Mixed Mode Small Crack Growth

The fatigue crack growth behavior of small part-through cracks in 1045 steel and Inconel 718 subjected to biaxial loading has been investigated. Experiments were performed on thin-wall tubular specimens loaded in tension, torsion and combined tension--torsion. Crack sizes analyzed ranged from 20 mu m to 1 mm and growth rates ranged from 10 exp --7 -10 exp --4 mm/cycle of 1045 steel and from 10 exp --5 -10 exp --2 mm/cycle for Inconel. Nucleation and the early growth of cracks occurs on planes of maximum shear strain amplitude for both of these materials even in tensile loading. An equivalent strain based intensity factor was employed to correlate the crack growth rate under mixed mode loading conditions. In loading conditions other than torsion, a transition from mode II to mode I was observed for 1045 steel. Principal strains were used to analyze mode I cracks. Cracks in Inconel 718 grow in mode II for the majority of the fatigue life. The maximum shear strain amplitude and the tensile strain normal to the maximum shear strain amplitude plane were used to calculate the strain based intensity factor for mixed mode loading. 39 ref.--AA