It was noted that, if the presence of crystal surfaces permitted some of the dipole content of a dislocation array to leave the crystal to form intrusions or extrusions at the surface, then the dipole strength of the array could change via glide processes. It was confirmed that the energy difference between vacancy and interstitial dipoles of equal height, due to non-linear elasticity, was a possible cause of the conversion of interstitial dipoles to vacancy dipoles by slip; with the consequent production of extrusions at the crystal surface. The energy difference increased with decreasing dipole height, and might account for the observed preponderance of small vacancy dipoles. However, the gain in energy which resulted from the conversion of interstitial dipoles to vacancy dipoles was off-set by the long-range elastic energy stored in the field of the so-called fiber stress. This was the average tensile stress in the band, resulting from the bias. This led to an equilibrium bias in dipole population, and caused the fiber stress to increase as cyclic straining reduced the height of the dipoles.

The Enumeration and Transformation of Dislocation Dipoles II. The Transformation of Interstitial Dipoles into Vacancy Dipoles in an Open Dislocation Array. L.M.Brown, F.R.N.Nabarro: Philosophical Magazine, 2004, 84[3], 441-50