On crack closure and S-N fatigue life
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abstract
The influence of crack closure on the fatigue life of common pressure vessel steels is investigated. In particular, experimental failure data used as the basis for the ASME Boiler and Pressure Vessel Code fatigue design criterion is evaluated. Generally, crack closure may influence crack propagation rates and thus the number of cycles-to-failure in fatigue specimens as well as in structural components. However, unlike the specimens, in-service components are subjected to assorted loadings, varying in both magnitude and type, as well as residual stresses, applied mean stresses and miscellaneous geometric constraints, all of which may influence crack closure. A bound on crack closure effects is established by estimating the cycles-to-failure of standard fatigue specimens assuming no crack closure occurs. Results indicate that fatigue life is significantly affected by an absence of crack closure and that mean stresses previously thought to be important only in the high cycle regime may be influential over the entire range of applied stresses or strains. Further work is continuing to quantify these effects for the general conditions important in designing components.
