Doctor of Philosophy, The Ohio State University, 2021, Materials Science and Engineering

Laser driven shockwaves are currently being used in an assortment of industrial applications and physics research. Although used in many studies, one of the most common and successful industrial applications is the process of laser shock peening (LSP). LSP has been a developing field of study since the 1970's but only experienced commercial success in the early 2000's. Despite the relatively long history, the physical impulses created by the process have been infrequently and incompletely investigated. This study was constructed to investigate the impulse loads created across the LSP tradespace parameters and evaluate how industry can better analyze LSP parameters and utilize the data in their own optimization. Using photon doppler velocimetry, peak pressures and magnitudes generated by LSP conditions are evaluated in titanium and aluminum alloys in this study. The studies are extended to be inclusive of opaque overlays on the target materials that act as thermal barriers and also modify the pressures generated. This data is critical to understanding and optimizing the LSP process for different material applications and LSP treatment purposes and has not been comprehensively investigated prior to this work. Extension of the pressure data to physical treatments was validated through measurements of residual stress with x-ray diffraction and simulation of the process with finite element simulations. Finite element studies were also used to define the converged boundaries for the newly defined impulse parameter space and demonstrated prediction of residual stresses in comparison to experimental datasets. Results of these studies are expected to provide additional understanding of the LSP process for both industrial use and extension to optimization studies of LSP treatments. It is the intent of these cumulative studies that a more thorough detailing of LSP impulse and simulation capabilities are available for those interested in evaluating the process.

Committee: Glenn Daehn (Committee Co-Chair); Stephen Niezgoda (Committee Co-Chair); Enam Chowdhury (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2018, Engineering and Applied Science: Materials Science

This study investigates the effects of laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM) on residual stress, near surface modification, and hardness of Inconel 718 (IN718) specimens manufactured by selective laser melting (SLM) techniques. IN718 is a nickel-based Ni-Cr-Fe superalloy. It has a unique set of properties that include good workability, corrosion resistance, high-temperature strength, favorable weldability, and excellent manufacturability. Additive manufacturing (AM) techniques, in particular, the laser assisted AM techniques have been developed and adopted in the industry in the past three decades. The LSP and UNSM are the recently developed mechanical surface treatment techniques that cause the severe plastic deformation on the surface. This in turn induces deep compressive stresses and forms a fine-crystalline surface layer in the specimen that improves the hardness, strength, and fatigue life. In this study, the SLM technique is used to manufacture IN718 super-alloy specimens. SLM parts are well known for their high tensile stresses in the as-built state, in the surface or subsurface region. These stresses have a detrimental effect on the mechanical properties, especially on the fatigue life. LSP and UNSM as a surface treatment method are applied on heat-treated specimens and as-built specimens. Heat-treated specimens are those samples which are fully annealed to relieve all the inbuilt stresses. They are heat treated at 955°C for one hour followed by furnace cooling. After LSP and UNSM treatment, optical microscope and electron back scattered diffraction (EBSD) is used to characterize the microstructures of both heat-treated and as-built specimens. A nanoindentation test is performed to determine the local properties like the hardness of as-built and heat-treated specimens. Afterward, the hardness along the distance from the LSP and UNSM treated surface is also defined. After UNSM treatment, compressive residual str (open full item for complete abstract)

Committee: Jing Shi Ph.D. (Committee Chair); Yao Fu (Committee Member); Vijay Vasudevan Ph.D. (Committee Member) Subjects: Materials Science

PhD, University of Cincinnati, 2024, Engineering and Applied Science: Materials Science

Co-Cr-Mo alloys are widely utilized in prosthetic devices due to their exceptional wear and corrosion resistance, mechanical properties, and biocompatibility. These attributes are highly dependent on the microstructure, primarily influenced by the γ-FCC to ε-HCP transformation, along with compositional control, processing, and heat treatment. This study investigates the γ-FCC to ε-HCP phase transformation in a laser powder bed fusion additively manufactured Co-29Cr-5Mo alloy through four distinct parts. In the first part, the γ-FCC to ε-HCP transformation was examined after direct aging heat treatment of the material in the as-built condition at temperatures between 750°C and 875°C for up to 100 hours. The microstructural changes were studied by XRD, SEM, and TEM. Crystallographic relations between the &gamma and ε phases were determined via EBSD in the SEM and electron diffraction in the TEM. The as-built material's microstructure was γ-FCC, characterized by a sub-micron dislocation cell structure and stacking faults, together with Cr23C6 carbides at the cell and grain boundaries. A rapid massive transformation to ε-HCP occurred within 0.5 hours at 1073 K, identified by composition invariance, grain face nucleation, and interface-controlled linear growth. This transformation is facilitated by reduced carbon content in the γ-FCC phase, partly due to carbide formation at grain and cell boundaries, and the varied grain structures and interfaces. The second part of the work focused on the kinetic and thermodynamic aspects of massive transformation. The LPBF process produced specimens with a distinctive microstructure featuring columnar grains oriented along the build direction, adorned with intergranular and intragranular precipitates, high dislocation density, and numerous stacking faults. Isothermal aging treatments transformed the γ-FCC phase to the ε-HCP phase at varying rates, with optimal kinetics observed at 850°C. The Johnson-Mehl-Avrami-Kolmogorov (JMA (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Dinc Erdeniz Ph.D. (Committee Member); Ashley Paz y Puente Ph.D. (Committee Member); Matthew Steiner Ph.D. (Committee Member) Subjects: Engineering

PhD, University of Cincinnati, 2021, Engineering and Applied Science: Materials Science

Aluminum 7xxx series alloys exhibit a combination of high mechanical strength as well as decent corrosion resistance and are widely utilized in aircraft structures. However, high strength 7xxx series alloys, like AA7075 in the T6 heat-treated condition, is susceptible to failures from fatigue, corrosion, stress corrosion cracking and corrosion-fatigue from the mechanical loading and saline environments these structures are exposed to during service. To address these shortcomings, the effects of advance surface treatment processes of Laser Shock Peening without coating (LSPwC) and Ultrasonic Nanocrystal Surface Modification (UNSM) on the mechanical behavior, corrosion properties and near-surface microstructure changes of Al 7075-T6 alloy were investigated. These treatments induce high compressive residual stresses which results in enhancement of the fatigue life of the material and has a positive impact on the corrosion resistance. A series of experiments were conducted to study the impact of these surface treatments on residual stress, microstructural evolution and, in turn, their effects on strengthening, fatigue, corrosion, and corrosion-fatigue properties. The near-surface microstructure in Al 7075-T6 alloy after these surface treatments were characterized by advanced electron microscopy techniques. LSPwC led to remarkable near-surface microstructure composed of a ~2 µm wide newly solidified matrix recast surface layer embedded with O-rich Al nanoparticles (NPs) with the same close-packed orientation relationship (OR) as the surrounding Al matrix, together with a nano-scale aluminum oxide layer formed on the outermost surface. The formation mechanism is associated with high-pressure surface ablation leading to melting, vaporization, and shock-assisted rapid solidification during the LSPwC process. The close-packed OR between NPs and matrix is believed to be due to surface energy minimization. These unique near-surface microstructural changes induced by LSPw (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Yao Fu Ph.D. (Committee Member); Ashley Paz y Puente Ph.D. (Committee Member); Matthew Steiner Ph.D. (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2018, Engineering and Applied Science: Materials Science

Stress Corrosion cracking of Aluminium 7075 has been an important concern in aerospace industry. AA7075 is one of the three alloys that contribute to 90 percent of the service failures in aluminium-based structures. Hence, there is a need to improve SCC resistance of this alloy. This can be done by introducing compressive residual stresses in the material to counteract the tensile stress arising from manufacturing processes and service conditions. Shot peening, water jet peening, low plasticity burnishing, Ultrasonic Nanocrystal Surface Modification (UNSM), Laser Shock Peening are examples of the surface treatments that induce compressive residual stresses in materials, together with changes in microstructure and properties. This study is an investigation for Laser Shock Peening (LSP) as a mitigation technique to improve corrosion and stress corrosion cracking resistance of Al 7075 - T6 alloy. Different experimental trials were conducted to determine the optimal parameters of LSP for the mitigation. The specimens peened using these optimal parameters were tested and compared with the unpeened (baseline) samples under similar conditions. Polarization tests show that there is 65.5% reduction in the corrosion rate of the laser peened material as compared to the baseline. Slow strain rate tests were conducted in sodium chloride environment to evaluate the stress corrosion cracking resistance of the baseline and laser peened specimens. LSP leads to a 14% increase in yield strength of the alloy which contributes to the improvement in stress corrosion cracking resistance of the material in sodium chloride environment. The increase in yield strength and corrosion resistance, along with microstructural changes induced by LSP could have a combined effect in improving the SCC resistance. Thus LSP is effective in improving the corrosion and stress corrosion cracking resistance of Al 7075 alloy.

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Ashley Paz y Puente Ph.D. (Committee Member); Matthew Steiner Ph.D. (Committee Member) Subjects: Materials Science

PhD, University of Cincinnati, 2015, Engineering and Applied Science: Materials Science

Stress corrosion cracking (SCC) of Alloy 600 has been a major problem in commercial light water reactor (LWR) nuclear power plants. Localized corrosion and intergranular SCC (IGSCC) have been observed in Alloy 600 in the high temperature (288-340 °C) pure water environment of LWRs. Additionally, IGSCC of Alloy 600 has been reported even at room temperature under certain conditions in thiosulfate and tetrathionate solutions. In general, SCC can be attributed to the presence of tensile stress, an aggressive environment and a susceptible microstructure. Therefore, SCC mitigation techniques address these factors by modifying the environment, metallurgical processing treatments and alleviating the tensile stresses by mechanical surface treatments/stress relief. This study investigated the application of laser shock peening (LSP) as a technique to mitigate SCC in Alloy 600. LSP induced large compressive residual stresses (-550 MPa) that decreases gradually through depth. The pressure pulse generated during the LSP treatment causes plastic deformation, resulting in high dislocation density, twins and formation of misoriented sub-grains/crystallites that have sizes in the range of 50-300 nm in the near-surface region. Slow strain rate tests (SSRTs) and constant load tests performed in tetrathionate solution at room temperature were used to evaluate the effect of LSP on the SCC behavior. LSP treated samples had a significantly longer time to failure and reduced susceptibility to SCC as compared with untreated sensitized Alloy 600. These improvements were attributed to LSP induced compressive residual stresses, increased yield strength (YS) and hardening caused by near-surface microstructural changes. SSRTs in simulated PWR environment also show similar results with higher YS, tensile strength and strain to failure. Additionally, the gage section shows fewer cracks and smaller crack lengths in the LSP treated samples as compared with the untreated samples. The other approa (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Seetha Ramaiah Mannava Ph.D. (Committee Member); Dong Qian Ph.D. (Committee Member); Rodney Roseman Ph.D. (Committee Member); Vesselin Shanov Ph.D. (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2013, Engineering and Applied Science: Mechanical Engineering

Ultrasonic Nano-Crystal Surface Modification (UNSM) is a relatively new material processing technology to enhance the operating service lives, or fatigue life, of engineering components. There is an increasing interest in extending this technology to metal parts such as aircraft engine turbine blades, compressor blades and medical implants such as spinal rods. In this process a ball made with tungsten carbide generates 20 - 40 KHz strikes of a few hundred Newton on the specimen surface. It works as a cold forging process; however the small ball that works itself across the specimen surface has a dynamic load added to the normal static load. The total striking force, feed, ball radius, amplitude of dynamic load and speed can vary to yield different results. UNSM induces severe plastic deformation and deep compressive residual stresses to increase surface hardness, improve surface roughness, and introduce nano-crystallization near the specimen surface. Currently there is no systematic approach to predict the material response under UNSM. Therefore the objective of this thesis is to develop a process model for predicting the material response as a result of the UNSM process. Before developing the model, experimental data is extracted from two UNSM treated coupons, one of Ti-6Al-4V and the other IN718 SPF. First a MATLAB code is developed to define the displacement history of the carbide ball on the surface of the specimen during the UNSM process. For titanium alloy (Ti-6Al-4V) a temperature, pressure, and rate dependent constitutive material model for is established to account for the high strain rates associated with UNSM. A semi-implicit forward tangent modulus algorithm is developed to implement the material and damage model, and this is linked with the FEM software LS-DYNA through a user-defined material subroutine. We use the Johnson Cook material model already within the LS-DYNA software to simulate IN718 SPF. The residual stress obtained from the s (open full item for complete abstract)

Committee: Dong Qian Ph.D. (Committee Chair); Seetha Ramaiah Mannava Ph.D. (Committee Member); Vijay Vasudevan Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy (PhD), Wright State University, 2009, Engineering PhD

Laser peening (LP) is a surface enhancement technique that has been applied to improve fatigue and corrosion properties of metals. The ability to use a high energy laser pulse to generate shock waves, inducing a compressive residual stress field in metallic materials, has applications in multiple fields such as turbomachinery, airframe structures, and medical appliances. In the past, researchers have investigated the effects of LP parameters experimentally and performed a limited number of simulations on simple geometries. However, monitoring the dynamic, intricate relationships of peened materials experimentally is time consuming, expensive, and challenging. With increasing applications of LP on complex geometries, these limited experimental and simulation capabilities are not sufficient for an effective LP process design. Due to high speed, dynamic process parameters, it is difficult to achieve a consistent residual stress field in each treatment and constrain detrimental effects. With increased computer speed as well as increased sophistication in non-linear finite element analysis software, it is now possible to develop simulations that can consider several LP parameters. In this research, a finite element simulation capability of the LP process is developed. These simulations are validated with the available experimental results. Based on the validated model, simplifications to complex models are developed. These models include quarter symmetric 3D model, a cylindrical coupon, a parametric plate, and a bending coupon model. The developed models can perform simulations incorporating the LP process parameters, such as pressure pulse properties, spot properties, number of shots, locations, sequences, overlapping configurations, and complex geometries. These models are employed in parametric investigations and residual stress profile optimization at single and multiple locations. In parametric investigations, quarter symmetric 3D model is used to investigate tempor (open full item for complete abstract)

Committee: Ramana Grandhi PhD (Advisor); Allan Clauer PhD (Committee Member); Robert Brockman PhD (Committee Member); Nathan Klingbeil PhD (Committee Member); Ravi Penmetsa PhD (Committee Member); David Stargel PhD (Other); Kristina Langer PhD (Other) Subjects: Engineering

MS, University of Cincinnati, 2012, Engineering and Applied Science: Materials Science

Solid implant rods made of Ti-6Al-4V (ELI) are currently used in spinal implant devices. Due to the high stiffness of these solid rods, the implant devices are unable the meet the needs for increased flexibility over a wide range of human activities owing to changes in lifestyles and in human work environments, thereby, necessitating the need for more flexible rods. Hence, designing, developing, manufacturing and testing flexible spinal implant rods is the aim of this project. In this project, increased flexibility has been achieved by reducing the cross-section of the solid rods. Since reduction in cross-section leads to reduced load bearing capacity, the fatigue endurance level of the flexible rods is lowered. In order to increase the endurance levels of these flexible rods, Laser Shock Peening (LSP) has been used. As a part of this project, the Ohio Center for Laser Shock Processing of Advanced Materials and Devices (LSP Center) has been set-up at the University of Cincinnati through the Ohio State Third Frontier Wrights Projects funding. This project is a collaborative effort between the LSP Center and X-Spine Systems Inc., an implant devices manufacturer based in Miamisburg, Ohio. All the experimental and modeling work pertaining to this project were carried out at the LSP Center. To begin with, flexible rods were designed using finite element modeling. After considering models like flat rods and flute rods, the final design was chosen as grooved rods. Based on this design, grooved rods were machined by X-Spine Systems Inc. Also, a fatigue model to predict the effect of LSP on the fatigue life of the flexible grooved rods was developed. Based on the model, fatigue experiments were designed and carried out. Per requirements outlined by X-Spine Systems Inc, the flexible implant rods were required to demonstrate endurance level (run-out) for 5 million cycles in construct fatigue testing at -160N. This load is higher than the current industrial standard of -150N f (open full item for complete abstract)

Committee: Vijay Vasudevan PhD (Committee Chair); Dong Qian PhD (Committee Member); Rodney Roseman PhD (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2012, Engineering and Applied Science: Mechanical Engineering

This thesis focuses on a physically based strain rate dependent plasticity model known as the Mechanical Threshold Stress (MTS) model proposed by Follansbee and Kocks. The objective is to develop an algorithm based on the tangent modulus method to resolve the constitutive equation represented by the MTS model and use it to analyze the material response under laser shock peening for the Ni alloy INCONEL 718 (IN718). A user defined subroutine has been developed and integrated with commercial software LS-DYNA. A parametric study is carried out to study the influence of various model parameters on the predicted residual stresses. The developed model is then applied in the study of residual stresses imparted on INCONEL 718 induced by laser shock peening (LSP) process. Finite element analysis is performed for the case of plate made of INCONEL 718 and the residual stress predictions are compared with experimental results. The model predictions are found to be in good agreement with the experimental results. To the best of author's knowledge, this is the first time that an MTS model has been developed for IN 718 with an integrated approach.

Committee: Dong Qian PhD (Committee Chair); Janet Dong PhD (Committee Member); Vijay Vasudevan PhD (Committee Member) Subjects: Mechanics

MS, University of Cincinnati, 2011, Engineering and Applied Science: Materials Science

Precipitation behavior in IN718Plus superalloy has been studied after heat treatment between 923K to 1123K upto 1000h. Spherical gamma prime (γ') precipitates transform to plate shaped eta phase (η) upon aging which is important for fatigue design of this alloy. The as received microstructure shows plate shaped grain boundary delta phase but after thermal aging titanium containing eta plates appear within the grains by transformation of γ' along the close-packed plane through a faulting mechanism. Confirmation was obtained using transmission electron microscopy (TEM) through convergent beam electron diffraction, orientation analysis and energy dispersive X-Ray measurements. The activation energy for γ' precipitate coarsening was calculated based on precipitate size data obtained through analysis of TEM micrographs. Coarsening kinetics of γ'and γ' are coupled and were analyzed based on activation energy analysis and are compared with microhardness results. Coarsening of γ' precipitates is found to deviate from LSW theory at small sizes but matches well for intermediate and bigger particles. In this study, a procedure to identify the range of validity of operative rate law is found based on JMA model. Small particle coarsening is found to exhibit a radius square dependence with time proposed [Ardell et al, Nature Materials 2005] for ordered alloys. Since number density of plate shaped precipitates increases progressively at expense of spherical γ' with time, a general condition is derived for such systems where the content of stable daughter phase is proportional to the metastable γ' phase which it is forming from. Gamma double prime (γ'') phase was not observed during above aging treatments. Finally, thermal relaxation of Laser Shock induced stresses in this material is compared based on SXRD, XRD and microhardness data. The main features of this study are: A. Advances knowledge of precipitation in this class of superalloy upon thermal ageing at and above service te (open full item for complete abstract)

Committee: Vijay Vasudevan PhD (Committee Chair); Rodney Roseman PhD (Committee Member); Raj Singh ScD (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2008, Engineering : Mechanical Engineering

Laser Shock Peening (LSP) is a relatively new material processing technology to enhance the operating service lives of engineering components. It has been applied to metal parts such as aircraft engine turbine blades, compressor blades and medical implants like human hip joints. In this process shock waves are generated in the material using a high powered laser beam which develops residual stresses in the material. It has been proved experimentally that the life cycle of laser shock peened components are higher than conventionally shot peened components. Although LSP process has been found to be effective, improper control of the process will lead to spallation in the material under certain conditions. The spallation within the material in the form of crack significantly reduces the operating life of the component. Currently there is no systematic modeling approach to predict the material response under LSP. As such, the objective of this thesis is to develop a comprehensive model for the predicting the material response as a result of the LSP process. More specifically a pressure model is first established to obtain the pressure loading based on laser pulse intensity. Using the pressure load as input the spallation response is simulated by developing a strain rate and temperature dependent material model. The material model is implemented along with a nucleation and growth based damage model. The material and dynamic fracture model is solved using a semi-implicit forward tangent modulus algorithm. A user defined subroutine (UDM) is written and combined with the analysis tool LS-DYNA. The new model is compared with reported experimental results and a parametric study is done to understand the influence of various processing parameters. The residual stress obtained from simulation is in good agreement with experimental results. The spallation results have not been verified due to lack of experimental data.

Committee: Dong Qian Dr (Advisor) Subjects: Mechanical Engineering

PhD, University of Cincinnati, 2013, Engineering and Applied Science: Materials Science

Laser shock peening (LSP) for improving fatigue life of IN718Plus superalloy is investigated. Fatigue geometry and LSP parameters were optimized using finite element method (FEM). Residual stress distributions estimated by FEM were validated using Synchrotron XRD and line focus lab XRD, and correlated with microhardness. An eigenstrain analysis of LSP induced edge deflections (measured with optical interferometry) was also conducted. Transmission electron microscopy (TEM) of single-spot LSP coupons shows sudden increase in dislocation density under LSP treated region. Total life fatigue was conducted at R=0.1 at 298K and 923K, with and without LSP. S-N curve endurance limit increases at both temperatures with FEM optimized LSP samples. Based on TEM of fatigue microstructure and LSP coupons, mechanistic description of observed fatigue improvement is attempted. Often need arises to weld components, and weld heat-affected-zone reaches near-solvus temperatures. To simulate this treatment, sub-solvus hot-rolled IN718Plus is aged at 923K. We observe precipitation of thin eta-Ni3(Al, Ti) plates after 1000 hours, making the material susceptible to cracks, and lowering fatigue life. Effect of LSP on fatigue crack growth (FCG) is studied following ASTM guidelines on M(T) geometry at R=0.1. Acceleration in FCG rate with LSP is observed for this geometry and LSP condition. Prior FEM optimization was not conducted for FCG tests, and may account for lower FCG resistance after LSP. FCG results were corroborated with COD compliance based analysis. Crack measurements were done using potential drop method, and crack closure was analyzed. Effect of LSP on overload FCG was investigated by single-cycle 100% overload followed by single-spot LSP on the crack-tip plastic zone. Crack retardation occurs after application of overload+LSP. Effective contribution of overload+LSP to crack retardation is estimated. Fractographic analysis of LSP treated fatigue samples suggests sub-surface (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Kristina Langer Ph.D. (Committee Member); Dong Qian Ph.D. (Committee Member); Rodney Roseman Ph.D. (Committee Member); Dale Schaefer Ph.D. (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2012, Engineering and Applied Science: Mechanical Engineering

There is a continuing interest in exploring magnesium and its alloys for biomedical applications due to its low density and high strength properties that are very similar to bone. However, the high corrosion rate of magnesium and its alloys while immersed in body fluid restrict the material from use as permanent implants. The ability to control the corrosion rate of a material is an important factor in the development of biodegradable implants. An emerging topic of study for controlling the corrosion rates of different material is through imparting high levels of compressive residual stress on the surface. One effective process of imparting high levels of compressive residual stress on a material is Laser Shock Peening (LSP). Laser Shock Peening is a material process in which a high-pulsed laser creates a pressure shock wave that travels through the depth of the material, leaving high levels of compressive residual stress through its depth. This thesis will present a study on the effects of compressive residual stresses imparted through laser shock peening on the corrosion characteristics of magnesium alloy AZ91D. In order to understand all aspects of this, an in-depth material characterization of the alloy is performed. Next, an LSP study is performed in which the optimal LSP parameters are determined. The experimental results are validated using Finite Element Analysis in LS-DYNA. Once LSP parameters have been optimized, corrosion tests are performed to link the relationship between compressive residual stress and corrosion rates of AZ91D.

Committee: Dong Qian Ph.D. (Committee Chair); Seetha Ramaiah Mannava Ph.D. (Committee Member); Mark Schulz Ph.D. (Committee Member); Vijay Vasudevan Ph.D. (Committee Member) Subjects: Mechanical Engineering

PhD, University of Cincinnati, 2012, Engineering and Applied Science: Materials Science

Laser shock peening (LSP) is a novel surface treatment process that generates deep compressive residual stresses and microstructural changes and thereby dramatically improves fatigue strength of critical metal aircraft engine parts. In the past, researchers have evaluated the mechanical effects of LSP experimentally through residual strain/stress measurements, microhardness measurements or fatigue life improvement. A number of microstructure characterizations have been done on variety laser shock peened materials. However, getting better view of how LSP brings about changes in the microstructure and establish quantitative relations between LSP parameters and residual strain/stress distributions, microstructure and texture evolution is still challenging. The present study was undertaken to develop a basic understanding of the effects of LSP on the residual strain/stress distributions, texture evolution and deformation microstructural changes in Ti-6Al-4V alloy. Scanning Electron Microscopy, Scanning Probe Microscopy, Conventional X-ray Diffraction, Synchrotron X-ray Diffraction, Electron BackScattered Diffraction, microhardness and nanoindentation have been used to characterize the laser shock peened Ti-6Al-4V alloy samples. The microstructure and surface modification of laser shock peened sample are outlined in terms of laser shock peening processing parameters. Naked laser peened samples show prominent evidence of surface melting and recasting. Little difference between the peened and virgin materials can be found in the taped laser peened samples surface microstructures. Depth-resolved characterization of the residual strains and stresses was achieved using high-energy synchrotron X-ray diffraction as well as by conventional X-ray diffraction. Compressive residual strain at peened surface and tensile residual strain in the interior of the sample are found in taped samples. Naked LSP-treated samples show tensile residual stresses at peened surfaces, then dramati (open full item for complete abstract)

Committee: Vijay Vasudevan Ph.D. (Committee Chair); Dong Qian Ph.D. (Committee Member); Rodney Roseman Ph.D. (Committee Member); Vesselin Shanov Ph.D. (Committee Member) Subjects: Materials Science

PhD, University of Cincinnati, 2012, Engineering and Applied Science: Materials Science

In this project effects of Laser shock peening (LSP) on two aero engine alloys, IN718 and IN718 SPF were studied. The primary goal of the program was to secure required fundamental knowledge of e impact of LSP process parameters on these two aero engine alloys and thereby advance the science and application base of this process to other materials and parts. The research program designed accordingly includes the following key elements: 1) Developing LSP process parameters for typical Ni base aero engine alloys; (2) characterization of surface and sub-surface macro and micro residual strains/stresses a function of LSP process parameters (3) characterization of microstructural changes as a function of LSP process parameters and; (4) Study Thermal relaxation of residual stresses and understand the underlying kinetics. Firstly, different LSP process parameters including: Power density, impact overlaps, ablative overlays and coverage were studied to impart deep compressive residual stresses to the near surface regions of peened coupons. A host of different techniques were then used to characterize distribution of residual stresses/strains, roughness, hardness, plastic strains and microstructure. Role of ablative layer was also investigated. Samples were peened using an ablative layer different ablative layers (black vinyl tape, aluminum tape) and without an ablative layer and compared in terms of topography, residual stress fields and microstructure. Two different diffraction based techniques were used to characterize residual stress fields: conventional X-ray diffraction and Synchrotron X-ray diffraction (SXRD). Conventional X-ray coupled with electro polishing offers a fast means of analyzing residual stresses, while SXRD enables high resolution, non-destructive characterization of strains/stresses. Experiments showed that higher power density lead to compressive residual stresses which were higher in magnitude in near surface regions. There is a saturation power dens (open full item for complete abstract)

Committee: Vijay Vasudevan PhD (Committee Chair); Seetha Ramaiah Mannava PhD (Committee Member); Dong Qian PhD (Committee Member); Rodney Roseman PhD (Committee Member) Subjects: Materials Science

PhD, University of Cincinnati, 2012, Engineering and Applied Science: Mechanical Engineering

High cycle fatigue (HCF) is a failure mechanism that dominates the design for many engineering components and structures. Surface treatments such as laser shock peening (LSP), ultrasonic nanocrystal surface modification (UNSM) and many others introduce significant residual stresses in the material, which drastically affects the fatigue life. Motivated by the need for effectively incorporating the residual stress effect in the fatigue life prediction, two approaches are developed in this thesis. In the first approach, a strain-life approach based model is implemented. Specifically, the effect of LSP induced residual stresses on fatigue life of dynamic spinal implant rods is studied. Strain-life model is applied to predict the fatigue lives of LSP treated spinal implant rods subjected to the bending fatigue loads. However, it is observed that, the traditional life prediction methods due to their empirical nature cannot effectively model residual stress relaxation. Both safe-life and damage tolerance approaches are based on limited loading conditions and specimen geometry in the test. Extrapolation of such test data to the complicated parts with multiaxial loading conditions becomes very difficult. Motivated by these limitations, a multiple temporal scale computational approach is developed to assess the fatigue life of structural components. This full-scale simulation approach is proposed in light of the challenges in employing the traditional computational method based on Finite Element Method (FEM) and semi-discrete schemes for fatigue design and analysis. Semi-discrete schemes are known to suffer from either the time-step constraints or lack of convergence due to the oscillatory nature of the fatigue loading condition. As such, simulating loading conditions with cycles on the order of hundreds of thousands and beyond is generally an impractical task for FEM. On the other hand, there is a great demand for such a computational capability as factors such as stress his (open full item for complete abstract)

Committee: Dong Qian PhD (Committee Chair); Janet Dong PhD (Committee Member); Samir Naboulsi PhD (Committee Member); David Thompson PhD (Committee Member); Vijay Vasudevan PhD (Committee Member) Subjects: Mechanics