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Slant fracture surface in 7075 aluminum alloy tensile specimens
2017 - LOBO, RAQUEL de M.; MORCELLI, APARECIDO E.; SOUZA, SAUL H.; PADILHA, ANGELO F.; ANDRADE, ARNALDO H.P. de
Rectangular specimen of a 7075 aluminum alloys in three different thermal treatment conditions were tested in a tensile equipment. The samples tested exhibit slant fracture surfaces. Some of the samples presented Portevin-Le Chatelier (PLC) effect. In fact, for aluminum alloys, the PLC phenomenon may occurs even at room temperature. It leads to strain localization and deformation band formation. In this work, the occurrence of slant fracture is studied by scanning electron microscopy and an attempt is done to correlate their presence with the PLC bands.
Mechanical anisotropy of ABS specimens 3D printed by FDM
2019 - ANDRADE, ARNALDO H.P. de; LOBO, RAQUEL de M.; BREDA, FRANCISCO J.; CASTAGNET, MARIANO
Additive manufacturing (AM) has been developed as a technique for fast fabrication of component parts through 3D printing, using a process of adding successive layers, one after another. The Fused Deposition Modeling (FDM) technique is a 3D printing process that generally uses a thermoplastic filament as the printing base material. The material is fed by a coil and the head of the extruder performing the process is heated. Printing can be done in several directions, depending on the purpose of the component part. In this work mechanical tests were done on ABS (Acrylonitrile-Butadiene-Styrene) tensile specimens built by FDM, in different orientations, to analyze its properties and to make a correlation between these properties and the influence of the printing direction in the final product. The tensile specimens in dog bone shaped were tested according to the ASTM D638 standards. The experiments were done at room temperature. The horizontal built specimens (H-specimens), fabricated in the z-direction, while the beads were layered along the x and y direction, parallel to the build table, showed the maximum resistance of 30 MPa. The specimens built with their gage length out of x-y plane (V-specimen and D-specimen, vertical and diagonal specimens respectively) presented lower ultimate strength, for instance 14 MPa for the Vspecimen. Additional investigation is on the way to understand the presence of defects (cavities and crazes) in the ABS microstructure since the literature points that the ability to manipulate it is the key to an improved performance of this type of structural material.
Fracture surface analysis of ABS samples printed by the FDM method
2019 - LOBO, RAQUEL de M.; MORCELLI, APARECIDO E.; BREDA, FRANCISCO J.; CASTAGNET, MARIANO; ANDRADE, ARNALDO H.P. de
FDM (Fused Deposition Modeling) is one of the most used technique in additive manufacturing (AM). It can be summarized as printing small components through a heated thermoplastic filament, which is deposited layer by layer through a 3D printer. The print head can be programmed to perform the job in different directions (X, Y and Z) even with predetermined slopes. In this work, flat tensile specimens were prepared in different directions using an ABS (Acrylonitrile-Butadiene-Styrene) filament with a processing temperature between 210-225 ° C. After tensile tests, the fracture surfaces were analyzed to get a better understanding of the deformation and fracture processes. The crosssection view of specimen´s morphology was examined with a scanning electron microscope at a very low accelerating voltage (1 kV). The specimens were coated with gold using a sputtering system. The specimens that showed the smaller ultimate strength (named V-specimen) presented numerous large cavities in its microstructure suggesting that these voids may have a major contribution to the mechanical performance of the material.The fracture surface also shows a possible pull-out between layers indicating a weak point in the microstructure of the built sample that’s requires further investigation.
Advances in the understanding of the mechanisms of iodine-induced SCC cracking in zirconium alloys
2019 - LOBO, RAQUEL de M.; ANDRADE, ARNALDO H.P. de
In pressurized water reactors (PWR) the fuel rod cladding is the first barrier against the spread of fission products. It is therefore essential to guarantee its use in the reactor. Sometimes the production of electricity requires that certain power plants operate in “network monitoring”. The fuel introduced into nuclear power reactors can then undergo so metimes significant power variations. Following a severe reactor power transient, clad failure can occur through a stress corrosion phenomenon (SCC), under the combined action of mechanical stresses and gaseous fission products generated by the fuel pellets. Among those iodine plays a major role, for it may induce SCC in zircaloy. In the early ages of water cooled reactors (PWRs, BWRs or CANDU), series of similar failures took place following sharp startups. Today power increase rates as well as instantaneous local power levels are limited. Indeed, it is well know that cladding failure by iodine induced stress corrosion cracking (I SCC) may occur under pellet cladding interactions (PCI) conditions during power transients in PWRs. In this paper we review the advances in the understanding of these SCC cracking mechanisms of the fuel rod cladding that would then allow better control of the integrity of the clad during the more severe demands related to the operating conditions of th e PWRs.
Monitoring of the ductile to brittle transition temperature of reactor pressure vessel steels by means of small specimens
2019 - ANDRADE, ARNALDO H.P. de; MIRANDA, CARLOS A.J.; LOBO, RAQUEL de M.
Neutron irradiation in nuclear power plants (NPPs) lead to microstructural changes in structural materials which induce a shift of the ductile to brittle transition temperature (DBTT) towards higher temperatures. Monitoring of the DBTT in NPP components receives therefore considerable attention. Small specimen testing techniques are developed for characterizing structural components with a limited amount of materials. One of the most used of these miniature testing is the small punch test (SPT) which is based on disc or square shaped specimens. SPTs may be performed from room to cryogenic temperatures, plotting the absorbed energy until rupture, against the test temperature. A ductile region (high energy) and a brittle region (low energy) with a transition between both zones are usually reported. The transition temperature thus obtained, DBTTSPT, is also related through empirical expressions to the transition temperature obtained in CVN tests, DBTTCVN, or in fracture toughness testing. Linear expressions such as DBTTSPT = α DBTTCVN have been used where α is a material characteristic constant. In all cases, the DBTTSPT temperature is much lower than that obtained in the CVN tests. In this paper, we present a short review of the literature on the determination of the DBTT for nuclear reactors pressure vessels steels by those two techniques analyzing the reason for the difference in their value as mentioned before. In dealing with irradiated materials, is a high priority to limit the exposure of the professional to irradiation. Therefore, the use of miniature specimens receives significant attention in the nuclear community. The high cost of irradiation experiments is a further incentive for using small specimen testing techniques.
Small specimen technique for assessing the mechanical properties of metallic components
2017 - LOBO, RAQUEL M.; ANDRADE, ARNALDO H.P.; MORCELLI, APARECIDO E.
Recent advances on charpy specimen reconstitution techniques
2017 - ANDRADE, ARNALDO H.P.; LOBO, RAQUEL M.; MIRANDA, CARLOS A.J.
Charpy specimen reconstitution is widely used around the world as a tool to enhance or supplement surveillance programs of nuclear reactor pressure vessels. The reconstitution technique consists in the incorporation of a small piece from a previously tested specimen into a compound specimen, allowing to increase the number of tests. This is especially important if the available materials is restricted and fracture mechanics parameter have to be determined. The reconstitution technique must fulfill some demands, among them tests results like the original standard specimens and the loaded material of the insert must not be influenced by the welding and machining procedure. It is known that reconstitution of Charpy specimens may affect the impact energy in a consequence of the constraint of plastic deformation by the hardened weldment and HAZ. This paper reviews some recent advances of the reconstitution technique and its applications.
Fractography and failure mechanisms in cfrp tubes submitted to burst testing
2016 - LOBO, RAQUEL de M.; DIAS, ALDISON D.F.; MARINUCCI, GERSON; ANDRADE, ARNALDO H.P. de
The characterization of the failure mechanisms of CFRP components has been expanded as its demand for structural members in several applications increases.Composite tubular elements are used as biaxial structural components in applications where high mechanical performance per unit weight is a vital design requirement.In composite materials it is well known that the interface between fiber and matrix or interphases in those regions influences the overall composite properties.Crack propagation between plies (delamination) is the most commonly observed damage in CFRP since the fracture energy for cracking through the resin layer is lower than that for cracking through the fibers.Composite materials are known to display complex failure modes, typically involving multiple interacting damage processes, occurring over various length-scales. Fiber breaks occur in the micrometer range, whereas delaminations may occur over millimeter to meter scales, depending on the structure´s size and geometry.In recent years, quantitative fractography has become a method by which one can analyze the evolution of the interaction between crack and microstructure.Fracture morphology reveals that the dominant failure surface features were found to be fiber breakage, pull-out, and matrix cleavage and hackle formation resulting from interfacial tensile or shear failure.In the present work, specimens extracted from filament wound tubes subjected to burst test at room temperature were examined by optical and scanning electron microscopies in order to understand the failure mechanisms of such tubes.
Deformation and fracture of an alpha/beta titanium alloy
2010 - ANDRADE, A.; MORCELLI, A.; LOBO, R.
Titanium alloys are used in the aero-spatial, energy and biomaterial industries among others and exhibit high specific strength and fracture toughness. Their mechanical properties show a strong dependence on the microstructure, especially on the size and morphology of the constituent phases. An experimental evaluation was done to a better understanding of that influence, using some techniques like as transmission electron microscopy (TEM), both low and high resolution (HR), scanning electron microscopy (SEM), coupled to electron back-scattering diffraction (EBSD), X-ray diffraction (XRD) and optical microscopy (OM). Some in-situ TEM deformation studies were also done. The alloy was submitted to two heat treatment conditions to get different phases distribution. An hcp phase (alpha) in coexistence with a bcc phase (beta) was observed after both treatments as well the occurrence of twins, stacking faults and dislocations arrangements. The work then, discusses the influence of these features on the overall alloy strength.
Tensile properties characterization of an stainless steel tube for LBB evaluation
2015 - PAES de ANDRADE, ARNALDO H.; CUNTO, GABRIEL G. de; LOBO, RAQUEL de M.; MONTEIRO, WALDEMAR A.

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