Abstract
The possible mechanisms causing an increase of electrical resistance during fatigue of a commercial bondable fatigue gauge are considered. Commercial users of the gauge would welcome a greater sensitivity than at present available. If it were possible to isolate the cause of the resistance increase a more sensitive type of material might be suggested. The present gauge material is annealed foil of composition Cu 43at% Ni, 2at% Mn. Previous work, reviewed under the broad headings - clustering in Cu-Ni alloys; electrical resistivity of defects and clusters; point defect mobility and fatigue and plastic deformation indicates that Cu-Ni alloys contain clusters of nickel atoms and that the formation or break-up of these clusters, which are believed to affect the electrical resistivity, can be brought about by appropriate heat treatment or deformation. Annealed foils of Cu, Ni, Cu 20.5, 50. 6 and 75.9at% Ni and commercial constantan (Telconstan - Cu 46.1at% Ni, 1.4at% Mn) were bonded to an aluminium alloy cantilever beam and fatigued in bending. Measurements of electrical resistance were made during fatigue and subsequent annealing. The dislocation structures at different stages of fatigue and annealing were followed by electron microscopy, while the formation and growth of fatigue slip bands and of cracks were observed by optical microscopy. The results indicate that the break-up of the nickel clusters may be the predominant cause of the resistance increase. This suggests that further work might usefully be carried out on other alloys known to show a large clustering effect.