Surface crack growth rate under tension and bending in aluminum alloys and steel

摘要

Fatigue surface crack growth is studied using FE-analysis and experiments for aluminum alloy D16 and low carbon steel under tension and bending. The subject for the study is central notched specimens with external semi-elliptical surface crack. Crack growth rate was determined on precracked specimens under tension and three point bend loadings. As experimental result the crack length increment on the specimen surface and crack edges opening displacement were obtained. A relationship between the crack form geometry and a number of loading cycles was obtained during tests by beach marks produced on each specimen. For the experimental surface crack paths in tested specimens the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of I_n-integral was calculated by finite element analysis along semi-elliptical crack front. The governing parameter of the elastic-plastic stress fields I_n-integral was used as the foundation of the elastic-plastic stress intensity factor. The plastic stress intensity factor approach was applied to the fatigue crack growth on the free surface of the specimens and in the deepest point of the semi-elliptical surface crack front. A significant influence of material properties and loading type on crack growth rate characteristics was established in the present study. The experimental and numerical results of the present study shows that the elastic-plastic stress intensity factor, which is sensitive to elastic-plastic material properties, is attractive for fatigue crack growth rate characterization.