Comprehension of chip formation in laser assisted machining

Laser Assisted Machining (LAM) improves the machinability of materials by locally heating the workpiece just prior to cutting. Experimental investigations have confirmed that the cutting force can be decreased, by as much as 40%, for various materials. In order to understand the effect of the laser on chip formation and on the temperature fields in the different deformation zones, thermo-mechanical simulations were undertaken. A thermo-mechanical model for chip formation was also undertaken. Experimental tests for the orthogonal cutting of 42CrMo4 steel were used to validate the simulation. The temperature fields allow us to explain the reduction in the cutting force and the resulting residual stress fields in the workpiece.Contrat Plan Etat Région (CPER) Pays de la Loir

Effects of the Herbicide Atrazine on Members of the Freshwater Genus Hydra

Atrazine is a commercially available herbicide with known effects on the hormone levels in frogs and several other organisms. This effect has not been significantly studied in invertebrates, even though observing invertebrate levels represent a very common assay the health of a freshwater ecosystem. To study the effects of atrazine in invertebrates, hydra were subjected to environmental (10 μg/L) and high (400 μg/L) concentrations of atrazine and observed for changes in fertility and reproduction over a period of four weeks. No significant changes in fertility or reproduction were found at high concentrations, and environmental concentration testing is currently in progress, Hydra bearing a symbiosis with chlorella algae demonstrated a significant loss of algae in their cells at high concentrations, and environmental concentration testing is currently in progress. The findings currently indicate that atrazine does not have immediate adverse effects on hydra, although atrazine is dangerous to the plant life that the hydra may interact with

Global existence for a hyperbolic model of multiphase flows with few interfaces

In this thesis we consider a hyperbolic system of three conservation laws modeling the one-dimensional flow of a fluid that undergoes phase transitions. We address the issue of the global in time existence of weak entropic solutions to the initial-value problem for large BV data: such is a challenging problem in the field of hyperbolic conservation laws, that can be tackled only for very special systems. In particular, we focus on initial data consisting of either two or three different phases separated by interfaces. This translates into the modeling of a tube divided into either two or three regions where the fluid lies in a specific phase. In the case of two interfaces this possibly gives rise to a drop of liquid in a gaseous environment or a bubble of gas in a liquid on

Global existence of solutions for a multi-phase flow: a drop in a gas-tube

In this paper we study the flow of an inviscid fluid composed by three different phases. The model is a simple hyperbolic system of three conservation laws, in Lagrangian coordinates, where the phase interfaces are stationary. Our main result concerns the global existence of weak entropic solutions to the initial-value problem for large initial data

Global weak solutions for a model of two-phase flow with a single interface

We consider a simple nonlinear hyperbolic system modeling the flow of an inviscid fluid. The model includes as state variable the mass density fraction of the vapor in the fluid and then phase transitions can be taken into consideration; moreover, phase interfaces are contact discontinuities for the system. We focus on the special case of initial data consisting of two different phases separated by an interface. We find explicit bounds on the (possibly large) initial data in order that weak entropic solutions exist for all times. The proof exploits a carefully tailored version of the front tracking scheme

Experimental and numerical analysis of micromechanical damage in the punching process for High-Strength Low-Alloy steels

Sequential sheet metal forming processes can result in the accumulation of work hardening and damage effects in the workpiece material. The mechanical strength of the final component depends on the “evolution” of these two characteristics in the different production steps. The punching process, which is usually in the beginning of the production chain, has an important impact on the stress, strain and damage states in the punched zones. It is essential that the influence of these mechanical fields be taken into account in the simulation of the forming sequence. In order to evaluate the evolution of each phenomenon, and in particular damage accumulation in the forming process, it is essential to characterize the punching process. The objective of this work is to understand and identify the physical damage mechanisms that occur during the punching operation and to establish relevant numerical models to predict the fracture location. The effect of the punch–die clearance on mechanical fields distribution is also discussed in this work

Experimental characterization and numerical modeling of micromechanical damage under different stress states

The use of HSLA steels for the manufacture of automotive components is interesting from an engineering point of view. This family of steels, while possessing high strength, also has good formability and can be used in forming manufacturing processes. In some forming processes such as blanking, shear strain localization occurs, which causes damage and results in the final fracture of the material. This paper presents an experimental study based on in situ tests to understand and identify the physical mechanisms of ductile damage under two stress states: tension and shear. Different macroscopic tests were performed to calibrate a damage model based on a micromechanical approach. This damage model is based on the Gurson–Tvergaard–Needleman theory and presents recent improvements proposed by Nahshon and Hutchinson and by Nielsen and Tvergaard so as to better predict fracture under a wide range of stress states, especially with low levels of stress triaxiality. These extensions have made the identification of the material parameter more complicated. In this work an identification strategy has been proposed using tests on specimens with different shapes. The identified parameter values are validated and the fracture model show good predictive capability over a wide stress state range

Global existence of solutions for a multi-phase flow: a bubble in a liquid tube and related cases

In this paper we study the problem of the global existence (in time) of weak, entropic solutions to a system of three hyperbolic conservation laws, in one space dimension, for large initial data. The system models the dynamics of phase transitions in an isothermal fluid; in Lagrangian coordinates, the phase interfaces are represented as stationary contact discontinuities. We focus on the persistence of solutions consisting in three bulk phases separated by two interfaces. Under some stability conditions on the phase configuration and by a suitable front tracking algorithm we show that, if the BV-norm of the initial data is less than an explicit (large) threshold, then the Cauchy problem has global solutions

Numerical integration of an advanced Gurson model for shear loading: Application to the blanking process

A new extension of the Gurson damage model has been proposed recently to predict ductile fracture under shear dominated loads. The aim of this work is to verify the ability of this approach to simulate, in an accurate way, the damage evolution in shearing processes. An implicit stress integration algorithm is then developed to implement the new model in a finite element code. The numerical procedure is checked through simulations of shear and uniaxial tension tests on a single elements. The extended Gurson damage model is tested and applied to the punching process to compare its predictive ability with the original approach. The obtained numerical results are in good agreement with experimental results of the punching process, showing better ductile fracture predictions compared to the original Gurson model

The Rhetoric of Passions in John Tillotson's Sermons

As the leading member of the Latitudinarian movement, scholars have often referred to John Tillotson as the father of the reform of ecclesiastical oratory that took place in the second half of the seventeenth century. His influence as a prose writer continued in the eighteenth century and his writings were appreciated by religious and lay critics. Although his style has been often described as 'impassionate', sober and reiterative, this article shows how he adapted his style to conquer the hearts of the congregation by exploiting two passions, self-love and fear, which are impossible to eradicate in Man