Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item EMBEDDED HIGH FREQUENCY SIGNAL EFFECTS ON FAILURE MECHANISMS AND MODELS(2022) Lara, Paul; Burck, Hugh; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Embedded high frequency signal effects can have a deleterious effect on the fatigue resistance of structures. For example, ship structures can be subject to many operational loads (wind, pressure, temperature, etc.), one of which is the structural effects from the surrounding sea environment. Typically, the wave environment applies an ordinary wave component, which drives the primary bending stress of the vessel, along with a more stochastically driven element that manifest itself as wave impacts. To account for these effects, designers have relied on simplified assumptions, such as safety factors and/or margins of safety. Existing academic research centered on capturing a simplified sinusoidal response associated with the primary loading event and the embedded high frequency response, but has not addressed logarithmic decay, signal frequency, or frequency of occurrence. All these factors have associated uncertainty and cause impact on fatigue life and failure mechanisms exhibited by structures. This research effort focuses on a more fundamental understanding of the effects of embedded high frequency loading on fatigue crack propagation in Aluminum 5xxx material. Carried out by accounting for the signal’s characteristics, and through an experimental evaluation assess its impact on the local failure mechanism and life cycle models. In particular, the use of Digital Image Correlation to quantify the effects of the embedded high frequency on the plastic zone that develops ahead of the fatigue crack, and the subsequent changes in crack growth. This enabled the following four primary contributions: (1) development of a unique test configuration protocol and process to investigate HF pulse effects on fatigue crack growth in small scale specimens, (2) measured a 35% decrease in COD due to crack closure from the residual stresses associated with a larger plastic zone caused by HF loading, (3) development of a unique model that couples crack kinking and retardation behavior, and (4) elucidation on the effects of sequencing of HF pulses on crack kinking and retardation. The findings of this research can be used in future investigative efforts to develop analytical models that address secondary material effects, such as welds, provide underpinnings for high fidelity numerical modelling, and to reduce the dependency of designers on the use of safety factors and enable them to account more rigorously for failure mechanisms in digital twins.Item New Methodology for Predicting Ultimate Capacity of One-Sided Composite Patch Repaired Aluminum Plate(2019) Hart, Daniel C; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Composite patch repairs are an alternative to traditional weld repair methods to address cracking in aluminum plates. Analytical and numerical design methods use linear elastic fracture mechanics (LEFM) and do not account for elastic-plastic crack tip behavior demonstrated in static tests of one-sided patch repaired ductile panels. This research used digital image correlation (DIC) and three-dimensional finite element analysis (FEA) with first order elements to study crack tip effects due to the one-sided composite patch applied to center crack tension (CCT) specimens loaded monotonically to failure. The measurable effects on crack tip behavior due to the composite patch were ultimate tensile load increase of more than 100% and a total achieved crack opening displacement (COD) increase of 20% over the unpatched behavior. Crack tip fracture behavior was found to be an intrinsic property of the aluminum and directly related to the COD independent of the one-sided composite patch. Increased capacity was related to accumulation of large-strain free surface area and through thickness volume ahead of the crack tip. Test data and numerical predictions correlated with measured load, strain, displacement fields, and J-integral behavior. Correlation of displacement fields with HRR and K fields established a state of small scale yielding prior to failure. Data and predictions indicated critical COD occurs when unpatched and patched large strain area is equivalent, which occurs before crack tip behavior transitions from small scale to large scale yielding and crack growth. Identifying a critical COD for both small and large scale one-sided patch repaired cracked ductile panels results in a predicted failure closer to the ultimate tensile load and 80% greater than predicted with LEFM methods. Observations and predictions demonstrated in this research resulted in three scientific contributions: (1) development of criteria to determine crack growth in cracked ductile panels repaired with a one-sided composite patch using a critical COD, (2) development of a three-dimensional FEA to study development of the plastic zone and evolution of the large-strain region ahead of the crack tip, and (3) development of a numerical methodology to predict ultimate tensile load capacity of cracked ductile panels repaired with a one-sided composite patch.Item MULTI-SCALE MECHANICS OF COMPOSITE SANDWICH STRUCTURES WITH BIOLOGICALLY INSPIRED FIBER REINFORCED FOAM CORES: A POTENTIAL TEMPLATE FOR DEVELOPING MULTIFUNCTIONAL STRUCTURES(2013) Haldar, Sandip; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes a novel multi-scale characterization and modeling approach for developing bio-inspired composite sandwich structures. The source of bio-inspiration was chosen to be Palmetto wood, a naturally occurring porous composite material with macrofiber reinforcement. Characterization of Palmetto Wood at multiple length scales revealed that the mechanical behavior is dominated by the stronger and stiffer macrofibers, while the porous cellulose matrix controls load transfer and failure between macrofibers. Shear dominated debonding and pore collapse mechanisms have been identified as the leading modes of failure mechanism. The role of macrofiber volume fraction and strain rate on macroscale response and damage evolution has been evaluated through experiments. It is seen that increase in macrofiber concentration increases the stiffness of the Palmetto wood, leading to a higher concentration of macrofiber in the outer region of the wood by evolution. A damage model has been developed to decouple the effect of the plastic strain and pore collapse on damage evolution. Using Palmetto wood as a template, prototype bioinspired sandwich composite structures have been fabricated using carbon fiber reinforcement in the foam core to translate the mechanics principles of Palmetto wood. The sandwich composite structures with bioinspired foam core and standard foam core have been characterized under quasi-static and dynamic three-point bending load. The model developed to study damage evolution in Palmetto wood has been applied to the behavior of bioinspired sandwich to quantify the parameters. The enhancement in mechanical behavior has been achieved by reinforcement of the carbon rods in the core like the macrofibers in the Palmetto wood. An increase in macroscale reinforcement in the core led to the behavior that tunes the material response to a better combination the flexural stiffness, energy absorbance and damage evolution characteristics. A Finite Element Analysis (FEA) model has been developed to numerically study the effects of reinforcement in the foam core on its flexural behavior as observed in the experimental characterization. The simulations performed using homogenized, isotropic properties from simulations of the bioinspired core affirm the experimental observations. The viability of developing multifunctional sandwich structures from the multiscale characterization and modeling of the bioinspired foam cores has also been investigated. Prototype sandwich battery structures were fabricated using copper coated fiberglass and Zn plate facesheet, a carbon foam core, and an adhesive of NH4Cl and ZnCl2 bound by HTPB and epoxy polymers. Very low power generation was demonstrated using the prototype batteries, however it was determined that the mechanical strength and energy absorbing capability were compromised, as expected from the model, indicating that the use of macrofiber reinforcement could potentially enhance multifunctional behavior.Item A Multi-Scale Approach for Characterizing the Mechanical Behavior of Pin-Reinforced Composite Sandwich Structures with Digital Image Correlation(2013) Brandveen, Bianca Renee; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the last 10 years, pin–reinforced composite sandwich structures have become an interesting research topic in aerospace and naval engineering because of their low weight and high compressive properties. Current models lack rigorous physical understanding of the mechanics of these structures and do not accurately predict their performance. This hybrid numerical–experimental research approach investigates the compressive and flexural mechanical behavior of these materials and also characterizes and models the mechanical response in the form of full–field displacements and strains using Digital Image Correlation (DIC). This thesis establishes an experimental mechanics characterization approach spanning several length scales, including: single pins, representative volume elements, contoured beams, and cylindrical shells with 6” radius of curvature. The previously assumed deformation response of pins within the composite was substantiated with 2D and 3D DIC and extrapolated to the macroscale for both straight and contoured composites.