Fatigue crack growth in the threshold region

Abstract

The fatigue behaviour of aerospace structural components is dominated by the time to grow from a small sub mm size flaw to a flaw of the order of a few mm. As such crack growth is dominated by the Region I growth of short cracks. Consequently management of the fleet requires that Region I short crack growth be both understood and predicted. Recent studies at the NASA Johnson Space Center have shown that the current ASTM testing standards can produce Region I da/dN versus K data that are a function of the test geometry and, as result, that similitude was not valid in Region I. As a result the NASA Johnson Space Center has abandoned these standards. Similar conclusions follow from recent Australian studies into cracking in 7050-T7451 aluminium alloy under variable amplitude loading. This research particularly focuses on the near threshold crack growth behaviour in 7050-T7451 aluminium Alloy, which is widely used in F/A-18 Hornet, the Super Hornet and the Joint Strike Fighter (F35). In order to understand the short crack growth behaviour, a fatigue test program in 7050-T7451 Aluminium Alloy specimens under repeated block loading was performed. The results show that the short crack growth rate (da/dN) versus K anomaly is due to forcing a relationship between them. It is also shown that this anomaly vanishes if crack growth is expressed using the Generalized Frost-Dugdale crack growth equation or the Hartman-Shijve crack growth equation. In order to investigate the effect of the stress concentration factor (Kt) on the crack growth rate, the growth of fatigue cracks from etched pits and machinery defects at a fastener hole under an operation RAAF load spectrum was analysed. The results of this study show that the Hartman-Schijve equation can be used to predict DSTO short crack growth tests data for both low and high Kt specimens. Crack growth under a representative combat and transport load spectrum was also studied. The characteristic stress intensity factor range Krms and the Effective Block Approach for analysing fatigue crack growth was conducted. Here it was found that the Krms effective block approach is a valid method for predicting crack growth under F-15 Eagle fighter aircraft and military transport spectra. The use of the fractal based crack growth equation was also evaluated. In this content it was found that fracture surfaces associated with the growth of cracks from small near micron initial defects can be treated as a fractal set and that crack growth conforms to a fractal crack growth equation. From this research, the growth of fatigue cracks from small pits, typically less than 20m, to long cracks, can be represented by the Generalized Frost-Dugdale crack growth equation and the Hartman-Schijve crack growth equation.