Calorimetric consequences of thermal softening in Johnson–Cook’s model

At high loading rates, the development of adiabatic shear bands in metals is conventionally attributed to the strong interactions induced by viscoplastic dissipation within the bands and thermal softening effects. The rheological equation proposed by Johnson and Cook takes both viscoplastic hardening and thermal softening into account. The present paper reviews and includes this equation into a thermodynamic framework in order to analyse the energy impacts of thermal softening. Indeed this latter implies the existence of a ther-momechanical coupling source, probably non-negligible and which must be considered when estimating temperature variations induced by shear band development

High Speed Blanking: An Experimental Method to Measure Induced Cutting Forces

Lien vers la version éditeur: http://link.springer.com/article/10.1007/s11340-013-9738-1A new blanking process that involves punch speed up to 10 ms −1 has obvious advantages in increased productivity. However, the inherent dynamics of such a process makes it difficult to develop a practical high speed punch press. The fracture phenomenon governing the blanking process has to be well understood to correctly design the machine support and the tooling. To observe this phenomenon at various controlled blanking speeds a specific experimental device has been developed. The goal is to measure accurately the shear blanking forces imposed on the specimen during blanking. In this paper a new method allowing the blanking forces to be measured and taking into account the proposed test configuration is explained. This technique has been used to determine the blanking forces experienced when forming C40 steel and quantifies the effect of process parameters such as punch die clearance, punch speed, and sheet metal thickness on the blanking force evolution

Very high cycle fatigue of copper: Evolution, morphology and locations of surface slip markings

The surfaces of commercially pure polycrystalline copper specimens subjected to interrupted 20 kHz fatigue tests in the very high cycle fatigue regime were investigated. The stress amplitude needed to form the early slip markings was found twice lower than the stress amplitude required to fracture which confirmed the results obtained by Stanzl-Tschegg et al. (2007). Three types of slip markings were classified according to their morphology and their location in the polycrystalline material. They are compared to slip markings observed during fatigue tests at frequencies lower than 100 Hz and numbers of cycles lower than 107. For 20 kHz fatigue tests, stress amplitudes ranging from 45 MPa to 65 MPa produce straight and long early persistent slip markings located along twin boundaries. Stress amplitudes lower than 45 MPa produce clusters of fine early persistent slip markings mainly located at triple junctions

Simulation of green wood milling with discrete element method

This work was carried out in LaBoMaP and PIMM at Arts et Metiers ParisTech. We acknowledge I2M laboratory for their technical support in Discret Element Method; Robert Collet, Louis Denaud, Guillaume Pot; PIMM and LaBoMaP technicians and Morgane Pfeiffer-Laplaud for their availability and advice.During the primary transformation in wood industry, logs are faced with conical rough milling cutters commonly named slabber or canter heads. Chips produced consist of raw materials for pulp paper and particleboard industries. The process efficiency of these industries partly comes from particle size distribution. However, chips formation is greatly dependent on milling conditions and material variability. Numerical simulation of chip fragmentation can allow some useful chip thickness prediction. In this complex situation in wood cutting, the utilization of the Discrete Element Method (DEM) is relevant. In this method, solids are modeled with spherical discrete elements linked by cohesive bonds. However the Discrete Element Method requires a previous calibration step with simple mechanical loading. For example the nature and the mechanical properties of the cohesive bonds must be determined. After an analysis of the different mechanical loadings in green wood milling, a complete study of green wood compression is carried out. This experimental study covers the strain rates range of 10-3 to 103 s-1 using a hydraulic compression machine and the Split Hopkinson Pressure Bar technique. Wood specimens at different moisture content states are compressed longitudinally. This study enables us to observe the viscoelastic and hygroscopic behaviour of wood. The experimental and qualitative simulation results show that elastic brittle beams are not well adapted to be used in quantitative green wood milling simulations

Protection biologique contre ravageurs aériens sur tomate sous abris : compte-rendu d'essai 2005

En agriculture biologique, les moyens de protection contre les ravageurs aériens sont coûteux et leur efficacité est parfois insuffisante. La pratique des brumisations sous abris réduit les températures et augmente l’hygrométrie, et peut améliorer la protection contre l’acarien Tetranychus urticae, comme l'ont montré les essais du GRAB conduits de 2002 à 2004 sur melon, aubergine et concombre. Cet essai a pour objectif d’étudier l’intérêt de la brumisation et de la lutte biologique contre l'ensemble des ravageurs aériens sur tomate : acariens, aleurodes, pucerons, thrips

POD preprocessing of IR thermal data to assess heat source distributions

International audienceInfrared thermography is a useful imaging technique for analyzing the thermomechanical behaviour of materials. It allows, under certain conditions, surface temperature monitoring and, via a diffusion model, estimation of heat sources induced by dissipative and/or thermally coupled deformation mechanisms. However, the noisy and discrete character of thermal data, the regularizing effect of heat diffusion and heat exchanges with the surroundings complicate the passage from temperature to heat source. The aim of this paper is to show that the prior use of reduced-basis projection of thermal data improves the signal-to-noise ratio before estimating the heat source distributions. The reduced basis is generated by proper orthogonal decomposition (POD) of physically-admissible thermal fields. These fields are solutions of ideal diffusion problems related to a set of putative heat sources. preprocessing is applied to different direct methods (finite differences, spectral solution, local least-squares fitting) already used in the past. The gain of this preprocessing is determined using a numerical penalizing benchmark test. The methods are finally compared using data extracted from a dynamic cyclic test on a pure copper specimen

Experimental studies of Portevin-Le Chatelier plastic instabilities in carbon-manganese steels by infrared pyrometry

The dynamic strain aging (DSA) phenomenon that occurs in some materials under certain temperature and strain rate conditions can cause plastic strain localization in the form of Portevin-Le Chatelier (PLC) bands. Carbon-manganese steels are used commonly and frequently in construction because of their ductility, low cost and ability to form mechanically. In these steels, the DSA phenomenon occurs for common quasi-static strain rates from 150 to 300 °C, which makes band observation complicated. PLC bands on a carbon-manganese steel that was sensitive to DSA were studied using an infrared camera. Specimen heating was achieved using an induction furnace (with an adapted coil inductor), which allows for temperature recording during tensile tests. Thermography with an infrared camera was used to estimate the band characteristics and increments in band plastic strain, which is an important parameter for material behavior identification necessary for DSA phenomenon modeling. This technique had been developed only for PLC phenomenon observation at ambient temperature on aluminum alloys. Band characteristics on the carbon-manganese steels have been compared with results obtained previously on aluminum alloys

A clarified position for solanum lycopersicum var. cerasiforme in the evolutionary history of tomatoes (solanaceae)

The natural phenotypic variability present in the germplasm of cultivated plants can be linked to molecular polymorphisms using association genetics. However it is necessary to consider the genetic structure of the germplasm used to avoid false association. The knowledge of genetic structure of plant populations can help in inferring plant evolutionary history. In this context, we genotyped 360 wild, feral and cultivated accessions with 20 simple sequence repeat markers and investigated the extent and structure of the genetic variation. The study focused on the red fruited tomato clade involved in the domestication of tomato and confirmed the admixture status of cherry tomatoes (Solanum lycopersicum var. cerasiforme). We used a nested sample strategy to set-up core collection maximizing the genetic diversity with a minimum of individuals. Results Molecular diversity was considerably lower in S. lycopersicum i.e. the domesticated form. Model-based analysis showed that the 144 S. lycopersicum var. cerasiforme accessions were structured into two groups: one close to the domesticated group and one resulting from the admixture of the S. lycopersicum and S. pimpinellifolium genomes. SSR genotyping also indicates that domesticated and wild tomatoes have evolved as a species complex with intensive level of hybridization. We compiled genotypic and phenotypic data to identify sub-samples of 8, 24, 32 and 64 cherry tomato accessions that captured most of the genetic and morphological diversity present in the entire S. lycopersicum var. cerasiforme collection. Conclusion The extent and structure of allelic variation is discussed in relation to historical events like domestication and modern selection. The potential use of the admixed group of S. lycopersicum var. cerasiforme for association genetics studies is also discussed. Nested core collections sampled to represent tomato diversity will be useful in diversity studies. Molecular and phenotypic variability of these core collections is defined. These collections are available for the scientific community and can be used as standardized panels for coordinating efforts on identifying novel interesting genes and on examining the domestication process in more detail

Flash temperature measurement during dry friction process at high sliding speed

An experimental method is presented in this paper to measure flash temperatures of sliding surfaces. High sliding velocities are reached by using a ballistic set-up equipped with a high speed camera. The temperature field on the friction surface was recorded during the process. Tests were conducted under dry sliding conditions by using an identical material for the rubbing bodies, which are of middle hard steel (C22). Experiments showed that the temperature distribution generated by frictional heating is made up of small hot spots that correspond to the friction of asperities located on the sliding surface during very short time. Deduced from observations, maximum local surface temperatures can exceed about 1100 °C around an area less than 100 μm in diameter

Identification of the heat input during dry or MQL machining

Machining with minimum quantity lubrication (MQL) has been the focus of many scientific investigations since the last 20 years. Nevertheless an acceptable method of predicting the thermal influences on the workpiece quality has still not been developed. This paper describes a simulation approach to estimate the distribution of the cutting energy during machining. During machining, the cutting power is measured to calculate the specific cutting power for each machining process – here drilling, tapping and milling of aluminum. In parallel, the warm-up of the part is measured by fast response thermocouples implanted closed of the machined zone. These thermocouples identify the cutting energy entering into the work piece