Riadh Elleuch

Viscoelastic behavior of HDPE polymer using tensile and compressive loading

Large containers for liquids, exposed to different static loadings, are mainly made of high-density polyethylene (HDPE). The viscoelastic response of HDPE under long-term tensile and compressive creep is investigated. Monotonic experiments under tension are performed over a wide range of strain rates. In these experiments, the transition in the damage mechanisms (development and propagation of contraction in the HDPE specimen) is analyzed. The monotonic tensile behavior of the HDPE is found to be nonlinear and depends on the strain rates. It is observed that both elastic modulus and plastic flow stress present an increase with displacement speed due to the viscoelastic behavior of HDPE. A similar observation can be made for monotonic compressive tests by developing a new experimental device that ensures accurate measurement of the strain. Such a device makes use of an extensometer of compressive displacement of the specimen. In addition, the long-term behavior of HDPE is evidenced through creep and relaxation tests at an imposed range respectively of lower stresses and strains. It is shown that the normalized curves, associated with these tests, can be represented by a single curve characterizing the compressive creep compliance or relaxation stresses versus time. The linearity of the viscoelastic behavior is confirmed within the linear domain of the monotonic compressive and tensile tests.

Analysis of Degradation and Failure Mechanisms that Develop in Hot Forging Die

A failed die, used to forge gas valves, was analyzed using macroscopic and microscopic equipment on some specimens. Fatigue cracks on the die were evidently observed on all the surfaces of the die. Many processes were found to be major contributors to have caused the die failure. Oxidation was observed in some areas, especially on plane surfaces and thin sections. Surface layers of tools at the tool-workpiece interface were not only exposed to high mechanical stresses but also to severe temperature cycles, which could have led to loss of strength and hardness. Metallographic analysis revealed that the oxide layers consisted of different diffusion layers which accelerated thermal fatigue mechanisms.

Analysis of friction, wear and oxidation behaviour of X40CrMoV5/Fe360B steel couple in an open-sliding contact

In hot-forming processes, the interface tool/product is important for the quality of the finished product surfaces. Consequently, it is important to better understand the phenomena, namely, oxidation, which occurs at the interface between tool and sheet in this kind of process. It is in this context that an experimental program was conducted to analyse friction, wear and oxidation behaviours on the tribological performance of the combination of tool and sheet. A high-temperature pin-on-disc tribometer, with an original open-sliding contact, was used. Pins, representing tools are X40CrMoV5 high-alloyed steel, and discs, representing drawing products, are Fe360B steel plates (EN 10025–S235JRG2). The friction and wear study of the combination was carried out at temperatures ranging from ambient (room temperature) to 800℃ and within a normal load range of 35–140 N. Regarding oxidation effect and wear mechanisms, they are examined and characterized by scanning electron microscope and energy-dispersive X-ray spectrometer. As for the experimental results, they showed that the increase in temperature involves the decrease in the friction coefficient due to the oxide scales for all the tests. Pre-oxidized discs are also tested. It was found that the wear process is characterized by an adhesive wear with a transfer of oxides on the pins.

Study of Failure Modes Affecting a Crimped Nut Related to Forging Process

In this current study, the problems caused by hot and cold forging—the two successive manufacturing process of a crimped nut—were investigated and resolved. The forged material is a leaded brass (CuZn39Pb2). Concerning the problem faced during hot forging (stamping), we chose a methodology based on the reproduction of industrial phenomena in laboratory scale. The study of this problem was conducted by micrographic and chemical analyses. For cold forging (crimping), a mechanical and microstructural characterization of the brass was performed in various conditions of heat treatment and shaping. Two items were proven in this study: (1) the use of graphite reduced the oxidation of the brass which was heated at the forging temperature to quarter; and (2) the heat treatment and annealing at 300°C avoided cracking during cold forging.