Transient Fatigue Crack-Growth Behavior and Damage Zones in Zr-Based Bulk Metallic Glass
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Transient Fatigue Crack-Growth Behavior and Damage Zones in Zr-Based Bulk Metallic Glass Peter A. Hess and Reinhold H. Dauskardt Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 ABSTRACT Fatigue crack propagation mechanisms of bulk metallic glasses (BMGs) are not well understood, limiting their use in safety-critical structural applications particularly where complex fatigue loading may occur. Accordingly, the present study examines the effects of variable amplitude fatigue loading associated with block loading and tensile overloads on fatigue crackgrowth rates in a Zr-based BMG. Crack growth studies were conducted on compact tension specimens using computer control of the applied stress intensity range, ∆K. Fatigue crack closure loads, which represent the initial contact of mating crack surfaces during the unloading cycle, were continuously monitored during testing. Abrupt drops in ∆K were found to significantly decrease fatigue crack-growth rates far below equilibrium values, arresting growth completely at a ∆K twice the nominal fatigue threshold ∆KTH. Conversely, an abrupt increase in ∆K was found to accelerate fatigue crack-growth rates. The effects of roughness-induced crack closure were assessed and found to be consistent with the suppression or acceleration of growth rates. However, in order to fully explain the observed transient growth rate response, other mechanisms that may be related to the fatigue mechanism itself were also considered. Specifically, the nature of the fatigue crack tip damage zone was also investigated. As BMGs lack distributed plasticity at low temperatures, the plastic zone differs greatly from that seen in ductile crystalline materials, and its role in fatigue crack propagation mechanisms is examined. INTRODUCTION Bulk metallic glasses offer interesting possibilities for use in many load-bearing applications due to their impressive monotonic mechanical properties [1-4]. However, these materials are easily damaged by cyclic loading. A low fatigue crack-growth threshold, ∆KTH, and endurance limit, σe, under cyclic loading are among the foremost concerns for structural applications. Most experimental studies on fatigue crack growth in BMGs have been performed on Zr41.2Ti13.8Cu12.5Ni10Be22.5 [2, 5-9]. These studies have investigated such effects as load ratio [2], elevated temperature[7, 9], environment [5], and partial crystallization [8]. Though steadystate fatigue testing provides a great deal of useful information, many structures in service conditions undergo random or variable-amplitude loading. Sudden changes in loading can result in transient crack-growth behavior for subsequent crack extension. In polycrystalline metals, such changes are generally attributed to changes in the level of premature contact of mating crack faces, known as crack closure. Mechanisms responsible for these transients include residual stresses in the plastic zone, surface asperity contact, and deflection of the crack tip, and the affected regions of crack g
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