Some Investigations on the Relation between the Internal Friction and the Scattering of Fatigue Strength of Steel with Internal Flaws

Abstract

The results of fatigue test concerning metals with various flaws usually scatter to such a extent, that they must be represented by means of a scatter band of considerable wide range, in cases when they are simply classified in groups from the view point of superficial flaws. On the contrary, the internal friction is supposed to have much closer connection with internal flaws of material, thus investigations were made on the relationship between the internal friction and the fatigue strength.
The internal friction is measured by means of period in which the amplitude of free damping vibration diminishes to one fourth of its original magnitude (quarter-value period). Test pieces are cut out along a radius of a forged block of about 600mm in diameter which has been previously inspected by supersonic wave to ascertain the inclusion of much flaws. Referring to the supersonic inspection, they are devided in three groups of layers, the first layer being near the cuticle and the third near the core. The superficial flaws of each specimen are investigated by means of magna-flux method. The test is carried on in such a way that each test piece is hung down in the air by thin threads at the nodes of transverse vibration of both ends free, so that the quarter-value period is measured.
The rotating bending tests are carried cut. The results obtained from specimens belonging to the second and third layers show scattering, as expected, to a great extent, which can not be explained by the number of superficial flaws. It is found, however, that the scattered results can be arranged in fairly good order, if they are classified by the quarter-value period.