New method for evaluation of the Manson-Coffin-Basquin and Ramberg-Osgood equations with respect to compatibility

Abstract

In the modern description of the fatigue behaviour of materials the stress-strain curve, described with Ramberg-Osgood equation, and the strain-life curve, described with Manson-Coffin-Basquin equation, are typically used. It is known that the assumption of equality of the plastic and elastic components in both equations leads to the so-called compatibility condition and connect the equations theoretically. The conventional method for evaluation of the fatigue parameters use one set of experimental data from strain-controlled uni-axial fatigue tests but they not ensure the compatibility conditions. The presented new method for determining the stress-strain and strain-life curves retains the mathematical and physical relationships between the considered curves. The method involves fitting the curve to experimental data points in a three-dimensional strain-stress-life space. With the plastic part of strain, stress and fatigue life as coordinates, a straight line is used for fitting the experimental data points. The material parameters are calculated directly from projections of the three-dimensional straight line on suitable planes. The results obtained from this new method using high-strength aluminium alloys subjected to different manufacturing conditions and different test temperatures are presented. These results are then compared to results obtained with a conventional method for determining the fatigue parameters. © 2008 Elsevier Ltd. All rights reserved.