Defect sensitivity of bulk PCMs composed of octet and Kagome trusses to mechanical behaviors subjected to compression

The Kagome truss has been attracting attention, because it has equivalent or even higher strength compared to the octet truss with the same material and density. In this work, another aspect, which seems to be important for its practical applications, that is, defect sensitivity of bulk PCMs (periodic cellular metals) composed of Kagome trusses to mechanical behaviors subjected to compression was compared with a counterpart composed of octet trusses. In order to investigate the mechanical characteristics of bulk PCMs composed of the two trusses, a hybrid approach was taken in this work. First, assuming perfectly uniform structure and deformation of WBK, the behavior of the bulk PCMs composed of infinite number of trusses was simulated by finite element analysis for a unit cell with periodic boundary conditions. From the results, the force-displacement response of a single strut composing the trusses in each bulk PCM was estimated. Then, the effects of geometric imperfections and the inhomogeneous material properties were evaluated by network analyses, in which the force-displacement responses were used to characterize mechanical behaviors of the networks. The imperfections were modeled to have Gaussian distributions, and the analysis results of the two bulk PCMs were compared to evaluate their defect sensitivities. For the geometric imperfections, the maximum strength of both bulk PCMs decreased gradually as the imperfection level increased. For material property imperfections, the maximum strength maintained nearly unchanged for the PCM composed of Kagome trusses. On the other hand, for that composed of octet trusses, it slightly dropped as the imperfection level increased. The octet truss PCM was found to be more sensitive to the property imperfections than the other.     

采用ANSYS（隐式）、LS-DYNA（显式）和两者共同分析法对薄壁结构进行有限元极限荷载分析

在分析了采用ANSYS隐式有限元分析和LS-DYNA显式分析的一般情况之后,得到显式时间积分法对准静态极限荷载分析进行瞬态分析的时间和方式。然后将重点关注隐式分析的优点,探讨如何更有效地将ANSYS和LS-DYNA进行组合,使其能具有更好的瞬态分析效果。最后,概述了模型的离散化,控制方案的选择,缺陷的考虑和结果校核。     

Single- and double-pass FSW lap joining of AA5456 sheets with different thicknesses

Friction stir welding of AA5456 aluminium alloy with different thicknesses was investigated for single- and double-pass lap joint configurations. The influences of tool tilt angle in first and second passes, and rotational and welding speeds in second pass on metallurgical structure and joint strength were studied. The results indicated that tilt angle significantly influences material flow and imperfection formation, and accordingly controls the weld mechanical properties. The best results were achieved by tilt angle of 5° for single pass, and tilt angle, rotational speed and traverse speed of 5°, 250?rev?min^??1 an 50?mm?min^??1 respectively for double-pass. The characteristics of hooking and cold lap defects were used as criteria to recognise the influence of processing conditions on joint performance.     

Influence of handling effects on the permeability of preforms

Handling manipulations during the process chain of dry fibre preforms starting with the production of semi‐finished products, throughout the transport and storage until the build‐up, lead to mechanical stress on the textile structure. The knowledge of the relation between the stress in preforms caused by handling processes and their effect on the quality of the subsequent composite product could be of great importance for the development of automated solutions. The reduction of permissible requirements in the handling of textiles would lead to significantly simpler and thus more cost‐effective automation solutions. Using effects of defects methodology, impacts on preforms due to handling processes and the resulting permeability will be investigated. The analysis of the preforming process chain shows various visible effects as a result of handling and storage loads on textiles. One representative effect is a spherical shaped buckling within handled textile layers. Therefore the buckling will be used as a representative imperfection in this analysis. To create measurable imperfections textile samples will be impacted with a defined and reproducible load. Followed by the measurement of permeability during vacuum infusion of imperfect textile samples the relation between handling effects and the quality of preforms will be shown. In der Prozesskette zur Herstellung von Vorformen werden textile Halbzeuge durch zahlreiche Handhabungsprozesse belastet. Von zentraler Bedeutung ist hierbei die mechanische Beanspruchung der textilen Halbzeuge bei deren Handhabung und Lagerung während des Prozesses zur Herstellung der Vorform.     

An analysis of the imperfections and defects inside composite bipolar plates using X-Ray computer tomography and resistivity simulations

With the increasing mass production, the quality control of bipolar plates (BPPs) becomes more important. A classification and understanding of imperfections and defects is necessary for the design of quality assurance measures within the manufacturing process. In this paper, a combined X-Ray computer tomography (CT) and resistivity simulation approach is used to investigate graphite composite BPPs. With this non-destructive approach, the morphology and composition of a detailed BPP section, as well as an entire flow field, can be analyzed for its characteristics and defects. The different detected imperfections occurring in injection-molded and compression-molded BPP samples include cracks, air bubbles and agglomerations of voids and foreign particles. The investigated flow field geometry has a major impact and increases the electrical resistance by about 19% compared to a bulk material body. The investigated imperfections inside the BPP material have a minimal impact on the electrical resistance of the BPP.     

钢结构在随机状态和模糊非确定性状态下的稳定性问题

概括所提议的概率法的总体思路和问题以及结构设计程序在欧洲规范中的应用,旨在对根据“欧洲规范3”设计的受压钢构件的极限状态进行概率分析。理论失效概率（可靠性指标）与永久和可变荷载比值的比较,采用Monte Carlo模拟法进行计算,并根据ENl990对失效概率的误差进行分析。初始缺陷一般都视为随机变量和随机域,采用非线性梁有限元方法,分析构件轴线上的初曲率形状和大小变动对载荷能力的影响。概率分析是对模糊分析的补充,而模糊分析主要研究不确定性因素对失效概率产生的影响。     

The single perturbation load approach applied to imperfection sensitive conical composite structures

The importance of taking into account geometric imperfections for cylindrical and conical thin-walled structures prone to buckling had been already recognized by the first authors dealing with new formulations. Nowadays, the analysts still use empirically based lower-bound methods such as the NASA SP-8007 guideline to calculate the required knock-down factors (KDFs), which does include important mechanical properties of laminated composite materials, such as the stacking sequence. New design approaches that allow taking full advantage of composite materials are required.The single perturbation load approach (SPLA), a new deterministic approach first proposed by Hühne, will be investigated with unstiffened composite conical structures varying the geometry, lamina and layup. The SPLA׳s capability for predicting KDF is compared with the NASA approach. The SPLA was applied to the geometrically perfect structures and to the structure with geometric imperfections of two types, mid-surface imperfections and thickness imperfections. The study contributes to the European Union (EU) project DESICOS, whose aim is to develop less conservative design guidelines for imperfection sensitive thin-walled structures.     

Effect of imperfections in the bond on the strength of FRP wrapped cylindrical concrete columns

Effect of imperfections at the interface between concrete and FRP on the strength of FRP confined axially loaded cylindrical concrete columns is investigated, experimentally and numerically. It is seen that the presence of imperfections facilitates localization of deformation, adversely affects the confining capacity of FRP, and reduces the failure load. The influence of size, location and orientation of imperfection on failure load is studied: the orientation and location are found to be more important than size. Critical locations and orientations of the imperfection are found and explained in terms of the mechanics of shear banding in pressure-sensitive elasto-plastic materials.     

The effects of long-term corrosion on the dynamic characteristics of ground based cylindrical liquid storage tanks

Results of numerical investigations on the effects of material degradation due to corrosion on the dynamic characteristics of ground-based, anchored, steel liquid storage tanks are presented. Internal corrosion is considered as a time-dependent constant thinning of the wall, at locations in contact with residual water, water condensate, atmospheric oxygen and acid gases. Dynamic analyses are performed on numerical tank-liquid models, having different aspect ratios and wall thicknesses at different stages of wall thinning at the specified locations. The aim of the analyses is to determine the corrosion effects on the natural periods and mode shapes of vibration. Steady-state, harmonic base excitation analyses are also carried out to determine the corrosion effects on the hydrodynamic pressures produced in the liquid. It is found that progressive corrosion has significant effects on the tank fundamental period and its associated mode shape of vibration as well as the magnitude and location of the maximum hydrodynamic pressure and that as design provisions should cover the service life of the tank, the errors associated with the current code provisions for design of such tanks cannot be ignored.