The merging of fatigue and fracture mechanics concepts: a historical perspective

摘要

The seventh Jerry L.Swedlow Memorial Lecture presents a review of some of the techical developments, that have occured during the past 40 years, which have led to the merger of fatigue and fracture mechanics concepts. This review is made from the viewpoint of "crack gropagation." As methods to methods observe the "fatigue" process have imaproved, the formation of fratigue micro-cracks have beenb observed earlier in life and the measured crack sizes have become smaller. These observations suggest that fatigue damage can now be characterized 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 nonlinear 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 stress-intenity factor range, has provided an engineering tool to predict small- and large-crack-growth rate behavior under sevice loading conditions. Theses mechamnisms have also provided a rationale for developing new, damage-tolerant materials. This review suggests that small-crack growth behavior sold be viewd as typical behavior, whereas large-crack threshold beavior should be viewed as the anomaly. Small-crack theor has unified "fatigue" and "fracture mechanics" concepts; and has bridged the gap between safe-life and durabilty/damage-tolerance design concepts.