SLOW BRITTLE FRACTURE C. J. McMahon, Jr. Department of Materials Science and Engineering University of Pennsylvania, Philadelphia, PA 19104 ABSTRACT In contrast to the catastrophic types of brittle fracture in which cracks propagate unstably at hundreds of meters per second, brittle fracture can occur in a stable fashion at speeds that are many orders of magnitude slower. This occurs as a result of surface contamination by a mobile embrittling element that is induced to diffuse into a solid, usually along grain boundaries, under the influence of an applied stress. Examples are common in various engineering structures, but it is not widely recognized that they all belong to the same generic form of brittle fracture. KEY WORDS: dynamic embrittlement, brittle fracture, intergranular fracture INTRODUCTION The term “brittle” is used here to denote fracture by decohesion that occurs apart from any plastic process. That is, plasticity may occur concomitantly, but it is not an essential part of the decohesion process. In the familiar fast brittle fracture in metals by transcrystalline cleavage or intergranular decohesion, plasticity is usually essential to crack nucleation, but is only incidental to crack propagation. The same applies here to slow brittle fracture. Slow brittle fracture involves the propagation of a crack in a solid the surface of which is contaminated by an element that, when concentrated in the solid, would reduce the local cohesive strength. The application of a tensile stress to a solid in which a crack has nucleated both stretches bonds and causes the embrittling element to diffuse inward at the tip of the crack. This allows the crack to spread at a rate dependent upon the rate of diffusion of the embrittling element from the surface into the region ahead of the crack tip. This process has been termed “dynamic embrittlement” [1,2]. The phenomenon of dynamic embrittlement can be categorized according to the available concentration of embrittling element at the crack tip. The three categories being studied systematically in alloys are the following: (i) Low concentration. The embrittling element comes by way of surface segregation from the alloy itself.
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