Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item A Strain-Based Experimental Methodology for Measuring Sectional Stiffness Properties of Composite Blades(2020) Sinotte, Tyler Mel; Bauchau, Olivier A.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Sectional stiffness properties of composite rotor blades, such as the axial, torsional, and bending stiffness, play an important role in the design and analysis of rotorcraft, as they impact the predicted rotor dynamics, structural loads, and stress and strain fields. Multiple numerical tools exist for predicting the sectional stiffness properties based on the cross-sectional geometries and materials; however, rotor blades are often made of composite materials and require complex manufacturing procedures, making it challenging to provide an accurate prediction of the inputs needed for these numerical tools. This dissertation therefore focuses on developing an experimental technique for calculating the sectional stiffness properties, based on the measurement of a detailed strain field using digital image correlation. Two primary features distinguish the developed methodology from currently existing techniques. First, this method is based upon measured strain fields, as opposed to measurements of displacements or frequencies that are traditionally used. The strain field provides a description of the local deformation of the blades, thereby allowing measurements of the sectional stiffness properties to be made at discrete spanwise locations along the blade and providing the capability to predict changes in properties due to features commonly associated with helicopter rotors, such as twist or taper. Second, this method can be used to calculate the full cross-sectional stiffness matrix based on a combination of experimental measurements and a numerical warping function, as opposed to only a subset of these properties that are typically measured in displacement or frequency based approaches. The developed methodology is first validated using numerical results from 3-D FEA for cross-sections that have received extensive study in literature. A test setup is then developed for experimental implementation of the proposed method and applied to five different beams. The material properties and geometries of these beams were selected to place an emphasis on the unique capabilities of the method, including the measurement of elastic coupling stiffness components and spanwise variations in the stiffness properties, with results compared against analytic solutions and predictions from SectionBuilder. A detailed uncertainty analysis is also implemented for estimating the impact of measurement errors on the stiffness properties.Item Determination of Mixed Mode Energy Release Rates in Laminated Carbon Fiber Composite Structures Using Digital Image Correlation(2012) Puishys, Joseph Francis; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon fiber composites have recently seen a large scale application in industry due to its high strength and low weight. Despite numerous beneficial attributes of composite materials, they are subject to several unique challenges; the most prevalent and troubling is delamination fracture. This research program is focused on developing an appropriate damage model capable of analyzing microscopic stress strain growth at the crack tip of laminated composites. This thesis focuses on capturing and identifying the varying stress and strain fields, as well as other microstructural details and phenomena unique to crack tip propagation in carbon fiber panels using a novel mechanical characterization technique known as Digital Image Correlation (DIC).Item Inverse Hybrid Method for Determining Explosive Loading on Plates Due to Buried Mines(2007-12-11) Bretall, Damien Carl; Fourney, William; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to the changing face of warfare there is an ever growing need to protect the underside of combat vehicles from mine blasts. This research effort presents a new method to better characterize the pressure profiles experienced by a plate as the blast develops. The explosive deformation of a small-scale plate was recorded using synchronized high-speed digital cameras, and then analyzed using 3D Digital Image Correlation software. Time-varying pressure profiles were input into an axisymmetric FEM simulation by fitting curves to data obtained from tests using Kolsky bars to measure pressures. These were then modified to find possible profiles that produce the measured deformations. It was discovered that the final deformation cannot be determined from only total impulse or peak pressures, it is very sensitive to the time and spatial decay of the pressures, and a deforming plate travels with greater initial velocity than a nondeforming plate of equal mass.Item NEW METROLOGICAL TECHNIQUES FOR MECHANICAL CHARACTERIZATION AT THE MICROSCALE AND NANOSCALE(2004-12-20) jin, huiqing; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)New metrological techniques have been developed for mechanical characterization at the microscale and nanoscale as follows: (1) Development of a control system and integrated imaging capability at the microscale and nanoscale for a new biaxial microtensile tester, (2) a new method for characterizing nonlinearity in AFM imaging using Digital Image Correlation (DIC), and (3) development of pointwise DIC technique. In the biaxial microtensile tester, loading of specimen is induced through the opposing motion of dual picomotor linear actuators in orthogonal directions with a displacement resolution of less than 30 nm. Using an optical microscope, in situ digital images are obtained and analyzed with DIC to determine the full field displacements at the microscale over an Area of Interest (AOI) in order to characterize the biaxial performance of the microtensile tester. An objective AFM has been integrated into the biaxial microtensile tester to obtain in situ digital images of topographic microstructural features at the nanoscale. These topographic images can then be converted to gray scale images with textures that are suitable for DIC to calculate full field displacements at the nanoscale. This measurement capability is demonstrated on a sputtered nanocrystalline copper film subjected to uniaxial loading in the microtensile tester. Since image quality is critical to the accuracy of the nanoscale DIC measurements, a new method was developed to calibrate the errors induced by the nonlinearity of AFM scanning. In this new method, the DIC technique was applied to AFM images of sputtered nanocrystalline NiTi films to calculate the displacement errors caused by the probe offset that must be eliminated from the apparent displacement field. The conventional DIC technique assumes a zero-order or first order approximation of the variation in displacement fields (i.e., displacement gradients) relative to the center of a subset of the image. In the case of displacement fields associated with the microstructure of a material, the displacement gradients can vary discontinuously, which violates the assumed nature of the displacement gradients in the conventional DIC. Therefore, a pointwise DIC technique has been developed to calculate displacements independently at each pixel location, eliminating the constraints imposed by the subset on the calculated displacements. Because of the potentially large number of unknown displacement variables that need to be determined using this approach, an efficient Genetic Algorithm (GA) optimization algorithm with a Differential Evolution (DE) method was investigated for optimizing the correlation function. To guarantee uniqueness of the optimized displacement field, the correlation function was modified using intensity gradients that had to be transformed from an Eulerian to Lagrangian reference frame using displacement gradients. The theoretical development of pointwise DIC is discussed in detail using ideal sinusoidal images, and its validation using real images is also presented.