LETICIA DOS SANTOS PEREIRA

LETICIA DOS SANTOS PEREIRA

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Investigação numérico-experimental de modelos de dano em estudo mecânico-energético da fratura dúctil de aços estruturais de alta tenacidade
2024 - PEREIRA, LETÍCIA dos S.
A avaliação de integridade estrutural é importante para a previsão de falhas em estruturas de grande responsabilidade. Contudo, com a melhoria das propriedades dos materiais, algumas teorias não conseguem mais descrever o processo de falha dúctil de materiais de alta tenacidade como o aço API X65. Para melhor compreender a falha em estruturas que utilizam esses materiais é necessário entender o processo de falha dúctil. Nesse sentido, são necessárias simulações não-lineares com trinca propagante para tentar reproduzir os resultados obtidos em laboratório utilizando o programa Abaqus (2020). Os ensaios realizados foram de tração (ASTM, 2022) e Charpy V-Notch (CVN) instrumentado (ASTM, 2018a; ASTM 2018b). Para o estudo de triaxialidade, os corpos de prova de tração foram modificados com diferentes entalhes com raios de 10, 5, 3, 2 e 1 mm. Já os ensaios CVN foram realizados em um pêndulo de 450J e nenhum dos 12 corpos de prova foi rompido, comprovando a alta tenacidade desse material. Por conta dessa alta tenacidade, houve a necessidade de verificação da adequação dos modelos de dano XFEM (Extend Finite Element) e GTN (Gurson-Tveegard-Needleman) para esse material. Além disso, uma metodologia de calibração do modelo de dano dúctil (Ductile Damage) foi proposta utilizando corpos de prova de tração modificados. Esse modelo foi o que melhor reproduziu as curvas experimentais e chegou mais próximo de reproduzir a morfologia de fratura do corpo de prova CVN. Por fim, utilizando uma rotina em Matlab, foi possível obter a energia de dano para todos os corpos de prova. Para esse estudo, foram consideradas as energias elástica e plástica nos elementos onde o dano já se iniciou. Essa parcela de energia, para todos os corpos de prova, é pequena quando comparada com a energia total de deformação, comprovando que utilizar a energia total absorvida em um ensaio CVN superestima a tenacidade à fratura. Além disso, observou-se um balanço entre fratura e colapso plástico. Para os corpos de prova sem entalhe, com tendência ao colapso plástico, a energia de dano é desprezível. Quanto maior a parcela de fratura dúctil, menor a energia de dano absorvida.
Correlations of mechanical properties by SPT (Small Punch Test) and conventional tensile test for Al 6061 – T6
2023 - LOBO, RAQUEL de M.; CASTAGNET, MARIANO; MIRANDA, CARLOS A. de J.; FALOPPA, ALTAIR A.; LIMA, JOSE R. de; MATTAR NETO, MIGUEL; PEREIRA, LETICIA dos S.; ANDRADE, ARNALDO H.P. de
The Small Punch Test (SPT) was development by nuclear industries to analyses mechanical properties of irradiated materials principally by small volume of the samples. This technique intend to evaluate the materials behavior during the time life of nuclear reactors, where yours properties changed by irradiation intensity and exposition time. It is considered an almost ¨non-destructive” method [2] due to small sample volume and its applications are spreading for use in situations where conventional methods do not apply. SPT consists of pressing a sphere, with a diameter equal to 2.5 mm, in a miniaturized sample of circular geometry (diameter d = 8 mm and thickness about 0.5 mm)[1], which has fixed edges, tested in conventional mechanical testing machines with the aid of a device developed for their achievement. In this work, mechanical properties of aluminum (Al 6061-T6) were abstained by two different methods: conventional tensile test and the small punch test (SPT). The SPT results depends on graph interpretations and discussions take place at now. Correlations of results guide us in choosing the most appropriated method for interpreting the force x displacement graph from SPT.
Assessment of the von Mises stresses and stress triaxiality in notches using modified tensile specimens
2023 - PEREIRA, LETICIA dos S.; DONATO, GUSTAVO H.B.; MATTAR NETO, MIGUEL
Complete understanding of the local stress triaxiality and stress concentration is essential to ensuring structural safety of several structures. A combination of mechanical tests with numerical simulations can be used to obtain this information. One way to study stress triaxiality is by modifying the standard tensile test geometry (ASTM E8) with a notch. Based on previous results from the literature, five notches were chosen: 10, 5, 3, 2, and 1 mm. These geometries were tested, and the results were numerically reproduced using the Abaqus/Explicit 2020 software. The models used were a non-linear model with the Gurson-Tvergaard-Needleman damage model to reproduce the failure. The numerical analyses allowed the assessment of the von Mises stress and stress triaxiality near the notch to compare with the standard smooth specimen. Two instants were considered as crack propagation onset; the instant of the maximum von Mises stress in the element at the center of the specimen, where the failure process begins; and the moment of maximum stress in the true stress x true strain curve. For the von Mises stress analysis, the difference between the curves was small. The stress triaxiality is a better variable to visualize the influences of the notch. When the strain is equal to a 0.07 (instant of the maximum force for the standard specimens), for the smaller notches (1 and 2 mm), there is a region where the effective plastic strain is zero. Consequently, the stress triaxiality is larger in this region than in the center. For the crack propagation onset instant, the plastic strain occurs along the whole transversal section. In this instant, the maximum value of stress triaxiality occurs in the center for all specimens. These results demonstrate that the stress triaxiality changes as the strain increases, i.e., varies with time.
Assessment of the von Mises stresses and stress triaxiality in notches using modified tensile specimens
2022 - PEREIRA, L.d.; DONATO, G.H.; MATTAR NETO, M.
Stress triaxiality is important in fracture mechanics to check the safety of several structures. Stress triaxiality is one of the main factors that influence the fracture process of high toughness steels. For example, a ductile fracture tends to be more predominant for a low constrain geometry with less plastic restriction. The configuration and loading of the structural components are different from those of the mechanical test specimens used to obtain the materials fracture properties. So, understanding the local stress triaxiality is essential to ensure structural safety. Combination of tests with numerical simulations is a way to assess this effect. Modifying the standard tensile test geometry (ASTM E8) with a notch causes a change in the stress triaxiality. Based on the literature information, two notches were chosen: 1 and 2 mm. These geometries were tested, and the results were numerically reproduced using a non-linear model with the GTN damage model in the software Abaqus/Explicit 2020. The properties (elastic and plastic) were obtained from the standard specimen. An axisymmetric finite element model was developed considering the symmetry in the specimen longitudinal direction, and a mesh with the smallest element having the dimensions of 0.2x0.4 mm. First, a test speed of 0.015 mm/s was applied in the specimen longitudinal direction and convergence problems occurred. Thus, the speed was increased to 100 mm/s to solve these problems. Finally, the nine GTN damage parameters were calibrated to describe numerically the experimental curve. The stresses were obtained for the centroid of the elements. All the analyses were done for two points, i.e.,. first is the plastic instability point for a standard specimen, and second is the maximum force of the load vs. displacement curve. The numerical results analysis allowed the assessment of the stress field and stress triaxiality near the notch to compare with the standard specimen. The notch influences the stress locally, but, after a short distance, approximately 45 mm in these specimens, the tendency was the same for three geometries. The evaluation of the triaxiality considered the stress in the specimen longitudinal direction and the hydrostatic stress. Before the point of plastic instability (first point), the stress triaxiality is low, practically an uniaxial stress state. To the second point, the stress state is no longer uniaxial. The notch increases the stress triaxiality across the cross-section, and the biggest value occurred in the center of the specimen. These specimens results can help to identify the region affected by the notches in structural components.
The use of miniaturized samples to determine mechanical properties of materials
2022 - LOBO, RAQUEL de M.; CASTAGNET, MARIANO; MIRANDA, CARLOS A. de J.; LIMA, JOSE R. de; FALOPPA, ALTAIR A.; MATTAR NETO, MIGUEL; PEREIRA, LETICIA dos S.; ANDRADE, ARNALDO H.P. de
The caracterization of irradiated materials through the SPT (Small Punch Test) technique uses miniaturized samples, with 8 mm in diameter and 0.5 mm in thickness, which has fixed edges, pressed by a sphere that has a diameter d=2.5 mm[1], tested in convencional mechanical testing machines, with the aid of a device developed for their achivement. This tecnique developed for nuclear industry can be used where conventional methods do not apply because it is considered an almost “non-destructive” method[2]due to the small sample volume. In this work two different devices were developed to perform tests at room and sub-zero temperature. The SPT tests will be carried out on standardized nuclear materials unirradiated (ferritic and stainless steels) for later correlation with conventional mechanical tests. Several mechanical properties will be obtained such as yield stress, tensile strength and fracture properties of the materials such as its toughness.
The effect of mass scaling and speed increase in explicit dynamic simulations using tensile test
2021 - PEREIRA, L.S.; DONATO, G.H.B.; MATTAR NETO, MIGUEL
Small punch tests with a recently developed device in IPEN
2021 - MIRANDA, C.A.J.; ANDRADE, A.H.P. de; CASTAGNET, M.; LOBO, R.M.; FALOPPA, A.A.; MATTAR NETO, M.; LIMA, J.R.; PEREIRA, L.S.
Small punch test devices in development at IPEN aiming to perform tests in RMB hot cells
2021 - MIRANDA, C.A.J.; LIMA, J.R.; FALOPPA, A.A.; ANDRADE, A.H.P. de; MATTAR NETO, M.; CASTAGNET, M.; LOBO, R.M.; PEREIRA, L.S.
Numerical analysis of the small punch test for different theoretical materials
2021 - PEREIRA, L.S.; MATTAR NETO, M.; MIRANDA, C.A.J.; LIMA, J.R.; FALOPPA, A.A.; ANDRADE, A.H.P. de; CASTAGNET, M.; LOBO, R.M.