30. REDUCING FATIGUE IN METALS USED FOR AEROPLANES

The selection of metals for the construction of the turbine and compressor blades in gas turbine engines for aircraft has to be made carefully, for failure of these components and parts can have disastrous consequences. One of the problems into which manufacturers have had to delve for some years is the destructive effects of corrosive atmosphere on the compressor blades of engines of aircraft used on trans-ocean routes. The compressor blades can be seriously affected by this.

General Motors Corporation has been working along new lines of research to cure this compressor blade trouble. Fatigue is the result of dislocations, which are lattice imperfections in the material, brought about by an excess or deficiency of atoms in an otherwise regular lattice. These .imperfections "migrate" until they are confronted with .barriers. They are then entrapped by solute atoms or, alternatively, are neutralised by encountering other imperfections.

General Motors research department formed the view that by subjecting the blades to a magnetic field of sufficient strength they would become magnetically saturated to such an extent that the dislocations would be locked in the metal while, at the same time, the residual magnetism would act as a brake on the migration of the dislocations along dominant glide planes to the surface.

Experiments were carried out in which stainless steel blades were placed in a salt fog atmosphere and a current of 1,500 amperes was used to magnetise them. The results were satisfactory, for the fatigue stress which brought about failure was 54 per cent greater than that needed to cause a non-magnetised blade of identical design to fail. Moreover, the magnetised blade was under test over three and a half times longer than the non-magnetised specimen.

(from New Scientist, 24th August, 1961)