THE INFLUENCE OF THE BAUSCHINGER EFFECT ON THE COMBINED STRESS INTENSITY FACTORS OF MULTIPLE LONGITUDINALLY COPLANAR CRACKS IN AUTOFRETTAGED PRESSURIZED CYLINDERS

摘要

The influence of the (Bauschinger Effect (BE) on the three dimensional, Mode I, Combined Stress Intensity Factor (SIF) distributions for arrays of longitudinal coplanar, surface cracks emanating from the bore of a fully or partially autofrettaged thick-walled cylinder is investigated. The combined SIFs, K_(IN), that depend on both the "realistic" -(Bauschinger Effect (Dependent Autofrettage (BEDA) and "ideal" - (Bauschinger Effect Independent Autofrettage (BEIA) are obtained and compared for crack to wall thickness, a/t=0.01-0.25; crack ellipticity, a/c=0.5-1.5; crack spacing ratio, 2c/d=0.25-0.75; and autofrettage level, e=30, 60 and 100%. The 3-D analysis is performed via the finite element (FE) method and the submodeling technique, employing singular elements along the crack front. Both autofrettage residual stress fields, BEDA and BEIA, are simulated using an equivalent temperature field. The K_(IN) is found to vary along the crack front with the maximum determined by the crack ellipticity, crack depth and crack spacing ratio. For a partially autofrettaged cylinder, the influence of the BE on the combined SIF, K_(IN) is considerably reduced as the level of overstrain becomes smaller. For some cases, when comparing like crack distributions, the K_(IN) values obtained from the BEDA model are found to be as much as 100% higher than the K_(IN) values that are computed using the BEIA model. A pressurized thick-walled cylinder withrn can be most dangerous when small cracks have small spacing ratio, I.e., when the cracks are farther apart. As crack length increases, or, for increased spacing ratio when the spacing between cracks is smaller, the SIFs increase. Though the differences in the BEDA SIF, K_(IA), between e = 100% and 60% are small (7-15%, in most cases), the increased level of autofrettage produces a 23-30% decrease in the combined SIF values, K_(IN). In certain cases, the BEIA model implies an infinite fatigue life, whereas the (BEDA model for the same parameters implies a finite life. Therefore, it is important to perform a full 3-D analysis to determine the real life cycle of the pressurized cylinder for materials that exhibit the Bauschinger effect.