Abstract: Traditionally, the fatigue crack growth rate is described by one parameter-the stress intensity factor range (PARIS model), or the effect stress intensity factor range (ELBER model). PARIS model can not explain stress ratio effect and variation amplitude loading history. ELBER’s crack closure model has been widely accepted as a dominant mechanism influencing fatigue crack growth behavior. But the opening load or closure load is determined based on different measurements, and there is an inaccuracy problem by the manual operation after experiment. It is generally accepted that the fatigue crack growth (FCG) depends mainly on the stress intensity factor range and the maximum stress intensity factor. Recently, KUJAWSKI proposed a two-parameter crack driving model, which does not invoke disputable crack closure data. This paper focuses on the physical meaning of compliance changes caused by plastic deformation at the crack tip, an improved two-parameter driving force model is proposed. The model is successfully verified using fatigue crack growth data obtained for Q345 steel welded joint under two stress ratio R=0.1 and 0.5. Presented results indicate that new model is equally effective or better than above three models from literature in correlating and predicting the R-ratio effects on fatigue crack growth rate.

Key words: Crack growth driving force model, Crack opening/closure, Fatigue crack growth, Hysteresis loop, Stress ratio