The Workpiece Material in Machining

In machining operations, much attention is paid to the improvement of the lifetime of tools by increasing the hardness of the tool whether or not it is combined with an optimised hard coating. Another method is to change the geometry of the tool by chip breakers and thus shorten the contact time and decrease the friction between chip and tool. Up to now, little attention has been paid to gaining an understanding of the influence of the workpiece material in machining. From forming operations, it is known that the fracture strain under different stress conditions can be predicted by using ductility curves. These curves can be determined easily by two simple material tests. It will be shown that these curves can also be used to understand chip breaking. The work clarifies a part of the role of the cooling liquid, and also the phenomenon that the tool life can sometimes be increased by using a harder workpiece material. A lower friction between chip and tool can be more effective than a harder tool material. Application of this idea teaches us the effect of Molybdenum sulphide (MoS2 ) as a tool coating and the fine cutting of steel with diamond tools. Another workpiece material property that influences the life-time of tools is its strain rate dependency on the flow stress. Its prediction is not easy and strongly depends on the material.     

A self‐tuning iterative learning controller for time variant systems

We consider the iterative learning control problem from an adaptive control viewpoint. The self‐tuning iterative learning control systems (STILCS) problem is formulated in a general case, where the underlying linear system is time‐variant and its parameters are all unknown and where its initial conditions are not constant and not determinable in various iterations. A procedure for solving this problem will be presented. The Lyapunov technique is employed to ensure the convergence of the presented STILCS. Computer simulation results are included to illustrate the effectiveness of the proposed STILCS. Copyright © 2008 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society     

Deformation and life assessment of high temperature materials under creep fatigue loading

The knowledge of mechanical long term behaviour under static and cyclic loading for high temperature components requires methodologies for life assessment in order to employ the full potential of materials. A phenomenological life time prediction concept which was developed for multi‐stage creep fatigue loading demonstrates the applicability of rules for synthesis of stress strain path and relaxation including an internal stress concept, as well as mean stress effects. Further, a creep fatigue interaction concept which was also developed covers a wide range of creep dominant loading as well as fatigue dominant loading. Service‐type experiments conducted at different strain rates and hold times for verification purposes demonstrate the acceptability of life prediction method for variation of conventional 1 %Cr‐steels as well as modern high chromium 9‐10 %Cr‐steels. Generally, the service life of components is influenced by multi‐axial behaviour. Multi‐axial experiments with e.g. notched specimens and with cruciform specimens accompanied by advanced methods for calculation of stress strain path and life time prediction stress conditions are of future interest.     

Tensilely Strained Germanium Nanomembranes as Infrared Optical Gain Media

The use of tensilely strained Ge nanomembranes as mid‐infrared optical gain media is investigated. Biaxial tensile strain in Ge has the effect of lowering the direct energy bandgap relative to the fundamental indirect one, thereby increasing the internal quantum efficiency for light emission and allowing for the formation of population inversion, until at a strain of about 1.9% Ge is even converted into a direct‐bandgap material. Gain calculations are presented showing that, already at strain levels of about 1.4% and above, Ge films can provide optical gain in the technologically important 2.1–2.5 μm spectral region, with transparency carrier densities that can be readily achieved under realistic pumping conditions. Mechanically stressed Ge nanomembranes capable of accommodating the required strain levels are developed and used to demonstrate strong strain‐enhanced photoluminescence. A detailed analysis of the high‐strain emission spectra also demonstrates that the nanomembranes can be pumped above transparency, and confirms the prediction that biaxial‐strain levels in excess of only 1.4% are required to obtain significant population inversion.     

BACTERIAL MUTAGENICITY OF DICYCLOPENTA-FUSED PYRENE CONGENERS IN FVT-PYROLYSATES: PARTIAL COMBUSTION EXHAUST MIMICS

The mutagenicity response of well-characterized flash vacuum thermolysis (FVT) pyrolysates that contain cyclopenta[cd]pyrene (1) and 1-ethynylpyrene (1b) (pyrolysate I), the dicyclopentapyrene congeners dicyclopenta[cd,mn]- (2), dicyclopenta[cd,fg]- (3), or dicyclopenta[cd,jk]pyrene (4) and their related bis-ethynyl- (2b–4b) and monocyclopenta-ethynylpyrene (2a–4a) analogues (pyrolysates II–IV, respectively), or cyclopenta[cd]- (1) and the three dicyclopentapyrenes (2–4) (pyrolysate V), respectively, was assessed using the standard protocol outlined by Ames et al. (Salmonella typhimurium strain TA98 ± S9-mix 4% (v/v)). It is shown that the mutagenic activity of the pyrolysates deviates from the weighed sum of the activity of the individual pyrolysate constituents. Hence, FVT-pyrolysates are proposed as model mixtures, that is, as partial combustion exhaust mimics, to establish and evaluate interactions (additive, synergistic, or antagonistic effects) that may occur among the constituents and affect the global mutagenicity response.     

Influence of stress on passive behaviour of steel bars in concrete pore solution

The influence of stress on passive behaviour of steel bars in concrete pore solution was studied with electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The passive ability of steel decreased as the applied load increased and higher load had much greater influence on passivation than repeated loading of small magnitude. A micro-crack model was presented to explain the damage of passive layer by loads. Lower load caused micro-cracks in the passive film which might be completely recovered after unloading. Under higher load more micro-cracks were produced in the passive film and some may penetrate the film, leading to irreversible damages.     

From Overcrowded Polycyclic Aromatic Enes to Semifullerenes

Abstract

Overcrowded polycyclic aromatic enes (1), e.g., bi-9H-fluoren-9-ylidene (2) and bi-4H-cyclopenta[def]-phenanthren-4-ylidene (3) are potential starting materials for the preparation of bowl-shaped fragments of fullerenes. Semiempirical MNDO-PM3 calculations of C₂₆ H_n and C₃₀ H_n (n = 12,14,16) species 2–14 are used to analyze energetic and steric effects on the dehydrocyclization and isomerization reactions of these molecules. the out-of-plane bending and pyramidalization in these species are ascribed to intramolecular overcrowding in the fjord and cove regions and to strain introduced by C5 rings in the PAH skeleton. Oxidative photocyclization reactions on Z-2,2′-bridged derivatives of 2 and 3 are briefly outlined.     

Effect of austenite stability on the pitting corrosion resistance of cold worked stainless steels

The influence of cold rolling on the pitting resistance of stainless steels is investigated with respect to the austenite stability. A maximum in pitting initiation frequency for 20% reduction is confirmed whatever the austenite stability, but the value of this maximum fairly correlates to the amount of deformation-induced martensite. A direct influence of dislocations piling-up and an indirect role of martensite as a pile-up stabilizer is postulated. In the potentiostatic regime, cold work is shown to lower the repassivation ability and to increase the number of stable pits. Then, addition of alloying elements (such as copper) affects the pitting resistance not only for chemical but also for mechanical reasons (such as their effect on the staking fault energy and austenite stability), confirming the decisive role of microstructure in pitting corrosion of industrial alloys.     

Unconstrained springback behavior of Al-Mg-Si sheets for different sitting times

In this work, the springback behavior of the commercial 6022 aluminum alloy in temper aging (T4) is investigated taking into account that the sheets, prior to deformation process, are initially pre-strained and then submitted to various sitting times at room temperature. The unconstrained cylindrical bending test based on the NUMISHEET2002 proceedings as presented by Yoon et al. [Yoon JW, Pourboghrat F, Chung K, Yang DY. Springback prediction for sheet metal forming process using a 3D hybrid membrane/shell method. International Journal of Mechanical Sciences 2002;44:2133-53] is selected as validation benchmark. For finite element simulations, the geometry is modeled by solid-shell finite elements using the formulation of Alves de Sousa et al. [Alves de Sousa RJ, Yoon JW, Cardoso RPR, Fontes Valente RA, Grácio JJ. On the use of a reduced enhanced solid-shell (RESS) element for sheet forming simulations. International Journal of Plasticity 2007;23:490-515; Alves de Sousa RJ, Cardoso RPR, Fontes Valente RA, Yoon J-W, Natal Jorge RM, Grácio JJ. A new one-point quadrature enhanced assumed strain (EAS) solid-shell element with multiple integration points along thickness: Part I—Geometrically linear applications. International Journal for Numerical Methods in Engineering 2005;62:952-77; Alves de Sousa RJ, Cardoso RPR, Fontes Valente RA, Yoon JW, Grácio JJ, Natal Jorge RM. A new one-point quadrature enhanced assumed strain (EAS) solid-shell element with multiple integration points along thickness: Part II—Nonlinear applications. International Journal for Numerical Methods in Engineering 2006;67:160-88]. The material behavior is described based on the work of Correia et al. [Correia JPM, Simões F, Gracio JJ, Barlat F, Ahzi S. A simple hardening rule accounting for time-dependent behavior in Al-Mg-Si alloys. Materials Science Engineering A 2007;456:170-9]. The results of conducted experiments and numerical simulations are compared. It can be concluded about the good agreement between experiments and simulations attesting the effectiveness of the material model utilized to describe the time-dependent hardening behavior.