Este trabalho apresenta uma avaliação dos efeitos de juntas soldadas dissimilares sobre forças motrizes de trinca e o desenvolvimento de procedimentos adaptados baseados em critérios mecânicos e micromecânicos para avaliação de defeitos em juntas soldadas incorporando efeitos de dissimilaridades mecânicas. A motivação central reside na forte expansão da indústria nacional de petróleo e a crescente demanda por aumento de produtividade aliado ao aumento na segurança operacional de sistemas dutoviários e estruturas correlatas. Para que tal intento seja atingido, é inicialmente apresentada uma descrição detalhada dos parâmetros de interesse da mecânica da fratura, fundamentos da metalurgia da soldagem e efeitos microestruturais e mecânicos oriundos da presença de soldas dissimilares. São então descritos os principais métodos experimentais de determinação de tenacidade à fratura, detalhados os procedimentos correntes de avaliação de integridade por meio de diagramas FAD e é estudada a teoria da tensão de Weibull. Tais estudos configuram o arcabouço conceitual necessário para o desenvolvimento de uma extensa variedade de ensaios experimentais em juntas soldadas e proposição de correções aos procedimentos correntes de determinação experimental de tenacidade, refinamentos na determinação de cargas limite e o desenvolvimento de um modelo de transferabilidade de tenacidades baseado em tensão de Weibull aplicável a juntas soldadas. Por fim, todas as contribuições são compiladas e incorporadas em um procedimento adaptado do tipo FAD especificamente adaptado à avaliação de estruturas soldadas constituídas de materiais dissimilares. Os resultados experimentais revelam forte e deletéria influência das juntas soldadas na tenacidade e capacidade de carga de estruturas. Por outro lado, o extenso compêndio de fatores eta e rotacionais plásticos rp desenvolvido permite a determinação experimental acurada de tenacidade à fratura para espécimes SE(B) homogêneos e bimateriais. Adicionalmente, a metodologia proposta de determinação de cargas limite incorpora o encruamento dos respectivos materiais e o modelo de transferabilidade baseado em tensão de Weibull permite a estimação acurada da tenacidade de componentes soldados dissimilares a partir dos dados do metal de base e poucas calibrações experimentais. Resultados de uma aplicação exploratória real indicam elevado grau de precisão e corroboram a validade das propostas apresentadas. Assim, os desenvolvimentos do presente trabalho mostram-se como potenciais ferramentas de projeto e avaliação de integridade estrutural. / This work presents an evaluation of weld strength mismatch effects on crack driving forces and proposes an extension of defect assessment procedures for welded structures to include effects of weld strength mismatch. The main motivation is based on the strong expansion of Brazilian petroleum industry and the increasing demand for productivity, safety and reliability of pipelines and correlated structures. In this context, it is initially presented a detailed description of fracture mechanics parameters, welding metallurgy fundamentals and mechanic and microestructural effects due to mismatched welds. Next, the main procedures for experimental evaluation of fracture toughness are addressed followed by a discussion on current integrity assessment standards based on FAD and the Weibull stress methodology. These studies provide the necessary background to conduct a wide variety of experimental tests in welded joints and the proposal of corrections for experimental fracture toughness evaluation in welded joints, corrections for limit load estimation schemes in mismatched structures and a new toughness scaling model based on Weibull stress applicable to welded mismatched joints. Finally, the resulting framework is incorporated into a modified FAD-based procedure specifically designed for integrity assessment of welded mismatched structures. The experimental results reveal strong deleterious effects of mismatched welds on loading capacity and fracture toughness of the analyzed specimens. However, the extensive body of calibrated eta and plastic rotational rp factors allows accurate experimental determination of fracture toughness for homogeneous and bimaterial SE(B) specimens. Additionally, the limit load estimation scheme proposed incorporates the materials hardening capacity while the Weibull-based toughness scaling model incorporating mismatch effects allows accurate estimation of the weldment toughness from base metal data and a simple experimental calibration. Results from an exploratory application indicate a high degree of accuracy and confirm the validity of the proposed methodology as a potential tool for design and integrity assessments of cracked weldments.

O objetivo desta Dissertação foi analisar o efeito do tratamento térmico de alívio de tensões pós-soldagem sobre a microestrutura e comportamento mecânico de soldas produzidas com arame tubular rutílico AWS E-81T1 Ni1. Foi soldada uma junta com 60 mm de espessura, na posição vertical e progressão ascendente. Esta junta foi dividida em três partes, sendo uma mantida como soldada e as outras duas tratadas termicamente por 6 horas em diferentes temperaturas, 585 °C e 650 °C. Avaliou-se o efeito do tratamento térmico sobre as propriedades mecânicas por meio de ensaio de dureza, tração e impacto Charpy-V. A caracterização microestrutural foi realizada por microscopia óptica e microscopia eletrônica de varredura. As observações experimentais mostraram uma diminuição da tenacidade ao impacto no material tratado termicamente, com uma tendência para uma queda maior com o aumento da temperatura do tratamento térmico. Este comportamento pode ser atribuído principalmente à precipitação e crescimento de carbonetos. Não foi observada a presença de mecanismos de fratura intergranular. Provavelmente o metal de solda estudado não possuía nível de impurezas suficiente para a ativação deste mecanismo. / The objective of this Dissertation was to analyze the effect of postweld heat treatment on microstructure and mechanical behavior of welds obtained by flux cored wire AWS E-81T1 Ni1. It was welded a joint with 60 mm thickness, on vertical position and ascendant progress. This joint was divided in three parts, one maintained as welded and two others heat treated for 6 hours on different temperatures, 585 °C and 650 °C. The effect of the heat treatment on mechanical properties was evaluated by means of hardness, tensile and impact Charpy-V tests. The characterization of the microstructure was undertaken utilizing optical and scanning electron microscopy. The experimental results revealed a decrease of impact toughness in heat treated material, with a tendency to a low impact toughness for high heat treatment temperature. This behavior may principally be attributed to an increase precipitation and growth of carbides. No intergranular fracture mechanism was observed. Probably the level of impurities in the weld metal studied was not high enough to provoke such phenomena.

O presente trabalho investiga a soldabilidade dos aços ARBL (Alta Resistência e Baixa Liga) API 5L X80) e a tenacidade da ZAC (Zona Afetada pelo Calor), assim como a sua correlação com o ensaio CTOD (Crack Tip Opening Displacement) nos processos de soldagem Eletrodo Revestido estudado por Silva [2009] e GMAW-STT® (Gas Metal Arc Welding Surface Tension Transfer®) no passe de raiz com FCAW (Flux Core Arc Welding) para os passes de enchimento e acabamento. Foram utilizados corpos de provas SENB em chapa com espessura de 19 mm e chanfro em ½ V, onde ficaria mais fácil a localização do entalhe na região de grãos grosseiros na zona afetada pelo calor (parte reta do chanfro). Ensaio de tração e impacto Charpy entalhe com 1/2 V foram utilizados para determinar as propriedades mecânicas e de impacto do material testado. Foram comparados os processos SMAW (Eletrodo Revestido), muito utilizado para a soldagem de gasodutos e oleodutos, com os processos GMAW-STT® e FCAW, que têm maior produtividade do que o SMAW. A soldabilidade dos aços ARBL está associada às regiões da ZAC (Zona Afetada pelo Calor) que, por sua vez, encontra-se correlacionado com o processo de soldagem, composição química e com os parâmetros de soldagem utilizados durante o processo de solda, sendo que a qualificação dos procedimentos geralmente não exige o ensaio de tenacidade, como o CTOD. / This work studies HSLA steels (High Strength Low Alloy) API 5L X80 weldability, HAZ (Heat Affected Zone) fracture behavior and their relationship with the CTOD test (Crack Tip Opening Displacement) on the welding processes SMAW (Shielded Metal Arc Welding) studied by Silva [2009] and GMAW-STT® (Gas Metal Arc Welding Surface Tension Transfer®) in the root pass with FCAW (Flux Core Arc Welding) others welding passes. There were used 19 mm width SENB plate samples with a half V groove, where would be easier to locate the crack in the HAZ coarse grain region. Conventional tensile and Charpy V-Notch were performed to determinate mechanical and impact properties of the tested materials. SMAW process, widely used for gas and oil pipelines welding, was compared with GMAW and FCAW processes which have a better productivity than the first one. The HSLA steels weldability is related with HAZ region which also depends on welding process, its parameters and material chemical composition. Normally is not necessary to perform toughness test as CTOD test to evaluate the procedures.

This document describes work performed related to the determination of how elemental additions affect the interfacial fracture toughness of thermal barrier coatings at the bond coat/thermally grown oxide interface in gas turbines. These turbines are exposed to cyclical thermal loading, therefore a simulation was designed to model this interface in a temperature cycle between 200 K and 1000 K that included oxide growth between 2 μm and 27 μm. The fracture toughness of this interface was then determined to elucidate the function of elemental additions. It was shown that minimal concentrations of atomic species, such as hafnium and yttrium cause notable increases in the toughness of the bond coat/thermally grown oxide interface, while other species, such as sulphur, can dramatically reduce the toughness. Furthermore, it was shown that, contrary to some empirical results, the addition of platinum has a negligible effect on the fracture toughness of this interface.

Fatigue crack growth behavior of AA7050 T7451 aluminum alloy under mode II loading condition in two orientations was investigated. Compact shear specimens were prepared in TL and LT directions. A loading frame for mode II type of loading was manufactured. Using the loading frame and the specimen, KIIC values and mode II fatigue crack growth rates were calculated. Fractographic analysis of the fracture surfaces of both mode II fracture toughness test specimens and mode II fatigue crack growth test specimens were done to examine the effects of mode II load. KIIC values were measured between 1.3 and 1.5 times the KIC values for this alloy. As for mode II fatigue crack growth rates, TL orientation shows the highest mode II fatigue crack growth resistance.

For roughly fifteen years the military has sought to use the properties of ceramics for armor applications. Current high-performance ceramics have extremely high compressive strengths and low densities. One ceramic that has been shown to be highly resistant under ballistic impact is silicon carbide (SiC). It has been found that even within the silicon carbides, those manufactured by certain methods and those with certain microstructural properties have advantages over others. In order to understand the microstructural reasons behind variations in ballistic properties, plate impact tests were conducted on two sintered silicon carbides with slightly different microstructures. Two variations of a silicon carbide with the trade name Hexoloy SA were obtained through Saint Gobain. Regular Hexoloy (RH) and Enhanced Hexoloy (EH) are pressureless sintered products having exactly the same chemistries. EH went through additional powder processing prior to sintering, producing a final product with a slightly different morphology than RH. Samples of each were characterized microstructurally including morphology, density, elastic wavespeeds, microhardness, fracture toughness, and flexure strength. The characterization revealed differences in porosity distribution and flexure strength. It was determined that the porosity distribution in EH had fewer large pores leading to an 18% increase in flexural strength over that for RH. The focus of the mechanics of materials community concerning dynamic material behavior is to pin down what exactly is happening microstructurally during ballistic events. Several studies have been conducted where material properties of one ceramic type are varied and the dynamic behavior is tested and analyzed. Usually, from one variation to the next, several properties are different making it hard to isolate the effect of each. For this study, the only difference in the materials was porosity distribution. Plate impact experiments were conducted at the Army Research Laboratory (ARL) using the gas gun facilities within the Impact Physics Branch. A VISAR was utilized to measure free surface velocities. Tests were performed on each material to determine the Hugoniot Elastic Limit (HEL) and spall strength. Spall strength was measured as a function of impact stress, and pulse duration.

No-flow underfill materials that cure during the solder reflow process is a relatively new technology. Although there are several advantages in terms of cost, time and processing ease, there are several reliability challenges associated with no-flow underfills. When micron-sized filler particles are introduced in no-flow underfills to enhance the solder bump reliability, such filler particles could prevent the solder bumps making reliable electrical contacts with the substrate pads during solder reflow, and therefore, the assembly yield would be adversely affected. The use of nano-sized filler particles can potentially improve assembly yield while offering the advantages associated with filled underfill materials. The objective of this thesis is to study the thermo-mechanical reliability of nano-filled epoxy underfills (NFU) through experiments and theoretical modeling. In this work, the thermo-mechanical properties of NFUs with 20-nm filler particles have been measured. An innovative residual stress test method has been developed to measure the interfacial fracture toughness. Using the developed residual stress method and the single-leg bending test, the mode-mixity-dependent fracture toughness for NFU-SiN interface has been determined. In addition to such monotonic interfacial fracture characterization, the interface crack propagation under thermo-mechanical fatigue loading has been experimentally characterized, and a model for fatigue interface crack propagation has been developed. A test vehicle comprising of several flip chips was assembled using the NFU material and the reliability of the flip-chip assemblies was assessed under thermal shock cycles between -40oC and 125oC. The NFU-SiN interfacial delamination propagation and the solder bump reliability were monitored. In parallel, numerical models were developed to study the interfacial delamination propagation in the flip chip assembly using conventional interfacial fracture mechanics as well as cohesive zone modeling. Predictions for interfacial delamination propagation using the two approaches have been compared. Based on the theoretical models and the experimental data, guidelines for design of NFUs against interfacial delamination have been developed.

Semi-circular specimens (SCB) under three point-bending which are commonly used for fracture testing of rocks were used here for fracture mechanics tests. A total of 65 specimens were tested by using Ankara andesite rock. Investigations including the effects of initial notch thickness, different loading span ratios (S/R), flattened loading end, and little dimensional variations when preparing the specimens were carried out. Stress intensity factors for specimens with different geometries were computed individually by using a 3D finite element program ABAQUS. Specimens with a preliminary notch thickness varying from 0.84 to 3.66 mm were tested under three point bending. For a second group of specimens loading span was changed and fracture toughness variation was studied. Another change in the specimen geometry was made by machining a flat loading end at the upper load application point. Fracture toughness values were computed using the stress intensity values computed from numerical modeling and failure loads from the experiments. It was found that up to 2 mm fracture toughness was not affected by variations in the thickness of preliminary notches. Fracture toughness was not affected by changing the loading span. For specimens with flat loading ends, fracture toughness was about 16% lower than the value found from regular SCB type specimens loaded at a point at the top by a steel roller. As a result of about 46 experiments average fracture toughness of Ankara G&ouml / lbasi andesite was found as 1.36 MPa .

Modified Ring specimens are of the shape of discs having a hole inside and flattened ends. These specimens are used for determination of Mode I fracture toughness. Finite element program, named ABAQUS, is used for numerical modeling for finding stress intensity factors. Varying disc geometries were used for the experiments and numerical modeling in which size of the flat ends, radius of the hole inside, and external radius of the specimen were varied. Experiments were done by using pink Ankara andesite. Effects of internal hole radius, external disc radius and size of the flat ends on both stress intensity factor and fracture toughness were studied. In order to compare the results, fracture tests with semi-circular specimens under three point bending (SCB) were also performed. From a similar previous study, fracture toughness values of gray andesite were recalculated and compared to the fracture toughness values of pink andesite for varying geometrical factors. Size effect studies were performed as well for varying diameter of core specimens.Fracture toughness values of andesite were found to increase with increasing specimen size. Fracture toughness of 100 mm specimens was determined as 1.11&plusmn / 0.07 MPa&amp / #8730 / m, whereas fracture toughness of 75 mm specimens was 0.96&plusmn / 0.08 MPa&amp / #8730 / m. 100 mm or larger diameter specimens were suggested for the fracture toughness determination with the modified ring tests.

Fracture toughness is an important rock property for rock fracturing and fragmentation applications. Theory and practice of opening mode (mode I) and shearing mode (mode II) fracture toughness tests are still in a developing stage for the cylindrical rock cores. A new circular plate type test specimen is used for mode II fracture toughness testing on rock cores. This involves a straight edge notched circular plate type core disc geometry under three-point bending load / new method and its associated specimen geometry is referred as straight edge notched disc bend (SNDB) specimen under three-point bending. Mode II fracture toughness results of the tests with this new geometry were compared to the results of the tests commonly employed for mode II fracture toughness testing. Specimen geometries were modeled and mode II stress intensity factors were computed by finite element modeling using ABAQUS program. For comparison purposes, mode II or shearing mode fracture toughness KIIc of two different rock types were determined by different testing methods commonly employed in recent practice. Core specimens of Ankara andesite and Afyon marble rock types were tested with cracked chevron notched Brazilian disc and cracked straight through Brazilian disc specimens under Brazilian type loading, semi-circular bend specimen and straight edge notched disc bending specimen geometries under three-point bending.For all testing groups, cylindrical cores with diameters varying from 7.5 cm to 12.5 cm were prepared with notch lengths changing from 1.5 cm to 2.6 cm. Effect of specimen thickness on mode II fracture toughness was investigated for three different testing methods. Fracture toughness values remained constant when thickness of the specimens was increased for cracked straight through Brazilian disc, semi-circular bend and straight notched disc bend methods. For cracked straight through Brazilian disc method KIIc values of Ankara andesite and Afyon marble were 0.99 MPa&radic / m and 0.86 MPa&radic / m, respectively. Mode II fracture toughness with semi-circular bend specimen was 0.43 MPa&radic / m for andesite and 0.46 MPa&radic / m for marble. When the results of the two three-point bending type tests were compared straight notched disc under three-point bending resulted in higher KIIc values (0.61 MPa&radic / m for andesite and 0.62 MPa&radic / m for marble) than the results found by semi-circular bend tests.