An overview and discussion of basic methodology for fatigue

Abstract

This paper broadly reviews the stress-based, strain-based, and crack growth approaches to fatigue life prediction, and it attempts to suggest some choices and variations of these that might enhance their inclusion in undergraduate education and their more routine use by practicing engineers. For the stress-based approach, emphasis should shift toward the use of data on actual components, and it should be recognized that damage below the usual fatigue limit may occur. Also, evaluation of mean stress effects by the modified Goodman diagram should be replaced by other methods. The usefulness of the strain-based approach for simple situations may be extended by adding empirical adjustments for surface finish and size. It may also be desirable to lower the long-life end of the strain-life curve to obtain agreement with limited component test data, producing a component-specific strain-life curve. Use of the crack growth approach is hampered by the lack of a widely accepted set of materials constants for describing da/dN versus ΔK curves. It is recommended that this situation be remedied by representing the intermediate growth rate region with a Paris-type exponent, an associated coefficient, and a third constant that characterizes the sensitivity to R-ratio according to the equation of Walker. Limits or asymptotic behavior for the low and high growth rate regions should then be handled separately.