The use of laser welding in the automotive industry in the past few decades has facilitated joining of hot-dip galvanized (HDG) steel sheets at high production rates and low cost. The recent development of tailor welded blanks (TWB) using laser welding allowed combinations of sheet grades and thicknesses to “tailor” the vehicle part for optimized design, structural integrity and crash performance but more importantly, reductions in weight. Welded blanks are further subjected to stamping or stretch forming prior to final assembly. Unfortunately, both welding and stretch forming cause the galvanized coating to deteriorate, and thereby, undermine the long term corrosion protection. Despite existing publications on zinc coated steel and advances in processing techniques, there is a lack of understanding on the influence of laser welding and stretch forming on the corrosion performance of HDG steel. Hence, the purpose of this study was to determine how welding speed and biaxial strain affect interstitial-free (IF) and high strength low allow (HSLA) steel coupons when they are subjected to continuous immersion and accelerated corrosion tests. The corrosion rates of the coupons were evaluated using electrochemical techniques and gravimetry. Changes in the galvanized coating were characterized using scanning electron metallography. It was observed that, the original zinc layer transformed into the delta and gamma Fe-Zn intermetallic phases locally in the heat affected zone (HAZ) after laser welding. The resulting microstructure was similar to that of a commercially galvannealed coating and exhibited superior corrosion resistance than that of pure zinc. Linear polarization resistance (LPR) measurements revealed that the zinc coating was able to protect a chemically exposed region of steel in 0.1 M NaCl solution. While the Nd:YAG laser welded coupons with narrow HAZs performed equally well as the non-welded ones, diode laser welded coupons, with a wide locally annealed coating in the HAZ, exhibited a decrease in the peak corrosion rate of zinc. Moreover, minimal amounts of rust were observed on the surface of the HAZ after testing. With biaxial strain, welded and deformed coupons generally demonstrated higher peak corrosion rates than that of undeformed welded ones. When subjected to cyclic corrosion testing according to SAE J2334, rust formed in the exposed region after one 24 hour test cycle due to wet-dry conditions. However, zinc corrosion products on the surface provided substantial corrosion resistance to the remaining zinc coating and to the steel substrate. Gravimetric measurements of welded coupons showed a linear increase in weight gain with increased exposed widths of the steel after 30 cycles but biaxial strain further increased the weight gain on deformed coupons. After 60 cycles, the trend became exponential for both welded and deformed coupons. There was a negligible difference between the corrosion performance of IF and HSLA steel. Using X-Ray diffraction and Raman spectroscopy, species of both iron and zinc corrosion products were identified. Without the application of paint coatings, zinc oxide (ZnO), zinc hydroxy chloride (ZnCl2[Zn(OH)2]4), and hydrozincite ([ZnCO3]2[Zn(OH)2]3) were responsible for passivating the surface and reducing the overall corrosion rate of the galvanized coating.

The use of laser welding in the automotive industry in the past few decades has facilitated joining of hot-dip galvanized (HDG) steel sheets at high production rates and low cost. The recent development of tailor welded blanks (TWB) using laser welding allowed combinations of sheet grades and thicknesses to “tailor” the vehicle part for optimized design, structural integrity and crash performance but more importantly, reductions in weight. Welded blanks are further subjected to stamping or stretch forming prior to final assembly. Unfortunately, both welding and stretch forming cause the galvanized coating to deteriorate, and thereby, undermine the long term corrosion protection. Despite existing publications on zinc coated steel and advances in processing techniques, there is a lack of understanding on the influence of laser welding and stretch forming on the corrosion performance of HDG steel. Hence, the purpose of this study was to determine how welding speed and biaxial strain affect interstitial-free (IF) and high strength low allow (HSLA) steel coupons when they are subjected to continuous immersion and accelerated corrosion tests. The corrosion rates of the coupons were evaluated using electrochemical techniques and gravimetry. Changes in the galvanized coating were characterized using scanning electron metallography. It was observed that, the original zinc layer transformed into the delta and gamma Fe-Zn intermetallic phases locally in the heat affected zone (HAZ) after laser welding. The resulting microstructure was similar to that of a commercially galvannealed coating and exhibited superior corrosion resistance than that of pure zinc. Linear polarization resistance (LPR) measurements revealed that the zinc coating was able to protect a chemically exposed region of steel in 0.1 M NaCl solution. While the Nd:YAG laser welded coupons with narrow HAZs performed equally well as the non-welded ones, diode laser welded coupons, with a wide locally annealed coating in the HAZ, exhibited a decrease in the peak corrosion rate of zinc. Moreover, minimal amounts of rust were observed on the surface of the HAZ after testing. With biaxial strain, welded and deformed coupons generally demonstrated higher peak corrosion rates than that of undeformed welded ones. When subjected to cyclic corrosion testing according to SAE J2334, rust formed in the exposed region after one 24 hour test cycle due to wet-dry conditions. However, zinc corrosion products on the surface provided substantial corrosion resistance to the remaining zinc coating and to the steel substrate. Gravimetric measurements of welded coupons showed a linear increase in weight gain with increased exposed widths of the steel after 30 cycles but biaxial strain further increased the weight gain on deformed coupons. After 60 cycles, the trend became exponential for both welded and deformed coupons. There was a negligible difference between the corrosion performance of IF and HSLA steel. Using X-Ray diffraction and Raman spectroscopy, species of both iron and zinc corrosion products were identified. Without the application of paint coatings, zinc oxide (ZnO), zinc hydroxy chloride (ZnCl2[Zn(OH)2]4), and hydrozincite ([ZnCO3]2[Zn(OH)2]3) were responsible for passivating the surface and reducing the overall corrosion rate of the galvanized coating.

In this work¡Awe studied the strain effects on heavy hole (hh) and light hole (lh) bands of Zn1-xMnxSe/GaAs by photoreflectance (PR) spectroscopy . The Zn1-xMnxSe epilayers were grown on GaAs substrates by the MBE technique . There is a biaxial compressive strain exist in the epilayer, due to the different lattice constants between epilayers and substrates .The biaxial strain will shift hh and lh bands and lift the hh-lh degeneracy. In our experiment ,we found that the splitting of the hh and lh transition energies is almost lineally proportional to the Mn ion concentrations. It can be ascribed to the strain in the epilayer . We have also measured the PR of Zn0.96Mn0.04Se/GaAs at various temperatures , and analyzed the transition energy of different temperatures in terms of Varshni relation.

The paper treats the question of feasibility of measuring intrinsic thin film stresses due to nanoindentation. A variety of different methods is proposed and analysed with respect to their applicability. As this accuracy-estimation results in boundary conditions for the measurements which can not be fulfilled yet, the whole topic is considered from a purely academic point of few. With the help of a special software package [25] the following methods are considered: 1. Taking the moment of beginning plastic flow within the substrate as indictor, 2. Taking the moment of beginning plastic flow within the film as indictor, 3. Applying mixed loads (normal and lateral forces), 4. Applying the concept of the effectively shaped indenter. While the methods 2 to 4 appear to be in principle feasible, method 1 can completely be ruled out as being of no practical use. The mentioned software package (FilmDoctor prototype) is part of the supplemental material of this study.

Electrical motors and/or hydraulics and pneumatics cylinders are commonly used methods of actuation in mechanical systems. Over the last two decades, due to arising market needs, novel self-independent mobile systems such as mobility assistive devices have emerged with the help of new advancements in technology. The actuation criteria for these devices differ greatly from typical mechanical systems, which has made the implementation of classical actuators difficult within modern assistive devices. Among the numerous challenges, limited energy storage capabilities by mobile systems have restricted their achievable operational time. Furthermore, new expectations for device weight and volume, as well as actuator structural compliance, have added to this quandary. Electroactive polymers, a category of smart materials, have emerged as a strong contender for the use in low-cost efficient actuators. They have demonstrated great potential in soft robotic and assistive device/prosthetic applications due to their actuation potential and similar mechanical behaviour to human skeletal muscles. Dielectric Elastomers, in particular, have shown very promising properties for these types of applications. Their structures have shown large achievable deformation, while remaining light-weight, mechanically efficient, and low-cost. This thesis aims to characterize, and model the behaviour of 3MTM VHB polyacrylic dielectric elastomer, in order to establish a foundation for its implementation in a proposed novel linear actuator concept. In this thesis, a comprehensive experimental evaluation is accomplished, which resulted in the better understanding of the elastomer’s biaxial mechanical and electro-mechanically coupled behaviours. Subsequently, a constitutive biaxial mechanical model was derived in order to provide a predictive design equation for future actuator development. This model proved effective in providing a predictive tool for the biaxial mechanical tensile response of the material. Finally, a simplified prototype was devised as a proof of concept. This first iteration applied experimental findings to validate the working principles behind the proposed actuator design. The results confirmed the proof of concept, through achieved reciprocal linear motion, and provided insight into the design considerations for prototype optimization and final actuator development.

abstract: There is a concerted effort in developing robust systems health monitoring/management (SHM) technology as a means to reduce the life cycle costs, improve availability, extend life and minimize downtime of various platforms including aerospace and civil infrastructure. The implementation of a robust SHM system requires a collaborative effort in a variety of areas such as sensor development, damage detection and localization, physics based models, and prognosis models for residual useful life (RUL) estimation. Damage localization and prediction is further complicated by geometric, material, loading, and environmental variabilities. Therefore, it is essential to develop robust SHM methodologies by taking into account such uncertainties. In this research, damage localization and RUL estimation of two different physical systems are addressed: (i) fatigue crack propagation in metallic materials under complex multiaxial loading and (ii) temporal scour prediction near bridge piers. With little modifications, the methodologies developed can be applied to other systems. Current practice in fatigue life prediction is based on either physics based modeling or data-driven methods, and is limited to predicting RUL for simple geometries under uniaxial loading conditions. In this research, crack initiation and propagation behavior under uniaxial and complex biaxial fatigue loading is addressed. The crack propagation behavior is studied by performing extensive material characterization and fatigue testing under in-plane biaxial loading, both in-phase and out-of-phase, with different biaxiality ratios. A hybrid prognosis model, which combines machine learning with physics based modeling, is developed to account for the uncertainties in crack propagation and fatigue life prediction due to variabilities in material microstructural characteristics, crack localization information and environmental changes. The methodology iteratively combines localization information with hybrid prognosis models using sequential Bayesian techniques. The results show significant improvements in the localization and prediction accuracy under varying temperature. For civil infrastructure, especially bridges, pier scour is a major failure mechanism. Currently available techniques are developed from a design perspective and provide highly conservative scour estimates. In this research, a fully probabilistic scour prediction methodology is developed using machine learning to accurately predict scour in real-time under varying flow conditions. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2016

Como conseqüência do atual desenvolvimento tecnológico dos materiais aço e concreto, várias pesquisas têm surgido com vistas a um melhor aproveitamento da capacidade desses dois materiais, exigindo um maior conhecimento sobre o comportamento dos elementos, inclusive sobre sua vulnerabilidade a estados limites últimos. Sendo a instabilidade um estado limite último possível de ocorrer em configurações de equilíbrio de peças de concreto armado submetidas a determinadas solicitações normais, seu estudo torna-se fundamental para que seja possível propor soluções estruturais seguras e economicamente viáveis. Este trabalho apresenta a análise de alguns aspectos que interferem no estudo da estabilidade de pilares esbeltos de concreto armado, através de uma abordagem envolvendo aspectos teóricos, como também aspectos práticos oriundos da resolução de exemplos. O estudo é feito com base nos métodos geral e do equilíbrio, com os processos exato e do pilar padrão, através de um software (SISTEMA FLEXOR), desenvolvido por CADAMURO Jr. (1997). O objetivo é testar a viabilidade desses métodos a fim de propor soluções e, com isso, tornar mais amplo e acessível o uso de pilares esbeltos de concreto armado, incentivando a adoção de projetos mais arrojados sem subutilização dos materiais. / As a consequence of the actual technological development of the materials steel and concrete, many researches have come up with proposal of better use of these two materials capacity, demanding a bigger knowledge about the behaviour of the elements, included your vulnerability to ultimate limit states. Being the instability a possible ultimate limit state of reinforced concrete columns equilibrium configuration, this study has been fundamental to make possible to propose safe and economically reasonable structural solutions. This work presents the analysis of some aspects that interfere in the study of slender reinforced concrete columns stability, through an approach involving theoretical and practical aspects derived from numerical solutions. The scheme is based in the General and Equilibrium Methods with the Exact Process and Model Column Method, through a software (SISTEMA FLEXOR) developed by CADAMURO Jr. (1997). The objective is to testify the practicability of these methods, to propose solutions and to render wider and more practical the use of slender reinforced concrete columns, stimulating the adoption of projects bolders without underutilisation of the materials.

Ultra-high performance concrete (UHPC) was evaluated as a potential material for future bridge deck designs. Material characterization tests took place to identify potential challenges in mixing, placing, and curing UHPC. Biaxial testing was performed to evaluate behavior of UHPC in combined tension and compression stress states. A UHPC bridge deck was designed to perform similarly to a conventional concrete bridge deck, and a single unit bridge deck section was tested to evaluate the design methods used for untreated UHPC. Material tests identified challenges with placing UHPC. A specified compressive strength was determined for structural design using untreated UHPC, which was identified as a cost-effective alternative to steam treated UHPC. UHPC was tested in biaxial tension-compression stress states. A biaxial test method was developed for UHPC to directly apply tension and compression. The influence of both curing method and fiber orientation were evaluated. The failure envelope developed for untreated UHPC with random fiber orientation was suggested as a conservative estimate for future analysis of UHPC. Digital image correlation was also evaluated as a means to estimate surface strains of UHPC, and recommendations are provided to improve consistency in future tests using DIC methods. A preliminary bridge deck design was completed for untreated UHPC and using established material models. Prestressing steel was used as primary reinforcement in the transverse direction. Preliminary testing was used to evaluate three different placement scenarios, and results showed that fiber settling was a potential placement problem resulting in reduced tensile strength. The UHPC bridge deck was redesigned to incorporate preliminary test results, and two single unit bridge deck sections were tested to evaluate the incorporated design methods for both upside down and right-side up placement techniques. Test results showed that the applied design methods would be conservative for either placement method. / Ph. D.

Les connecteurs clips utilisés pour assembler les tubes de riser pour le forage pétrolier offshore subissent un chargement cyclique dû aux vagues. 90% de la durée de service est passée en mode connecté, avec une contrainte moyenne élevée, alors que 10% est passé en mode déconnecté, avec une faible contrainte moyenne. Des calculs numériques montrent qu’un chargement cyclique de tension biaxiale en phase prévaut dans la zone critique de la structure. Les effets de contrainte moyenne et de biaxialité doivent tous deux être pris en compte pour un design approprié. Les critères de fatigue multiaxiale de la littérature sont basés sur des données de traction-torsion et ne discriminent pas bien l’influence de la tension biaxiale de celle d’une contrainte moyenne. Un des objectif de cette étude est donc de caractériser séparément ces deux effets.Pour étudier les effets de contrainte moyenne, des essais de fatigue uniaxiale ont été menés avec différents rapports R. Les durées de vie diminuent avec l'augmentation de R, et la limite d'endurance suit la parabole de Gerber. À faible contrainte moyenne et amplitude de contrainte élevée, les fissures s’amorcent en surface, tandis que pour des rapports R élevés et des amplitudes faibles, les fissures s’amorcent à partir de défauts internes ou coupant la surface. Cette transition est analysée à partir de calculs élasto-plastiques des champs de contrainte et déformation autour des défauts. Les fissures internes se propagent sous un faible ΔK indépendant de R, ce qui est attribué à la quasi absence d’effets de fermeture.Pour étudier l'effet de biaxialité, des essais cycliques de tension et pression interne combinées en diverses proportions ont été effectuées à rapport R fixe (0,25). Un taux de biaxialité modéré (B = 0,25 et 0,5) a un effet bénéfique, attribué à un retard de l'amorçage des fissures, alors que la tension équibiaxiale a un effet légèrement nuisible, attribué à un "pseudo effet de taille" (probabilité plus grande qu’une microfissure se propage le long de deux plans principaux équivalents, au lieu d’un seul).De facettes intergranulaires révélatrices de fragilisation par l'hydrogène ont été observées sur les surfaces de rupture. L’évolution de leur fraction avec ΔK et la biaxialité suggèrent une réduction de la vitesse de fissuration à B≤0.5, mais l'effet néfaste de la tension équibiaxiale ne peut être attribué à une accélération de la propagation.Plusieurs des critères de fatigue existants échouent à décrire toutes les données de cette étude. Les critères d'endurance avec un terme linéaire de contrainte moyenne ou de tension hydrostatique ne parviennent pas à prédire les variations de la limite d'endurance. Un nouveau critère de fatigue a été proposé sur la base de la parabole de Gerber. Il décrit bien les effets combinés d'une contrainte moyenne et d’un taux de biaxialité positif.Des essais biaxiaux ont également été effectués dans l’eau salée (3.5% NaCl) pour étudier l'influence de ce milieu sur les durées de vie en fatigue, en corrosion libre et avec une protection cathodique qui amplifie le dégagement d’hydrogène.En corrosion libre, l'eau salée réduit fortement les durées de vie et supprime la limite d'endurance. Cela est dû à la formation de piqûres de corrosion qui favorisent l’amorçage précoce et multiples de fissures. La tension équibiaxiale n’accentue pas l’effet nocif de l'eau salée, malgré des mécanismes de fissuration différents: décohésion fragile transgranulaires en tension uniaxiale, mais principalement intergranulaire en tension biaxiale.La protection cathodique annule l'effet néfaste de l'eau salée pour tous les taux de biaxialité, en dépit d'un net accroissement de la fragilisation par l’hydrogène des joints de grains. Les surfaces de rupture deviennent presque entièrement intergranulaire, tandis qu’à l'air, le taux de rupture intergranulaire ne dépasse pas 45%. / The clip connectors used to join the riser tubes for offshore oil drilling undergo cyclic loading due to sea waves. 90% of the service life is spent in the “connected mode” with a high mean stress and 10% in the “disconnected mode” with a lower mean stress. Finite element computations revealed in-phase biaxial tension in the critical areas of the clip connector along with high mean stresses. Thus, both the mean stress effect and the biaxiality effect need to be addressed for proper design of these structures. However, most of the multiaxial fatigue criteria are based on tension-torsion fatigue data and do not discriminate the influence of biaxial tension from that of a mean stress. This study investigates separately these two effects.For investigating the mean stress effect, uniaxial fatigue tests were run on Cr-Mo steel with various R ratios (σmin/σmax). The fatigue lives, as well as the slope of the S-N curves were found to decrease with increasing R, and the endurance limit to follow Gerber’s parabola. At low R ratios and thus relatively high stress ranges, fatigue cracks initiated from the surface, while for high R ratios, and thus low stress ranges, cracks initiated from internal or surface-cutting defects. This transition was analyzed based on elastic-plastic computations of stress-strain fields around the defects. The threshold for internal fatigue crack growth from defects was found to be quite low and independent from the R ratio. This was attributed to a nearly closure-free propagation.To investigate the effect of positive stress biaxiality, combined cyclic tension and internal pressure tests with various proportions of each loading were run on tubular specimens, at fixed R ratio (0.25). Moderate stress biaxialities (B= 0.25 and 0.5) had a beneficial effect on fatigue lives, attributed mainly to a retardation of crack initiation, while equibiaxial tension had a slightly detrimental effect, attributed to a “pseudo size effect” (higher probability for an incipient crack to grow along two possible planes, compared to a single one).Intergranular facets associated with temper and H2 embrittlement were observed on the fracture surfaces. The evolutions of their surface fraction with ΔK and load biaxiality suggested a possible reduction in crack growth rate at moderate biaxialities, but the detrimental effect of equibiaxial tension could not be explained in terms of crack growth rate.Several popular fatigue criteria failed to describe all fatigue data. Endurance criteria that include a linear mean stress term or contain a hydrostatic tension term fail to predict the variations of the endurance limit of this material with the R ratio and biaxiality ratio. Thus, a new fatigue criterion based on Gerber’s parabola was proposed. It captured the evolution of the endurance limit under the combined effects of positive mean stress and biaxiality.Similar tests were run to investigate the influence of salt water (3.5% NaCl) on fatigue lives under two types of test conditions: 1) free corrosion and 2) cathodic protection.In free corrosion, salt water strongly reduced the fatigue lives and suppressed the endurance limit. This was due to the formation of corrosion pits that favor early, multiple crack initiations. The detrimental effect of salt water was not enhanced by equibiaxial tension, which did not modify the size and density of corrosion pits. Fatigue lives in uniaxial and biaxial tension were nearly the same, although the crack growth mechanism was different: transgranular brittle decohesion in uniaxial loading and mostly intergranular in biaxial tension.Cathodic protection cancelled the detrimental effect of salt water for all biaxialities, in spite of a clear enhancement of H-induced embrittlement of the grain boundaries. The fracture surfaces were nearly fully intergranular, irrespective of load biaxialities, while in air the proportion of intergranular fracture was less than 45%.

This work deals with fatigue life and fractographycal analysis of plasma-nitrided and virgin specimen made of the low-alloy high-strength steel. Specimens were subjected to in-phase combined bending-torsion loading. The work is focused on three groups of problems. The first group of problems was associated with the fatigue life and its prediction. The objective was to examine a relationship between the presence of the nitrided layer and fatigue life. The application of surface layers has significantly improved the fatigue life in the high-cycle region. An extended comparison between classical and advanced multiaxial criteria was performed. Criterions proposed by Marin and Matake were found to be most precise. The second objective of this work was to examine a relationship between the loading ratio ( is the bending amplitude and is the torsion amplitude ) and the fracture surface topography in the highcycle fatigue. Properties of fracture surfaces were quantified by many roughness and fractal parameters. Many roughness parameters start to incrise rapidly above a critical value of the loading ratio zc = 0,5. The last part of this work deals with the fractographical analysis of fish-eye cracks that were formed in plasma-nitrided steel specimens under symetrical bending, symetrical torsion and biaxial in-phase bending-torsion combinations.