Thermoplastic Polymers

Abstract:  In mechanical testing empirical correlations are often used for estimating yield stress values from hardness values. Such empirical correlations are only valid within particular classes of materials and in the case of polymers they may be dependent on deformation rate, temperature and kind of deformation (tensile or compressive). For a better understanding of these complex correlations, instrumented indentation tests, tensile tests and compressive tests were performed on different semicrystalline and amorphous thermoplastic polymers. Indentation hardness and Martens hardness values were compared with tensile and compressive yield stress. The ratio of hardness values to tensile yield stress depends very strongly on the basic deformation mechanism (crazing, voiding or shear yielding) of the polymers. Therefore, under tensile load there is no general linear relationship between hardness and yield stress. In contrast, in the case of compressive loading hardness and yield stress are much better linearly correlated.     

Effect of Pallet Deckboard Stiffness on Corrugated Box Compression Strength

The packaging industry has long considered pallets to be rigid structures. However, in a unit load, the weight of the product produces compressive forces that are distributed across the pallet causing the top deckboards to deflect. Corrugated paperboard boxes are highly susceptible to changing support conditions; therefore, the deckboard deflection directly impacts the vertical compression strength of the box. Therefore, the objective of this study is to evaluate the effect of pallet deckboard stiffness on the vertical compression strength and deflection of corrugated paperboard boxes. Additional treatments included gaps between the deckboards and location of the box relative to the pallet stringers. Copyright © 2016 John Wiley & Sons, Ltd.     

Mechanics Modeling of the Compressive Stiffness and Strength of Open-Celled Aluminum Foams

This article presents the results of analytical modeling of the strength and stiffness of open-celled aluminum foams under compressive loading. The analytical models are based on a four-strut structure unit that was extracted from the real open-celled aluminum foam structure. Deformation and strength were first studied in the idealized structure unit bounds. The effects of asymmetric factors were also considered. The models predicted ranges in foam strength and stiffness as functions of relative densities, length ratio, and strut orientation. The predictions from the models were in good agreement with the experimentally measured strengths stiffnesses for open-celled aluminum foams.     

High-Temperature Strength of Copper-Based Condensed Materials

Strength of Materials - The temperature dependences of Vickers microhardness, ultimate strength, and offset yield point in the temperature range between 290 and 1070 K were determined for the...     

三维四步编织复合材料刚度和强度的数值模拟

为了研究三维四步法编织复合材料的力学性能,利用ANSYS有限元软件对材料的细观体胞模型进行数值模拟,计算三维编织复合材料的宏观弹性常数,讨论了纤维编织角和体积比对弹性常数的影响。采用不同的强度准则分别对纤维束和基体材料进行强度校核,从而得到材料发生破坏时失效单元的体积百分比。根据失效单元的分布情况分析材料的破坏机理,进而预报材料的拉伸强度。模拟计算结果与实验值吻合较好。