Stress Intensity Incubation Periods for the Al-hg Couple Subjected to Lme

When in the presence of liquid metal environments, structural materials can potentially lose the ability to deform and plastically flow. In the case of a ductile material, the result of this reduction in flow ability is a transition from ductile to brittle behavior, resulting in a brittle-like failure. This phenomenon is known as liquid metal embrittlement (LME) and is a subset of the more commonly known family of environmentally assisted cracking (EAC). Both EAC and LME have a significant negative impact on structural materials that are designed to behave elastically. Previous research in all facets of EAC, including stress corrosion cracking (SCC), corrosion fatigue (CF) and LME, has revealed that structural materials subjected to loading will generate and propagate cracks at stresses and stress intensities well below the critical values for that material. Additionally, crack tip velocities have been predicted and observed to be orders of magnitude greater than in ambient environments, with velocities in the range of tens to hundreds of centimeters per second. A variety of experimental routines have been used to characterize the interaction and develop microstructural failure mechanism in LME; however, uncertainty still surrounds the true failure mechanism. In a novel experimental approach, the dependence of the stress intensity factor (SIF) on crack propagation in the presence of a liquid metal was observed. Fracture mechanics specimens machined from Al7075-T651 in the S-L orientation were fatigue pre-cracked and incubated under load while submersed in liquid mercury. The result was the observation of rupture times over a range of stress intensity factors. It was noted that any stress concentration could provide the necessary criterion for crack initiation and propagation, regardless of the presence of a crack. Critical stresses and critical microstructural orientations dictated rupture paths more so than a pre-formed fatigue crack. Further experimentation, involving original and novel methods, has been conducted to determine the relationship between the stress intensity factor, stress concentration and microstructural orientation. Ultimately, the goal to confirm, extend or reject current microstructural failure mechanisms can be achieved through continued experimental routines. INTRODUCTION The susceptibility of metals to attack by the surrounding environment has been observed to significantly affect the load carrying capability of the metal component. Through a chemical reaction, a reduction in the strength of structural components occurs, with the possibility of premature failure. Cracks that initiate and propagate in components subjected to various environments fall under the umbrella of phenomena known as environmentally assisted cracking (EAC). Environmentally assisted cracking includes several subsets, including: hydrogen embrittlement (HE), corrosion fatigue (CF), stress corrosion cracking (SCC) and liquid metal embrittlement (LME). Liquid metal embrittlement, although not as prevalent as other branches, has led to the failure of several components in service [1-3]. Consequences are severe and catastrophic, with the loss of life a real possibility. A prime example was that of an ethylene plant, located in Texas, that experienced a failure in an aluminum pipe weld due to exposure to liquid mercury in 1987 [4]. It was noted that mercury does not wet aluminum well as a result of oxide formation, but in the event of corrosion, fatigue and other events, wetting can occur. Additional examples of failures have been previously discussed and provide thorough detail on the failure modes. Of particular importance has been the underlying microstructural failure mechanism that drives cracks in the presence of liquid metals. Understanding the mechanisms behind LME could provide engineers, designers and technicians adequate knowledge needed at all phases in the life of a component. Acknowledging the presence of liquid metals, either present in the design, by the liquefaction of a solid metal or generation of liquid metals through chemical reactions, there 1 Copyright © 2010 by ASME Proceedings of the ASME 2010 International Mechanical Engineering Congress & Exposition IMECE2010 November 12-18, 2010, Vancouver, British Columbia, Canada