Papers by Author: Fabrizio D'Errico

Abstract: This paper concerns about a failure analysis of an electric all aluminum alloy conductor (AAAC) damaged and broken for fretting fatigue phenomena induced by aeolian vibrations. Life of electric conductors is often reduced by various degradation mechanisms such as repeated bending, fluctuating tension, distortion, fatigue, wear and corrosion phenomena. However the main limiting factor of the electrical conductors is related to aeolian vibrations in the high frequency range (between 5 to 50 Hz). Conductor oscillations may lead to fretting fatigue problems (otherwise called fretting wear) caused by wind excitation, mainly in the suspension clamp regions, spacers or other fittings. The induced aluminium wire fracture imply a drastic reduction in the transmission line service. Vibration dampers are considered the most effective method to extend service life of electric conductors, as they are the means to reduce fretting damage of aluminium wires. The aim of the present work is to investigate the failure of an AAAC conductor of a 400kV overhead transmission line (twin conductors) located in Touggourt Biskra (Algeria); the damaged and broken conductors were operated in-service only for six months without spacers or dampers. Three different types of conductors have been taken as experimental samples: the in-service broken conductor, another in-service damaged conductor and a new conductor from warehouse as terms of comparison. Samples have been analysed to identify the root cause of the failure and to verify the conformity of the conductor elements to the international standards. The investigation has outlined the morphology of the fretting damage: in all cases the fractured wires have shown typical static deformation marks and dynamic fretting wear tangential marks associated with intense presence of Al2O3 debris.

Abstract: The Thixomolding® process has a 20-year track record of research with commercial applications beginning in 1992. The apparatus machine is similar to those used for an injection plastic molding: it transforms in a single step magnesium chips heated to a semisolid slurry (inside a barrel and screw) into precision molded parts. Originally employed by Asian manufacturers for consumer electronics and consumer hardware applications (such as notebook computer cases, digital camera bodies and chain saw housings, etc.), the industrial uses of Thixomolding® has been widely extending from last years to other parts suppliers in the Americas, Europe and Asia. Nowadays thixomolded parts there are in automotive and sport equipment as automotive shift cams, bike saddle, fishing reels, snowboard bindings, etc. Thanks to the semisolid and thixotropic state of magnesium, the slurry moves in laminar flow inside the mould. The absence (or very low) of turbulence allows to reduce drastically (and often to eliminate) residual porosity which is typical of die-casting. In this work a thixomolded AZ91D has been characterized by tensile and axial fatigue testing (fatigue ratio R=0.1) both in the as-supplied condition and after a Tx heat treatment.

Abstract: A deep comprehension of the damage mechanisms involved in contact fatigue should optimize material and heat treatment choice for a specific application. In this work rolling disc-on-disc contact fatigue tests have been performed on a hardened and tempered UNI EN 42CrMo4 . The adopted test method creates the best conditions in order to develop micro-pitting on disc surface. Extensive micro-fractographic examinations have been carried out, on the damaged surfaces, through a scanning electronic microscope (SEM). For this steel, loaded with Hertzian pressure of 1000 MPa, the failure mode is always micro-pitting which begins at the surface, and it is not a sub-superficial damaging. If micro-pits develop, they will coalesce in larger craters. By this way, the probability that micropitting will degenerate into sub-superficial destructive pitting is very high.