AFIT School of Engineering Contributions to Air Force Research and Technology. Calendar Year 1971

This report contains abstracts of Master of Science theses and Doctoral Dissertations completed during the 1971 calendar year at the School of Engineering, Air Force Institute of Technology

Reliability-based maintenance optimization of corrosion preventive designs under a life cycle perspective

[EN] Sustainability is of paramount importance when facing the design of long lasting, maintenance demanding structures. In particular, a sustainable life cycle design for concrete structure exposed to aggressive environments may lead to significant economic savings, and to reduced environmental consequences. The present study evaluates 18 different design alternatives for an existing concrete bridge deck exposed to chlorides, analyzing the economic and environmental impacts associated with each design as a function of the maintenance interval chosen. Results are illustrated in the context of a reliability-based maintenance optimization on both life cycle costs and life cycle environmental impacts. Maintenance optimization results in significant reductions of life cycle impacts if compared to the damage resulting from performing the maintenance actions when the end of the service life of the structure is reached. The use of concrete with 10% silica fume has been shown to be the most effective prevention strategy against corrosion of reinforcement steel in economic terms, reducing the life cycle costs of the original deck design by 76%. From an environmental perspective, maintenance based on the hydrophobic treatment of the concrete deck surface results in the best performance, allowing for a reduction of the impacts associated with the original design by 82.8%.The authors acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness, along with FEDER funding (Project: BIA2017-85098-R).Navarro, I.; Martí Albiñana, JV.; Yepes, V. (2019). Reliability-based maintenance optimization of corrosion preventive designs under a life cycle perspective. Environmental Impact Assessment Review. 74:23-34. https://doi.org/10.1016/j.eiar.2018.10.001S23347

Hydrogen Embrittlement of Automotive Ultra-High-Strength Steels: Mechanism and Minimisation

Automotive manufacturers are increasingly using ultra-high-strength steels in vehicle components to facilitate mass reduction via downgauging. Unfortunately, as the strength of steels increases, so does susceptibility to ‘hydrogen embrittlement’, a process in which ductility is significantly impaired by ingress of hydrogen. Mechanisms and environmental conditions by which this degradation occurs are not fully understood. In this work, 2 fully-ferritic, 2 fully-martensitic boron, and 2 ferrite-martensite dual-phase, ultra-high-strength steels, were assessed for susceptibility to hydrogen embrittlement via 3 key characteristics: firstly, with particular regard to hydrogen evolution under corrosion conditions, through well-established open circuit potential and potentiodynamic polarisation experiments. Exacerbation of hydrogen evolution through galvanic corrosion of a zinc coating was assessed by scanning vibrating electrode technique (SVET), and an attempt made to quantify increased risk of hydrogen evolution during crevice corrosion through a novel time-lapse photography experiment. Secondly, hydrogen diffusivity was assessed via permeation experiments. Finally, degradation in mechanical properties due to diffusing hydrogen was evaluated through slow strain rate tests (SSRT), whereby susceptibility to embrittlement was equated to reduction in ductility of hydrogen-charged test specimens. The fully-ferritic steels showed the greatest resistance to mechanical degradation, attributed to micro-alloy nano-precipitates within their microstructure acting as ‘traps’, leading to lower diffusivity compared to dual-phase steels of equivalent strength. Indeed, lower diffusivity showed a strong correlation with lower levels of embrittlement across all steels. 1000 MPa dual-phase steel showed the greatest degradation in mechanical properties, with fully-martensitic boron steels also found to be particularly susceptible. 1000 MPa dual-phase steel also showed the largest increase in hydrogen evolution reaction in response to polarisation, thought to result from the inherent potential difference between ferrite and martensite phases. Galvanic corrosion of a damaged zinc coating was found to polarise the exposed steel substrate, triggering sufficient hydrogen evolution to reach critical concentrations for embrittlement

Corrosion and Protection of Metals

Introduction and Scope—During the last few decades, an enormous effort has been made to understand corrosion phenomena and their mechanisms, and to elucidate the causes that dramatically influence the service lifetime of metal materials. The performance of metal materials in aggressive environments is critical for a sustainable society. The failure of the material in service impacts the economy, the environment, health, and society. In this regard, corrosion-based economic losses due to maintenance, repair, and the replacement of existing structures and infrastructure account for up to 4% of gross domestic product (GDP) in well developed countries. One of the biggest issues in corrosion engineering is estimating service lifetime. Corrosion prediction has become very difficult, as there is no direct correlation with service lifetime and experimental lab results, usually as a result of discrepancies between accelerated testing and real corrosion processes. It is of major interest to forecast the impact of corrosion-based losses on society and the global economy, since existing structures and infrastructure are becoming old, and crucial decisions now need to be made to replace them. On the other hand, environmental protocols seek to reduce greenhouse effects. Therefore, low emission policies, in force, establish regulations for the next generation of materials and technologies. Advanced technologies and emergent materials will enable us to get through the next century. Great advances are currently in progress for the development of corrosion-resistant metal materials for different sectors, such as energy, transport, construction, and health. This Special Issue on the corrosion and protection of metals is focused on current trends in corrosion science, engineering, and technology, ranging from fundamental to applied research, thus covering subjects related to corrosion mechanisms and modelling, protection and inhibition processes, and mitigation strategies

Reducing CO2 and Corrosion: Insights from Thermodynamic Descriptors Calculated with Density Functional Theory

Catalytic reaction mechanisms can be extremely complex, and it is difficult to determine all the factors that control reaction rates. Fortunately, complex chemical phenomena can frequently be described by thermodynamic properties (such as molecular pKas and reaction overpotentials) that correlate with catalytic reaction rates. While these properties can be difficult or time intensive to measure experimentally, they can be easily computed using Kohn-Sham density functional theory (KS-DFT). We have developed a thermodynamic descriptor-based model that uses molecular pKas and redox potentials calculated with KS-DFT to predict the electrochemical conditions at which aromatic N-heterocycle (ANH) molecules could facilitate multi-proton and multi-electron reduction reactions. By automating this procedure using the ADF modeling suite, we can rapidly screen through potential catalysts with minimal user input. To establish a baseline procedure for studying the chemical reduction of CO2 via hydride transfers from ANH molecules, we characterized the chemical reduction of CO2 by hydride transfers from sodium borohydride. We located hydride transfer pathways with nudged elastic band calculations and obtained free energy barriers from potentials of mean force derived from constrained molecular dynamics simulations along the reaction pathways. These simulations provided reaction energetics at realistic operating conditions and highlighted the potential pitfalls of only studying reaction pathways at 0 K. Cathodic reduction reactions can limit galvanic corrosion rates in atmospheric environments. To help guide the design of titanium alloys that resist galvanic corrosion, we used density functional theory to predict dopants that inhibit cathodic reduction reaction kinetics on oxide surfaces. We calculated overpotentials for the oxygen reduction reaction (ORR) occurring on metal dopants in an amorphous TiO2 surface. These overpotential trends successfully predicted six dopants that have been experimentally verified to inhibit ORR activity by up to 77% (Sn, Cr, Co, Al, Mn, and V). Next, we used this approach to study the native oxides of Ti-6Al-4V, a Ti alloy with improved corrosion resistance. We used Behler-Parrinello neural networks to create defective and amorphous surface models for TiAl2O5 (the oxide that forms on Ti-6Al-4V surfaces in addition to TiO2) and predicted how ORR activity was altered by different complex oxide surface morphologies

Technology of Welding and Joining

In this book, you will find information on new materials and new welding technologies. Problems related to the welding of difficult-to-weld materials are considered and solved. The latest welding technologies and processes are presented. This book provides an opportunity to learn about the latest trends and developments in the welding industry. Enjoy reading

Graphene-Polymer Composites II

Graphene-polymer nanocomposites continue to gain interest in diverse scientific and technological fields. Graphene-based nanomaterials present the advantages of other carbon nanofillers, like electrical and thermal conductivity, while having significantly lower production costs when compared to materials such as carbon nanotubes, for instance. In addition, in the oxidized forms of graphene, the large specific area combined with a large quantity of functionalizable chemical groups available for physical or chemical interaction with polymers, allow for good dispersion and tunable binding with the surrounding matrix. Other features are noteworthy in graphene-based nanomaterials, like their generally good biocompatibility and the ability to absorb near-infrared radiation, allowing for the use in biomedical applications, such as drug delivery and photothermal therapy.This Special Issue provides an encompassing view on the state of the art of graphene-polymer composites, showing how current research is dealing with new and exciting challenges. The published papers cover topics ranging from novel production methods and insights on mechanisms of mechanical reinforcement of composites, to applications as diverse as automotive and aeronautics, cancer treatment, anticorrosive coatings, thermally conductive fabrics and foams, and oil-adsorbent aerogels

Materials Science and Technology - Nuclear Materials, Advanced Course - Kon-67.5100 Postgraduate Seminar on Engineering Materials - Seminar papers 8 October, 2015

The Engineering materials research group of Department of Engineering Design and Production of Aalto University arranged a postgraduate course on nuclear materials. The course consisted of three day long lecture session given in April 20-22, 2015. Lectures were given by professionals from nuclear power related research institutes (Aalto and VTT), nuclear industry and authority. The course also included a seminar session held October 8, 2015. The seminar session was targeted to postgraduate students, who prepared articles from their field of expertise. This proceeding is the collection of these seminar articles