TH EMERGING OF FATIGUE AND FRACTURE-MECHANICS CONCEPTS - A HISTORICAL-PERSPECTIVE

In this review, some of the technical developments that have occurred during the past 40 years are presented which have led to the merger of fatigue and fracture mechanics concepts. This review is made from the viewpoint of ''crack propagation''. As methods to observe the ''fatig ue'' process have improved, the formation of fatigue micro-cracks have been observed earlier in life and the measured crack sizes have becom e smaller. These observations suggest that fatigue damage can now be c haracterized by ''crack size''. In parallel, the crack-growth analysis methods, using stress-intensity factors, have also improved. But the effects of material inhomogeneities, crack-fracture mechanisms, and no nlinear behavior must now be included in these analyses. The discovery of crack-closure mechanisms, such as plasticity, roughness, and oxide /corrosion/fretting product debris, and the use of the effective stres s-intensity factor range, has provided an engineering tool to predict small- and large-crack-growth rate behavior under service loading cond itions. These mechanisms have also provided a rationale for developing new, damage-tolerant materials. This review suggests that small-crack growth behavior should be viewed as typical behavior, whereas large-c rack threshold behavior should be viewed as the anomaly. Small-crack t heory has unified ''fatigue'' and ''fracture mechanics'' concepts; and has bridged the gap between safe-life and durability/damage-tolerance design concepts. (C) 1998 Elsevier Science Ltd. All rights reserved.