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This book offers a concise introduction to fatigue crack growth, based on practical examples. It discusses the essential concepts of fracture mechanics, fatigue crack growth under constant and variable amplitude loading and the determination of the fracture-mechanical material parameters. The book also introduces the analytical and numerical simulation of fatigue crack growth as well as crack initiation. It concludes with a detailed description of several practical case studies and some exercises. The target group includes graduate students, researchers at universities and practicing engineers.
Fatigue failure is a multi-stage process. It begins with the initiation of cracks, and with continued cyclic loading the cracks propagate, finally leading to the rupture of a component or specimen. The demarcation between the above stages is not well-defined. Depending upon the scale of interest, the variation may span three orders of magnitude. For example, to a material scientist an initiated crack may be of the order of a micron, whereas for an engineer it can be of the order of a millimetre. It is not surprising therefore to see that investigation of the fatigue process has followed different paths depending upon the scale of phenomenon under investigation. Interest in the study of fatigue failure increased with the advent of industrial ization. Because of the urgent need to design against fatigue failure, early investiga tors focused on prototype testing and proposed failure criteria similar to design formulae. Thus, a methodology developed whereby the fatigue theories were proposed based on experimental observations, albeit at times with limited scope. This type of phenomenological approach progressed rapidly during the past four decades as closed-loop testing machines became available.
Volume is indexed by Thomson Reuters CPCI-S (WoS). The book covers a wide range of topics: Fracture Mechanics, Failure analysis, Composites, Multiscale Modelling, Micromechanics, Structural Health Monitoring, Damage Tolerance, Corrosion, Creep, Non-linear problems, Dynamic Fracture, Residual Stress, Environmental effects, Crack Propagation, Metallic and Concrete Materials, Probabilistic Aspects, Computer Modeling Methods (Finite Elements, Boundary Elements and Meshless), Microstructural and Multiscale Aspects.
This dissertation is composed of three papers and related unpublished work providing a foundation that helps understand near threshold fatigue crack growth in both vacuum and environment conditions in the atomic scale. All studies are made possible by utilization of a recent implementation of the multiscale coupled atomistic and discrete dislocation (LF-CADD) methodology. First, near threshold fatigue crack growth of a ductile material in vacuum is investigated via atomistic modeling of dislocation motions and slip interactions. The simulation results indicate that sustained fatigue crack growth in vacuum requires emitted dislocations to change slip planes prior to their reabsorption into the crack on the opposite side of the loading cycle. This finding assesses the validity of long-hypothesized material separation mechanisms using state-of-the-art computational resources, conforms to reports of crack growth below experimentally assisted fatigue crack growth thresholds, and opens the door for improved prognosis and the design of novel fatigue resistance alloys. Second, the role of material dissolution is studied for intrinsically ductile and semi-brittle materials isolated from other environmentally assisted fatigue crack growth mechanisms. The multiscale simulations and subsequent analysis suggest that dissolution can be dual natured depending on the material. For a ductile material, dissolution at crack tip acts to blunt the crack with increasing crack tip opening displacement (CTOD). On the other hand, dissolution will work as a driving force for a slow growing in a brittle and semi-brittle material where the crack remains atomically sharp. The latter will eventually trigger brittle, unstable crack propagation as the applied load exceeds a critical threshold. Third, a crack with surface film is studied under mode I fatigue loading. Using our concurrent multiscale LF-CADD model, we conduct a series of simulations with varying loading conditions and find that the crack grows via film rupture mechanism, which requires film thickness and brittleness. Surface film swelling is needed to produce experimentally observed R-effect on $\Delta K$ threshold. In order to achieve sustained fatigue crack growth, a critical K ($K_{max}$) is necessary to overcome the compressive stress ahead of the crack tip at the bottom of the loading cycle due to film swelling.