Fatigue crack growth from foreign object damage under combined low and high cycle loading. Part II: A two-parameter predictive approach

An approach based on a two-parameter fatigue theory has been developed to predict FOD-induced small crack growth under combined low and high cycle fatigue loading conditions. In this approach, both ΔK and Kmax are recognized as being important to the fatigue crack growth process and therefore included in a crack growth formulation. An elastic stress intensity factor is not appropriate for the small cracks studied here, particularly due to the influences of residual stresses and stress concentration produced at FOD indentations. The El Haddad approach was used as a correction factor, providing a modified stress intensity range ΔKmod and maximum stress intensity Kmod,max. The levels of stress concentration and residual stresses were calculated by a general finite element code ABAQUS. The results show that small crack growth data compare well with those of long cracks when correlated with a new two-parameter crack driving force (ΔKmod)n′(Kmod,max)p′(1-ΔKmod,th/ΔKmod)0.25, where n′ and p′ are material constants derived from long-crack fatigue tests. This means that FOD-induced small crack behaviour may be predicted using long-crack experimental results.