塔式钢框架模态试验及半刚度模拟

文章对螺栓连接的塔式钢框架结构进行了模态实验研究,并利用有限元软件进行了模拟,通过对其节点及边界条件的半刚度模拟,使模型更加合理,分析结果更加可靠。在此基础上对具有局部损伤的结构作了模态分析,进一步验证了半刚度模拟的合理性。     

Development of nonlinear spectral correlation between ultrasonic modulation components

When a damaged structure is exposed to ultrasonic waves at two distinct frequencies, nonlinear wave modulation resulted from a damage such as a fatigue crack can be observed in the corresponding response, offering an opportunity for early damage detection. This study attempted to improve nonlinear wave modulation-based damage detection by applying statistically weak-linked inputs over two distinct frequencies to the target structure and then conducting a cyclostationarity analysis of the corresponding structural response. The cyclostationary nature of the structural response produces a statistical variation over time, allowing the structural response to be processed using a spectral correlation function. The spectral correlation can thus be used to transform the structural response into two dimensions characterized by their cyclic and spectral frequencies. Then, the damage-induced nonlinear modulation can be detected by studying the spectral correlation values for the specific cyclic and spectral frequencies defined by the modulation frequencies. This premise forms the basis for nonlinear spectral correlation, which is a new damage feature that is superior in terms of its sensitivity for nonlinear damage and improved robustness against noise compared to a conventional spectral density function. The performance of the proposed technique was validated by conducting an experiment with aluminum plates containing real fatigue cracks. The results of this experiment showed that the proposed technique could detect damage even under severely noisy conditions.     

Quantitative evaluation of orientation-specific damage using elastic waves and probability-based diagnostic imaging

Different from the damage with relatively smooth boundaries or edges such as a through-thickness hole or delamination which scatters elastic waves omnidirectionally, orientation-specific damage of sizable length in a particular dimension (e.g., a crack or a notch) often exerts strong directivity to elastic wave propagation. As a consequence the damage-scattered waves may not be captured efficiently by sensors at certain locations, posing a challenging issue to elastic-wave-based damage identification. In this study, the influence of damage orientation on Lamb wave propagation was quantitatively scrutinised. Based on the established correlation between damage parameters (location, orientation, shape and size) and extracted signal features, a probability-based diagnostic imaging approach was developed, in conjunction with use of an active sensor network in conformity to a pulse-echo configuration. Relying on enhancive signal features including both the temporal information and signal intensity, this imaging approach is capable of indicating the orientation of individual damage edges clearly and further shape/size of the damage. The effectiveness of the approach was demonstrated by predicting orientation-specific damage cases including a triangular through-thickness hole (through finite element simulation), a through-thickness crack and an L-shape crack (through experiment) in aluminium plates. With the assistance of a two-level synthetic image fusion scheme, all damage cases were visually and quantitatively highlighted in the probability images.     

抽水蓄能电站巴氏合金上导轴瓦损坏原因分析

由于冷却油供油路中出油不均匀,使得油槽中存在大量漩涡或空气,造成轴瓦上半部分的油膜无法形成或形成不均匀,从而在运行过程中产生了油膜压力波动。同时,由于轴瓦轴向摆动的灵活性较差,轴瓦上半部分与滑转子间隙较小,轴瓦只有上半部分与滑转子接触,致使轴瓦表面单位面积受力过大。上述两种原因造成的瓦面上半部分应力幅值增加,使轴瓦产生了疲劳破坏。为此,利用光学显微镜和直读光谱仪对轴瓦巴氏合金的组织和成分进行了分析,在此基础上,提出了适宜的改进措施。实践表明,轴瓦金相组织中再无区域性偏析、聚集和组织分层现象出现,Cu6Sn5分布均匀,Sn Sb未发生区域性偏析,组织形貌与两种硬相Sn Sb和Cu6Sn5的总量符合要求。通过对轴瓦衬垫结构进行改进,使得轴瓦摆动的灵活性加大,轴瓦和滑转子之间形成了稳定的油膜,从而使轴瓦受力均匀,运行平稳。     

Prevention of Internal Cracks in Forward Extrusion by Means of Counter Pressure: A Numerical Treatise

In the context of forward bulk extrusion, where product defects are frequently observed, the effect of counter pressure on damage accumulation materializing a Continuum Damage Mechanics (CDM) approach is presented. A Lemaitre variant damage model accounting for unilateral damage evolution coupled with a multiplicative finite plasticity is utilized for this purpose. After a presentation of the crack governing mechanism, it is demonstrated that application of counter pressure introduces a marked decrease in the central damage accumulation, which in turn increases the formability of the material through keeping the tensile triaxiality in tolerable limits. It is also shown that, for a crack involving process, through systematic increase of the counter pressure, the crack sizes diminish; and at a certain level of counter pressure chevron cracks can be completely avoided.     

3D-FE-Analysis of CT-specimens including viscoplastic material behavior and nonlocal damage

In order to describe the time-dependent inelastic behavior of steel structures up to failure, a material model is developed within the framework of thermodynamics considering viscoplastic material behavior, isotropic hardening, softening, and isotropic ductile damage. Since softening and damage lead to localization of deformations, the model is extended by a non-local implicit gradient formulation. An additional gradient equation is formulated for the local damage variable in order to achieve results independent of the finite-element discretization. The approach is verified by 3D-structural analysis of CT-specimens.     

Effects of radiation pretreatments on the rubber adsorption power and reinforcing properties of fillers in rubber compounds

Radiation damage to fillers such as carbon black, graphite and silica induced by high doses of γ‐radiation or neutrons dramatically increases their ability to adsorb rubber irreversibly. In fact, the ‘bound rubber’, ie the amount of non‐extractable rubber which remains irreversibly linked to the filler matrix, increases dramatically in radiation‐treated fillers. The increased adsorption power of radiation‐damaged fillers has been attributed to the formation of a higher concentration of surface defects in the form of trapped free radicals, fullerene‐like structures and other kinds of defects. The mechanical properties of rubber compounds filled with radiation‐treated carbon blacks show a significant increase in their reinforcing effects, in line with the increased ability to form ‘bound rubber’. © 2001 Society of Chemical Industry     

Risk of human fatality in building fires: A decision tool using Bayesian networks

The Netherlands is the most densely populated country of the European Union, which makes space very expensive. This leads to increasing complexity of the cities’ layout and other public spaces, together with a large number of people involved. Authorities would like to know whether new and innovative building designs ensure an appropriate level of safety of people in case of fire, before the accident happens, and to be prepared for the so-called “low probability–high consequences” accidents. Therefore, they need a tool to help them estimate the extent of a fire in a building, given any combination of possible conditions and any unexpected course of events during an emergency. This paper discusses the possibility of using Bayesian belief nets for this task. Using this approach, the people in charge can take decisions at different stages of the design process of a building regarding the location, the structure, the loading of the building, the types of fire protection systems inside the building, as well as the characteristics of the fire brigade that fights the possible fire. In the current study, usefulness of the approach is investigated using a small example. This will show the feasibility of the approach for the Netherlands situation and give authorities involved confidence that building a large comprehensive model would fulfil their needs for a support tool in the planning process. The effort to gather real data therefore was restricted as demonstration of fitness for purpose was the primary objective.     

Extending the inelastic thermal spike model for semiconductors and insulators

The inelastic thermal spike framework was extended to incorporate an additional balance equation for the carrier density. Temporal and spatial evolution of carrier density, electronic and lattice temperatures were solved for silicon using a finite difference method. Calculated track radii for a range of electronic stopping powers are presented. The model allows us to fit the electron-phonon coupling to experimental data of amorphised track radii. We compare the methodology of this framework to an earlier inelastic thermal spike model, which is based on the two-temperature model for non-equilibrium processes in metals, and discuss its contribution to the understanding of microscopic processes following a swift ion irradiation event in band gap materials.