Quantification of Slip Lines in Fatigue

Although fatigue, even low-cycle fatigue, can occur at stresses below the engineering yield strength of a material, plasticity is involved because slip lines may appear on the surface of a test material. These slip lines increase in number in the early stages of fatigue and are indeed the precursors of ultimate failure. In a cyclic stress test, the initial rapid increase in number of slip lines produces slip saturation, after which other phenomena such as crack initiation become predominant. New slip lines are added at a decreasing rate.

Quantitative measurements have been made of slip lines in fatigue specimens and compared with the number in a tensile test of the same material. In fatigue, the equation describing the rate of addition of new slip lines requires that the stress be described in terms of how much the test stress exceeded a quantity, characteristic of the material, which can be thought of as a “yield strength” in fatigue. This quantity corresponded closely to the measured endurance limit of the material.

A comparison is made with similar measurements in a tensile test.