THE DELTA-K-EFF-CONCEPT - A UNIQUE AND COMPLETE DESCRIPTION OF FATIGUE-CRACK PROPAGATION

Fatigue crack propagation (FCP) rates were obtained by R-constant (AST M E647) and K-max-constant test methods for a spectrum of R-ratios and K-max values on the Ni-base alloy Nicrofer 5219 Nb (annealed) and Al 7075-T7351. Furthermore, non-propagation Conditions Delta K-th, as a l imit for the FCP regime, were determined for these materials. The part itioning point K-op dividing Delta K into Delta K-eff had been experim entally determined for Nicrofer 5219 Nb (annealed) and Al 7475-T7351 [ 1]. The K-op was expressed as a polynomial equation of R-ratio and K-m ax. These K-op data were applied to the FCP data of Nicrofer 5219 Nb ( annealed) and Al 7075-T7351. It is shown that the Delta K-eff concept is a unique and complete description of FCP. Even the wedging action o f the wake zone associated with a fatigue crack, which frequently occu rs when testing at low R-ratios, is easily accounted for by the Delta K-eff concept. Copyright (C) 1996 Elsevier Science Ltd