In this work the author presents simulation procedures (mathematical models) with the aim to help determining and analysing the mechanical properties of coating-substrate-systems and finding an “optimal” coating structure which should protect the compound from inelastic deformation under a given range of load conditions. Such procedures may be used as a tool to minimise the search field for experimental work. For this purpose one would need a mathematical model which allows one to calculate the complete elastic field with all its displacement and stress components within a multilayer film on a substrate under given mechanical loading and intrinsic stress conditions. Due to copyright restrictions the author is not allowed to publish the Part II of his habilitation thesis at this place. It concerns the references in meta data. / In der Arbeit werden mathematische Modelle zur Berechnung der mechanischen Eigenschaften geschichtet aufgebauter Materialien unter unterschiedlichsten Lastbedingungen (Kontakt- und intrinsische Beanspruchung) vorgestellt und diskutiert. Auf Grund von Schutzrechtsbestimmungen ist eine Veröffentlichung der in der Habilitation angegebenen Literatur im Teil II an dieser Stelle nicht möglich. Der interessierte Leser wird gebeten die Arbeiten in den entsprechenden Journalen einzusehen. Dies betrifft die in den Metadaten angegebenen Veröffentlichungen des Autors.

Composite materials with laminated structure provide advantages which are utilised during component design. Low density, temperature and chemical stability are the profitable properties predetermining application of ceramic laminates. The main obstacle for wide spread of ceramic materials is their inherent brittleness. Therefore, in this thesis, the crack propagation in ceramics laminates has been extensively studied. Laminated structures with various volume fractions of components (alumina and zirconia) were prepared by electrophoretic deposition. Evaluation of crack propagation through the interface and determination of basic mechanical properties was conducted on the basis of extensive literature search. Crack deflection originated in both presence of internal stresses and differences in elastic modulus during the crack interface passing was monitored. A special type of specimen geometry was employed with the aim to set arbitrary angle between crack and interface. It was experimentally found that the degree of crack deflection is dependent on entering angle and volume fraction of components. Higher crack deflection was already found in the bulk of the test piece comparing to the test piece surface. The 3D fracture surface reconstruction generated using laser confocal microscopy was used in this detailed crack propagation study. Further basic elastic and strength characteristics of laminates were determined and compared to those obtained from monolithic materials. The validity of the mix rule for elastic characteristics was confirmed by comparing of elastics modulus. The most reliable method for elastic modulus determination was marked the dynamic resonance method due to low scatter and consistency in measurement. The flexural strength of all laminates tends to be close to the flexural strength of the weakest component. Therefore the mix rule is not applicable for flexural strength estimation on the contrary of elastic characteristics. The change of component volume fraction leads only to change of flexural strength scatter. Thanks to gained knowledge about crack propagation and basic characteristic determination will be possible to design ceramic laminates more efficiently for given needs of application.

This comprehensive study is the first to quantify the fatigue performance, failure loads, and microstructure of resistance spot welding (RSW) in 6061-T6 aluminum (Al) alloy according to welding parameters and process sensitivity. The extensive experimental, theoretical and simulated analyses will provide a framework to optimize the welding of lightweight structures for more fuel-efficient automotive and military applications. The research was executed in four primary components. The first section involved using electron back scatter diffraction (EBSD) scanning, tensile testing, laser beam profilometry (LBP) measurements, and optical microscopy(OM) images to experimentally investigate failure loads and deformation of the Al-alloy resistance spot welded joints. Three welding conditions, as well as nugget and microstructure characteristics, were quantified according to predefined process parameters. Quasi-static tensile tests were used to characterize the failure loads in specimens based upon these same process parameters. Profilometer results showed that increasing the applied welding current deepened the weld imprints. The EBSD scans revealed the strong dependency between the grain sizes and orientation function on the process parameters. For the second section, the fatigue behavior of the RSW’ed joints was experimentally investigated. The process optimization included consideration of the forces, currents, and times for both the main weld and post-heating. Load control cyclic tests were conducted on single weld lap-shear joint coupons to characterize the fatigue behavior in spot welded specimens. Results demonstrate that welding parameters do indeed significantly affect the microstructure and fatigue performance for these welds. The third section comprised residual strains of resistance spot welded joints measured in three different directions, denoted as in-plane longitudinal, in-plane transversal, and normal, and captured on the fusion zone, heat affected zone and base metal of the joints. Neutron diffraction results showed residual stresses in the weld are approximately 40% lower than the yield strength of the parent material, with maximum variation occurring in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. In the final section a theoretical continuum modeling framework for 6061-T6 aluminum resistance spot welded joints is presented.

[pt] A fotoelasticidade é uma ferramenta poderosa para análise de tensões em problemas bi e tridimensionais. Nos últimos anos a técnica vem ganhando renovado interesse da comunidade científica devido à adaptação de modernas técnicas de aquisição e processamento digital de imagens. Uma das linhas mais promissoras é a Fotoelasticidade RGB, que entende a cor como uma combinação única dos níveis de intensidade das componentes Red, Green e Blue. Tratase de uma metodologia poderosa, porém de fácil compreensão e aplicação em ambientes industriais. Uma aplicação tradicional da fotoelasticidade é na inspeção de tensões residuais em componentes de vidro, utilizando-se a Norma ASTM F218. Esta Norma estabelece dois procedimentos para a determinação da ordem de franja isocromática: um quantitativo e outro qualitativo. O vidro tem birrefringência muito baixa, o que faz com que a resposta ótica seja basicamente em tons de cinza. No seu procedimento qualitativo a Norma ASTM F218 recomenda a utilização de um filtro retardador de onda inteira. Com esse simples procedimento, a resposta ótica se modifica. No lugar de tons de cinza tem-se cores em torno da passagem do vermelho para o azul, o que simplifica muito a análise de birrefringência por um operador. A aplicação da Fotoelasticidade RGB para componentes de vidro, transforma este método qualitativo em quantitativo: a cor deixa de ser um parâmetro abstrato, e passa a ser um número. Este trabalho analisa os procedimentos necessários para aplicar a Fotoelasticidade RGB à indústria do vidro. A Norma ASTM F218 recomenda que o filtro de onda inteira, seja inserido com sua direção principal alinhada à direção principal de cada ponto analisado. Foi pesquisada a diferença entre os resultados obtidos, quando a norma é seguida, e quando é utilizado um procedimento simplificado, ou seja, é utilizada apenas uma posição do filtro para analisar todos os pontos. Além disso, o método foi aplicado a dois casos: um esteme e um bulbo de lâmpada. Nos dois casos, foram analisados um componente considerado bom e outro considerado ruim, por seu fabricante, procurando mostrar seus estados de tensões residuais através da Fotoelasticidade RGB. / [en] Photoelasticity is a powerful tool for analyzing stress in bidimensional and three-dimensional problems. In the last years the technique renewed the scientific community s interest due to adaptation of modern acquisition and image digital processing techniques. One of the most promising field is called RGB Photoelasticity. It quantifies the color as an unique combination of the intensity levels of Red, Green and Blue components. It is a powerful methodology,even so of easy understanding and application in industrial enviroment.A traditional application of the photoelasticity is in the inspection of residual stresses in glass components, using the specification ASTM F218. That specification establishes two procedures for the determination of the isocromatics fringe order: one of them quantitative and anotehr one qualitative. The glass has very low birefringence, so that its optic response is basically in gray tones. In the qualitative procedure the specification F218 recommends the use of a full - wave plate. With this simple procedure the optical answer is changed. It is possible to obtain colors near the passage of the red to blue, what simplifies the analysis too much. The application of RGB photoelasticity to glass transforms the qualitative method in quantitative: the color is not more an abstract parameter, but a number. This dissertation analyzes the necessary procedures to apply RGB photoelasticity to the glass industry. The specification F218 recommends that the full wave plate must be inserted with its principal direction aligned to the principal direction of each analyzed point. A implification to that procedure was proposed: to use only one position of the filter to analyze all the points. The difference among the two procedure was researched; the results obtained following the specification and using the simplified procedure were compared. The method has been applied to two cases: a stem and a lamp bulb. In the both cases, a component in the acceptable condition and another in the non-acceptable condition were analyzed, in order to show their states of residual stresses using the RGB photoelasticity.

Ultra-high temperature ceramics (UHTCs), such as ZrB2-based ceramic composites, have been identified as next generation candidate materials for leading edges and nose cones in hypersonic air breathing vehicles. Mechanical performance of ceramic composites play an important role in the ultra-high temperature applications, therefore SiC is added to ZrB2 as a strengthening phase to enhance its mechanical performance. The high melting temperatures of both ZrB2 and SiC, as well as the ability of SiC to form SiO2 refractory oxide layers upon oxidation make ZrB2-SiC ceramics very suitable for aerospace applications. Thermal residual stresses appearing during processing are unavoidable in sintered ZrB2-SiC ceramic composites. Residual microstresses appear at the microstructural level (intergranular microstresses) or at the crystal structure level (intragranular microstresses). These microstresses are of enormous importance for the failure mechanisms in ZrB2-SiC ceramics, such as ratio of the trans- and intergranular fracture; crack branching or bridging, microcracking, subcritical crack growth and others, as they govern crack propagation–induced energy dissipation and affect the toughness and strength of the ceramic material. Therefore, understanding the evolution of residual stress state in processed ZrB2-SiC ceramic composites and accurate measurements of these stresses are of high priority. In the present research the ZrB2-17vol%SiC, ZrB2-32vol%SiC, and ZrB2-45vol%SiC ultra-high temperature particulate ceramic composites were sintered using both Hot Pressing (HP) and Spark Plasma Sintering (SPS) techniques. The mechanical performance of the ZrB2-SiC composites was investigated using 3- and 4-point bending techniques for measurements of instantaneous fracture strength and fracture toughness. Resonant Ultrasound Spectroscopy was used for measurement of Young's, shear, and bulk moduli as well as Poisson's ratio of the composites. The distribution of thermal residual stresses and the effect of the applied external load on their re-distribution was studied using micro-Raman spectroscopy. Piezospectroscopic coefficients were determined for all compositions of ZrB2-SiC ceramic under study and their experimentally obtained values were compared with the piezospectroscopic coefficients both published in the literature and calculated using theoretical approach. Finally, the novel ZrB2-IrB2-SiC ceramic composites were also produced using Spark Plasma Sintering (SPS), where IrB2 powder was synthesized using mechanochemical route. It is expected that the IrB2 additive phase might contribute to the improved overall oxidation resistance of ZrB2 based ultra-high temperature ceramic composites.

This project was initiated by the Cardcrock Division of the Naval Surface Warfare Center (CDNSWC) and the Office of Naval Research to study the effects of various processing parameters on the residual stress state of HSLA-100 bead-on-plate weldments. Three groups of samples were provided to Virginia Tech by CDNSWC. The first (GPX) was a sample of unwelded, as-received base material; the second group (RS-) consisted of an experimental matrix of differently processed bead-on-plate weldments which were all ground prior to welding in order to prepare the surface for welding; the third group (SR-) was a pair of weldments vacuum annealed at 1200°F for one and two hours, and then welded (without grinding) using welding parameters identical to weldments from the second group. X-ray diffraction was used to measure the surface residual stress state of all samples. It was found that the surface residual stress states of the GPX plate and the unwelded SR-group plates showed no statistically significant difference in magnitude, though the variation of the stress state over the surface of the plates seemed to decrease with increasing annealing time. The severe, non-uniform grinding was determined to play a very large role in the residual stresses generated in the welds, sometimes changing both the magnitude and the shape of the stress patterns. Residual stresses in plates that were ground before welding were always more tensile than those that were not ground. Grinding also caused a large compressive-to-tensile stress gradient in the transverse direction. The grinding made it difficult to determine the effects of different welding parameters on the residual stress state. Assuming that the stresses closest to the weld bead are exclusively residual stresses due to welding, preheat temperature reduced the tensile nature, or increased the compressive nature, of the residual stresses. This is due to the preheat reducing the effect of shrinkage stresses induced after the austenite transformation upon cooling of the weldment. Because of the effects of grinding and the small sample sizes, no definitive conclusions could be drawn about the effects of heat input and plate thickness. It was shown that grinding was the dominant parameter on the residual stress state in these HSLA-100 bead-on-plate weldments. Because the angle and force of grinding are purely at the discretion of the operator, it is very difficult to determine the effects of different welding parameters on the residual stresses generated in bead-on-plate weldments ground prior to welding. / Master of Science

Lightweighting of automotive and heavy-duty vehicle components is an important task that does not need any further motivation or background. It can be read in a large part of the technical papers in the field. A common approach for finding lighter solutions is to increase the material grade while decreasing the material thickness. Often in combination with design changes. For perfectly smooth components this is not an issue, but when cut edges from manufacturing processes are present the situation changes. One topic to address is that increased material grade often means increased notch and surface damage sensitivity. This has implications both on forming and fatigue. The reason for selecting a higher strength material is to allow for higher stresses in design. It has however been shown that for a given stress level the fatigue performance of a higher strength material could be worse than for a lower strength counterpart if punched holes or trimmed edges are present. This means that in the search of lower weight there is a risk of increasing stresses, and at the same time selecting a material that is less suited to handle this increase. Hence, engineers and developers are put in a position where these effects must be quantified to find the most efficient solution. This quantification is a cumbersome and expensive task, often including a considerable amount of testing. Important sources of fatigue life reduction in this context are the residual stresses in the loading direction and the surface roughness in the cut edge. This thesis aims to present an overview of metal fatigue in the context of shear cut components. Necessary knowledge regarding the shear cutting process is provided along with a description of numerical methods and considerations for process simulations. These findings are then applied to the presented papers where the first introduces a simplified approach for numerical simulation of shear cutting to obtain residual stresses. In this approach the simplification mainly lies in the failure model calibration. The second paper studies the possibility of using the obtained residual stresses together with measured values of surface roughness to quantify fatigue life reduction of shear cut specimens.

Advances in computer automation and control, compact and portable x-ray sources, and reliable and efficient detector systems over the last ten years have allowed X-Ray Determination of Residual Stress (XRDRS) measurements to become a viable method of evaluating the state of stress in metals, alloys, and ceramics. However, problems associated with incorrect XRDRS equipment operation and poor experimental technique are prevalent, necessitating better operator training and education. Therefore, an interactive computer workstation, called RS/hyper, was developed to lead the operator towards correct operating procedures and reliable experimental technique. RS/hyper teaches the operator proper machine setup, machine maintenance, radiation safety, experimental technique, theoretical understanding, and provides limited data evaluation. Graphical aids are used extensively to avoid confusion and misinterpretation during the learning process. Since RS/hyper is interactive, the operator may select the desired level of understanding on a particular topic. These topics are linked, through a hypertext interface, so that the information is presented coherently and efficiently. Compared to written texts and references, RS/hyper has been shown in preliminary tests to reduce XRDRS training and problem solving time by a factor of 16. RS/hyper will train novice users of XRDRS equipment so that the data acquired from such machines will be reliable in an industrial environment. Since the software educates the user, the results of the data will be more accurately represented before interpretation. The experienced user should find RS/hyper useful as a reference of XRDRS and related information. / Master of Science

A series of experiments was performed in an attempt to measure the effects of surface residual stresses on the stress corrosion cracking/hydrogen embrittlement (SCC/HE) behavior of AISI 4340 steel. Stress corrosion tests were performed under load control on cylindrical and notched tensile specimens in acidified 3.5% NaCl solution. The electrochemical potential of the specimens was maintained at -0.7 V versus a saturated calomel reference electrode. Time to failure for specimens tested at various applied and residual stress levels was measured. Stress relieved specimens as well as specimens containing mechanically induced residual stresses were tested. Residual stresses were estimated using Neuber's rule and were measured using an x-ray diffraction technique. In all cases, the sum of the applied and residual stresses was greater than zero. Test results showed the initiation of SCC/HE cracks to be insensitive to the effects of surface residual stresses under the conditions evaluated. This is probably a result of the total time to failure criterion used to evaluate the SCC/HE tests. The extremely aggressive environment used in these experiments apparently led to rapid crack initiation, even in specimens containing compressive residual stresses. Another possible explanation of the insensitivity of this series of tests is crack initiation in the interior of the specimens below the depth of the mechanically induced residual stresses. / Master of Science

The research presented here investigates the effects of laser trimming on the state of stress in alumina Al₂O₃ hybrid microelectronics substrates. Evaluation of stress was performed using x-ray diffraction residual stress analysis and dynamic strain measurements using strain gages before and after laser trimming. X-ray diffraction measurements were carried out in both the longitudinal and transverse directions on the front and back sides of the substrates. The dynamic strain measurements were performed in situ with strain gages attached to the bottom of the substrates while the substrates were trimmed with a 400 watt YAG laser. The substrates were characterized using optical microscopy, scanning electron microscopy / energy dispersive x-ray analysis (SEM/EDAX), electron probe microanalysis (EPMA) and electron spectroscopy for chemical analysis (ESCA). The results from these characterization steps gave results for fractography (optical), surface and bulk composition (SEM/EDAX), chemical composition (ESCA) and phase analysis (EPMA). Results show that laser trimming produces stress gradients which are generally tensile in nature and could have deleterious effects on the mechanical integrity of the substrates if used in hybrid microelectronic applications. Furthermore the stress distribution across the substrates was found to be uniformly distributed showing no peak stresses near the heat affected zone (HAZ) boundary. Phase analysis determined that the substrates contained a magnesium aluminum spinel phase (MgAl₂O₄) and that the glass and pore phases are randomly distributed in the substrates. This could have some overall effect on the state of residual stress in the substrates after they have been laser trimmed. / Master of Science