Research Officer, Technology Planning & Research Division, Central Electricity Generating Board, Central Electricity Research Laboratories, Leatherhead, Surrey
Pages: 27 Published: Jan 1988
This review considers the standard low-cycle fatigue (LCF) specimen, the area of the component it represents, the empirical crack growth relations measured on that specimen, and their ranges of validity. It also presents some recent observations on the effects of crack shape, strain rate, weld material, notches, and aging upon cyclic propagation rates at high temperature.
In applying LCF growth results to estimate crack behavior in service, size effects must be considered. From crack growth tests at constant plastic strain on LCF specimens of differing width, it is shown in both cases that the cracks accelerated, then decelerated after a critical depth which occurred sooner in the smaller specimens. Accompanying variations in the shape of the hysteresis loop and crack closure effects (all of which have implications for service loading) are also reported.
Beyond a certain depth in the specimen, crack growth is no longer describable by the bulk plastic strain, so other parameters (cyclic J integral and equivalent stress intensity) are compared and their ranges of validity discussed.
The survey next considers a constant-load test where, despite equal tension and compression ranges, ratchetting of the specimen occurred, leading to quite different conditions for crack closure. The crack growth data nevertheless agree with LCF data when the correct stress range is taken into account.
Finally, a high temperature lifetime assessment route is briefly described, where the user is permitted to enter a permissible defect path which utilizes many of the experimental short crack data mentioned in this review.
high temperature alloys, thermal cycling, low-cycle fatigue, empirical cyclic crack growth relation, accelerating and decelerating growth, linear elastic fracture mechanics, equivalent stress intensity, Δ, J, parameter, strain rate (frequency) effect, integrated endurance, aging effect, austenitic weld material, notch effect, crack shape, crack monitoring, shallow and deep cracks, laboratory specimen size, component size, plastic zone depth, hysteresis loop, stress drop with crack penetration, crack closure, crack opening, strain- or load-controlled test, ratchetting, total and plastic strain range, tension dwell, negative , R, ratio, assessment procedure
Paper ID: STP24484S