Chapter 2 Characterization of High Frequency Pulse Loading on Fatigue ofMetals Paul A. Lara, Hugh A. Bruck, and Edda C. Müller Abstract Aluminum materials of various grades are utilized across many industries, spanning from the cycling, automotive, aerospace, and the marine industry. In the marine grade, aluminum materials are utilized to construct entire vessels of various lengths or portions of them by taking advantage of the lower weight characteristics of the materials and impact on stability of the structures. In particular, 5xxx series aluminum materials are relied on by the marine industry for these purposes, taking advantage of the ability of these series to resist marine corrosive environments. However, during its lifetime, a marine vessel will experience a multitude of variable amplitude loading conditions, with occasional overloads and underloads depending on the operation and environmental conditions. In some cases, these overloads/underloads can result in the failure of the structures by reaching its ultimate capacity, but in other instances, they can systematically affect the growth rate of localized cracks. Existing models, like the Wheeler, Willenborg, plus variations of these, have been utilized to predict the crack growth behavior with varying degrees of success. We created an experimental matrix to explore the effects of overload/underload combinations on fatigue crack growth in 5xxx aluminum. Both visual inspection of crack tip location and digital image correlation (DIC) characterization of the crack tip deformation fields were used to characterize the crack growth in center crack tension (CCT) panel specimens. DIC also enabled additional analysis of strain fields to elucidate on the conditions responsible for change in the crack growth behavior. This chapter outlines some of the ongoing results of this work, which built on past experimental work conducted. Future phases of this work will utilize this data to develop new models for fatigue crack growth, and application of multiple pulses sequences. Keywords Fatigue crack growth · High frequency pulse · Overloads/underloads · Plastic zone · Crack kinking 2.1 Introduction Embedded high frequency (HF) signal effects derived from wave impacts on ships can affect failure mechanisms on the structures and have an adverse impact on the fatigue life of the vessel. While operating in a sea environment, ship structures can be subject to many operational loads (wind, pressure, temperature, etc.), one of which is the structural effects derived from the surrounding sea environment. Typically, the wave environment applies an ordinary wave component which drives the primary bending stress of the vessel, along with a more stochastically driven element that manifest itself as wave impacts. This dynamic wave impacts results in a high frequency vibratory response signals load applied to ship structures. The vibratory nature of these high frequency responses makes the design of structures increasingly complex in nature due to their uncertainty; designers and naval design rule societies have relied on methods such as safety factors and/or margins of safety to account for its effects. A typical wave impact on a naval structure imparts a time-dependent pulse load that exhibits a higher frequency logarithmic decaying sinusoidal response when measured via experimental means and captured utilizing P. A. Lara Naval Surface Warfare Center, Carderock Division, West Bethesda, MD, USA e-mail: lara@umd.edu H. A. Bruck ( ) Mechanical Engineering Department, University of Maryland, College Park, MD, USA e-mail: bruck@eng.umd.edu E. C. Müller Düsseldorf University of Applied Science, Düsseldorf, Germany e-mail: edda.mueller@study.hs-duesseldorf.de © The Society for Experimental Mechanics, Inc. 2021 S. Xia et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-60959-7_2 7
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