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- Caraterização mecânica do aço S 355 a temperatura elevadaPublication . Ramalho, Armando; Antunes, F.V.; Nobre, Telmo; Ferreira, J.A.M.No presente trabalho pretende-se efetuar a caracterização mecânica do aço S 355 AR, EN 10025-4, a alta temperatura. A avaliação rigorosa da variação da tensão de cedência, do módulo de Young, do módulo de encruamento, do calor específico e do coeficiente de dilatação térmica, com a temperatura é essêncial na simulação numérica da distorção e das tensões residuais induzidas por campos térmicos. Embora esta informação esteja disponível em alguns códigos estruturais, como o Eurocódigo 3 parte 1.2, ela não inclui o efeito da composição química e da microestrutura. O aço S 355 AR é um material bastante utilizado em estruturas soldadas, onde a avaliação de distorções e tensões residuais é um problema usual. A caracterização mecânica do aço S 355 AR, EN 10025-4, foi obtida através de ensaios de tração isotérmicos realizados a temperaturas até 600 °C, por Calorimetria Exploratória Diferencial (DSC) realizada até à temperatura de 950 °C e por Análise Termomecânica (TMA) conduzida até aos 1350 °C. Os resultados obtidos diferem significativamente dos valores apresentados na parte 1.2 do Eurocódigo 3, para um aço ferrítico genérico.
- Crack growth in simulated residual stress fields on tungsten inert gas dressed welded joints: a 2d approachPublication . Ramalho, Armando; Antunes, F.V.; Ferreira, J.A.M.In this paper is presented a 2D finite element model to predict the residual stresses generated by Tungsten Inert Gas (TIG) dressing at the weld toe of a T-joint. The welded T-joints, made of S355AR steel, are obtained by covered electrode process and improved with TIG dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress results obtained using X-ray diffraction. Over the estimated residual stress field is simulated the growth of fatigue cracks. The simulation is performed with the virtual crack closure technique, implemented in the MSC.Marc software. The obtained results are compared with the integration solutions of the Paris-Erdogan law, without influence of residual stresses, where the stress intensity factor is obtained using the Mk factor proposed by Bowness and Lee, included in BS 7910 standard. The numerical predictions are also compared with experimental results.
- Measuring the dynamic elastic properties of anisotropic materials to characterize the mechanical behaviour of products manufactured by additive processesPublication . Ramalho, ArmandoThe use of additive processes in the manufacture of prototypes and structural components, or machine parts, had a great growth in the last decade. The use of new materials adapted to the requirements of use, with environmental sustainability and enabling the production of complex geometries, has leveraged additive manufacturing processes. The layer-by-layer slicing sequence of these manufacturing processes are prone to introducing anisotropy into the materials. The mechanical characterization of anisotropic materials through classical tests is not always the most suitable for this purpose, given the economic aspects, the time required, precision requirements and, sometimes, the technological difficulties of the tests. The ASTM E 1876 – 01 standard presents a method for determining the dynamic elastic properties of materials by impulse excitation of vibration, at room temperature. Although the presented methodology is specifically suitable for isotropic materials, it is also mentioned that the methodology can be adopted for the use of anisotropic and non-homogeneous materials, requiring in this case additional care in the interpretation of the results. This speech will present the methodology adopted to obtain the dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio by impulse excitation of vibration, of materials obtained by 3D printing. The obtained properties are used in the simulation of the mechanical behaviour of components under in-service conditions, through the finite element method.
- Magnetoresistive sensors and piezoresistive accelerometers for vibration measurements: a comparative studyPublication . Dionísio, Rogério Pais; Torres, Pedro; Ramalho, Armando; Ferreira, Ricardohis experimental study focuses on the comparison between two different sensors for vibration signals: a magnetoresistive sensor and an accelerometer as a calibrated reference. The vibrations are collected from a variable speed inductor motor setup, coupled to a ball bearing load with adjustable misalignments. To evaluate the performance of the magnetoresistive sensor against the accelerometer, several vibration measurements are performed in three different axes: axial, horizontal and vertical. Vibration velocity measurements from both sensors were collected and analyzed based on spectral decomposition of the signals. The high cross-correlation coefficient between spectrum vibration signatures in all experimental measurements shows good agreement between the proposed magnetoresistive sensor and the reference accelerometer performances. The results demonstrate the potential of this type of innovative and non-contact approach to vibration data collection and a prospective use of magnetoresistive sensors for predictive maintenance models for inductive motors in Industry 4.0 applications.
- A numerical study of non-linear crack tip parametersPublication . Antunes, F.V.; Branco, R.; Correia, Luís M.; Ramalho, ArmandoCrack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the ΔKeff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid.
- Enhanced reconditioning of heavy-duty pulp industry equipment: advantages of abrasive waterjet cuttingPublication . Ramalho, Armando; Silva, Agostinho; Gaspar, Marcelo; Capela, Carlos; Celorrio-Barragué, Luis; Dan, GlăvanWithin a pulp industry production process, a drop in the performance of a wood log rotary debarker was identified. Such loss of performance was due to the occurrence of excessive wear of the device’s cutting blades, requiring their regular and premature replacement. The material used to manufacture the cutting blades, Hardox 500, has characteristics considered adequate for the required use. However, it was concluded that the blade manufacturing process degraded its mechanical properties, leading to its premature wear when placed in service. The present study intends to propose, characterize, and validate an alternative cutting process to manufacture the debarker’s cutting blades. Abrasive waterjet cutting technology was proposed to produce the cutting blades. The suitability of this manufacturing process is discussed considering the specified geometry and the material characteristics of the resulting cutting blades.
- O novo contexto de formação nas instituições de ensino superiorPublication . Ramalho, ArmandoRecentemente o ensino superior politécnico viu reconhecida uma das suas mais justas e ambicionadas aspirações, a faculdade deste subsistema de ensino conferir o grau de Doutor. Retirada a limitação meramente institucional, a ministração de ciclos de estudo de doutoramento passa a estar dependente da existência de ambientes próprios de investigação de elevada qualidade, designadamente considerando os resultados da avaliação das unidades de Investigação e Desenvolvimento, regularmente realizada pela Fundação para a Ciência e a Tecnologia, e a integração alargada dos docentes desse ciclo de estudos em unidades com classificação mínima de Muito Bom na área científica correspondente. Todas as instituições universitárias ou politécnicas, desde que tenham capacidade científica, poderão ministrar cursos de doutoramento.
- Residual stresses analysis in plasma dressed welded jointsPublication . Ramalho, Armando; Ferreira, J.A.M.; Branco, C.M.The presence of residual stresses in a structure influences his behaviour. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses, of flaws and notch sharpness at the weld toe. Plasma dressing at the weld toe is frequently used to remove flaws and to lessen the notch sharpness at the weld toe. With this re-melt, the stress field in the weld toe is also profoundly modified. The Finite Element Method (FEM) proves to be effective to simulate the stress fields generated by welding. The simplified two-dimensional models (2D) are efficient in this estimation. This paper presents a 2D finite element model to predict the residual stresses generated by Plasma dressing at the weld toe of a T-joint. The welded T-joints are made in St 52-3 steel, are obtained by covered electrode process and improved with plasma dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress results obtained using X-ray diffraction
- The influence of layer height in the orthotropic elastic properties of PLA material obtained by additive processesPublication . Gonçalves, Luís; Couto, Gonçalo; Ramalho, ArmandoPolylactic acid (PLA) is a biodegradable thermoplastic polyester used extensively in 3D printing, that can be obtained from renewable resources with low production costs and low carbon emissions. The extrusion temperature of PLA is lower, and its tensile strength and elastic modulus are higher than that of other common polymeric thermoplastic materials. To assess the structural integrity of parts obtained by additive manufacturing, especially in more complex geometries, the finite element method is extensively used, being necessary, for this purpose, to characterize the constitutive model of the material. From the printer manufacturing parameters, one of the most affecting the elastic and strength properties is the layer height. The layer-by-layer slicing sequence of additive manufacturing processes can introduce anisotropy into the materials, whereby, in most applications, materials obtained by these processes are considered orthotropic. The mechanical characterization of anisotropic materials through classical tests is not always the most suitable for this purpose, given the economic aspects, the time required, precision requirements and, sometimes, the technological difficulties of the tests. The ASTM E1876-21 standard presents a method for determining the dynamic elastic properties of materials by impulse excitation of vibration, at room temperature. In this article, the influence of the layer height in the dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio obtained by impulse excitation of vibration (ASTM E1876-21 standard), of Tough PLA is analyzed.
- Simulação do crescimento de fendas em juntas soldadas: influência de campos de tensões residuaisPublication . Ramalho, Armando; Antunes, F.V.; Ferreira, J.A.M.Neste artigo é apresentado um modelo de elementos finitos tridimensional para prever o crescimento de fendas no pé do cordão de juntas soldadas em T. O modelo é desenvolvido sobre o software MSC.marc. A vida de fadiga é estimada por integração da lei de Paris-Erdogan. Os fatores de intensidade de tensão são obtidos pela técnica do fecho virtual de fenda (“virtual crack closure technique”), VCCT, implementada no software MSC.marc. É analisada a influência das tensões residuais geradas por deformação plástica no pé do cordão, na velocidade de propagação da fenda. Observou-se que a existência de campos de tensões residuais de compressão provoca um retardamento do crescimento de fenda.