M. Ben Jemaa

Impact of Viscoelasticity on the Tribological Behavior of PTFE Composites for Valve Seals Application

In this article, we studied and explored the impact of viscoelasticity on the friction and wear behavior of pure polytetrafluoroethylene (PTFE), carbon–graphite PTFE composite, and glass fiber–MoS2 PTFE composite. Tests were carried out using a specific reciprocating tribometer for valve seal application. The worn surfaces of the PTFE composites and the transfer films formed on the counterface were examined with a scanning electron microscope (SEM). Experimental results revealed that the addition of filler materials was effective in reducing the wear volume in all composites studied. In addition, the friction coefficient and wear resistance showed high sensitivity to the viscoelastic behavior of the PTFE seal. SEM investigation showed that the incorporation of particulate fillers into the PTFE matrix could dramatically reduce and stabilize the transfer films to the counterface, so they largely decreased the wear of the PTFE composites.

Design of a New Tribometer for Tribological and Viscoelasticity Studies of PTFE Valve Seats

This paper presents a new tribometer developed for a study of the tribological and the viscoelastic behaviour of seat materials used in ball valve. This device simulates the contact configuration and the loading conditions produced during service process (opening–closing valve). The originality of such tribometer lies in the possibility of coupling/decoupling the stress relaxation from the tribological behaviour that allows the evaluation of the interaction between tribology and viscoelastic behaviour. Besides, the tribometer concept and manufactured solution of the same subassemblies as well as the measure system and software for data acquisition and processing results are described. This paper also reports the results of a very first set of experiments conducted on polytetrafluoro ethylene materials under dry rolling conditions. The results obtained show a good correlation with the results found in literature, which encourages more investigation into seats ball valve.

Multicast routing protocol for Ad Hoc Networks with a Quality of Service

The evolution of telecommunications is characterized by an increase of availability of bandwidth and the user mobility. It takes to a revolution in offers of the service quality. The notion of quality of service (QoS) becomes then an important concept if one wants to transport information with a maximum of reliability. The most important metrics of QoS are the rate, the delay (as well as the jig) and the rate of losses. In this paper, a routing protocol that integrates this concept of QoS is conceived and is given back efficient in NS-2 to study its performances. The initial objective of our work was to conceive a reliable protocol that distributes packets sent by one or several senders to several recipients with an elevated percentage of success. Qos-AODV is conceived in order to extricate profit of that is made in the AODVprotocol. It uses the different tables of neighbors, of topology and for routing, in order to minimize the delay and the traffic of control. The results show that QoS is essential for multicast routing in ad hoc networks.

Une cause inhabituelle de céphalées

A 64 year old woman presented to the emergency department with a history of sudden onset severe generalised headache after waking, associated with nausea. She had no medical history. On examination, the pain had not eased but she appeared alert with no other discernible features. She underwent a computed tomography scan of the head (fig 1). What is the diagnosis, and are further investigations warranted?