Doctor of Philosophy, University of Toledo, 2016, Mechanical Engineering

Thermoplastic composites are suitable alternatives to metals in some load-bearing applications such as in the automotive industry due to a large number of advantages they present. These include light weight, ease of processing for complex geometries at high production rate, outstanding cost to performance ratio, ability to reprocess, and corrosion resistance. Addition of fillers such as talc or reinforcements such as short glass fibers can improve the mechanical performance of unreinforced thermoplastics to a high degree. Components made of thermoplastic composites are typically subjected to complex loadings in applications including static, cyclic, thermal, and their combinations. These applications may also involve environmental conditions such as elevated temperature and moisture which can dramatically affect their mechanical properties. This study investigated tensile, creep, fatigue, creep-fatigue interaction, and thermo-mechanical fatigue (TMF) behaviors of five thermoplastic composites including short glass fiber reinforced and talc-filled polypropylene, short glass fiber reinforced polyamide-6.6, and short glass fiber reinforced polyphenylene ether and polystyrene under a variety of conditions. The main objectives were to evaluate aforementioned mechanical behaviors of these materials at elevated temperatures and to develop predictive models to reduce their development cost and time. Tensile behavior was investigated including effects of temperature, moisture, and hygrothermal aging. Kinetics of water absorption and desorption were investigated for polyamide-6.6 composite and Fickian behavior was observed. The reductions in tensile strength and elastic modulus due to water absorption were represented by mathematical relations as a function of moisture content. In addition to moisture content, aging time was also found to influence the tensile behavior. A parameter was introduced for correlations of normalized stiffness and strength with different aging t (open full item for complete abstract)

Committee: Ali Fatemi Dr. (Advisor); Mohamed Samir Hefzy Dr. (Committee Member); Saleh Jabarin Dr. (Committee Member); Joseph Lawrence Dr. (Committee Member); Efstratios Nikolaidis Dr. (Committee Member) Subjects: Engineering; Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 2024, Civil Engineering

This dissertation presents a comprehensive study on the bond behavior of sand coated glass fiber-reinforced polymer (GFRP) bars embedded in concrete, utilizing a series of pull-out tests to evaluate the effects of various bonded lengths and bar diameters on bond performance. The study includes five bonded lengths (5db, 10db, 20db, 30db, and 40db) and three nominal bar diameters (6 mm, 12 mm, and 20 mm). The experimental setup involves measuring loaded-end displacement using three linear variable displacement transformers (LVDTs), and for some specimens, free-end displacement with two additional LVDTs. The results are presented as load responses, including applied load versus machine stroke, loaded-end slip, and free-end slip for different bonded lengths. The average shear stress is plotted against the bonded length, and the maximum axial stress from pull-out tests is compared with predictions from ACI 440.1R-15. This comparison provides a basis for evaluating design guidelines against experimental data. In addition to static pull-out tests, the fatigue behavior of the bond between sand-coated GFRP bars and normal strength concrete is examined. Pull-out fatigue tests are conducted on 6 mm and 12 mm bars, with varying load ranges and loading frequencies. The study compares the fatigue load responses and post-fatigue pull-out test results for different bar diameters at similar load range percentages of their tensile strengths. Bond stiffness and loaded-end slip are analyzed in relation to the number of fatigue cycles. Finally, this study presents a finite element analysis (FEA) of GFRP bars embedded in concrete. The interaction between the concrete and GFRP bars is modelled using a surface-based cohesive behavior model, which is based on a calibrated local shear stress-slip model. The analysis compares isotropic and orthotropic properties of the bar, highlighting differences in stress distribution. Mesh convergence has been carried out. Results align well with exper (open full item for complete abstract)

Committee: Christian Carloni (Advisor); Xiong Yu (Committee Member); Tommaso D’Antino (Committee Member); Elias Ali (Committee Member); Anna Samia (Committee Member); Yue Li (Committee Member) Subjects: Civil Engineering

Master of Science in Engineering, University of Akron, 2018, Mechanical Engineering

In this thesis document, the results and interpretations of an experimental study aimed at investigating and rationalizing the mechanical behavior of a conventionally processed high strength aluminum alloy is presented and discussed. The aluminum alloy chosen for this study was the high strength Al-Cu-Mg alloy, designated as 2024 by the Aluminum Association of America (Washington, D.C., USA). In this study, a few mechanical tests, to include: tension, compression, hardness, shear and cyclic stress-controlled fatigue, were conducted in synergism with microstructural characterization and macroscopic observation and record of the nature of fracture with the objective of establishing the role of alloy microstructure in governing the macroscopic mechanical response and fracture behavior of the chosen aluminum alloy. The mechanical tests were conducted in accordance with procedures detailed in the standards of ASTM. The test specimens for each test were precision machined from the as provided wrought alloy stock. For the cyclic fatigue tests, the presence of a notch, conforming to specifications detailed in ASTM Standard, on cyclic fatigue life is presented. The test results are presented and briefly discussed with specific reference to nature of loading and microstructural influences.

Committee: Tirumalai Srivatsan Dr. (Advisor); Anil Patnaik Dr. (Committee Member); Craig Menzemer Dr. (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

PhD, University of Cincinnati, 2017, Engineering and Applied Science: Mechanical Engineering

Fatigue failure is a major reason behind the failure of mechanical components and machine parts. In turbine engines and related applications, the components are subjected to cyclic loading at elevated temperatures. Superalloys have high strength and environmental resistance to perform under extreme high temperatures and stress conditions. Improvement in the strength, fatigue life, and/or temperature capabilities of these superalloys will yield huge economic benefits. To address these challenges, surface treatment techniques are implemented to improve the fatigue behavior of currently used superalloys at elevated temperatures. This study investigates Ultrasonic Nano-crystal Surface Modification (UNSM) and Laser Shock Peening (LSP) as techniques to improve strength and fatigue behavior of ATI 718 Plus (718Plus) at room and elevated temperatures. The effect of temperature and strain rate on the strength, ductility, and failure behavior of 718Plus was investigated. The results showed that with the increase of temperature at slow strain rate, there is a small reduction in the yield strength, a large drop in ductility, and a change in fracture mode from ductile transgranular to brittle intergranular cracking. Analysis of the microstructure showed that the driving mechanism at higher temperatures and slower strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism and that the d precipitates on the grain boundaries are facilitators. Increase of strain rate at 704 °C caused a small increase in the yield strength, a huge increase in the ductility, and a change in fracture mode from brittle to ductile failure. This showed that the driving mechanism at higher strain rates was Portevin–Le Chatelier effect. Finally, 718Plus has superior fatigue behavior at its operation temperature (650 °C) compared to room temperature due to the strengthening of the ?' precipitates which increased its endurance limit by ~20% (~145 MPa). The repetitive strikes (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Woo Kyun Kim Ph.D. (Committee Member); Yijun Liu Ph.D. (Committee Member); Dong Qian Ph.D. (Committee Member); Jing Shi Ph.D. (Committee Member) Subjects: Mechanical Engineering; Mechanics

Doctor of Philosophy, University of Akron, 2014, Civil Engineering

A heavy duty riveted steel grating is an open grid deck system used in movable bridge construction and rehabilitation projects. They are lightweight and easy to install when compared to conventional slab systems and is thus preferred when the load carrying capacity of an existing bridge needs to be increased. Empirical methods have been used in the past due to limited information about their design and behavior. Open grid decks have been used in a number of bridges with the majority being welded decks. A major problem encountered with these decks is the development of fatigue cracks resulting in increased maintenance cost. Observations in the field when heavy duty riveted steel gratings are used and results from experiments indicate better fatigue performance than welded decks. The fatigue characterization of the heavy duty riveted grating has not been established and there are no provisions to govern the design in the AASHTO LRFD Specifications. The current research examines the fatigue behavior of heavy duty riveted steel decks under AASHTO H20 truck loading and also establishes an effective width to be considered during design. Preliminary tests were conducted on two large panels of 37R5 lite to investigate the static behavior and the nature of stress distribution on major components of the grating. A 3D finite element model was calibrated to laboratory data to simulate experimental tests and used for parametric studies in estimating stresses in various components. Fatigue testing of six structural panels with simulated H20 design truck tire loads and of 26 smaller panels at stress ranges of 20ksi, 25ksi, 30ksi and 35ksi was performed. A fracture mechanics approach was used to estimate the fatigue life of the gratings. Results showed that the primary strip width provided in the AASHTO LRFD specifications for the design of open grid decks under predicted the stresses on main bearing bars. An effective width is proposed and involves the length or width of the t (open full item for complete abstract)

Committee: Craig Menzemer Dr. (Advisor); Anil Patnaik Dr. (Committee Member); David Roke Dr. (Committee Member); T.S. Srivatsan Dr. (Committee Member); Desale Habtzghi Dr. (Committee Member) Subjects: Civil Engineering; Design; Engineering; Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 1992, Macromolecular Science

Environmental stress cracking (ESC) of polyethylene (PE) in the presence of active liquid environments results in fracture under applied stress conditions much lower than for failure in the absence of the ESC agent. However, recent studies have shown that the fatigue lifetime of high density polyethylene (HDPE) shows a marked increase in pure ESC liquids and their aqueous solutions. The aim of this present study is to investigate in more detail the effects of ESC agents on the fatigue behavior of HDPE. The existence of craze-like voids inside the halo region in the main craze damage zone occurs in both air and ESC environments. Secondary crazes outside of the main damage zone are observed only in the presence of liquid ESC environments. These secondary crazes enhance the fatigue lifetime, since their formation serves as an additional mechanism to dissipate the applied cyclic load. Concurrent plasticization results in a blunted crack tip which decreases fatigue crack propagation (FCP) rates.SEM analysis offers evidence that Igepal plasticizes the base of fibrillated structure as well as crack tips and broken fibrillated structures, resulting in "dimple-like" structures. The base region contains highly disordered crystalline/amorphous material which is more easily penetrated by the ESC agents. A stress-enhanced diffusion process apparently is operative at higher stress/strain rates resulting in facilitated formation of secondary crazes and blunted crack tip. In air environment, HDPE exhibits a strong dependence on experimental variables, such as test frequency, waveforms, and stress/strain rates. In contrast, when the samples were tested in Igepal, the same experimental variables did not affect the fatigue testing results at all. Methyl cellosolve is a more efficient cracking agent than Igepal. Secondary crazes and a blunted crack tip also were observed in the presence of methyl cellosolve. This behavior, which is the same as in the case of Igepal, should result in a (open full item for complete abstract)

Committee: Charles Rogers (Advisor) Subjects: Engineering, Materials Science

Doctor of Philosophy, Case Western Reserve University, 1992, Materials Science and Engineering

MB85 (Al-3.5%Cu-2.4%Mg Alloy reinforced with SiC particulates) and 2014 (Al-4.2%Cu-1.5%Mg alloy reinforced with Al2O3 particulate) composites were heat-teated to underaged and overaged conditions followed by low cycle fatigue testing with total strain control. The results include three aspects: (i) Bauschinger effect and internal residual stress, (ii) Cyclic work hardening behavior, and (iii) Fatigue life and low cycle fatigue damage processes. X-ray diffraction techniques were used to measure the residual internal stress to compare with the Bauschinger measurements. SEM and TEM were used to characterize the microstructures before, during, and after the test. In light of a literature review on the above aspects, the possible mechanisms occuring during cyclic deformation and fatigue are discussed. The main results are as follows: (1) Bauschinger parameters (σb and β1) increase with increasing plastic prestrain (εp). (2) Reinforcements and heat treatment conditions have strong effects on the BEPs: BEPOA > BEPUA in the same material; BEPCP > BEPCT in the same heat treatment; BEP20%SiC > BEP15%SiC in the same heat treatment. (3) Internal stress may be measured both by Bauschinger Effect and by x-ray diffraction. Both results are comparable. (4) Cyclic deformation behavior is strongly affected by the reinforcements and heat treatment conditions. For a given composite material and a fixed heat treatment condition, the cyclic deformation behaviors is determined by the initial plastic strain. (5) The Manson-Coffin expression can be used to predict the fatigue life of the composites.

Committee: John Lewandowski (Advisor) Subjects:

Doctor of Philosophy, Case Western Reserve University, 1991, Mechanical Engineering

Metal Matrix Composites (MMC) and Intermetallic Matrix Composites (IMC) have been identified as potential material candidates for advanced aerospace applications. They are especially attractive for high temperature applications which require a low density material that maintains its structural integrity at elevated temperatures. High temperature fatigue resistance plays an important role in determining the structural integrity of a material. There are several fundamental issues that surface when considering high temperature fatigue response of MMC's and IMC's. Among them are test technique, failure criterion and life prediction. This study attempts to examine the relevance of these concepts as they pertain to an IMC material, specifically unidirectional SiC fiber reinforced titanium aluminide. As a part of this study, a series of strain- and load-controlled fatigue tests were conducted on unidirectional SiC/Ti-24Al-11Nb (atomic %) composite at 425 and 815°C. Several damage mechanism regimes were identified by using a strain-based representation of the data, Talreja's fatigue life diagram concept. Results from these tests were then used to address issues of test control modes, definition of failure and testing tech niques. Finally, a strain-based life prediction method was proposed for an intermetallic matrix composite (IMC) under tensile cyclic loadings at elevated temperatures. Styled after the "Universal Slopes" method, the model utilizes the composite's tensile properties to estimate life. Factors such as fiber volume ratio (V f), number of plys and temperature dependence are implicitly incorporated into the model through these properties. The model parameters were determined by using fatigue data at temperatures of 425 and 815°C. Fatigue life data from two independent sources were used to verify the model at temperatures of 650 and 760°C. Cross-ply life data from specimens with ply lay-ups of (0/90) 2s and (0/±45/90) 2s at 760°C were also predicted. Correlation (open full item for complete abstract)

Committee: T. Kicher (Advisor) Subjects: