Plane strain deformations of an elastoplastic material are studied using numerical methods. In the first chapter, a meshless formulation of the static small strain elastic-plastic problem is formulated using the meshless local Petrov-Galerkin method. The code is validated against the small strain plasticity routines in the commercial finite element code ABAQUS for two basic configurations with loading, unloading, and reloading. The results are found to agree within 5%. The validated code is then used to analyze the stress intensity factor (SIF) in a double edge-cracked plate. Deformations of the plate are studied both with and without exploiting the symmetry conditions. The penalty method is used to enforce the essential boundary condition in the former case. When analyzing the deformations of the entire plate, the diffraction method is employed in order to introduce the discontinuity in the displacement field across the crack faces. The log-log and a higher order extrapolation technique due to Dally and Berger (1996) are used to calculate the SIF. It is found that the penalty method was inadequate to enforce the essential boundary conditions in the vicinity of the crack tip and that in this region the deformations were oscillatory. Consequently, the SIF calculation using the higher order technique was not accurate. It is also found that for a small plastic zone (3% of the cracked length) the SIFs do not differ significantly from their values for the corresponding linear elastic problem. In the second chapter, a finite element formulation of the dynamic deformations of a micro-porous thermoviscoplastic solid is formulated. The heat conduction in a material is assumed to be governed by a hyperbolic heat equation; thus thermal and mechanical waves propagate with finite speeds. The formation and propagation of an adiabatic shear band (ASB) inplane strain tensile deformations is studied for eleven materials. The ASB is assumed to form when the maximum shear stress has been reduced to 80% of its peak value at a point and it is deforming plastically. The materials are ranked according their susceptibility to the formation of an ASB. A parametric study of the effect of the initial defect strength where the defect is assumed through an initially inhomogeneous distribution of porosity, the thermal conductivity, the thermal wave speed, and the applied strain-rate upon the ASB initiation and propagation is conducted. It is found that the susceptibility ranking for this configuration differs somewhat from that previously found for simple shear and torsion of thin-walled tubes. It is also found that thermal conductivity influences ASB initiation and propagation only for materials with large values of · and that for such materials an adiabatic model may not be adequate. The effects of initial defect strength and the nominal strain-rates are both found to be consistent with simple shearing studies except that the ASB propagation speed was found to decrease with increasing nominal strain-rate. It is found that the criterion employed for ASB initiation accurately predicts the onset of the collapse of the total axial load applied to the body. In the final chapter, the formulation from the previous chapter is modified to permit the formation and propagation of brittle and ductile fracture. Deformations of the impact loaded double edge-crack specimen of Kalthoff and Winkler (1987) are studied. The brittle to ductile failure mode transition with increasing impact speed was found. Previous studies have focused on identifying the transition speed and did not allow for crack propagation. In this study, crack propagation is achieved through a nodal release algorithm and interpenetration of the crack surfaces is prevented using stiff-spring contact elements. Brittle fracture is assumed to occur when the maximum tensile principal stress achieves a critical value and the ductile fracture is assumed to occur when the effective plastic strain reaches a critical value. It is found that the transition speed for 4340 steel is approximately 54 m/s. For the brittle failure, the stress field is found to be significantly modified by the propagating crack and in the vicinity of the propagating crack the field is mode-I dominant. The crack formed through brittle fracture is found to completely propagate through the plate. For the ductile failure, the distribution of effective plastic strain about the crack tip is not significantly altered by the formation of the crack. The temperature rise in the vicinity of the ductile crack is found to be approximately 45% of the melting temperature of the material. / Ph. D.

Using experimental data obtained from standard fracture test configurations, theoretical and numerical tools are developed to mathematically describe non-self-similar progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in the fracture mechanics literature as the Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture-mechanics-based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and a lower surface that are connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or a mixed mode. The initiation of fracture is determined by the interfacial strength and the progression of the crack is determined by the critical energy release rate. The adhesive is idealized with an interfacial surface material to predict interfacial fracture. The interfacial surface material is positioned within the bulk material to predict discrete cohesive cracks. The interfacial surface material is implemented through an interface element, which is incorporated in ABAQUS using the user defined element (UEL) option. A procedure is established to formulate a rate dependent model based on experiments carried out on compact tension test specimens. The rate dependent model is incorporated into the interface element approach to capture the unstable crack growth observed in experiments under quasi-static loading conditions. The compact tension test gives the variation of the fracture toughness with the rate of loading, this information is processed and a relationship between the fracture toughness and the rate of the opening displacement is established. The cohesive-decohesive zone model is implemented through a material model to be used in an explicit code (LS-DYNA). Dynamic simulations of the standard test configurations for Mode I (Double Cantilever Beam) and Mode II (End Load Split) are carried out using the explicit code. Verification of these coupon tests leads to the crash analysis of realistic structures like the square composite tube. Analyses of bonded and unbonded square tubes are presented. These tubes shows a very uncharacteristic failure mode: the composite material disintegrates on impact, and this has been captured in the analysis. Disadvantages of the interface element approach are well documented in the literature. An alternative method, known as the Extended Finite Element Method (XFEM), is implemented here through an eight-noded quadrilateral plane strain element. The method, based on the partition-of-unity, is used to study simple test configuration like the three-point bend problem and a double cantilever beam. Functionally graded materials are also simulated and the results are compared to the experimental results available in the literature. / Ph. D.

O presente trabalho desenvolve uma formulação do Método dos Elementos de Contorno para análise de problemas tridimensionais de fraturamento no regime transiente. Utilizam-se as soluções fundamentais da elastostática para obter a matriz de massa, empregando-se o Método da Reciprocidade Dual e a discretização do domínio por células tridimensionais. Para a integração no tempo são utilizados os algoritmos de Newmark e Houbolt. O fenômeno do fraturamento é abordado através da consideração de um campo de tensões iniciais, introduzindo-se o conceito de dipolos de tensão. Os tensores desenvolvidos que se relacionam aos dipolos, derivados das soluções fundamentais, são também apresentados. É utilizado o modelo de fratura coesiva. O contorno é discretizado utilizando-se elementos triangulares planos com aproximação linear, e elementos constantes para a superfície fictícia de fraturamento. São feitas várias aplicações cujos resultados obtidos confirmam a importância e a adequação da formulação apresentada para os problemas propostos. / This work presents a Boundary Element Method (BEM) formulation for analysis of three-dimensional fracture mechanics transient problems. Elastostatics fundamental solutions are considered in order to obtain the mass matrix, using both Dual Reciprocity Method and three-dimensional cell discretization. Newmark and Houbolt algorithms are employed to evaluate the time integrals. The fracture effects are captured by using dipoles of stresses, derived from an initial stress field. The tensors related to those dipoles, developed in the present work, are presented. The cohesive crack is the adopted model. Body boundary is discretized though linear flat triangular elements and the fracture surfaces are approximated by constant flat triangular elements. Some applications are processed to show the efficiency of presented BEM formulations.

Os ensaios mecânicos são mundialmente empregados para caracterização e implementação de diversos materiais. Dentre a vasta gama de ensaios existentes, o ensaio de impacto convencional é consagrado no meio científico, industrial e na indústria do óleo e gás onde a sua função básica é determinar a energia total absorvida na fratura de um corpo-de-prova entalhado conforme as recomendações da norma ASTM E23. Entretanto, a energia global absorvida possui um valor de uso muito limitado e ela normalmente não é aceita como um indicador quantitativo da resistência à fratura do material. Assim, com a utilização de uma máquina de ensaios de impacto instrumentada, torna-se possível a aquisição de um maior número de informações obtidas através de um ensaio de impacto clássico, como por exemplo, o estudo detalhado da integridade estrutural de materiais utilizados na fabricação de dutos. Desta maneira, o presente trabalho tem como objetivo avaliar a tenacidade de um duto de aço API 5L X70 sem costura, com 11 polegadas de diâmetro externo e ½ polegada de parede, frente a uma falha dinâmica, através da tenacidade a fratura, comparando carregamento estático com dinâmico através dos resultados de J0,2 e Jm. Entretanto a tenacidade a fratura é consideravelmente afetada para altas taxas de deformação, com redução de aproximadamente 30% nas amostras com orientação de entalhe X-Y (defeito lateral passante) e de aproximadamente 40% nas amostras com orientação de entalhe X-Z (defeito superficial), demostrados através do JΔinit. / Mechanical testing are worldwide employed for materials characterization and implementation. Among the large group of the existing tests, the conventional impact test has been used for industry and academic fields, especially oil and gas engineering in order to evaluate the energy absorbed of a notched sample, according to the ASTM E23 standard. However, the energy absorbed has a limited use and is not usually accepted as a quantitative indicator of the fracture thougness. Thus, by using an instrumented impact test, there will be an increasing in the amount of information obtained than the traditional impact test and, thefore, a detailed study of the structural integrity of materials can be carried out for manufacturing of pipes. Hence, this study aims to characterize a pipe steel API 5L X70 seamless, with 11 inches outside diameter and ½ inche of wall, against dynamic failure through fracture toughness, comparing static loading with dynamic through the results of J0,2 and Jm. However, fracture toughness is considerably affected at high deformation rates, with a reduction of approximately 30% in X-Y notch (through thickness defect) and approximately 40% in X-Z notch (surface defect) samples, demonstrated for JΔinit.

Os ensaios mecânicos são mundialmente empregados para caracterização e implementação de diversos materiais. Dentre a vasta gama de ensaios existentes, o ensaio de impacto convencional é consagrado no meio científico, industrial e na indústria do óleo e gás onde a sua função básica é determinar a energia total absorvida na fratura de um corpo-de-prova entalhado conforme as recomendações da norma ASTM E23. Entretanto, a energia global absorvida possui um valor de uso muito limitado e ela normalmente não é aceita como um indicador quantitativo da resistência à fratura do material. Assim, com a utilização de uma máquina de ensaios de impacto instrumentada, torna-se possível a aquisição de um maior número de informações obtidas através de um ensaio de impacto clássico, como por exemplo, o estudo detalhado da integridade estrutural de materiais utilizados na fabricação de dutos. Desta maneira, o presente trabalho tem como objetivo avaliar a tenacidade de um duto de aço API 5L X70 sem costura, com 11 polegadas de diâmetro externo e ½ polegada de parede, frente a uma falha dinâmica, através da tenacidade a fratura, comparando carregamento estático com dinâmico através dos resultados de J0,2 e Jm. Entretanto a tenacidade a fratura é consideravelmente afetada para altas taxas de deformação, com redução de aproximadamente 30% nas amostras com orientação de entalhe X-Y (defeito lateral passante) e de aproximadamente 40% nas amostras com orientação de entalhe X-Z (defeito superficial), demostrados através do JΔinit. / Mechanical testing are worldwide employed for materials characterization and implementation. Among the large group of the existing tests, the conventional impact test has been used for industry and academic fields, especially oil and gas engineering in order to evaluate the energy absorbed of a notched sample, according to the ASTM E23 standard. However, the energy absorbed has a limited use and is not usually accepted as a quantitative indicator of the fracture thougness. Thus, by using an instrumented impact test, there will be an increasing in the amount of information obtained than the traditional impact test and, thefore, a detailed study of the structural integrity of materials can be carried out for manufacturing of pipes. Hence, this study aims to characterize a pipe steel API 5L X70 seamless, with 11 inches outside diameter and ½ inche of wall, against dynamic failure through fracture toughness, comparing static loading with dynamic through the results of J0,2 and Jm. However, fracture toughness is considerably affected at high deformation rates, with a reduction of approximately 30% in X-Y notch (through thickness defect) and approximately 40% in X-Z notch (surface defect) samples, demonstrated for JΔinit.

Os ensaios mecânicos são mundialmente empregados para caracterização e implementação de diversos materiais. Dentre a vasta gama de ensaios existentes, o ensaio de impacto convencional é consagrado no meio científico, industrial e na indústria do óleo e gás onde a sua função básica é determinar a energia total absorvida na fratura de um corpo-de-prova entalhado conforme as recomendações da norma ASTM E23. Entretanto, a energia global absorvida possui um valor de uso muito limitado e ela normalmente não é aceita como um indicador quantitativo da resistência à fratura do material. Assim, com a utilização de uma máquina de ensaios de impacto instrumentada, torna-se possível a aquisição de um maior número de informações obtidas através de um ensaio de impacto clássico, como por exemplo, o estudo detalhado da integridade estrutural de materiais utilizados na fabricação de dutos. Desta maneira, o presente trabalho tem como objetivo avaliar a tenacidade de um duto de aço API 5L X70 sem costura, com 11 polegadas de diâmetro externo e ½ polegada de parede, frente a uma falha dinâmica, através da tenacidade a fratura, comparando carregamento estático com dinâmico através dos resultados de J0,2 e Jm. Entretanto a tenacidade a fratura é consideravelmente afetada para altas taxas de deformação, com redução de aproximadamente 30% nas amostras com orientação de entalhe X-Y (defeito lateral passante) e de aproximadamente 40% nas amostras com orientação de entalhe X-Z (defeito superficial), demostrados através do JΔinit. / Mechanical testing are worldwide employed for materials characterization and implementation. Among the large group of the existing tests, the conventional impact test has been used for industry and academic fields, especially oil and gas engineering in order to evaluate the energy absorbed of a notched sample, according to the ASTM E23 standard. However, the energy absorbed has a limited use and is not usually accepted as a quantitative indicator of the fracture thougness. Thus, by using an instrumented impact test, there will be an increasing in the amount of information obtained than the traditional impact test and, thefore, a detailed study of the structural integrity of materials can be carried out for manufacturing of pipes. Hence, this study aims to characterize a pipe steel API 5L X70 seamless, with 11 inches outside diameter and ½ inche of wall, against dynamic failure through fracture toughness, comparing static loading with dynamic through the results of J0,2 and Jm. However, fracture toughness is considerably affected at high deformation rates, with a reduction of approximately 30% in X-Y notch (through thickness defect) and approximately 40% in X-Z notch (surface defect) samples, demonstrated for JΔinit.

O presente trabalho desenvolve uma formulação do Método dos Elementos de Contorno para análise de problemas tridimensionais de fraturamento no regime transiente. Utilizam-se as soluções fundamentais da elastostática para obter a matriz de massa, empregando-se o Método da Reciprocidade Dual e a discretização do domínio por células tridimensionais. Para a integração no tempo são utilizados os algoritmos de Newmark e Houbolt. O fenômeno do fraturamento é abordado através da consideração de um campo de tensões iniciais, introduzindo-se o conceito de dipolos de tensão. Os tensores desenvolvidos que se relacionam aos dipolos, derivados das soluções fundamentais, são também apresentados. É utilizado o modelo de fratura coesiva. O contorno é discretizado utilizando-se elementos triangulares planos com aproximação linear, e elementos constantes para a superfície fictícia de fraturamento. São feitas várias aplicações cujos resultados obtidos confirmam a importância e a adequação da formulação apresentada para os problemas propostos. / This work presents a Boundary Element Method (BEM) formulation for analysis of three-dimensional fracture mechanics transient problems. Elastostatics fundamental solutions are considered in order to obtain the mass matrix, using both Dual Reciprocity Method and three-dimensional cell discretization. Newmark and Houbolt algorithms are employed to evaluate the time integrals. The fracture effects are captured by using dipoles of stresses, derived from an initial stress field. The tensors related to those dipoles, developed in the present work, are presented. The cohesive crack is the adopted model. Body boundary is discretized though linear flat triangular elements and the fracture surfaces are approximated by constant flat triangular elements. Some applications are processed to show the efficiency of presented BEM formulations.

This diploma work deals with relationship between composition and mechanical properties of polymeric composite with polypropylene matrix and magnesia hydroxide as a filler. The influence of content of filler (20,40,60 w.%) and temperature (-30°C +60°C) on dynamic fracture toughness is observed. The regresion function describing the dependence of studied mechanical properti on temperature for all experimental materials where found. Experimental data were confront with data gained on the same materials then 10 years ago and influence of time on mechanical properties of follow - up composites is evaluate.

Diversos critérios de tenacidade à fratura translaminar dinâmica foram determinados para o laminado híbrido metal-fibra TiGra, empregando-se conceitos e metodologias da Mecânica da Fratura Elástica Linear MFEL (fator-K) e da Mecânica da Fratura Elasto-Plástica MFEP (integral-J). Verificou-se que as tenacidades de iniciação elasto-plástica, Jid, e de carga máxima, Jmd, do TiGra são controladas pelo desenvolvimento, ou supressão de delaminações. Os resultados indicaram que o emprego deste material se justifica mais pela sua resistência à propagação de danos (caracterizada por Jmd) do que à iniciação da fratura dinâmica (por Jid). De modo geral, os requisitos de validade de Jid como verdadeira propriedade do material (JId) foram satisfeitos, embora para Jmd boa parte das restrições quanto ao tamanho mínimo do corpo-de-prova tenha sido violada. Mais freqüentemente, velocidades mais rápidas de impacto beneficiaram as tenacidades-J do TiGra, enquanto que temperaturas mais elevadas afetaram negativamente estas propriedades. Quanto à MFEL, a tenacidade KJd do TiGra foi beneficiada pelo incremento na taxa de carregamento sob temperaturas mais elevadas, enquanto que a tenacidade Kid foi negativamente afetada pela taxa de deformação em todas as temperaturas avaliadas. Temperaturas mais altas também degradaram as propriedades de tenacidade-K do TiGra. Em oposição às tenacidades-J, os critérios KJd e Kid não satisfizeram em absoluto os mais exigentes critérios de contenção de plasticidade estabelecidos pela MFEL, se comparados aos propostos pela MFEP. Por fim, o desempenho mecânico do laminado TiGra foi severamente comprometido quando do cômputo da densidade específica para a determinação das tenacidades J e K por unidade de massa, sendo nesta ocasião o laminado híbrido facilmente superado por vários laminados convencionais da classe dos Carbono-Epóxi. / Several dynamic translaminar fracture toughness criteria have been determined for TiGr hybrid fiber-metal laminate through Linear Elastic (K-factor) and Elastic-Plastic (J-integral) Fracture Mechanics (LEFM and EPFM, respectively) concepts and methodologies. Instrumented Charpy impact testing was carried out over a wide range of temperatures under two loading rates. It has been discovered that the elastic-plastic initiation toughness, Jid, and the toughness at maximum load, Jmd, of TiGr are controlled by either delamination favoring or suppression. Impact tests revealed that the in-service use of TiGr must rely on its resistance to dynamic fracture propagation (as characterized by Jmd) rather than on fracture initiation (by Jid). In a broad sense, the requirements for Jid data validity as a material property (JId) were fulfilled, whereas many restrictive demands in regard to the minimum testpiece size were violated by the Jmd criterion. Generally, higher impact velocities were beneficial to TiGrs J-toughnesses, inasmuch as higher temperatures impaired these properties. Regarding the LEFM approach, KJd toughness of TiGr laminate was imparted by faster impacts at higher temperatures, whilst the strain rate negatively influenced the Kid toughness over the whole temperature range tested. Higher temperatures also degraded the K-toughness properties of TiGr hybrid laminate. Differently from J-toughnesses values, the KJd e Kid criteria did not satisfy at all the more stringent criteria set forth by the LEFM approach with regard to plastic constraint, as compared to those established by EPFM. Finally, the mechanical performance of TiGr laminate was overwhelmingly compromised as the materials specific gravity was taken in account to obtain K and J toughness values by unit weight, so that TiGr was by far exceeded in this regard by conventional Carbon/Epoxy composite laminates.

A utilização do eletrodo tubular no processo de soldagem a arco submerso leva a um aumento na produtividade com um custo relativamente baixo, pois não requer investimentos complementares em equipamentos. A mudança do eletrodo sólido para o tubular acarreta variações microestruturais no metal de solda que influenciam as propriedades mecânicas, tais como dureza, tração e tenacidade ao impacto. Este trabalho tem como objetivo principal estudar a tenacidade à fratura dinâmica do metal de solda, mostrando que o uso do eletrodo tubular diminui a propagação de trincas ao impacto, além de melhorar as propriedades de dureza e tração. Para a confecção da junta soldada foi utilizado o aço ASTM-A516 com espessura de 37,5 mm; para o metal de solda, o eletrodo sólido AWS EM12K, com diâmetro de 4,0 mm, e fluxo AWS F6A4 e o eletrodo tubular AWS E71T-5, com diâmetro de 4,0 mm, fabricado em caráter experimental, untamente com os fluxos AWS F6A4 e AWS F7A8 (Fluxo Neutro). Os ensaios de impacto, dureza e tração foram realizados em corpos de prova com e sem alívio de tensão, e o ensaio Charpy instrumentado em corpos de prova entalhados e em corpos de prova entalhados e com trinca por fadiga, nas temperaturas de 25, 200, 400 e 600OC. O metal de solda utilizando eletrodo sólido foi o que mostrou menor percentual de ferrita acicular (54%), enquanto que o utilizando eletrodo tubular e fluxo AWS F6A4 foi o que mostrou maior percentual de ferrita acicular (89%). Os ensaios de dureza mostraram uma influência do microconstituinte ferrita acicular: maior dureza para o maior percentual de ferrita acicular. Os limites de resistência e de escoamento também sofreram o efeito deste microconstituinte, apresentando maiores valores quando os percentuais de ferrita acicular foram maiores. Para a análise da tenacidade para o início de propagação da trinca foram utilizados os métodos da variação da taxa da flexibilidade elástica, o método da energia da carga máxima revisada e o método da energia da carga máxima. Foram calculados os valores de JId e de KJd (K derivado de J). Os únicos valores validados, de acordo com a norma, foram os de JId calculados pelo método da variação da taxa da flexibilidade elástica. Os resultados encontrados, utilizando este método, mostraram, à 600OC, a influência da ferrita acicular, sendo que o metal de solda com maior percentual deste microconstituinte forneceu maior valor de tenacidade. Os valores de JId utilizando o método da variação da taxa da flexibilidade elástica para os corpos de prova com trinca por fadiga ficaram próximos aos resultados encontrados quando o ensaio foi realizado em corpos de prova somente entalhados. Os resultados mostraram que a substituição do eletrodo sólido pelo tubular levou à maior tenacidade e a propriedades mecânicas superiores. / The cored wire application in the submerged arc welding process leads to a increase productivity with relative low cost, because it doesn’t require complementary investments in equipments. Changing from solid to cored wire promotes microstructural modification in the weld metal that enhances mechanical properties, such as hardness, stretching and impact toughness. The principal aim of this work is to study the dynamic fracture toughness of the weld metal. The use of cored wire reduces the impact crack propagation, as well as hardness and strength properties. The welded plate was composed of ASTM-A516 steel with 37.5 mm thickness and for making the weld metal was utilized AWS EM12K wire, with 4.0 mm diameter, and AWS F6A4 flux, and AWS E71T-5 cored wire, with 4.0 mm diameter, making in experimental mode, with AWS F6A4 and AWS F7A8 (Neutral Flux) fluxes. Impact, hardness and tensile tests were carried out in specimens with and without stress relief. The instrumented Charpy tests were carried out at notch and notch plus fatigue crack specimens, at 25, 200, 400 and 600OC temperatures. The solid wire weld metal produced 53.9% of acicular ferrite, while the cored wire weld metal and AWS F6A4 flux produced 88.8% of acicular ferrite. The hardness values were influenced by acicular ferrite and showed higher hardness for acicular ferrite higher values. Tensile strength and yield stress data suffered the same effect of this micro constituent also, they had bigger amount when the acicular ferrite percentiles were bigger. For toughness analysis to crack propagation start were utilized the compliance changing rate method, the energy revised method, and the maximum load energy method. JId and KJd (K derived of J) values were calculated according to the standards. The results met for this method had acicular ferrite influence at 600OC, where the weld metal with the biggest micro constituent had bigger toughness value. The JId values utilized the compliance changing rate method for the precracked specimen got values near to the notched specimen. The change solid wire by the cored wire had better toughness, and it gets better the other mechanical properties.