Commercially available polycrystalline Ni (Ni200; grain size: 32 μm) and electrodeposited nanocrystalline Ni (grain size: 57 nm), Ni-2.6%Fe (grain size: 25 nm) and Ni-8.5%Fe (grain size: 20 nm) were analyzed for the phenomena of stress-induced heat generation and strain localization during plastic deformation at room temperature (i.e. 250C). Tensile specimens according to ASTM E8 standard dimensions were tested at strain rates of 10-2/s and 10-1/s, respectively, to record the amount of heat dissipated and the change of localized strain using a high resolution infrared (IR) detector and digital image correlation (DIC) camera, respectively. Results have shown that the maximum temperatures that were recorded in nanocrystalline Ni and Ni-Fe alloys were at least 300C lower than the onset temperatures for subgrain coalescence previously measured through differential scanning calorimetry. It can be concluded that thermally activated grain growth during tensile testing of nanocrystalline Ni and Ni-Fe alloys is not likely to occur.

Commercially available polycrystalline Ni (Ni200; grain size: 32 μm) and electrodeposited nanocrystalline Ni (grain size: 57 nm), Ni-2.6%Fe (grain size: 25 nm) and Ni-8.5%Fe (grain size: 20 nm) were analyzed for the phenomena of stress-induced heat generation and strain localization during plastic deformation at room temperature (i.e. 250C). Tensile specimens according to ASTM E8 standard dimensions were tested at strain rates of 10-2/s and 10-1/s, respectively, to record the amount of heat dissipated and the change of localized strain using a high resolution infrared (IR) detector and digital image correlation (DIC) camera, respectively. Results have shown that the maximum temperatures that were recorded in nanocrystalline Ni and Ni-Fe alloys were at least 300C lower than the onset temperatures for subgrain coalescence previously measured through differential scanning calorimetry. It can be concluded that thermally activated grain growth during tensile testing of nanocrystalline Ni and Ni-Fe alloys is not likely to occur.

Mn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1→6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1→6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch.

In an automotive crash event, hot stamped, die quenched martensitic structural components have been shown to provide excellent intrusion resistance. These alloys exhibit only limited ductility, however, which may limit the overall impact performance of the component. The introduction of lower strength and more ductile “tailored” properties within some regions of a hot stamped component has the potential to improve impact performance. One approach being applied to achieving such tailored properties is through locally controlling the cooling rate within the stamping die. The primary motivation for the current work is to understand the role of cooling rate on the as-quenched mechanical response of tailored hot stampings, which has required characterization of the high strain rate mechanical behaviour of tailored hot stamped boron steel. The effect of cooling rate and resulting microstructure on the as-quenched mechanical behavior of USIBOR® 1500P boron steel at strain rates between 10-3 and 103 s-1 was investigated. Specimens quenched at rates above the critical cooling rate (~27 °C/s) exhibited a fully martensitic microstructure with a UTS of ~1,450 MPa. Sub-critical cooling rates, in the range 14°C/s to 50 °C/s, resulted in as-quenched microstructures ranging between bainitic to martensitic, respectively. Tension tests revealed that predominantly bainitic material conditions (14 °C/s cooling rate) exhibited a lower UTS of 816 MPa compared to 1,447 MPa for the fully martensitic material condition (50 °C/s cooling rate) with a corresponding increase in elongation from 0.10 to 0.15 for the bainitic condition. The reduction in area was 70% for the bainitic material condition and 58% for the martensitic material conditions which implied that a tailored region consisting of bainite may be a desirable candidate for implementation within a hot stamped component. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the measured flow stress curves were used to develop a strain rate sensitive constitutive model, the “Tailored Crash Model (TCM)”. The TCM accurately reproduced the measured flow stress curves as a function of effective plastic strain, strain rate and Vickers hardness (or area fraction of martensite and bainite). The effect of deformation during quenching and the associated shift in the CCT diagram on the subsequent constitutive response was also examined for this material. Specimens were simultaneously quenched and deformed at various cooling rates to achieve a range of as-quenched microstructures that included ferrite in addition to martensite and bainite. Tensile tests conducted on these specimens at strain rates ranging from 0.003 s-1 to ~80 s-1 revealed that the presence of ferrite resulted in an increase in uniform elongation and n-value which increased overall energy absorption for a given hardness level. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the TCM constitutive model was extended to account for the presence of ferrite. This extended constitutive model, the “Tailored Crash Model II (TCM II)”, has been shown to predict flow stress as a function of effective plastic strain, strain rate and area fraction of martensite, bainite and ferrite. As a validation exercise, uniaxial tension test simulations of specimens extracted from the transition zone of a hot stamped lab-scale B-pillar with tailored properties [1] were performed. The measured hardness distribution along the gauge length of the tensile specimens was used as input for the TCM constitutive model to define the element constitutive response used in the finite element (FE) models. The measured stress versus strain response and strain distribution during loading (measured using digital image correlation) was in excellent agreement with the FE models and thus validated the TCM constitutive model developed in this work. Validation of the TCM II version of the model is left for future work.

The thesis examines Thomas De Quincey’s opium use as a product of social strain. De Quincey’s collection of work provides evidence that he felt alienated from society prior to his addiction and that his feelings of inadequacy contributed to his dependence on drugs. Utilizing Robert K. Merton’s strain theory, this thesis delineates De Quincey’s aspirational references and perceived failures through an examination of his imagery and interprets his perceptions of human life as a catalyst for his compulsions to cope with opium. De Quincey, strained by the aspirations of an industrial and imperialistic society, looked for several avenues of escape. The Romanticism of William Wordsworth presented De Quincey with a method for alleviating social strain; however, when De Quincey failed to discover the transcendence evident in Lyrical Ballads he turned to the intoxicating effects of opium and retreated from English society.

Das hier vorgestellte Vorhaben hat die Entwicklung eines neuartigen Sensor- und Messtechnikkonzepts zur Erfassung der Maximalwerte mechanischer Belastungen von Bauteilen bestehend aus Kohlenstofffaserverbundwerkstoffen mit polymerer Matrix (CFK) zum Ziel. Dazu wurden Untersuchungen zur Anwendbarkeit der spontanen Magnetisierung durch die martensitische Gefügeumwandlung metastabiler Stähle als möglicher Sensoreffekt durchgeführt. Als ein einfaches passives Sensorverfahren hat die Ausbildung einer spontanen Magnetisierung durch die Gefügeumwandlung ein denkbares Anwendungspotenzial. Es ist keine permanente Energieversorgung des Messsystems erforderlich, der Sensor ist vollständig und ohne Kontakt zur Umgebung im Bauteil einbettbar und eine äquivalente Information der maximalen Last ist unauslöschbar im Sensor eingeprägt. Durch kontakt- und zerstörungsfreie Verfahren kann das Auslesen erfolgen. Um die Eignung des Effekts zu überprüfen, erfolgten verschiedene Untersuchungen. Die Ergebnisse von Zugversuchen, durchgeführt an eingebetteten und nicht eingebetteten Sensorproben, und die Messwerte magnetischer Messungen der Sensormagnetisierung wurden zur Ermittlung einer Abhängigkeit zwischen dem Grad der Belastung und der Änderung der Magnetisierung infolge der martensitischen Transformation korreliert. Mittels metallografischer Untersuchungen sowie durch Röntgenbeugung erfolgte zudem der Vergleich zwischen dem Martensitgehalt und den Messergebnissen zur spontanen Magnetisierung. Der vorliegende Artikel beschreibt die Grundlagen des ausgewählten Sensorprinzips, die durchgeführten Arbeiten und erläutert die Ergebnisse der Untersuchungen.

Methods for measuring deformation and motion of the human heart in-vivo are crucial in the assessment of cardiac function. Applications ranging from basic physiological research, through early detection of disease to follow-up studies, all rely on the quality of the measurements of heart dynamics. This thesis presents new improved magnetic resonance imaging methods for acquisition, image reconstruction and visualization of cardiac motion and deformation.As the heart moves and changes shape during the acquisition, synchronization to the heart dynamics is necessary. Here, a method to resolve not only the cardiac cycle but also the respiratory cycle is presented. Combined with volumetric imaging, this produces a five-dimensional data set with two cyclic temporal dimensions. This type of data reveals unique physiological information, such as interventricular coupling in the heart in different phases of the respiratory cycle.The acquisition can also be sensitized to motion, measuring not only the magnitude of the magnetization but also a signal proportional to local velocity or displacement. This allows for quantification of the motion which is especially suitable for functional study of the cardiac deformation. In this work, an evaluation of the influence of several factors on the signal-to-noise ratio is presented for in-vivo displacement encoded imaging. Additionally, an extension of the method to acquire multiple displacement encoded slices in a single breath hold is also presented.Magnetic resonance imaging is usually associated with long scan times, and many methods exist to shorten the acquisition time while maintaining acceptable image quality. One class of such methods involves acquiring only a sparse subset of k-space. A special reconstruction is then necessary in order to obtain an artifact-free image. One family of these reconstruction techniques tailored for dynamic imaging is the k-t BLAST approach, which incorporates data-driven prior knowledge to suppress aliasing artifacts that otherwise occur with the sparse sampling. In this work, an extension of the original k-t BLAST method to two temporal dimensions is presented and applied to data acquired with full coverage of the cardio-respiratory cycles. Using this technique, termed k-t2 BLAST, simultaneous reduction of scan time and improved spatial resolution is demonstrated. Further, the loss of temporal fidelity when using the k-t BLAST approach is investigated, and an improved reconstruction is proposed for the application of cardiac function analysis.Visualization is a crucial part of the imaging chain. Scalar data, such as regular anatomical images, are straightforward to display. Myocardial strain and strain-rate, however, are tensor quantities which do not lend themselves to direct visualization. The problem of visualizing the tensor field is approached in this work by combining a local visualization that displays all degrees of freedom for a single tensor with an overview visualization using a scalar field representation of the complete tensor field. The scalar field is obtained by iterated adaptive filtering of a noise field, creating a continuous geometrical representation of the myocardial strain-rate tensor field.The results of the work presented in this thesis provide opportunities for improved imaging of myocardial function, in all areas of the imaging chain; acquisition, reconstruction and visualization.

No description available.

The present work focuses on the formulation of a Continuum Damage Mechanics model for nonlinear analysis of masonry structural elements. The material is studied at the macro-level, i.e. it is modelled as a homogeneous orthotropic continuum. The orthotropic behaviour is simulated by means of an original methodology, which is based on nonlinear damage constitutive laws and on the concept of mapped tensors from the anisotropic real space to the isotropic fictitious one. It is based on establishing a one-to-one mapping relationship between the behaviour of an anisotropic real material and that of an isotropic fictitious one. Therefore, the problem is solved in the isotropic fictitious space and the results are transported to the real field. The application of this idea to strain-based Continuum Damage Models is rather innovative. The proposed theory is a generalization of classical theories and allows us to use the models and algorithms developed for isotropic materials. A first version of the model makes use of an isotropic scalar damage model. The adoption of such a simple constitutive model in the fictitious space, together with an appropriate definition of the mathematical transformation between the two spaces, provides a damage model for orthotropic materials able to reproduce the overall nonlinear behaviour, including stiffness degradation and strain-hardening/softening response. The relationship between the two spaces is expressed in terms of a transformation tensor which contains all the information concerning the real orthotropy of the material. A major advantage of this working strategy lies in the possibility of adjusting an arbitrary isotropic criterion to the particular behaviour of the orthotropic material. Moreover, orthotropic elastic and inelastic behaviours can be modelled in such a way that totally different mechanical responses can be predicted along the material axes. The aforementioned approach is then refined in order to account for different behaviours of masonry in tension and compression. The aim of studying a real material via an equivalent fictitious solid is achieved by means of the appropriate definitions of two transformation tensors related to tensile or compressive states, respectively. These important assumptions permit to consider two individual damage criteria, according to different failure mechanisms, i.e. cracking and crushing. The constitutive model adopted in the fictitious space makes use of two scalar variables, which monitor the local damage under tension and compression, respectively. Such a model, which is based on a stress tensor split into tensile and compressive contributions that allows the model to contemplate orthotropic induced damage, permits also to account for masonry unilateral effects. The orthotropic nature of the Tension-Compression Damage Model adopted in the fictitious space is demonstrated. This feature, both with the assumption of two distinct damage criteria for tension and compression, does not permit to term the fictitious space as “isotropic”. Therefore, the proposed formulation turns the original concept of “mapping the real space into an isotropic fictitious one” into the innovative and more general one of “mapping the real space into a favourable (or convenient) fictitious one”. Validation of the model is carried out by means of comparisons with experimental results on different types of orthotropic masonry. The model is fully formulated for the 2-dimensional case. However, it can be easily extended to the 3-dimensional case. It provides high algorithmic efficiency, a feature of primary importance when analyses of even large scale masonry structures are carried out. To account for this requisite it adopts a strain-driven formalism consistent with standard displacement-based finite element codes. The implementation in finite element programs is straightforward. Finally, a localized damage model for orthotropic materials is formulated. This is achieved by means of the implementation of a crack tracking algorithm, which forces the crack to develop along a single row of finite elements. Compared with the smeared cracking approach, such an approach shows a better capacity to predict realistic collapsing mechanisms. The resulting damage in the ultimate condition appears localized in individual cracks. Moreover, the results do not suffer from spurious mesh-size or mesh-bias dependence. The numerical tool is finally validated via a finite element analysis of an in-plane loaded masonry shear wall.

There exists considerable motivation to reduce vehicle weight through the adoption of lightweight materials, such as aluminum alloys, while maintaining energy absorption and component integrity under crash conditions. To this end, it is of particular interest to study the crash behaviour of lightweight tubular hydroformed structures to determine how the forming behaviour affects the axial crush response. Thus, the current research has studied the dynamic crush response of both non-hydroformed and hydroformed EN-AW 5018 and AA5754 aluminum alloy tubes using both experimental and numerical methods. Experiments were performed in which hydroforming process parameters were varied in a parametric fashion after which the crash response was measured. Experimental parameters included the tube thickness and the hydroformed corner radii of the tubes. Explicit dynamic finite element simulations of the hydroforming and crash events were carried out with particular attention to the transfer of forming history from the hydroforming simulations to the crash models. The results showed that increases in the strength of the material due to work hardening during hydroforming were beneficial in increasing energy absorption during crash. However, it was shown that thinning in the corners of the tube during hydroforming decreased the energy absorption capabilities during axial crush. Residual stresses resulting from hydroforming had little effect on the energy absorption characteristics during axial crush. The current research has shown that, in addition to capturing the forming history in the crash models, it is also important to account for effects of material non-linearity such as kinematic hardening, anisotropy, and strain-rate effects in the finite element models. A model combining a non-linear kinematic hardening model, the Johnson-Cook rate sensitive model, and the Yld2000-2d anisotropic model was developed and implemented in the finite element simulations. This combined model did not account for the effect of rotational hardening (plastic spin) due to plastic deformation. It is recommended that a combined constitutive model, such as the one described in this research, be utilized for the finite element study of materials that show sensitivity to the Bauschinger effect, strain-rate effects, and anisotropy.