Master of Science, The Ohio State University, 2021, Mechanical Engineering

Teeth of a gear undergo cyclic forces as they rotate in and out of the gear mesh contact zone. The resultant contact and bending stresses produce fatigue damage, which can lead to failure through contact surface degradation and tooth breakage through the root fillet. The fatigue process is not instantaneous, and damage accumulates at different rates throughout the fatigue life. Many proposed diagnostic techniques to detect the onset of gear failure rely on changes in mechanical properties due to crack growth. Moreover, theoretical studies to predict fatigue crack growth rate rely on only a few experimental measurements for their validation with little extension to gears. The focus of this study is to develop an optics-based system for measuring surface crack length coupled with an acoustic emission system for measuring the release of elastic stress waves generated in the tooth root of a fatiguing spur gear, which may be related to early life fatigue damage. A gear single-tooth bending machine, a high-speed camera, and an acoustic emission sensor are utilized in unison to demonstrate the methodology. A digital image correlation technique is used to compute crack length during each load cycle to obtain cyclic crack growth rate, and acoustic emission signals are examined for signature behaviors.

Committee: Ahmet Kahraman PhD (Advisor); Isaac Hong PhD (Advisor) Subjects: Engineering; Mechanical Engineering

Doctor of Philosophy, University of Akron, 2013, Mechanical Engineering

This thesis sought to investigate and develop valid numerical approaches to predict ductile fracture under different stress state and loading conditions. As the first portion of this work, the plastic flow and fracture behaviors of three aluminum alloys (5083-H116, 6082-T6 and 5183 weld metal) under the effects of strain rate and temperature were studied through a series of experiments and finite element analyses. The fracture behavior under the influential factor of stress triaxiality was also studied. The applicability of the Johnson-Cook plasticity and fracture models were investigated with mixed results. For all three materials, the dependency of the failure strain on triaxiality is adequately described.The stress state effect on plasticity and ductile fracture behaviors was further explored for aluminum alloy 5083-H116 through tests on plane strain specimens and torsion specimens, focusing on the third deviatoric stress invariant (lode angle). A stress state dependent plasticity model, J2-J3 model, together with the Xue-Wierzbicki fracture criterion which defined the damage parameter as a function of the stress triaxiality and the Lode angle, was implemented and calibrated with the test data. The calibrated model was utilized to study the residual stress effect on ductile fracture resistance, using compact tension specimens with residual stress fields generated from a local out-of-plane compression approach. Fracture tests with positive and negative residual stresses were conducted on the C(T) specimens. Both experimental and finite element results showed significant effect of residual stress on ductile fracture resistance.In an attempt to predict ductile fracture under shear-dominated conditions, this study combined the damage mechanics concept with the Gurson-Tvergaard-Needleman porous plasticity model that accounts for void nucleation, growth and coalescence. The GTN model was extended by coupling two damage parameters, representing volumetric damage and she (open full item for complete abstract)

Committee: Xiaosheng Gao Dr (Advisor); Shing-Chung Wong Dr. (Committee Member); Gregory Morscher Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Mechanical Engineering; Mechanics

Doctor of Philosophy (PhD), Wright State University, 2020, Engineering PhD

This research focuses on the hierarchical structure of bone and associated mechanical properties at different scales to assess damage accumulation leading to premature failure, with or without instrumentation. In this work, an attempt was made to develop a framework of macro, micro, and nano damage accumulation models and implementing them to derive mechanical behavior of the bone. At macrolevel, retrospective evaluation of 313 subjects was conducted, and the damage of bone tissue was investigated with respect to subject demography including age, gender, race, body mass index (BMI), height and weight, and their role in initiating fracture. Experimental data utilized 28 human femoral bones implanted with cephalomedullary nails were used to develop damage prediction models. Investigation of three real life medical device failures identified the mechanical and clinical bases of bone failure. At the micro level, microdamage accumulation of the bone was investigated in 307 subjects and new effective morphological parameters at microscale were proposed. At the nano level, molecular dynamics simulation was performed to investigate the effect of interaction, orientation, and hydration on the atomic models of the bone composed of collagen helix and hydroxyapatite crystal. The results showed that bone density and maximum von Mises stress decreased drastically in elderly patients, implying fixation devices and implants used by the young cannot be used. The results also showed that the two-dimensional representation of the morphological parameters of the bone at microscale does not provide a realistic description of bone structure. Therefore, in this work, three-dimensional representations at microscale indicated that bone interconnectivity is higher in female patients than in male patients. Gender has a significant effect on microdamage distribution in the bone. More precautions should be taken into consideration for older female patients. Race should also be considered during (open full item for complete abstract)

Committee: Tarun Goswami D.Sc. (Advisor); Caroline GL Cao Ph.D. (Committee Member); Arnab K. Shaw Ph.D. (Committee Member); Partha P. Banerjee Ph.D. (Committee Member); Richard T. Laughlin M.D. (Committee Member); Jennie J. Gallimore Ph.D. (Committee Member) Subjects: Biomechanics; Biomedical Engineering; Biomedical Research; Engineering; Industrial Engineering; Molecular Biology; Nanoscience; Nanotechnology

Doctor of Philosophy, The Ohio State University, 2018, Civil Engineering

Extreme hazards such as earthquakes, floods, and hurricanes can significantly affect the performance and serviceability of structures and infrastructure systems during their lifetime. Recent prominent examples include the 2017 earthquake in the vicinity of Iran-Iraq border and the 2017 earthquake in Mexico that led to hundreds of fatalities. Hurricane Matthew (2016), Harvey (2017), Irma (2017), and Jose (2017) caused significant damage to critical infrastructure systems in a number of south-eastern states in the U.S. Such hazards can occur multiple times during the lifetime of infrastructure systems. Each event is accompanied by a set of adverse consequences including, among others, human casualties, physical damage, and downtime due to the repair of damage and restoration of the functionality of the system. In addition, as infrastructure assets are exposed to environmental stressors and service loads, they undergo gradual aging and deterioration over their lifetime. The subsequent degradations in the capacity of the systems increase their vulnerability against hazards over time. These compounding effects, among others, pose a tremendous challenge for evaluating the performance of structures and infrastructure systems, and managing their performance. In the light of such challenges and budget limitations, it is important to evaluate the lifecycle cost of infrastructure systems in order to minimize the potential losses over their service lifetime. For structures or infrastructure systems that are exposed to multiple hazards during their lifetimes, damage accumulation is a critical issue. As supported by historical records, the accumulation of damage from prior events can considerably increase the vulnerability of these systems to future hazards. However, this phenomenon is either disregarded or addressed inadequately in existing risk management frameworks. Additionally, these frameworks do not incorporate effects of gradual deterioration on the reduced capacity of i (open full item for complete abstract)

Committee: Abdollah Shafieezadeh (Advisor); Rabi Mishalani (Committee Member); Halil Sezen (Committee Member); Can Emre Koksal (Committee Member); Steve Hovick (Committee Member) Subjects: Civil Engineering