The compatibility of crack closure and K-max dependent models of fatigue crack growth

There has been continued debate over the relative importance of crack closu re and other sources of K-max dependence in metallic fatigue crack growth. Crack closure is believed to be a very important physical mechanism that is a source for the observed dependence of fatigue on load ratio or K-max. It is unclear whether the difficulty in observing some hypothesized crack clo sure is the result of the very small distances over which contact may occur , or whether new mechanisms which operate in front of the crack tip are aff ecting crack growth. From the existing experimental fatigue data, two drivi ng force models have been developed, one based on crack closure and a Delta K-effective term, and a second based on two terms: Delta K and K-max. We s how how these two driving force models are mathematically compatible with e ach other, examine what these models say about microstructural mechanisms o f fatigue crack growth, and discuss the implications of each model on mecha nical design of structures, materials design, and materials science. If the re is to be continued debate over the relative importance of closure behind the crack tip and of the effects of stresses in front of the crack tip, we believe the debate should focus on the physical micro-mechanisms involved, rather than on analysis of the mechanical driving forces of crack growth o r far field estimates of crack closure. (C) 1999 Elsevier Science Ltd. All rights reserved.