The overall goal of this work is to predict the mechanical response of an as-built Ti-6Al-4V directed energy deposition component by a dislocation mechanics-based internal state variable model based on the component's geometry and processing parameters. Previous research has been performed to connect additive manufacturing (AM) process parameters including laser power and scanning strategy to different aspects of part quality, such as porosity, mechanical properties, fatigue life, microstructure, residual stresses, and distortion. The lack of predictive capabilities to fully estimate residual stresses and distortion within parts produced via AM have hindered part qualification; however, modeling the AM process can aide in process and geometry optimization compared to traditional trial-and-error methods. The presence of unwanted thermally induced residual stresses and distortion can lead to tolerancing issues, reduced fatigue life, and decreased mechanical performance compared to similar components fabricated with traditional manufacturing methods such as casting and machining. A three-dimensional thermomechanical finite element model calibrated using dual-wave pyrometer thermal image datasets along with temperature- and strain rate-dependent mechanical data is utilized for this work. The purpose of this work is to understand the relationship between a component's temperature history and its resultant distortion and residual stresses.

Additive manufacturing (AM) of parts using a layer by layer approach has seen a rapid increase in application for production of net shape or near-net shape complex parts, especially in the field of aerospace, automotive, etc. Due to the superiority of manufacturing complex shapes with ease in comparison to the conventional methods, interest in these kinds of processes has increased. Among various methods in AM, laser powder bed fusion (LPBF) is one of the most widely used techniques to produce metallic components. As in all manufacturing processes, residual stress (RS) generation during manufacturing is a relevant issue for the AM process. RS in AM are generated due to a high thermal gradient between subsequent layers. The impact of residual stresses can be significant for the mechanical integrity of the built parts and understanding the generation of RS and the effect of AM process parameters is therefore important for a broader implementation of AM techniques. The work presented in this licentiate thesis aims to investigate the influence of build orientation on the RS distribution in AM parts. For this purpose, L-shaped Inconel 718 parts were printed by LPBF in three different orientations, 0°, 45°, and 90°, respectively. Inconel 718 was selected because it is a superalloy widely used for making gas turbine components. In addition, IN718 has in general good weldability which renders it a good material for additive manufacturing. Residual stress distributions in the parts removed from the build plate were measured using neutron diffraction technique. A simple finite element model was developed to predict the residual stresses and the effect of RS relaxation due to the separation of the parts and build plate. The trend of residual stress distribution predicted was in good agreement with experimental results. In general, compressive RS at the part center and tensile RS near the surface were found. However, while the part printed in 0° orientation had the least amount of RS in all three principal directions of part, the part built in 90° orientation possessed the highest amount of RS in both compression and tension. The study has shown that residual stress distributions in the parts are strongly dependent on the building process. Further, it has shown that the relaxation of RS associated with the removal of the parts from the build plate after printing has a great impact on the final distribution of residual stress in the parts. These results can be used as guidelines for choosing the orientations of the part during printing.

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Thermal barrier coatings (TBCs) have been widely used in gas turbine engines to protect the underlying metal from high operating temperature so as to improve the durability of the components and enhance the engine efficiency. However, since the TBCs always operate in a demanding high-temperature environment of aircraft and industrial gas-turbine engines, a better understanding of this complex system is required to improve the durability and reliability. The objective of this study is to investigate the effects of surface modification for the NiCoCrAlY bond coats on the thermal cycling lifetime and failure characteristics of TBCs. Parameters of modification for the bond coats included as-sprayed, barrel-finished, hand-polished and pre-oxidation heat treatment at 1100[degrees]C in P=10O2-8 atm up to 4 hours, carried out prior to the electron beam physical vapor deposition (EB-PVD) of ZrO2-7wt% Y2O3 (7YSZ) ceramic topcoat. The resulting characteristics of the bond coat and the thermally grown oxide (TGO) scale were initially documented by surface roughness, phase constituents of the TGO scale, and residual stress of the TGO scale. The thermal cycling test consisted of 10-minute heat-up to 1121°C, 40-minute hold at 1121°C, and 10-minute forced air-quench. As-coated and thermally-cycled TBCs were characterized by optical profilometry (OPM), photo-stimulated luminescence spectroscopy (PSLS), optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and scanning/transmission electron microscopy (TEM/STEM) equipped with high angle annular dark field (HAADF) and X-ray energy dispersive spectroscopy (XEDS). TBC specimens for TEM/STEM analysis were prepared by focused ion beam (FIB) in-situ lift-out (INLO) technique. Superior thermal cycling lifetime was observed for TBCs with as-sprayed bond coats regardless of pre-oxidation heat treatment, and TBCs with hand-polished bond coats only after pre-oxidation heat treatment. With pre-oxidation heat treatment, relative photostimulated luminescence intensity of the equilibrium α-Al2O3 increased. Thus, the improvement in TBC lifetime can be correlated with an increase in the amount of α-Al2O3 in the TGO scale, given a specific surface modification/roughness. The lifetime improvement due to pre-oxidation was particularly significant to TBCs with smooth hand-polished bond coats and negligible for TBCs with rough as-sprayed bond coats. Spallation-fracture paths depended on the lifetime of TBCs. Premature spallation of TBCs occurred at the interface between the YSZ and TGO. Longer durability can be achieved by restricting the fracture paths to the TGO/bond coat interface. Small particulate phase observed through the TGO scale was identified as Y2O3 (cubic) by diffraction analysis on TEM. While small addition of Y in the NiCoCrAlY bond coat helps the adhesion of the TGO scale, excessive alloying can lead to deleterious effects.

Thermal Barrier Coatings have been used for decades to impose a thermal gradient between the hot combustion gases and the underlying superalloy substrate in engine turbine blades. Yttria Stabilized Zirconia (YSZ) is an industry standard high temperature ceramic for turbine applications. The protective coating is adhered to the substrate using a nickel based alloy bond coat. Through exposure to high temperature, a Thermally Grown Oxide (TGO) layer develops at the bond coat-YSZ interface. Large residual stresses develop in these layers due to thermal expansion mismatch that occurs during cool down from high temperature spraying and cyclic operating conditions. Despite their standard use, much is to be determined as to how these residual stresses are linked to the various failure modes. This study developed techniques to monitor the strain and stress in these internal layers during thermal gradient and mechanical conditions representing operating conditions. The thermal gradient is applied across the coating thickness of the tubular samples from infrared heating of the outer coating and forced air internal cooling of the substrate. While thermal and mechanical loading conditions are applied, 2-dimensional diffraction measurements are taken using the high-energy Synchrotron X-Rays and analyzed to provide high-resolution depth-resolved strain. This study will include fatigue comparisons through use of samples, which are both 'as-coated' as well as aged to various stages in a TBC lifespan. Studies reveal that variations in thermal gradients and mechanical loads create corresponding trends in depth resolved strains with the largest effects displayed at or near the bond coat/TBC interface. Single cycles as well as experiments targeting thermal gradient and mechanical effects were conducted to capture these trends. Inelastic behavior such as creep was observed and quantified for the different layers at high temperatures. From these studies more accurate lifespan predictions, material behaviors, and causes of failure modes can be determined. The work further develops measurement and analysis techniques for diffraction measurements in internal layers on a coated tubular sample which can be used by various industries to analyze similar geometries with different applications.

Manufacturing induced shape distortions is a common problem for composite materials. Due to the non-isotropic nature of carbon fibre reinforced polymers (CFRP) unavoidable deformations occur during part production. During fabrication of polymer composites, the material obtains its final shape at elevated temperatures. The curing process involves a transition from the liquid state to the solid, glassy state, allowing bonding between fibres and matrix. As the material cools the mismatch in thermal expansion coefficients and cure shrinkage obtained during the matrix polymerization leads to residual stresses on the mechanical level within composite part. There is a great interest from the aircraft and automotive industries, to increase the ability to understand development of shape distortions and residual stresses during the cure, since these deformations often lead to dissatisfaction of tolerances and it is essential to predict the deformations beforehand in order to compensate time and cost.  In this context, a study of residual stresses during the curing process of thermosetting resin composites is presented. A methodology is proposed for predicting the formation and development of manufacturing- induced residual stresses. The present project reports on a comprehensive experimental study on the dependency of different short curing cycles on the build-up of residual stresses in a carbon fibre/fast-curing epoxy system and evaluate of post-curing methods through induction heating and oven post-curing with unidirectional [904] and unsymmetrical [9020] laminates. It includes characterization in thermo-elastic properties and degree-of-cure of the material by Thermal bending test, thermal expansion test, mechanical tensile test and Differential Scanning Calorimetry (DSC) in non-post-cured and post-cured laminates. The results showed slight variation in the thermal properties and not effect in the mechanical properties at different cure and post-curing conditions. Analytical data by Laminate Analysis program validated the experimental thermo-elastic data with analytical simulations. In addition, it is shown improvements in the temperature distributions in the post-curing by induction heating with different experimental set-ups, however, oven post-curing showed a more systematic system, higher heat efficient a low cure temperature, with more consistent mechanisms of shape distortions and residual stresses compared to induction heating. These findings are relevant for the future development of prediction methods for process induced deformations of Fast Curing Epoxy Resins (FCER).

The polymer laminated sheet metal (PLSM) is a layered material which involves a sheet metal substrate, a thin polymer film and an adhesive layer between the film and the substrate. The adhesion properties between the bonded materials are among the most important issues in PLSM forming operations. In this thesis, the main focus has been devoted to characterizing and improving the adhesion properties of the PLSM system for forming applications. Metallic surface roughness evolution and residual stress development in polymer adherends are two consequences of the plastic deformation of the PLSMs. In chapter 2, the effect of these factors on interfacial adhesion strength between metallic substrate and polymer adherend (polymer film with a thin uniform pressure-sensitive adhesive layer on one side) is investigated by devising a new experimental methodology. This methodology is based on two different protocols for preparation of peel sample, one involving pre-straining in uniaxial tension of the metallic substrate prior to lamination and the other involving post-lamination pre-straining of the PLSM. In chapter 3, the peel test results of two different types of PLSMs at different peel speeds are analyzed with two different approaches common in cohesive zone modeling in the literature, namely linear elastic stiffness approach and critical maximum stress approach. The modeling results revealed the significance of the peel speed in determining the interface strength between the adhesive and metallic substrate. In chapter 4, two mechanical treatment techniques of grinding and knurling are implemented to alter the metallic substrate surface roughness before lamination. Peel strength of these samples are investigated at different peel speeds and at different peel loading directions with respect to the grinding and knurling directions. / Thesis / Doctor of Philosophy (PhD) / The polymer laminated sheet metal (PLSM) is a layered material which involves a sheet metal substrate, a thin polymer film and an adhesive layer between the film and the substrate. In this thesis, the main focus has been devoted to characterizing and improving the adhesion properties of the PLSM system for forming applications. A new experimental methodology has been devised for analyzing the effects of deformation-induced surface roughness of metallic substrate and deformation-induced residual stress in polymer adherends on interfacial peel properties of PLSMs. A novel interpretation of the results obtained from rate-independent cohesive zone modeling of peel test has revealed the significance of peel speed in determining the interface strength between the adhesive and the metallic substrate. In another part of this thesis, the effects of two substrate surface alteration techniques, grinding and knurling, on peel properties of PLSMs have been studied.

No description available.

The extracellular matrix (ECM) of the aorta is a complex meshwork of elastin, collagen, and glycosaminoglycans (GAG). It also modulates the mechanical properties of the aorta, which in turn dictate lethal ruptures such as those caused by aneurysm and dissection. Amongst other roles, aortic stiffness controls the aorta’s ability to expand and recoil, and residual stresses, which are those existing in the absence of load, affect the magnitude and distribution of the mechanical stresses throughout the aortic wall. Mechanical stresses can be predicted via complex computer models, powerful tools that can also provide insight regarding the risk of rupture, given that ruptures occur when the mechanical stresses exceed the strength of the aorta. While this dissertation is primarily focused on the effect of GAG on residual stresses, other ECM (collagen, elastin) and mechanical (stiffness) factors are considered to expand our understanding of the structure-mechanics relationship in the aorta. This is important because the ECM undergoes extensive remodelling during aging and disease, but it is also critically important, as mentioned, in the context of aortic rupture. We first explored the mechanical roles of GAG in a finite element model by studying both the transmural residual stresses and the opening angle (an indicator of circumferential residual stresses) in ascending (AS) aortic ring models. Both were shown to be modulated by the GAG content, gradient, and the nature of the transmural distribution. While a heterogeneous GAG distribution led to the development of residual stresses which could be released by a radial cut, this was not the case when a homogeneous distribution was prescribed. Because the GAG distributions used in the first study were based on assumptions, and to get an in vitro understanding of the ECM role in modulating residual stresses, biomechanical mechanisms were explored in thoracic aortas from 5- to 6-month-old pigs. In a second study, we generated new detailed data on the distributions of collagen, elastin and GAG, throughout the aortic wall in the AS, arch (AR), and descending thoracic (DT) regions, and established correlations between the ECM constituents and the opening angle. The strongest correlations were observed between the opening angle and the total collagen:GAG ratio as well as the total GAG content. In line with our first in silico work, this in vitro investigation revealed that the GAG content and gradient modulate circumferential residual stresses and suggested that the interaction between GAG and the ECM fibers also plays a role in regulating residual stresses. In a third study, we examined the extent of contribution of GAG to circumferential residual stresses and to the radial compressive stiffness of the aortic wall, as well as the underlying mechanism through which GAG contribute to the mechanical properties using enzymatic GAG depletion. GAG depletion was associated with a decrease in the opening angle, by approximately 25%, 32%, 42% in the AS, AR, and lower DT regions respectively, and an increase in the radial compressive stiffness of the AS aorta. Glycation was also associated with a decrease in the opening angle, in which GAG depletion also had a similar effect. A small loss of water content was detected after GAG depletion, and the AS region was also associated with a significant loss of compressive deformation in the inner layer of the aorta following GAG depletion, suggesting that GAG interact with ECM fibers in their effect on aortic mechanics. The garnered experimental geometrical data and intramural GAG distributions were finally used to simulate animal-specific aortic rings from the AS, AR, and DT regions. The opening angle response was evaluated in solid matrices assuming one layer, and two layers to capture the different mechanical behaviors of the intima-media and the adventitia. A Holmes-Mow constitutive relationship was used and material parameters were obtained by curve fitting experimental stress-strain curves obtained from biaxial tests. Numerical results were evaluated by comparing simulated and experimental opening angles, revealing a notable overall agreement between the two.

No description available.