Stress Intensity Incubation Periods for the Al-Hg Coupled Subjected to LME

Abstract

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.