Service load fatigue damage - a historical perspective

Although a general understanding of many aspects of fatigue crack growth be havior was established in the early 1960s, a specific 'accumulation of dama ge model' for computation of growth under a wide variety of service loads w as lacking. The control of growth rates by K, the crack tip stress intensit y factor and its reversing plastic zone, was well understood but somehow cr ack closure was overlooked until W. Fiber's astute observations in the late 1960s. Various 'service load damage models' were proposed in the 1970s whi ch at best successfully augmented extensive testing under simulated service load conditions. More recently, the crack closure/finite element model of J. Newman has shown the greatest promise. However, the precise cyclic load crack growth data of J.K. Donald has shown limitations of the 'crack openin g based Delta K' as the most appropriate damage criterion. A simple partial crack closure model will be explored herein to attempt to better understan d the near-threshold effects observed. Although this new model, as an augme ntation of Newman's model, shows great promise, both of these models requir e measurements of opening loads which are avoided by Donald's 'adjusted com pliance ratio' and other approaches. Further possibilities for modeling wil l be discussed in this historical context to attempt to anticipate future d evelopments. Certainly, some simplification through physical understanding is still required. The recent development of a simplified view of cyclic cr ack growth behavior by R.W. Hertzberg will be cited as an example of ultima te relevant simplification. Optimism that better understanding will lead to yet better and simpler 'accumulation of service load damage models' will b e emphasized. (C) 1999 Published by Elsevier Science Ltd. All rights reserv ed.