Mechanical Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/2263
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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 FRACTURE BEHAVIOR AND THERMAL CONDUCTIVITY OF POLYCRYSTALLINE GRAPHENE(2014) Fox, Andrew Oliver; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the effect of grain boundaries (GBs) in polycrystalline graphene on the tensile fracture behavior and thermal conductivity of the graphene sheets. Current techniques to fabricate large-scale graphene intrinsically introduce defects, e.g., GBs, resulting in polycrystalline graphene sheets. Though GBs in graphene are expected to affect the mechanical properties of graphene, mechanistic understanding and quantitative determination of such effects are far from mature. For example, existing studies on the effect of GBs on the tensile behavior of graphene only focus on a twin GB perpendicular to the tensile loading direction. However, GBs in a polycrystalline graphene sheet under uniaxial tension could be subject to tension in any arbitrary directions, depending on the GB and grain orientation in the graphene sheet. In this dissertation, we focus on the effect of GBs on the tensile and thermal response of polycrystalline graphene. The fracture process of polycrystalline graphene sheets under uniaxial tension was studied using molecular dynamics (MD) simulations to determine how GBs affects the ultimate strength and critical failure strain of the graphene. We also study the flow of heat through polycrystalline graphene to determine the effect of GBs on the thermal conductivity of graphene. A comprehensive study including 24 GB misorientation angles ranging from 2.1° to 54.3° and the whole range of loading angle (i.e., that between a GB and in-plane tensile loading direction, ranging from 0° to 90°) was carried out to quantitatively determine the effect of GBs. Stress-strain data were generated from the MD simulations and the failure strength and critical strain were analyzed. A theoretical model combining continuum mechanics theory and disclination dipole theory was introduced to predict the failure strength of the polycrystalline graphene sheets, which was shown to be in good agreement with the MD simulation results. Various failure modes of polycrystalline graphene under tension were also analyzed. The thermal conductivity of polycrystalline graphene as a function of GB misorientation angle and thermal loading angle was also quantitatively determined through systematic simulations. The quantitative findings from this dissertation could potentially bridge the knowledge gap toward a better understanding of defects and their effects on two-dimensional materials, and also shed light on possible defect control and engineering to achieve desirable properties of graphene in applications.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).