Growth of fatigue cracks from casting defects in nickel–aluminium bronze

Abstract

The initiation and early growth of fatigue cracks from casting defects has been studied in a nickel-aluminium bronze, to provide a basis for the prediction of fatigue lives of marine propellers operating in a service environment of sea water. Observations have been made on the growth of fatigue cracks from surface shrinkage cavities produced in the casting process. It was found that the cracks irzltiated very easlly and, wnen short, showed anomalously high growth rates. Three regimes of behaviour have been identified: (1) very short cracks; these cracks showed extremely high growth rates, which are related to the effects of stress concentration and interaction with the microstructure; (2) slightly longer cracks; these cracks showed anomalqus growth behaviour similar to that reported by other workers; and (3) longer cracks, whose propagation behaviour was the same as that exhibited by long through-cracks in single-edge-notched bend specimens. These short-crack studies were supplemented by work on long cracks which investigated the entire Paris curve from threshold to fast fracture. Effects of environment (air and sodium chloride solution), frequency, and mean stress were investigated, and particular interest was taken in near-threshold behaviour. Fatigue lives of components in service can be estimated from propagation data by integrating the Paris curve with respect to the number of cycles. This integration procedure has been examined critically and its major sources of inaccurac yare discussed. From an understanding of the early growth behaviour of cracks and of the effect of service variables on the propagation threshold, it has been possible to use the integration method to estimate the fatigue lives of ships’ propellers made from cast nickel-aluminium bronze and to comment on the desirable frequency of NDT inspection.