PSI - Issue 43

John Campbell / Procedia Structural Integrity 43 (2023) 234–239 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction It is salutary to consider how a crack can initiate in a metal. Whereas the plastic deformability of metals is now elegantly and powerfully explained by dislocation theory, there appears to be no equivalent intrinsic mechanism to explain how metals can fail by crack formation (Campbell 2020). We can examine some of the more widely circulated suppositions as examples. The widely quoted dislocation pile-up theory, in which a crack can be formed by the pile-up, is now known to be wrong. Pile-ups are common, of course, and have been intensively studied. However, they appear never to initiate a crack. This emphasises the role of the immense inter-atomic forces, which militate against the pulling apart of atoms in the lattice. This is corroborated by the theory of condensation of vacancies, which, analogously as solute atoms might condense to a precipitate, might be supposed to condense to voids, which, in turn, might be of a sufficient size to initiate failure by cracking. However, this is also not true. Many studies have confirmed that vacancies condense only to completely collapsed structures of near lattice perfection such as dislocation rings and stacking fault tetrahedra. Similarly, Cottrell’s elegant dislocation theory, in which slip on two intersecting lattice planes creates a crack, has also never been demonstrated despite much research. In the absence of the design flaw or accidental hacksaw cut, metals should never fracture. The fact that metals commonly fracture in a number of conditions, including for instance, tensile testing, fatigue, creep, hot tearing etc. requires explanation. This is the purpose of this paper. 2. Crack Generation by Casting Liquid metals usually react with their environment, such as air or (partial) vacuum, and so develop a covering of oxide film. Under the microscope, the top surface of the film is dry and rugged, like sandpaper, whereas the underside of the film is wetted, in perfect atomic contact with the liquid from which it has grown. Unfortunately, when stirred or poured, droplets and splashes come into contact by the mutual impingement of the dry surfaces of their oxide films. As a result, the two masses of metal form a double oxide film between them, but which is unbonded at its interface. This pair of unbonded films, as a double film, becomes now suspended in the liquid as a crack (figure 1). This defect is known for convenience as a ‘bifilm’. Turbulent pouring of liquid metals can create a snowstorm of bifilms, filling the liquid with cracks (Campbell 2015).

Figure 1. The nature of surface turbulence, and bifilm crack creation.

Because, when pouring, splashes occur within milliseconds, the metal surface expanding to create turbulence reveals fresh metal which has little time for its oxide to form and thicken, with the result that many bifilms are only nanometers thick. They are therefore often not easily seen during casual observation, and seem to have escaped notice for the past 6000 thousand years.