Fiber Bragg grating sensors have been constantly researched for the last ten years and have finally begun to find use in the commercial market. However, one of the major factors limiting their widespread use is their system cost. Their lightweight, flexibility, electromagnetic immunity, and small size make fiber Bragg grating (FBG) sensors feasible in hostile environments where electrical and mechanical sensors may not function effectively. These sensor systems utilize moderately expensive light sources and detectors at telecommunication wavelengths of 1300 nm and 1550 nm. These are the center wavelengths of the mass-produced FBGs and FBG phase masks. This thesis addresses the development of a lower cost short wavelength fiber Bragg grating strain sensor system using gratings written at 790 nm and 850 nm with the modified phase mask method recently developed at Oregon State University. Short wavelength gratings allow the use of less expensive semiconductor sources and silicon detectors, greatly reducing the overall cost of a strain sensor system from approximately $1600 for a 1300 nm system to $1000 for a 790 nm system. First, the fundamental properties and historical background of fiber Bragg gratings were reviewed. Followed by a literature review of the structures, fabrication methods, and applications of FBGs including sensor applications. The design, manufacture, and assembly of the new short wavelength strain sensor system were described including the production of pigtailed super-luminescent edge emitting light emitting diodes (SELED) from commercial laser diodes, a fiber recoater, and multiple attempts to write a fiber Bragg grating in the 750-850 nm wavelength region. Finally, the short wavelength strain sensor system was compared with a 1300 nm strain sensor detailing the potential cost savings with the short wavelength system. / Graduation date: 2003

We have developed a new digital image correlation (DIC) algorithm for non-contact, two-dimensional, whole-field displacement and strain measurement. Relative to existing algorithms, our algorithm substantially reduces the calculation expense by using neighborhood information while processing the data to determine the displacement field in a sub-region of interest. The new algorithm also uses higher-order interpolations of the displacement field, allowing for better accuracy in estimating strain distributions when the deformation field is non-homogeneous. Numerically-generated digital images are used to show that the new algorithm accurately reproduces the imposed displacement fields. The algorithm is also tested on actual images from deformed specimens from a variety of experiments, and shown to perform satisfactorily. / Singapore-MIT Alliance (SMA)

A nonlinear finite element model was developed to study the behaviour of unstiffened steel plate shear walls. The model was validated using the results from quasi-static and dynamic experimental programs. With the validated finite element model, the performance of 4-storey and 8-storey Type D (ductile) and Type LD (limited-ductility) steel plate shear walls with moment-resisting beam-to-column connections was studied under spectrum-compatible seismic records. A design procedure that aims to achieve optimal seismic behaviour for steel plate shear walls was proposed. The proposed method uses the concepts of indirect capacity design principles of CAN/CSA-S16-01 to identify the infill plates that are likely to yield in the design earthquake. The proposed method was used for the design of two 4-storey and one 8-storey shear walls. Design axial forces and moments in the boundary columns for the shear walls were shown to be in good agreement with nonlinear seismic analysis results. Results also showed that some of the other capacity design methods available generally underestimate the maximum design forces in the columns, while others can be overly conservative. The effect of loading rate on the dynamic behaviour of steel plate shear walls was also investigated, as was the P-Delta effect in terms of its influence on seismic demand in shear and flexure. A shear strength model of the infill plate with circular openings at any location was developed based on a strip model where all the strips with perforations were partially discounted. A design method for steel plate shear walls with perforations was introduced. The method was applied for the design of boundary columns of a 4-storey steel plate shear wall with perforations. The predicted design forces in the columns for the 4-storey perforated shear wall agreed well with the forces obtained from nonlinear seismic analysis. Finally, an improved simple formula for estimating the fundamental period of steel plate shear walls was developed by regression analysis of the period data obtained from frequency analysis of series of steel plate shear walls. In addition, the effectiveness of a shearflexure cantilever formulation for determining fundamental periods and P-Delta effects of steel plate shear walls was studied. / Structural Engineering

The application of cellular therapies for the treatment of myocardial infarction has provided encouraging evidence for the possibility of cellular therapies to restore normal heart function. However, questions still remain as to the optimal cell source, pre-conditioning methods and delivery techniques for such an application. Here I propose the use of a unique population of stem cells arising from the embryonic neural crest. These cells were shown to express neural crest markers as well as pluripotency-associated markers. Furthermore, the cells were shown to express proteins essential to the formation of gap junctions and to possess a cardiomyogenic differentiation potential by several means. Furthermore, I explore the use of mechanical strain as an inducer of cardiomyogenesis and possibly pre-conditioning stimulus for the better engraftment of the cells while in the heart. Mechanical strain was shown to elicit a cardiomyogenic response from the cells following just a couple of hours of stimulation. The mode in which mechanical strain elicited these responses was demonstrated to be via the mediation of the reactive oxygen species (ROS) pathways. Given the results presented here, the use of these periodontal ligament-derived stem cells (PDLSC) in combination with mechanical strain preconditioning of the cells prior to their delivery into the heart may pose a valuable alternative for the treatment of myocardial infarction and merits further exploration for its capacity to augment the already observed beneficial effects of cellular therapies.

<p align='justify'>This thesis is devoted to the micromechanical study of the size-dependent strengthening in Transformation Induced Plasticity (TRIP) steels. Such grades of advanced high-strength steels are compelling for the automotive industry, due to their improved mechanical properties. Among others, they combine a good strength versus ductility balance. In this context, many research works have been carried out to study these grades of steels. In particular, from a numerical point of view, earlier studies within the framework of classical plasticity do not properly reproduce the strengthening levels characterizing TRIP steels and obtained experimentally.</p> <p align='justify'>In this study, the strain gradient plasticity theory presented by Fleck and Hutchinson (2001) is chosen to account for the strengthening effect resulting from the phase transformation. A two-dimensional embedded cell model of a simplified microstructure composed of small cylindrical metastable austenitic inclusions, partially undergoing the phase transformation, within a ferritic matrix is used.</p> <p align='justify'>First, the single-parameter version of the strain gradient plasticity theory under small strain assumption is used for the simulations. The impact of the higher order boundary conditions is assessed. It is shown that, when the plastic flow is unconstrained at the elasto-plastic boundaries, the transformation strain has no significant impact on the overall strengthening. The strengthening is essentially coming from the composite effect with a marked inclusion size effect resulting from the appearance during deformation of new boundaries (at the interface between parent and product phases) constraining the plastic flow.</p> <p align='justify'>Second, the multi-parameter version of the strain gradient plasticity theory, incorporating separately the rotational and extensional gradients in the formulation, is employed under small strain assumption. The effect of the plastic strain gradients resulting from the transformation strain is better captured. In particular, the results show a significant influence of the shear component of the transformation strain. An implicit confinement effect is revealed at the elasto-plastic boundaries which is partly responsible for the transformation strain effect. Size effects on the overall strengthening are also revealed, due to a combined size dependent effect of the transformation strain and of the evolving composite structure.</p><p align='justify'>Third, the extension of the strain gradient plasticity theory to a finite strain description is applied. A significant effect of the transformation strain is obtained with the multi-parameter version of the theory as well as an optimal austenite grain size improving the damage resistance of the martensite, in agreement with the typical grain size of the current TRIP-assisted steels (Jacques et al., 2007).</p>

Polymer matrix composites are widely used as structural components in the aerospace industry and wind turbine industry etc. to take advantage of their unique mechanical properties and weight saving ability. Although there have been considerable developments in analyzing delamination growth and effects of voids on certain mechanical properties of composites, none of the present literatures investigates the effects of voids on delamination growth under compression. In this research, a parametric study is performed to investigate the effects of voids on delamination growth in composite laminates under compression. In composite structures, delamination would be created by eccentricities in structural load path, structural discontinuities, and during manufacturing and maintenance processes. Also, the service damage such as the impact of foreign objects may also result in delamination. In the Finite Element model developed, a through-width surface delamination is assumed, and void is placed in critical locations ahead of crack tip. Strain Energy Release Rate (SERR) is calculated by the Virtual Crack Closure Technique (VCCT) in order to study the delamination growth. It is found that the delamination front experiences a mixed-mode delamination behavior when local out-of-plane buckling occurs. During the loading, Mode II SERR increases monotonically while Mode I SERR increases first and then decreases as the delamination front starts to close. Meanwhile, Mode II SERR is found to be much larger than the Mode I component. The presence of void does not significantly alter the transverse displacement of the delaminated part. However, the presence of void increases the Mode II SERR, as well as the total SERR, and this increase depends on the size and location of void. For Mode I SERR, the effect of void is not that prominent.

The brain undergoes structural changes in response to new experiences like learning a new skill. Skilled motor movements depend greatly on the primary motor cortex for their execution. Recent studies describe rat strain differences in motor performance related to differential synaptic efficacy in the motor cortex of rats. Previous studies identified differences in motor performance related to differential dendritic morphology and strain related differences in synaptic function in the motor cortex. Strain differences are one way of investigating anatomical organization and behaviour of the motor system. The object of this research was to examine strain related differences in dendritic morphology in layer II / III pyramidal cells of the forelimb area of the sensory motor cortex in both Long-Evans and Fischer 344 rats after reach training. This research also examined whether changes in reaching behaviour could be attributed to changes in dendritic morphology. Rats were trained once a day for 30 days to reach for a food pellet through a slot in a reaching box. Pyramidal cells in the motor sensory forelimb (MSF) cortex were stained with the Golgi Cox method. Subsequent analysis of Sholl and branch order data of cell drawings determined that there were no significant differences in any measure of dendritic length or dendritic length at branch order 3, 4, 5 of pyramidal cells in layer II/III of the MSF cortex between the Long Evans and Fischer 344 rat strain. The only significant strain related difference was that the Fischer 344 strain exhibited fewer reaches for each food pellet obtained, demonstrating greater reaching proficiency than similarly trained Long-Evans rats. These findings suggest that further research examining strain comparisons is required to understand the neural mechanisms underlying the differences in motor behaviour observed in these rat strains. / October 2006

In an effort to improve the fuel efficiency of automobiles, car designers are investigating new materials to reduce the overall vehicle weight. Magnesium alloys are good candidates to achieve that weight reduction due in part to their low density and high specific strength. To support their introduction into vehicle body structures, the dynamic behavior of magnesium alloys must be determined to assess their performance during a crash event. In this work, the tensile high strain rate behavior of AZ31B magnesium alloy sheets was characterized. Two different temper conditions were considered: AZ31B-O (fully annealed) and AZ31B-H24 (partially hardened). Three different sheet thicknesses were considered for the O temper condition, 1.0, 1.6 and 2.5 mm, while the H24 temper was 1.6 mm in thickness. The sheet condition of the magnesium alloys implies an in-plane anisotropy induced by the rolling process. Therefore, both the rolling and transverse directions were investigated in the current research. In order to characterize the constitutive behaviour of AZ31B-O and AZ31B-H24 magnesium alloy sheets, tensile tests were performed over a large range of strain rates. Quasi-static experiments were performed at nominal strain rates of 0.003s-1, 0.1s-1 and 1s-1 using a servohydraulic tensile machine. Intermediate strain rate experiments were performed at 30s-1 and 100s-1 using an instrumented falling weight impact (IFWI) apparatus, and high strain rate experimental data at 500s-1, 1000s-1 and 1500s-1 was collected using a tensile split Hopkinson bar (TSHB) apparatus. Elevated temperature experiments (up to 300°C) were also performed at high strain rates using a radiative furnace mounted on the TSHB apparatus. The tensile experiments show a significant strain rate sensitivity of the constitutive behavior of both the O and H24 temper conditions. The two tempers exhibit an average increase of stress level of 60-65 MPa over the range of strain rates considered. As the strain rate increases, the strain rate sensitivity of both tempers also increases. The strain rate has a different effect on the ductility of the two material conditions. The ductility of AZ31B-O is significantly improved under high strain rate deformations, whereas the AZ31B-H24 exhibits similar ductility at low and high strain rates. Both material conditions presented a strong in-plane anisotropy, with an average stress level in the transverse direction higher than in the rolling direction by 15 MPa and 35 MPa for the O and H24 tempers, respectively. The thermal sensitivity for both tempers at high strain rates was obtained. The two material conditions exhibit a clear thermal softening. From room temperature to 250°C, the loss in strength at 5% plastic strain was found to be 55 MPa and 125 MPa for the AZ31B-O and AZ31B-H24 materials, respectively. The thickness of the AZ31B-O sheets has a mild effect on the measured constitutive behavior. The flow stress increases with increasing thickness. An average difference of 10-15 MPa was seen between the flow stress of the 1.0mm and 2.5mm sheets. However, similar strain rate sensitivity was seen for the three thicknesses. The experimental data was fit to three constitutive models: the Johnson-Cook model, its modified version with a Cowper-Symonds strain rate sensitivity formulation, and the Zerilli-Armstrong model. The three models were evaluated by numerical simulation of the TSHB experiment under various testing conditions. It was found that the Zerilli-Armstrong model was the most accurate in predicting the flow stress of the different material conditions. However, finite element models incorporating the three constitutive fits failed to predict necking in the specimen.

Shape memory alloys (SMAs) are a class of alloys that display the unique ability to undergo nonlinear deformations and return to their original shape when heat is applied or the stress causing the deformation is removed. This unique shape memory characteristic is a result of a martensitic phase-change, which can be temperature induced (shape memory effect) or stress induced (superelastic effect). In this study, the cyclical behavior of NiTi, a binary shape memory alloy, is compared to the cyclical behavior of NiTiCr, a ternary SMA. The purpose of this study is to compare the behavior of a 0.085-in. diameter NiTiCr wire with the behavior of the same size NiTi wire to determine whether ternary SMAs are more viable ways to take advantage of the unique properties of SMAs for seismic applications. The experimental results showing the superelastic behavior of these alloys under cyclical tensile loading are summarized with attention to the effects of annealing temperature, strain rate, and cyclical training on the stress-strain hysteresis, maximum recoverable strain and equivalent viscous damping.

In recent years, multiferroics was one of the most popular materials and were widely studied by many scientists. Among all we interested the most is BiFeO3, which exist a room temperature ferromagnetic and ferroelectric properties with high ferromagnetic transition temperature Tc that can provide various kind of applications. However, the drawback of the BiFeO3 is difficult to synthesize the pure phase and to eliminate the leakage of the current. According to others¡¦ reports, with a proper doping can reduce the evaporation of Bi atoms and stabilize the crystal structure. Therefore, we choose Pb as the mixed elements and wish to reduce the unstable oxygen vacancies around the Bi atoms and to increase the dielectric property of Bi1-xPbxFeO3. In our experiment, SrRuO3 is chosen as the buffer layer material, which can grow as a strain relaxed film on the substrate. It is found that the strain relaxation transforms the SrRuO3 crystal structure to a nearly cubic one which has a lattice matching with Bi1-xPbxFeO3. As a result of this, we might improve the leakage problems of Bi1-xPbxFeO3.