INVESTIGATIONS AND MODELLING OF BENDING FATIGUE BEHAVIOR OF A LOW CARBON STEEL II MATHEMATICAL DESCRIPTION AND ANALYSES

摘要

Growth of short cracks on hour-glass specimen surfaces has been observed in rotation-bending and pure-bending fatigue tests of a rolled low-carbon steel ROLCLAS 09G2 with microstructure of long and uniform pearlite and ferrite bands repeated in sequence. Two models have been proposed describing the presentation of growth rate of surface cracks against their length in the first model, and against a specially introduced energy function in the second one. The graphical presentation of the first model is a System of parabolic-linear functions and of the second model - Simple straight line. The Straight line model can be treated as a model presenting the natural fatigue tendency of a given material. The proposed models are supported by a reasonable agreement between the calculated by them and experimental fatigue lifetimes.Investigations include short fatigue crack rotation-bending experiments and length measurements of propagating crack on hour-glass specimens divided into two groups - of basic specimens and of specimens with modified surface. In the second group the basic specimens have modified surface obtained at special conditions of electrical-discharge treatment in electrolyte. The pure bending is performed on basic flat specimens forming a third group of specimens especially chosen for investigations on crack-microstructure interactions.