| A finite element program was adapted to analyze a crack growing from a rounded notch under cyclic loading conditions at 1200(DEGREES)F. Plasticity induced closure was modeled by the use of 1-D "springs" attached to nodes along the crack surface. Analysis of the crack provided information on crack tip stresses, strains, closure effects, and crack opening displacements. These results were used to evaluate experimental crack growth rates under conditions which included notch induced plasticity.Testing of notched specimens showed that high crack growth rates resulted when a short crack was subjected to plastic loading conditions, while short cracks at load levels where there was no notch plasticity exhibited growth rates comparable to long crack data for the same specimen. None of the parameters examined was able to correlate the high crack growth rates for a short crack in a notch plastic zone. An El Haddad correction to the elastic stress intensity solution accounts for the initially high growth rate for short cracks. However, because of its inability to match the details of the observed growth rate variations, this method gave conservative life estimates. Due to the averaging effects of the high and low crack growth rates in testing, conventional crack growth calculations using the elastic stress intensity and long crack da/dN data provided the best estimate of crack growth life. |
