Modified NASGRO Equation for Short Cracks and Application to the Fitness-for-purpose Assessment of Surface-treated Components

Abstract

Fatigue cracks in cyclically loaded components usually initiate in areas of high stress concentration. Such stress concentrations do not only occur due to the geometrical design of a component, but also near material inhomogeneities or Foreign Object Damage (FOD). In the context of a fitness-for-purpose (FFP) assessment, it is important to estimate the fatigue crack growth rate as accurately as possible, e.g., for estimating inspection intervals for public transportation. However, the crack growth rate depends not only on the applied load and crack length, but also on residual stress fields introduced intentionally (surface treatment) or unintentionally (FOD or inappropriate handling). In this work, an analytical model for describing the fatigue crack growth rate of short cracks (provided the conditions of LEFM are fulfilled) as well as of long cracks is developed. Also the influence of residual stresses on the crack growth behavior is investigated. This permits to assess the combined influence of load stresses and residual stresses together with the build-up of crack closure, and leads to a simple but effective modification of the NASGRO equation for fatigue crack growth. The approach is validated experimentally, and its application to the fitness-for-purpose assessment of surface-treated components is discussed.