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Dynamic fracture of metals is a fascinating multiphysics-multiscale problem that often results in brittle and/or ductile fracture of structural components. Additionally, under high strain rates such as impact or blast loads, a failure phenomena known as shear banding may also occur, which is a common precursor to fracture. Both fracture and shear banding are instability processes leading to strong discontinuities and strain localization, respectively. Namely, shear bands are zones of highly localized plastic deformation, while brittle/ductile cracks are material discontinuities due to cleavage and/or void coalescence. Furthermore, while fracture events are mostly driven by triaxial tensile loading, shear bands are driven by shear heating caused by inelastic deformations and high temperature rise. In this work, fracture is modeled through a phase field formulation coupled to a set of equations that describe shear bands. While fracture is governed by a strong length scale that propagates at a fast time scale, shear bands are dominated by a weak length scale and propagate slower. These are two different failure modes with distinct spatial and temporal scales. This thesis is aimed at the development of analytical and numerical methods to determine the onset of both shear band localization and fracture. The main contribution of this thesis is the formulation of analytical criteria, based on the linear perturbation method, for the onset of fracture and shear band instabilities. We first propose a stability framework for shear bands that account for a non-constant Taylor Quinney coefficient. In addition, we apply the linear perturbation method to the phase field formulation of fracture to study the onset of unstable crack growth. The derivations lead to an analytical, energy based criterion for the phase field method in linear elastic and visco-plastic materials. The stability criterion not only recovers the critical stress value reported in the literature for simple elastic cases but also provides a criterion for visco-plastic materials with a general degradation function and fracture induced by cold-work. Finally, we analyze the physical stability of both failure modes and their interaction. The analysis provides insight into the dominant failure mode and can be used as a criterion for mesh refinement. Several numerical results with different geometries and a range of strain rate loadings demonstrate that the stability criterion predicts well the onset of failure instability in dynamic fracture applications. For the example problems considered, if a fracture instability precedes shear banding, a brittle-like failure mode is observed, while if a shear band instability is initiated significantly before fracture, a ductile-like failure mode is expected. In any case, fracture instability is stronger than a shear band instability and if initiated will dominate the response. Another contribution of this thesis is the development of numerical type stability methods based on the discretized model which can be employed within any finite element method. In this approach, a novel methodology to determine the onset of shear band localization is proposed, by casting the instability analysis as a generalized eigenvalue problem with a particular decomposition of the element Jacobian matrix. We show that this approach is attractive, as it is applicable to general rate dependent multidimensional cases and no special simplifying assumptions ought to be made. Furthermore, this technique is also applied to the fully coupled dynamic fracture problem and is shown to agree well with the analytical criteria. Finally, we propose an alternative for identifying the instability point following a generalized stability analysis concept. In this framework, a stability measure is obtained by computing the instantaneous growth rate of the vector tangent to the solution. Such an approach is more appropriate for non-orthogonal problems and is easier to generalize to difficult dynamic fracture problems.

Quantitative relationships among processing parameters, microstructure, and material properties are of considerable interest in the context of development of robust processing routes that optimize the required material properties. As a result, the scientific literature contains a large number of experimental and theoretical studies on microstructure-properties relationships. Fracture sensitive mechanical properties such as ductility, ultimate tensile strength, fatigue life, and fracture toughness depend on the average microstructural parameters as well as the distributions of microstructural parameters and their extrema.Development of quantitative relationships between such material properties and microstructural distributions and extrema has received considerably less attention, particularly in the wrought metals and alloys. Accordingly, an important objective of this research is to perform a systematic investigation in this direction. The dependence of the fracture-sensitive mechanical properties on the microstructural distributions and extrema often leads to substantial variability in these properties: a set of specimens having the same average chemistry, the same average processing history, and the same average microstructural parameters such as volume fractions of different constituents can exhibit substantially different material properties. The present research (i) is concerned with high strength (~ 1000 MPa) high martensite (>50%) dual phase steel where the martensite is a topologically continuous phase (matrix) containing a dispersion of islands of ferrite, and (ii) focuses on understanding the microstructural origins of the variability in fracture sensitive mechanical properties, in particular variability in the room temperature uniaxial tensile ductility. The research involves quantitative microstructure characterization using stereology and digital image processing and quantitative fractography using scanning electron microscopy (SEM) and fracture profilometry. The analysis of the quantitative fractographic and microstructural data obtained in this research leads to useful guidelines for reducing the variability in the tensile ductility of the dual phase steel under investigation.

Orientador: Antonio José Ramírez Londoño / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-17T12:50:46Z (GMT). No. of bitstreams: 1 UnfriedSilgado_Jimy_D.pdf: 31862670 bytes, checksum: f798572d3f744b9a4460a24f795ffa4d (MD5) Previous issue date: 2010 / Resumo: A fratura por queda de ductilidade (FQD) é um tipo de falha que ocorre em temperatura elevada e que afeta adversamente diversos materiais metálicos com estrutura cristalina cúbica de faces centradas (CFC), tais como, ligas de Níquel, Cobre e aços inoxidáveis. Na FQD é observada uma forte redução da ductilidade e a ocorrência de fratura intergranular no intervalo de temperatura homologa entre 0,4 e 0,8, e sob aplicação de esforços de tensão. Diversos autores têm proposto alguns fenômenos metalúrgicos, tais como o escorregamento ao longo dos contornos de grão e a precipitação de carbonitretos e carbonetos, como fatores que influenciam o comportamento da FQD em estrutura bruta de solidificação de ligas de Níquel. Não obstante, o mecanismo fundamental operante neste tipo de fratura ainda não está totalmente esclarecido. Neste trabalho é estudado o mecanismo operante na FQD em função do papel da precipitação de carbonitretos e das características microestruturais na estrutura bruta de solidificação de ligas Ni-Cr-Fe endurecidas por solução sólida. Foram fabricadas e avaliadas cinco ligas experimentais baseadas na composição da liga 690 com e sem adições de Nb, Mo e Hf, as quais foram projetadas com o suporte do método Calphad. A avaliação da FQD foi realizada a través da aplicação iterativa de técnicas de caracterização baseadas em microscopia eletrônica, combinada com a determinação experimental da energia de falha de empilhamento (EFE) usando radiação sincrotron e o uso de ensaio termomecânico in situ em temperatura elevada acoplado a um microscópio eletrônico de varredura (MEV), cujos resultados permitiram realizar mapeamento da deformação a partir de imagens digitais. O teste in situ facilitou o acompanhamento, em tempo real, do fenômeno de FQD dentro do intervalo de temperaturas entre 500 °C e 1000 °C, evidenciando a ocorrência de escorregamento ao longo dos contornos de grão, o que por sua vez permitiu a identificação das etapas do fenômeno com suas respectivas características. Nas ligas experimentais com adições de Nb e Hf foram obtidos contornos de grão fortemente ondulados devido à alta freqüência de precipitados primários intergranulares finos. A adição de Mo nas ligas experimentais juntamente com as adições de Nb e Hf contribuíram para uma forte diminuição da EFE. Os contornos de grão ondulados foram relacionados com o bloqueio mecânico do escorregamento que ocorre ao longo dos mesmos, como sugerido por diversos autores. A presença de Mo na rede cristalina e a baixa EFE contribuíram na restrição da mobilidade de discordâncias em temperaturas elevadas. As características anteriores foram relacionadas com o aumento da resistência a FQD. Finalmente, baseado na evidencia experimental obtida neste trabalho é proposto um mecanismo fundamental de ocorrência da FQD em estruturas brutas de solidificação por soldagem de ligas Ni-Cr-Fe similar ao mecanismo de fluência sem domínio da difusão com escorregamento ao longo dos contornos de grão proposto por Rachinger. / Abstract: Ductility-dip cracking (DDC) is a high temperature fracture phenomenon, which affects several face centered cubic (FCC) metallic materials, such as Nickel alloys, Copper alloys, and stainless steels. DDC is observed as a drastic reduction of ductility that leads to intergranular fracture at homologous temperature range between 0,4 and 0,8 under tensile stress. Diverse theories related to the grain boundary sliding and to carbides and carbonitrides precipitation were proposed to describe DDC behavior in Ni-alloys; however, the fundamental mechanism of DDC is not clear yet. In this work is investigated the fundamental mechanism of DDC, as well as the role of carbonitride precipitates and metallurgical characteristics on this phenomenon in aswelded solid-solution strengthened Ni-Cr-Fe alloys. Experimental alloys were designed by means of Calphad methodology using the alloy 690 chemical composition as the start point. Five compositions with Nb, Mo and Hf additions were subsequently fabricated and evaluated. The DDC evaluation was performed using electron microscopy characterization techniques, experimental measurements of stacking fault energy (SFE) using synchrotron radiation, and a scanning electron microscopy thermo-mechanical in situ test that allows a strain mapping from digital images. The in situ test has allowed obtaining at real time information about of DDC phenomenon on the temperature range between 500 °C and 1000 °C. Evidences of grain boundary sliding (GBS) were obtained through high temperature experiments, consequently allowing the recognition of DDC stages characteristics. Wavy grain boundaries were obtained in Ni-Cr-Fe alloys with Nb and Hf additions due to the high frequency and homogeneous distribution of fine intergranular primary precipitates. Mo, Nb, and Hf additions contributed for a perceptible SFE reduction. Several authors suggested that wavy grain boundaries block GBS, while the Mo presence in the crystal lattice leads to SFE reduction, which is related to the restriction of dislocations mobility at high temperature and to the increase of DDC resistance. Finally, a new fundamental mechanism of DDC is proposed based on experimental evidences for as-welded structures of Ni-Cr-Fe alloys, which is similar to the creep mechanism without diffusion, involving grain boundary sliding mechanism proposed by Rachinger. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

The difficulty in developing bolted connection designs lies in the limitations in existing methods to characterize their strength and typically nonlinear response due to the complex interaction of the bolts and structural components. Yet it is necessary for the engineer to be able to determine the three main connection response characteristics: stiffness, strength, and ductility to account for their influence on the overall structural response behavior. The need for better connection response characterization becomes even more crucial in a performance based design approach or when designing partially-restrained moment frames. Several welded moment resisting frame connections were found to have serious failures following the 1994 Northridge earthquake leading to more interest and research on bolted connections as an alternative. In this study a refined three dimensional nonlinear finite element modeling approach to accurately simulate the response of bolted connections is presented. Sensitivity studies of modeling parameters are also performed. A nonlinear response dataset of over 400 connection cases is generated using this approach with a parametric bolted angle connection model. The use of a parametric Richard-Abbott type function and a neural network, calibrated using the response dataset, as practical tool to model the nonlinear stiffness response of bolted connections under monotonic loading is demonstrated and assessed. Failure criteria that can be practically used in conjunction with the refined three dimensional finite element models without any additional modeling requirements are developed. The stress modified critical strain (SMCS) criterion based on the void growth and coalescence mechanism initiating ductile fracture in steel is used for determining failure in the connection member. The bolt failure criterion developed is a mechanics based model using the elliptical interaction of the tensile and shear capacity envelope. The failure criteria and bolted angle response dataset is combined to assess in detail the impact of geometry and topography of the bolted angle connections on the following response characteristics: strength, initial stiffness, plastic stiffness, and absolute ductility or the displacement capacity. Finally, using the dataset of bolted angle connection response, along with their capacities and failure modes determined using the developed failure criteria, the prying strength models in the AISC LRFD Specifications, Eurocode, and a hybrid model are assessed and found to be very conservative for some cases. Based on these results a modified Eurocode and hybrid prying strength model is proposed which greatly improves the prying strength prediction. These prying models are assessed and verified using experimental data found in literature.