This study investigates the reliability of SnPb and SnAg solder joints in semiconductor packages subjected to thermal cycling. More specifically, solder joint crack growth and life are experimentally measured, and FEM models are run to explain the test results. Ultimately a life-prediction model is proposed for both SnPb and SnAg solder joint packages. Motorola 357-plastic ball grid array packages on printed wiring boards were thermal cycled with the following test parameters: SnPb and SnAg solders, three post-process conditions (aged, aircooled and quenched), four package layouts on the printed circuit boards (singledense, single-sparse, double-alternating, and double-dense), three accelerated thermal cycling protocols (0°C to 100°C, -40°C to 125°C, and -55°C to 125°C), and tests run at Motorola and the University of Texas. At predetermined thermal cycles, packages were removed from the environmental chambers, dyepenetrated, packages removed to expose the solder joints, and optical images taken. Images were processed to measure crack area, shape, orientation and length to show crack growth. Selected joints were sectioned and polished to investigate microstructure and failure modes. Selected boards were connected to an ANATECH event detector to monitor life from joint failures. FEM crack initiation and propagation models were developed to better understand failure mechanisms. Major experimental results are: 1) SnPb joints have about 50% faster crack growth rates than SnAg joints, subsequently SnPb joints have half the life of SnAg joints, 2) air-cooled and quenched packages had similar failure characteristics, but aged SnPb joints had lower life and aged SnAg joints had significantly longer life than the comparable nonaged joints, 3) double-dense package layout significantly decreased life (by 75%) over the other package layouts, which were similar to each other, 4) the test results at the two locations (UT and Motorola) were similar for SnPb solder joints, but significantly different for SnAg solder joints, and 5) the largest cracks occurred at the corners of joints just under the die edge. Major FEM simulation results are: 1) the crack initiation life of SnAg joints is approximately 100% longer than SnPb joints, 2) shear load is a major cause of crack growth, but the contribution of tensile load increases as the cracks grow, 3) primary cracks at the board interface appear to reduce the propagation rate of the primary crack on the package interface, 4) secondary cracks are suppressed when compressive stresses prevent voids from nucleating, 5) the double-dense configuration shows no PWB warping due to symmetry, and its stresses are larger than for the other package layouts, and (6) the stresses and strains for single-dense, single-sparse, and double-alternating package layouts are similar because the stresses/strains are dominated by local effects due to the CTE mismatch between the die and board. Based upon the experimental results and FEM simulations, a lifeprediction model based upon a severity metric was proposed. The metric estimates damage to the solder joints and links material properties and parameters associated with package layout and thermal test conditions to the time-dependent creep, time-independent plastic deformation, and a time-dependent and geometric effective stress of the solder. The severity metric predicted life very well for most of the data tested and was more accurate than the industry-standard life-prediction models for SnPb solder joints.

The development of structural monitoring systems is a critical research area because of the age and sustainment costs associated with many aircraft in use today. Specifically, integrated structural health monitoring (SHM) systems are advantageous because they allow for automated, near real-time assessment of the state of the structure, where the automation improves both the accuracy of the measurements and allows for more frequent system interrogation than possible with traditional nondestructive evaluation methods. Ultrasonic techniques are particularly well-suited for SHM systems because of their potential to detect and track damage well before structural failure using in situ sensors. The research problem considered in this thesis is detection and tracking of fatigue cracks emanating from fastener holes in metallic structural components. The sensing method utilizes attached ultrasonic transducers, and tracking of damage is achieved by employing a state estimation framework that incorporates a well-known empirical model for crack growth and a measurement model relating the ultrasonic response to crack size. The state estimation process is preceded by an automated crack detection algorithm, and can be followed by a prediction of remaining life assuming future usage. The state estimation framework provides a better estimate of crack size than either the ultrasonic measurement model or crack growth model alone. Although the example application is monitoring of fastener holes, the general approach is applicable to a variety of SHM problems.

Devido à resistência dos fabricantes de automóveis quanto à utilização de engrenagens de FNA estudos tem sido feitos para averiguar o desempenho deste material em engrenagens. Dentro deste contexto esta dissertação tem como objetivos principais o desenvolvimento e construção de uma máquina de ensaios de fadiga de contato em engrenagens e a comparação quanto à resistência ao desgaste de engrenagens cilíndricas de dentes retos fabricadas em aço liga 8620 (cementado e temperado) e em ferro fundido nodular austêmperado (FNA). Estas engrenagens foram fornecidas em dois estados de acabamento superficial (shaving ou por fresamento). O equipamento desenvolvido funciona com o princípio de recirculação de potência e é comumente chamado de FZG. Os experimentos de desgaste têm sido feitos com dois estágios de torque: 135 N.m (running-in) e 302 N.m (steady-state) e as engrenagens são de perfil modificado. Após a realização dos ensaios, foi possível verificar que: (1) a área danificada dos flancos, devido ao fenômeno de pitting, das engrenagens de FNA foi maior do que das de aço AISI 8620; (2) a comparação entre os parâmetros de rugosidade nos estágio de fornecimento e após cada estágio do experimento mostra que as engrenagens com acabamento por shaving apresentam uma melhor distribuição da carga e um menor desgaste, (3) o regime de lubrificação na região do diâmetro primitivo durante os ensaios foi o limite (ou limítrofe) para ambos os processos de fabricação, porém o parâmetro de espessura de filme (λ) é maior para engrenagens fabricadas por shaving, fazendo com que a espessura do filme lubrificante seja maior, (4) o efeito da redução da área resistente aos esforços de contato, devido à presença dos nódulos de grafita, na resistência a fadiga foi evidenciado, porém o acabamento superficial mostrou-se mais relevante do que a distribuição e tamanho dos nódulos. / Due to the resistance of automotive manufactures on apply ADI gear studies have been made to verify the performance of this material. Inside this context this work has a main objective the construction of contact fatigue test machine in gears and the comparison, related to the wear resistance, between spur-gears made of carburized and quenched AISI 8620 low steel and austempered ductile iron (ADI). These gears were supplied in two states of finish surface (by shaving or by machining). The developed equipment functions with the principle of the power recirculation and commonly called FZG. The wear experiments were performed in two stages of torque: 135 N.m (running-in) and 302 N.m (steady-state) and the gears have modified profile. After the accomplishment of the experiments, was possible verify that: (1) The damaged area of flanks, due to the phenomenon of pitting, at ADI gears was bigger than AISI 8620 low-alloy steel gears. (2) The comparisons between the roughness parameter in the supply state and after each period of the experiment shows that gears with surface finish by shaving have better load distribution and minor wear. (3) Lubricant regime at pitch diameter was the boundary for both manufacture process, but the film thickness parameter λ is higher for shaved gears, making the lubricant film thickness has major value, (4) The reduction effect the resistive area of contact due the presence of the graphite nodules in the contact fatigue resistance was evidenced, however surface finishing was more relevant than the distribution and size nodules.

Devido à resistência dos fabricantes de automóveis quanto à utilização de engrenagens de FNA estudos tem sido feitos para averiguar o desempenho deste material em engrenagens. Dentro deste contexto esta dissertação tem como objetivos principais o desenvolvimento e construção de uma máquina de ensaios de fadiga de contato em engrenagens e a comparação quanto à resistência ao desgaste de engrenagens cilíndricas de dentes retos fabricadas em aço liga 8620 (cementado e temperado) e em ferro fundido nodular austêmperado (FNA). Estas engrenagens foram fornecidas em dois estados de acabamento superficial (shaving ou por fresamento). O equipamento desenvolvido funciona com o princípio de recirculação de potência e é comumente chamado de FZG. Os experimentos de desgaste têm sido feitos com dois estágios de torque: 135 N.m (running-in) e 302 N.m (steady-state) e as engrenagens são de perfil modificado. Após a realização dos ensaios, foi possível verificar que: (1) a área danificada dos flancos, devido ao fenômeno de pitting, das engrenagens de FNA foi maior do que das de aço AISI 8620; (2) a comparação entre os parâmetros de rugosidade nos estágio de fornecimento e após cada estágio do experimento mostra que as engrenagens com acabamento por shaving apresentam uma melhor distribuição da carga e um menor desgaste, (3) o regime de lubrificação na região do diâmetro primitivo durante os ensaios foi o limite (ou limítrofe) para ambos os processos de fabricação, porém o parâmetro de espessura de filme (λ) é maior para engrenagens fabricadas por shaving, fazendo com que a espessura do filme lubrificante seja maior, (4) o efeito da redução da área resistente aos esforços de contato, devido à presença dos nódulos de grafita, na resistência a fadiga foi evidenciado, porém o acabamento superficial mostrou-se mais relevante do que a distribuição e tamanho dos nódulos. / Due to the resistance of automotive manufactures on apply ADI gear studies have been made to verify the performance of this material. Inside this context this work has a main objective the construction of contact fatigue test machine in gears and the comparison, related to the wear resistance, between spur-gears made of carburized and quenched AISI 8620 low steel and austempered ductile iron (ADI). These gears were supplied in two states of finish surface (by shaving or by machining). The developed equipment functions with the principle of the power recirculation and commonly called FZG. The wear experiments were performed in two stages of torque: 135 N.m (running-in) and 302 N.m (steady-state) and the gears have modified profile. After the accomplishment of the experiments, was possible verify that: (1) The damaged area of flanks, due to the phenomenon of pitting, at ADI gears was bigger than AISI 8620 low-alloy steel gears. (2) The comparisons between the roughness parameter in the supply state and after each period of the experiment shows that gears with surface finish by shaving have better load distribution and minor wear. (3) Lubricant regime at pitch diameter was the boundary for both manufacture process, but the film thickness parameter λ is higher for shaved gears, making the lubricant film thickness has major value, (4) The reduction effect the resistive area of contact due the presence of the graphite nodules in the contact fatigue resistance was evidenced, however surface finishing was more relevant than the distribution and size nodules.

Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena. In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism. Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method. / Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Structural components that have service life in the VHCF regime include wheels and axles of high speed trains, gas turbine disks, and components of paper production machinery. Safe and reliable design, and the longevity, of these components depends on the knowledge of their VHCF properties. The overall aim of the experimental portion of this thesis was to gain in-depth knowledge of the VHCF properties of stainless steels. Fatigue test data in the VHCF regime was generated for different stainless steel grades using ultrasonic fatigue testing. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. Theoretical modelling of complex crack geometries involving kinks and branches was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The main aim of this development was to compute the stress intensity factors and to analyse the stress state around the cracks. The results showed that accuracy of 2% was attainable compared to the solutions obtained by Finite Element Method (FEM). / <p>Artikel 4 publicerad i avhandlingen som manuskript</p>

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Quantification of inclusions is important since it is correlated to the steel’s fatigue properties. One method that could be further developed for detection of inclusions in steel is ultrasonic testing (UST). The aim of this study is to investigate what type of inclusions, in terms of size, morphology and chemical composition, that can be detected with 25 MHz UST, and what type of inclusion that cannot be detected. This was done by firstly scanning 74 steel samples with 25 MHz UST, and then fatigue test the same samples until fracture. The inclusion that caused the fracture was then analysed with microscopy and compared with the results from the 25 MHz UST. It was found that Mn-Mg-sulphides, Ca-sulphides, oxy sulphides and complex oxides are difficult to detect with 25 MHz UST. Globular oxides can be detected with 25 MHz UST, at least down to an area of 8300 μm2 and if they are not too fragmented due to rolling. The results indicate that oxy sulphide stringers can be detected with 25 MHz UST if the inclusion have oxides in direct contact with the steel matrix, rather than oxides encapsulated by sulphides. / Kvantifiering av inneslutningar är viktigt då det korrelerar med stålets utmattningsegenskaper. En metod som kan utvecklas för detektering av inneslutningar i stål är ultraljudstestning (UT). Syftet med den här studien är att undersöka vilken typ av inneslutningar, med avseende på storlek, morfologi och kemisk sammansättning, som kan detekteras med 25 MHz UT, och vilken typ av inneslutningar som inte kan detekteras. Detta gjordes genom att först skanna 74 stålprover med 25 MHz UT och sedan utmattningstesta samma prover tills de gick till brott. Inneslutningen som orsakade brottet analyserades sedan med mikroskopi och jämfördes med resultaten från 25 MHz UT. Det visade sig att Mn-Mg-sulfider, Ca-sulfider, oxisulfider och komplexa oxider är svåra att upptäcka med 25 MHz UT. Globulära oxider kan upptäckas, åtminstone ner till en area på 8300 μm2 och om den inte har blivit för fragmenterad av valsning. Resultaten indikerar att oxisulfider kan detekteras om oxiderna är i direkt kontakt med stålmatrisen, i stället för att oxiderna är inkapslade av sulfider.

O aço SAE 1552 modificado é um desenvolvimento recente da família do aço carbono manganês ligado ao silício, sendo utilizado para confecção de grampo U que tem como finalidade prender o feixe de molas no eixo do veículo. A somatória das fases de produção dessa matéria prima introduz os limites de resistência mecânica final necessária para atender a classe de resistência normativa. A peça produzida com esse aço tem alcançado crescimento de utilização na indústria automotiva devido à mesma apresentar propriedades mecânicas que atendem a requisitos normativos de classe de resistência e com vantagem de evitar tratamento térmico na fase de confecção do grampo e sendo esse processo realizado a frio em todas as suas fases. O objetivo desse trabalho foi estudar falha de grampo na etapa do processo de dobramento em forma de U e aplicar ensaios como: programa de simulação de tratamento térmico Stecal 3.0, ultrassom, fadiga, teste de cela e análise de fratura para solucionar essas falhas. Também foi realizado ensaios para prever fraturas catastróficas como: elementos finitos através de programa de computador Abaqus, ensaio de extensometria, tenacidade a fratura e medições de tensões residuais através de técnica de difração de raios-x. Foi concluído através dos resultados dos estudos de microestrutura resultante de tratamento térmico da matéria prima que o processo com resfriamento controlado em esteira é mais adequado para a produção do aço para confecção de grampo. O ensaio de ultrassom antes e após ensaio de fadiga possibilitou dimensionar o crescimento da profundidade da trinca em cotovelo de grampo e através de elementos finitos e extensometria associado com mecânica da fratura foi possível conhecer as tensões em ponto de estudo e entender o motivo de não ocorrer falha catastrófica. O ensaio de difração de raios-x permitiu o entendimento das tensões residuais introduzidas na peça de estudo. / SAE 1552 steel modified is a recent development of manganese carbon steel group linked to silicon, which is used to manufacture u-bolt that aims to fix leaf spring at vehicle axle in the back part. The sum of production stages of this raw material introduces the final mechanical resistance limits to meet class rules resistance. The piece produced with this steel has achieved growth of use in the automotive industry due to the mechanical properties it presents, which meet regulatory requirements for strength class and the advantage of avoiding heat treatment during manufacturing of the u-bolt and all the phases of this process are carried out in cold. This work aims to study the u-bolt failure during the folding process in the form of U and apply tests such as heat treatment simulation -Stecal 3.0, ultrasonic test, fatigue test, cell testing and analysis of crack. Other tests have been carried out to predict catastrophic fractures such as: finite element through computer program called Abaqus, extensometry testing, toughness testing for fracture and residual stress measurement by X ray diffraction technique. Results of heat treatment studies, by microstructure analysis, allowed choosing appropriate process for steel production. Ultrasonic testing before and after fatigue testing enabled to measure growth of crack depth on u-bolt elbow, and through finite element and extensometry testing associated with Mechanical of Fracture it was possible to know the stress concentrated at a point and to understand why catastrophic failure did not occur. Residual stress understanding has provided overall vision of u-bolt studied and contributed to have precision in measurement at inner and outer part of the u-bolt elbow.

Two failure mechanisms in a double lap joint are investigated. Analytical models of net-section and gross-section failure modes are proposed to describe these mechanisms. The effects of lamping force, interference fit, maximum axial load and WDCP on fatigue performance of the joint are included in the models. The effect of WDCP is assumed to give a reduction in friction coefficient. Three types of stress reduction factors are proposed in the net-section failure model to account for these parameters. The stress reduction factors modify stress range that is used in crack growth calculation. If there are no effects of these parameters, the stress reduction factors are equal to one. Two types of fretting stress are introduced in gross-section failure model to describe either sliding contact or incipient sliding contact on faying surface. The fretting stress is combined with body stress to modify stress range. The net-section failure model predicts that fatigue life is increasing as interference fit, clamping force and friction coefficient increase. The gross-section failure model predicts that fatigue life is decreasing as clamping force and friction coefficient increase. Both models predict that fatigue life is decreasing as maximum axial load increases. Transition of the failure mode occurs earlier as friction coefficient and interference fit increase, while it is delayed as maximum axial load increases. A transition parameter is proposed to establish a relationship between the four main parameters. The transition parameter is expressed in a polynomial equation. It gives an optimum combination of the four main parameters in order to achieve relatively higher fatigue life by having gross-section failure mode. Finite element analysis and fatigue testing are performed to validate the models. The finite element and the analytical models show that stress concentration factor at the edge of the hole is decreasing as clamping force increases. The rate of decrease of stress concentration factor is increasing as friction coefficient increases. While stress concentration factor on the faying surface is increasing as clamping force and friction coefficient increase. Fatigue testing reveals that the fatigue life of the joint is in good agreement with the predicted fatigue life of the proposed models.

[pt] Nesta dissertação foram desenvolvidas técnicas de controle por aprendizado acelerado e Neuro-Fuzzy, aplicadas em um sistema servo-hidráulico para ensaio de fadiga. Este sistema tem o propósito de fazer ensaios em materiais para prever a resistência à fadiga dos materiais. O trabalho envolveu quatro etapas principais: levantamento bibliográfico, desenvolvimento de um controle por aprendizado acelerado, desenvolvimento de um controle por aprendizado Neuro-Fuzzy, e implementação experimental dos modelos de controle por aprendizado proposto em uma máquina de ensaios de materiais. A implementação do controle por aprendizado acelerado foi feita a partir do modelo de controle desenvolvido por Alva, com o objetivo de acelerar o processo de aprendizagem. Esta metodologia consiste em fazer um controle do tipo bang-bang, restringindo a servo-válvula a trabalhar sempre em seus limites extremos de operação, i.e., procurando mantê-la sempre completamente aberta em uma ou outra direção. Para manter a servo-válvula trabalhando em seus limites de seu funcionamento, os instantes ótimos para as reversões são obtidos pelo algoritmo de aprendizado, e armazenados em tabelas específicas para cada tipo de carregamento. Estes pontos de reversão dependem de diversos fatores, como a amplitude e carga média da solicitação, e são influenciados pela dinâmica do sistema. Na metodologia proposta, a lei de aprendizado inclui um termo de momentum que permite acelerar a aprendizagem dos valores das tabelas constantemente durante a execução dos testes, melhorando a resposta a cada evento. O desenvolvimento de um controle por aprendizado Neuro-Fuzzy foi motivado pela necessidade de ter um agente com a capacidade de aprendizado e armazenamento dos pontos ótimos de reversão. Este modelo de controle também consiste na implementação de um controle do tipo bang-bang, trabalhando com a servo-válvula sempre nos seus limites extremos de operação. O instante de reversão é determinado pelo sistema Neuro-Fuzzy, o qual tem como entradas a gama (dobro da amplitude) e o valor mínimo do carregamento solicitado. O processo de aprendizado é feito pelas atualizações dos pesos do sistema Neuro-Fuzzy, baseado nos erros obtidos durante a execução dos testes, melhorando a resposta do sistema a cada evento. A validação experimental dos modelos propostos é feita em uma máquina servohidráulica de ensaios de fadiga. Para este fim, o algoritmo de controle proposto foi implementado em tempo real em um módulo de controle CompactRIO da National Instruments. Os testes efetuados demonstraram a eficiência da metodologia proposta. / [en] In this thesis, accelerated learning and Neuro-Fuzzy control techniques were developed and applied to a servo-hydraulic system used in fatigue tests. This work involved four main stages: literature review, development of an accelerated learning control, development of a Neuro-Fuzzy control, and implementation of the learning control models into a fatigue testing machine. The accelerated learning control was implemented based on a learning control developed in previous works, introducing a faster learning law. Both learning control methodologies consist on implementing a bang-bang control, forcing the servovalve to always work in its operational limits. As the servo-valve works in its operational limits, the reversion points to achieve every peak or valley in the desired history are obtained by the learning algorithm, and stored in a specific table for each combination of minimum and mean load. The servo-valve reversion points depend on a few factors, such as alternate and mean loading components, while they are as well influenced by the system dynamics. In the proposed accelerated methodology, the learning law includes one momentum term that allows to speed up the learning process of the table cell values during the execution of the tests. The developed Neuro-Fuzzy control also consists on a bang-bang control, making the servo-valve work in its operational limits. However, here the instant of each reversion is determined by the Neuro-Fuzzy system, which has the load range and minimum load required as inputs. The learning process is made by the update of the Neuro-Fuzzy system weights, based on the errors obtained during the execution of the test.The experimental validation of the proposed models was made using a servo-hydraulic testing machine. The control algorithm was implemented in real time in a C-RIO computational system. The tests demonstrated the efficiency of the proposed methodology.