Dynamic crack propagation with cohesive elements: a methodology to address mesh dependency

In this paper, two brittle fracture problems are numerically simulated: the failure of a ceramic ring under centrifugal loading and crack branching in a PMMA strip. A three‐dimensional finite element package in which cohesive elements are dynamically inserted has been developed. The cohesive elements' strength is chosen to follow a modified weakest link Weibull distribution. The probability of introducing a weak cohesive element is set to increase with the cohesive element size. This reflects the physically based effect according to which larger elements are more likely to contain defects. The calculations illustrate how the area dependence of the Weibull model can be used to effectively address mesh dependency. On the other hand, regular Weibull distributions have failed to reduce mesh dependency for the examples shown in this paper. The ceramic ring calculations revealed that two distinct phenomena appear depending on the magnitude of the Weibull modulus. For low Weibull modulus, the fragmentation of the ring is dominated by heterogeneities. Whereas many cracks were generated, few of them could propagate to the outer surface. Monte Carlo simulations revealed that for highly heterogeneous rings, the number of small fragments was large and that few large fragments were generated. For high Weibull modulus, signifying that the ring is close to being homogeneous, the fragmentation process was very different. Monte Carlo simulations highlighted that a larger number of large fragments are generated due to crack branching. Copyright © 2003 John Wiley & Sons, Ltd.     

Sliding wear of austenitic and austenitic-ferritic stainless steels

The dry sliding behavior of a 304L austenitic stainless steel and a duplex 2205 austenitic-ferritic stainless was investigated. The evolution of wear was characterized by the existence of a sliding-distance transition. In particular, wear passed from delamination to tribo-oxidation, with a reduction in wear rate. The occurrence of such a transition was interpreted with reference to a theory of sliding wear based on the formation, by subsurface plastic deformation, of a tribological layer and its detachment during sliding. It has been found that the transition is controlled by the ability of the tribological system to form, on its outer part, a protective oxide-rich scale. This introduces a kinetic limitation, which is particularly important in the case of the duplex 2205, because of its lower ductility in comparison to the 304L steel. In this frame, the influence of sliding velocity, the particular frictional behavior, the role of chromium in the oxidative wear, and the surface temperature evolution during sliding could be explained.     

HIV, health care workers and patients: how to ensure safety in the dental office

What can be done to preserve the high standards of patient care, give reasonable assurances to patients and acknowledge the rights of health care workers? Conscientious adherence to infection control is the most responsible option available.     

Degradation of lincomycin in aqueous medium: Coupling of solar photocatalysis and membrane separation

The photocatalytic oxidation of a common antibiotic, the lincomycin was carried out in aqueous suspensions of polycrystalline TiO₂ Degussa P25 irradiated by sunlight. In order to improve the performance of the lincomycin degradation a hybrid system consisting of a solar photoreactor with the photocatalyst in suspension coupled with a membrane module, used to confine both photocatalyst and pollutants in the reaction environment, was tested.A preliminary study was carried out in order to determine some kinetics parameters of the drug photodegradation. The influence of initial substrate concentration on the lincomycin photooxidation rate was investigated. The photooxidation rate follows a pseudo-first order kinetics with respect to the lincomycin concentration under the used experimental conditions. The presence of the membrane reactor allows the catalyst separation and to operate in continuous mode as the membranes rejection for lincomycin and its oxidation products was quite high.     

An ensemble evolutionary constraint-based approach to understand the emergence of metabolic phenotypes

Constraint-based modeling is largely used in computational studies of metabolism. We propose here a novel approach that aims to identify ensembles of flux distributions that comply with one or more target phenotype(s). The methodology has been tested on a small-scale model of yeast energy metabolism. The target phenotypes describe the differential pattern of ethanol production and O₂ consumption observed in “Crabtree-positive” and “Crabtree-negative” yeasts in changing environment (i.e., when the upper limit of glucose uptake is varied). The ensembles were obtained either by selection among sampled flux distributions or by means of a search heuristic (genetic algorithm). The former approach provided indication about the probability to observe a given phenotype, but the resulting ensembles could not be unambiguously partitioned into “Crabtree-positive” and “Crabtree-negative” clusters. On the contrary well-separated clusters were obtained with the latter method. The cluster analysis further allowed identification of distinct groups within each target phenotype. The method may thus prove useful in characterizing the design principles underlying metabolic plasticity arising from evolving physio-pathological or developmental constraints.     

Reaction sintering of Fe-Al-Si alloys

The present study concerns the production of multiphase alloys of the Fe-Al-Si system using a powder metallurgical approach. Several compositions have been considered based on α-Fe, α′-FeAl; and α″-Fe₃Al intermetallic phases. Elemental powder mixtures were compacted and sintered in a dilatometer. In this way, the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperature. The effect of heating rate has been considered. The characterization of the final products was based mainly on x-ray diffraction (XRD) analyses, particularly as concerns the quantification of the crystalline phases present in the final products. Grain and porosity morphologies were characterized by scanning electron microscopy (SEM). In this way, clear indications of the reaction and densification processes occurring during the sintering treatments were obtained.     

A new thermomechanical model of cutting applied to turning operations. Part I. Theory

In this paper, an analytical approach is used to model the thermomechanical process of chip formation in a turning operation. In order to study the effects of the cutting edge geometry, it is important to analyse its global and local effects such as the chip flow direction, the cutting forces and the temperature distribution at the rake face. To take into account the real cutting edge geometry, the engaged part in cutting of the rounded nose is decomposed into a set of cutting edge elements. Thus each elementary chip produced by a straight cutting edge element, is obtained from an oblique cutting process. The fact that the local chip flow is imposed by the global chip movement is accounted for by considering appropriate interactions between adjacent chip elements. Consequently, a modified version of the oblique cutting model of Moufki et al. [Int. J. Mech. Sci. 42 (2000) 1205; Int. J. Mach. Tools Manufact. 44 (9) (2004) 971] is developed and applied to each cutting edge element in order to obtain the cutting forces and the temperature distributions along the rake face. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening, the thermomechanical coupling and the inertia effects are taken into account in the modelling. The model can be used to predict the cutting forces, the global chip flow direction, the surface contact between chip and tool and the temperature distribution at the rake face which affects strongly the tool wear. Part II of this work consists in a parametric study where the effects of cutting conditions, cutting edge geometry, and friction at the tool–chip interface are investigated. The tendencies predicted by the model are also compared qualitatively with the experimental trends founded in the literature.     

A new thermomechanical model of cutting applied to turning operations. Part II. Parametric study

In Part I of this work, Molinari and Moufki [Int. J. Mach. Tools Manufact., this issue], an analytical model of three-dimensional cutting is developed for turning processes. To analyse the influences of cutting edge geometry on the chip formation process, global effects such as the chip flow direction and the cutting forces, and local effects such as the temperature distribution and the surface contact at the rake face have been investigated. In order to accede to local parameters, the engaged part in cutting of the rounded nose is decomposed into a set of cutting edge elements. Thus each elementary chip, produced by a straight cutting edge element, is obtained from an oblique cutting process defined by the corresponding undeformed chip section and the local cutting angles. The present approach takes into account the fact that for each cutting edge element the local chip flow is imposed by the global chip movement. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening, the thermomechanical coupling and the inertia effects are considered in the modelling. A detailed parametric study is provided in this paper in order to analyse the effects of cutting speed, depth of cut, feed, nose radius and cutting angles on cutting forces, global chip flow direction and temperature distribution at the rake face. The influence of friction at the tool–chip interface is also discussed.