A new method of increasing the fatigue strength of metals

High-intensity ultrasonics at 23 kHz was applied to mild-steel specimens. No change in hardness could be detected at a temperature of 20°C, but when the temperature was allowed to increase by energy-absorption the hardness increased in a subsurface layer. This change in hardness was accompanied by changes in density and in the arrangement of dislocations, and resulted in an increase of 20–30% in the low-frequency fatigue limit. An explanation of this phenomenon is attempted.