Cyclic behaviour and plastic strain memory effect of 55NiCrMoV7 steel under low cycle fatigue

International audienceCyclic plastic behaviour of tempered martensitic tool steel 55NiCrMoV7 with four different initial hardness levels was studied under tensile-compress low cycle fatigue (LCF) in the temperature range from room temperature up to 873 K. Cyclic behavior tests and strain memory effect tests were performed in symmetrical tensile-compression strain loading with a triangular waveform. The results show that steel represents cyclic softening behaviour. The cyclic stress response generally shows an initial exponential softening for the first few cycles, followed by a gradual softening without saturation. Cyclic stress response depends on strain rate. The steel represents cyclic viscoplasticity. The steel shows the plastic strain memory effects at each test temperature, the cyclic stress and cumulated plastic strain depends on the history of cyclic loading. If strain amplitude increases after a previous linear softening is achieved, a new rapid non-linear cyclic softening appears. In the opposite, if strain amplitude decreases from higher one to lower, softening remains linear, and moreover sigma-p curve goes along the previous way at the previous same strain loading level. It was discussed that the influences of initial hardness, fatigue temperature, strain rate and cyclic strain amplitude on cyclic plasticity of the steel

XRD Synchrotron Study of Carbide Precipitation in Martensitic Steels During Tempering

International audienceIn order to improve the knowledge of the precipitation mechanism in martensitic steels containing carbon, XRD synchrotron experiments were performed. Firstly, the influence of Ni, Co and Al were studied and it was found that the precipitation of iron carbides occurs in same way as in Fe-C steel. However, with the addition of molybdenum and chromium in same steels, XRD synchrotron investigations clearly showed alloyed carbides directly precipitate, thereby preventing the iron carbides formation

Identification and validation of a continuum damage model applied for the lifetime prediction of H11 tool steel

Extrait de : Proceedings of 7th International tooling conference : tooling materials and their applications from research to market / sous la dir. de M. ROSSO, M. ACTIS GRANDE et D. UGUES. - 7th International tooling conference on tooling materials and their applications from research to market, Torino, ITALIE, 2-5 May 2006International audienceThis paper describes the work performed to identify and validate a non-isothermal continuum damage model for a widely used hot-work tool steel H11 at a hardness of 47 HRc. The investigation is based on an extensive high-temperature low-cycle fatigue database performed under strain rate controlled tests with and without dwell times in the range of 300°C-600°C. The approach gives a good description of the thermomechanical behaviour of H11 tool steel under various test conditions and constitutes a very good prediction tool for a very large amount of fatigue tests. This work is divided into three stages. First, isothermal fatigue tests are performed on flat specimens in order to identify lifetime model parameters. Then, thermo mechanical fatigue tests are considered in order to validate the model in a non-isothermal case. Lastly, low-cycle fatigue tests are carried out on notched specimens in order to investigate the model capabilities in a multiaxial condition

Superplasticity in Fine Grain Ti-6Al-4V Alloy: Mechanical Behavior and Microstructural Evolution

Titanium Ti-6Al-4V alloys are known to exhibit interesting superplastic properties for different conditions of temperature and strain rate, depending on the initial grain size. Even if superplasticity is generally explained in terms of grain boundary sliding (GBS) accompanied by several accommodation mechanisms, it appears that the micromechanisms of superplasticity are still controversial especially at the grain scale and even more at lower scale. These micromechanisms, involving microstructural evolution, depend also on the SPF conditions (temperature, strain rate and initial microstructure). In this study, the flow stress in the Ti-6Al-4V alloy is investigated for different strain rate and for temperature in the range of the α/β transformation. The preferred orientation evolution of alpha phase grains for different percentage of deformation is studied for a non-optimal SPF regime (920°C-10-4 s-1) in order to highlight the microstructural evolution and so the deformation mechanisms involved. For that, mechanical interrupted test combined with Scanning Electron Microscopy (SEM) and Electron Back Scatter Diffraction (EBSD) are used

Influence of strain rate and temperature on the deformation mechanisms of a fine-grained Ti-6Al-4V alloy

Depending on their initial microstructure, titanium alloys, as the Ti-6Al-4V may have activation of different deformation mechanisms during hot forming processes. In this work, interrupted tensile tests and heat treatments are used to improve the understanding of the mechanical and microstructural behaviour of a fine-grained Ti-6Al-4V alloy at two temperatures (750 and 920 ) and so for two different phase fractions. The microstructural features like, grain size and phase fraction, were determined by Scanning Electron Microscope (SEM) and image analysis. Moreover, evolution of the preferred crystallographic orientation of grains and local misorientations between and inside grains were obtained by Electron Backscatter Diffraction (EBSD). The strain rate sensitivity parameter as well as the activation energy were deduced from mechanical tests. It appears, from all these microstructural and mechanical data, that several mechanisms are activated depending on the strain level and on the temperature range. At 750 , for a high strain rate, the deformation is mainly controlled by dislocations activity in the phase (texture changes, dynamic recrystallization) and, at very low strain rate, by probably GBS accommodated with dislocations activity into (recovery) and . On the contrary at 920 , a clear decrease of the overall texture intensity associated with a high value suggests that GBS is the dominant mode of deformation. Nevertheless, as the volume fraction is around 48 %/52 % at this temperature, not only the phase but also the phase as well as and boundaries might contribute to the flow behaviour. During long deformation time (low strain rate and high temperature), dynamic coarsening behaviour (both into and ), that is controlled by bulk diffusion, can occur and modify the type, the distribution and decrease the number of , and boundaries. This can be partly related to the flow hardening observed at 920 and 10−4 s−1

Research and Design of a Routing Protocol in Large-Scale Wireless Sensor Networks

无线传感器网络，作为全球未来十大技术之一，集成了传感器技术、嵌入式计算技术、分布式信息处理和自组织网技术，可实时感知、采集、处理、传输网络分布区域内的各种信息数据，在军事国防、生物医疗、环境监测、抢险救灾、防恐反恐、危险区域远程控制等领域具有十分广阔的应用前景。 本文研究分析了无线传感器网络的已有路由协议，并针对大规模的无线传感器网络设计了一种树状路由协议，它根据节点地址信息来形成路由，从而简化了复杂繁冗的路由表查找和维护，节省了不必要的开销，提高了路由效率，实现了快速有效的数据传输。 为支持此路由协议本文提出了一种自适应动态地址分配算——ADAR（AdaptiveDynamicAddre...As one of the ten high technologies in the future, wireless sensor network, which is the integration of micro-sensors, embedded computing, modern network and Ad Hoc technologies, can apperceive, collect, process and transmit various information data within the region. It can be used in military defense, biomedical, environmental monitoring, disaster relief, counter-terrorism, remote control of haz...学位：工学硕士院系专业：信息科学与技术学院通信工程系_通信与信息系统学号：2332007115216

Outillage

International audienc

Caractérisation et modélisation de la séquence de précipitation de carbures au cours du traitement thermique d'aciers martensitiques alliés

Les aciers martensitiques revenus à 5% de chrome, principalement utilisés pour les outillages de forge et de fonderie sous pression présentent des durées de vie limitées en raison des sollicitations thermomécaniques sévères en service. La tenue en service à haute température est directement liée à la stabilité des carbures alliés de taille nanométrique qui se forment au-delà de 450C au cours du revenu. L'étude vise à caractériser les carbures alliés dès les premiers stades du revenu et de déterminer de quelle façon la séquence de précipitation est modifiée. En ce sens, des éléments carburigènes (V, MO) ou influençant la précipitation (NI, CO) ont été ajoutés pour modifier la précipitation d'un acier de référence à 5% de chrome notamment à des températures voisines du pic de durcissement secondaire.Martensitic steels containing 5% chromium, mainly used for forging and high-pressure die casting tools show limited lifetime due to severe thermo-mechanical working conditions. The resistance to stress at high temperatures is directly related to the stability of nanometer sized alloyed cabides which are formed beyond 450C during tempering. The study aims to characterize alloyed carbides in the early stages of tempering and determine how the sequence of precipitation is changed. In that way, carbides forming elements (V, MO) as well as influencing the precipitation (NI, CO) were added to change the precipitation of a low-silicon AISI H11 steel previously studied, especially at temperatures near the peak hardness.TOULOUSE-ENSEEIHT (315552331) / SudocALBI-ENSTIMAC (810042301) / SudocSudocFranceF

Mécanismes de précipitation dans les aciers martensitiques au carbone : caractérisation par DRX synchrotron et simulation thermocinétique

JA 2012 - Journées Annuelles de la SF2M 2012=SF2M Annual Meeting 2012, 29-31 octobre 2012, Paris, FRANCENational audienc

Developing cementite-free martensitic steels : a relevant route to obtain high strength-high toughness steels for tools applications

International audienc