Embrittlement: A Crack Tip View

Normally tough structural materials may  fail catastrophically when contaminated with even minuscule quantities of ubiquitous elements such as sulfur, phosphorus or hydrogen.  These so called embrittling elements exert their influence at the tip of atomically sharp cracks where applied stress is concentrated to a degree given by the stress concentration factor.  When the concentrated stress exceeds the strength of the bonds across the crack tip, the crack will run.  Hence, an element may  embrittle a host either by decreasing bond strength and/or increasing the stress concentration.  While the effects of embrittling elements on the earlier have been extensively studied, less attention has been directed to their effects on the latter.  Here we exploit the nearsightedness of electronic matter principle to provide a measure of the stress concentration factor and study its changes in the presence of a known embrittling element.  We find that for the well studied system of copper embrittled by dilute quantities of bismuth, bismuth atoms increase the  crack tip stress concentration  by more than 60%