Gláucio Soares da Fonseca

The Feasibility of Structural Health Monitoring Using the Fundamental Shear Horizontal Guided Wave in a Thin Aluminum Plate

Structural health monitoring (SHM) is emerging as an essential tool for constant monitoring of safety-critical engineering components. Ultrasonic guided waves stand out because of their ability to propagate over long distances and because they can offer good estimates of location, severity, and type of damage. The unique properties of the fundamental shear horizontal guided wave (SH0) mode have recently generated great interest among the SHM community. The aim of this paper is to demonstrate the feasibility of omnidirectional SH0 SHM in a thin aluminum plate using a three-transducer sparse array. Descriptions of the transducer, the finite element model, and the imaging algorithm are presented. The image localization maps show a good agreement between the simulations and experimental results. The SH0 SHM method proposed in this paper is shown to have a high resolution and to be able to locate defects within 5% of the true location. The short input signal as well the non-dispersive nature of SH0 leads to high resolution in the reconstructed images. The defect diameter estimated using the full width at half maximum was 10 mm or twice the size of the true diameter.

Strain-Induced Martensite Formation of AISI 304L Steel Sheet: Experiments and Modeling

Metastable austenitic stainless steels are prone to strain-induced martensite transformation (SIMT) during deformation at room temperature, as in the case of sheet metal forming processes. The SIMT is influenced by the chemical composition, grain size, temperature, deformation mode or stress state and strain-rate. In this work, interrupted and continuous uniaxial tensile tests were performed in AISI 304L sheet to evaluate the SIMT as a function of strain and strain-rate effects. The SIMT was evaluated by feritscope and temperature in-situ measurements and both XRD and optical microscopy techniques. The SIMT kinetics was also investigated by means of thermo-mechanical finite element simulations using a phenomenological model. In the small strain range, the yield stress increases with the strain-rate whereas in the large strain domain a cross-effect in the stress-strain curve is observed given that the SIMT is inhibited due to the specimen heat generation. A very good correlation between XRD and feritscope measurements was found from the interrupted uniaxial tensile testing. The finite element numerical simulations allowed to identify the parameters of a phenomenological model which describes the SIMT kinetics of AISI 304L steel sheet as a function of plastic-strain, strain-rate and temperature effects.

Grain Boundary Pinning by Particles

Grain boundary pinning by particles is widely used to prevent grain growth during heat treatment in a variety of commercial alloys. Its practical relevance is matched by a considerable amount of theoretical work that has been devoted this problem. A key issue of boundary pinning is the particle/interface interaction mechanism and its associated pinning force. According to Ashby et al. an interface may interact with a particle in two ways: either it goes through the particles or, more usually, bends round and envelopes the particle. Based on these mechanisms one may derive quantitative expressions relating the characteristics of the particle dispersion and a critical or limiting grain radius. Thus, Zener expression assumes that the interface goes through the particles whereas Rios expression assumes that the interface bends round and envelopes the particle. Both the mechanisms and the resulting expressions are discussed here in detail and compared with available experimental data.

Effects of Heat Input Conditions on the Local Thermophysical Properties of Super Duplex Stainless Steels

The thermal properties of the super duplex stainless steels are strongly affected by the thermal history when welding procedure are applied leading to substantial changes on the mechanical properties of the welding region. The controlled dual phase microstructure (ferrite and austenite) guarantee excellent mechanical properties such as mechanical strength and corrosion resistance, in addition to small thermal expansion coefficient and high thermal conductivity. In this research a model able to predict the thermal history of the welding pieces coupled with local mechanical properties developed during welding procedure is developed. The model was verified by measured temperature profile and used to predict local properties such as grain size evolution, hardness and mechanical strength. An inverse method was implemented to obtain the parameters fitting for the grain growth evolution, hardness and yielding strength compatible with the final microstructure and grain size measured using SEM images and stereological techniques.

Correlation between Microstructure and Mechanical Properties of Austempered Ductile Irons

Austempered ductile cast irons (ADI) have received great attention in last years because their combined properties of good ductility, high strength and fracture toughness, good fatigue strength, good wear properties and low production cost. Such combination of properties can be reached because of their microstructures consist of a mixture acicular ferrite (bainite), residual austenite with a high carbon content and nodular graphite. In this work, the effect of austempering heat treatment on the microstructure of a commercial alloy to produce three different grades of ADI, with different strength level, is analyzed. Microstructure characterization has been performed using techniques of optical microscopy, scanning electron microscopy and x-ray diffraction. Mechanical properties were evaluated from tensile and impact tests at room temperature. In addition, the residual stress due to heat treatment was evaluated. The results of this study show that there is a strong relationship between austempering temperatures and mechanical properties. The highest tensile and yield strength obtained were 1599 and 1427 MPa, respectively, for the sample austempered at 280°C. The sample austempered at 320°C presented the highest Charpy absorption energy (99,90 J) and highest volume fraction of austenite (27%).

Analysis of the Iron Ore Pellet Mechanical Behavior under Biaxial Compression

The increasing global demand for iron ore pellets has made the pelletizing companies to step up their investments. The mechanical strength of the pellets, as well as its wear resistance are important factors to characterize the mechanical behavior. These properties are influenced by the type and nature of the ore or concentrate, the additives and the subsequent heat treatment used. This paper develops a numerical finite element model in order to characterize the mechanical behavior of iron ore pellets. The biaxial compressive stress was analyzed in this study. The results show that the pellet subjected to biaxial stress supports higher levels of stress and strain when compared to uniaxial efforts. Accuracy increase of the simulation results can be obtained with the implementation of a failure criterion for brittle materials in the numerical model. Finally, could be seen that the pellet had higher levels of deformation under biaxial symmetric strain, when compared with the uniaxial compression results.

ANÁLISE TERMODINÂMICA E EXPERIMENTAL DA FORMAÇÃO DA FASE SIGMA EM UM AÇO INOXIDÁVEL DUPLEX SAF 2205

Ana Carolina Martins Silva Fabiane Roberta Freitas da Silva Wesley Luiz da Silva Assis Gláucio Soares da Fonseca Resumo Os aços inoxidáveis dúplex ferríticos-austeníticos fazem parte de uma classe de materiais com microestrutura bifásica, composta por uma matriz ferrítica e ilhas de austenita, com frações volumétricas aproximadamente iguais dessas fases. Essa classe de materiais é caracterizada por apresentar interessante combinação de elevadas propriedades mecânicas e de resistência à corrosão e, por isso, é considerada bastante versátil, sendo utilizados nas indústrias química e petroquímica, de papel e celulose, siderúrgicas, alimentícias e de geração de energia. Este trabalho objetiva caracterizar as microestruturas do Aço Inoxidável Duplex SAF 2205, revelando as fases que se apresentam após tratamento térmico a 800oC, em diferentes tempos, estudar a relação entre a dureza do material e a precipitação da fase sigma, além de determinar a cinética de precipitação dessa fase. Após obtidos os dados experimentais, dados de VV e composição de equilíbrio da fase sigma foram obtidos via Thermo-Calc. Ao final do trabalho, os resultados experimentais e de simulação são comparados e conclui-se que a nucleação da fase sigma é por difusão de Cromo na matriz. Palavras-chave: Aço Inoxidável duplex; fase sigma; cinética; Thermo-Calc.

Evaluation of Martensite Fraction in 1026 Steel by Infrared Thermography Combined with the Koistinen-Marburger Model

In the present work, the martensite formation during heat treatment of 1026 steel was studied in order to acquire process knowledge and reinforce the effectiveness of infrared thermography method to evaluate the temperature distributions. Several tests were carried out and monitored by an infrared camera and thermocouples. Martensite fraction was evaluated with the aid of the Koistinen-Marburger model and adequate parameters describing phase transformations were obtained for 1026 steel samples. This research revealed the need of model adjustment in order to accurately describe the martensite transformation kinetics according to experimental results.

Three-Dimensional Reconstruction of the Porosity of Pellets of Iron Oxide and Coal Powders by Serial Sectioning

The self-reducing agglomerates produced from powders generated within the electric arc furnace and LD converters is of special interest in process of recycling due to the amount of iron and other metals of high economical value. However, the reducibility and inner pore structures play important role on the processing technology of these materials. Aiming at investigating the influence of the agglomerate structure, some two-dimensional metallographic techniques have been used to evaluate the porosity in clusters with inaccurate results, essentially due its three-dimensional features. From the processing technological point of view, the shape and distribution of inner porosity of the powders agglomerates are of fundamental importance due to their effects on the reaction rates involving the present phases taking place into the reduction stage. In this study a 3D serial sectioning imaging reconstruction is proposed to determine local inner porosities and detailed measurements of parameters of pores connections and tortuosities. The averaged porosities results are discussed and compared with traditional measurements based on pycnometry method.