Effect of High-Strain Fatigue Cycles on the Brittle-Ductile Transition of Two Mild Steels

Abstract

Low endurance fatigue curves were obtained for grades A and D mild steel under the following axial conditions:

reversed load cycling—plain specimens;

repeated tensile load cycling—plain specimens (grade A only);

reversed strain cycling—plain material;

reversed strain cycling—notched material.

Some results were also obtained for reversed strain cycling in bending of notched specimens. The fatigue results were generally as expected, the grade D steel showing a slightly superior fatigue strength.

The major part of the programme was then concerned with the effect of plastic strain cycling on the brittle fracture transition behaviour. Prior axial strain cycling initiated fairly blunt fatigue cracks after only a few cycles in all notched specimens and the Tipper test transition was lowered by about 10°C for the grade A steel, but there was little effect on the grade D material. The Charpy transition temperature was raised by various amounts (20°C maximum), depending on the cyclic strain range, number of cycles and type of steel.