Omar Maluf

Charpy impact toughness of conventional and advanced composite laminates for aircraft construction

A weight-based analysis was made of the translaminar Charpy impact toughness performance of conventional and advanced composite materials for aircraft fabrication. The materials were carbon-epoxy (C-Ep) and hybrid fiber-metal TiGr (Titanium-Graphite) laminates. 5 mm-thick three-point bend specimens were tested over a temperature range of -70 to 180 oC to reproduce typical in-service conditions of supersonic jetliners. The energies required for the processes of damage initiation (Ei), damage propagation (Ep), and whole fracture (Et = Ei + Ep), were evaluated at two loading rates, namely, 2.25 and 5.52 m/s in an instrumented Charpy impact testing machine. C-Ep laminates with unidirectional fiber tapes arranged in cross-ply architecture consistently showed the best performance in terms of damage initiation toughness, whereas the hybrid fiber-metal laminate TiGr excelled in terms of propagation toughness. On the other hand, the overall performance of bi-directional fabric C-Ep laminates was very disappointing. The impact behavior of composite laminates was substantiated by a qualitative analysis of topographic aspects of fracture surfaces.

Effect of surface rolling on fatigue behavior of a pearlitic ductile cast iron

Surface rolling is a mechanical treatment usually used in parts fabricated with steel and ductile cast iron, specifically in stress concentration regions, to improve fatigue properties. This process hardens and introduces compressive residual stresses to the surface of the material through the application of controlled strains, thus provoking a reduction of resulting tensile stress at its surface under cyclic loading. This work deals with the effect of surface rolling on high cycle fatigue behavior of a pearlitic ductile cast iron used in crankshafts by the automotive industry. Rotating bending fatigue tests were performed in both smooth and notched specimens, the latter either with or without a surface rolling treatment. Compressive residual stresses and heavy plastic deformation imposed on the surface grains due to cold work made difficult the nucleation and propagation of the crack at the rolled surface of the notch. As a consequence, surface-rolled notch testpieces presented a higher endurance limit (478 MPa) than both smooth (299 MPa) and notched (166 MPa) testpieces did. The surface rolling apparatus developed for this work proved to be very efficient and simple, providing good control of parameters involved in the process (i.e., rolling load, frequency, and number of revolutions).

Residual analysis applied to S-N data of a surface rolled cast iron

Surface rolling is a process extensively employed in the manufacture of ductile cast iron crankshafts, specifically in regions containing stress concentrators with the main aim to enhance fatigue strength. Such process hardens and introduces compressive residual stresses to the surface as a result of controlled strains, reducing cyclic tensile stresses near the surface of the part. The main purpose of this work was to apply the residual analysis to check the suitability of the S-N approach to describe the fatigue properties of a surface rolled cast iron. The analysis procedure proved to be very efficient and easy to implement and it can be applied in the verification of any other statistical model used to describe fatigue behavior. Results show that the conventional S-N methodology is able to model the high cycle fatigue behavior of surface rolled notch testpieces of a pearlitic ductile cast iron submitted to rotating bending fatigue tests.

Thermomechanical and Isothermal Fatigue Behavior of Gray Cast Iron for Automotive Brake Discs

At the end of the 19th century, in the wake of railway transportation and the beginning of automotive vehicle production, new technology-based materials became necessary for the manufacture of brake systems to provide safer and more effective braking of vehicles transporting heavy loads at higher speeds. These devices serve to decelerate vehicles by friction, transforming most of the kinetic energy into thermal energy, which is dissipated by the brake system during the braking process [IOMBRILLER, 2002]. Many parts contribute actively or passively to a vehicle’s satisfactory performance, but safety is closely linked to the efficiency of the brake system, which is subjected to relatively high thermal and mechanical stresses during regular braking action. Therefore, a crucial factor is the precision of the analysis and development of brake systems taking into account all the aspects involved in their thermal and dynamic behavior [MAZUR et al., 2005]. During severe deceleration by braking, the temperature of the brake system may reach up to 650oC and overheating of the brake discs may lead to serious consequences that reduce the vehicle’s safety [IOMBRILLER, 2002]. This temperature variation causes thermal shock and localized overheating points, changing the behavior of the metal involved due to metallurgical transformations, as well as crack nucleation in the disc in response to plastic flow of the surface metal and inducing stresses after cooling [MAZUR et al., 2005]. Even disregarding the presence of thermal shock, a few braking cycles with abrupt deceleration still suffice to produce small cracks in the usable part of brake discs. The study of the mechanical behavior and fracture mechanisms of these materials is essential to allow for the design and rational use of these components. Figure 1 illustrates the failure in front brake rotor in a disc submitted to penetrating liquid inspection to reveal cracks. The cyclic stresses resulting from the continuous use of vehicles can cause fatigue, propagate cracks and fracture of the brake component [IOMBRILLER, 2002]. This mechanism may cause crack nucleation and growth in the material when subjected to cyclic strain. As cyclic loading conditions in brake discs are induced mainly by temperature gradients, thus essentially strain-controlled tests were planned for this study. In this way, it

Dissimilar TIG Welding of HSLA to Austenitic High-Mn TRIP Steels

Abstract The microstructure and mechanical properties of dissimilar butt-joints of high-strength low-alloy steel to austenitic high Mn TRIP steels produced by manual TIG welding with AISI 309L filler metal were investigated. The heat input was varied by changing the welding current between 40 and 70 A. A fully austenitic microstructure with nearly equiaxed grains and without any iron/manganese carbide is observed in the high Mn TRIP base metal, whereas polygonal ferrite and pearlite colonies oriented along the rolling direction are found in the high-strength low-alloy steel base metal. Coarse and refined heat-affected zones (HAZ) were observed in the high-strength low-alloy steel, whereas no significant HAZ was detected at the TRIP side. Bainite and/or martensite grew in the coarse HAZ of the high-strength low-alloy steel steel joined with highest current. The weld seam was formed by austenitic dendrites with interdendritic ferrite and martensite. The best welding condition was achieved with an intermediate current of 50 A, where the hardness transition was smooth, fracture occurred in the HSLA base metal and the joint ductility was higher.