Influence of specimen orientation and loading history on s.c.c. in an aluminium alloy

A contoured double cantilever beam specimen developed for linear elastic fracture mechanics (LEFM) studies was used in a stress corrosion cracking study of the Al Alloy RR 58. Subcritical s.c.c. tests were performed, in which a 3.5% NaCl solution, at 21°C, was drip-fed on to the crack surfaces with the crack propagating in the S-L orientation.Initial results showed three distinct regions of crack propagation when plotted as crack growth rate, da/dt, as a function of stress intensity K. Additional comparative tests showed that a faster-crack growth rate was obtained when the specimens had their longitudinal axis positioned vertically rather than horizontally, due to the better access of the corrodent to the crack tip. The present results were compared with data obtained under similar conditions on a range of 2000 series A1 alloys.The influence of loading history on the crack growth rate was studied. When the level of K was increased a temporary increase in crack growth rate was observed. Tests under decreasing K values showed an increase of the incubation time with decreasing values of the stress intensity. This latter effect is discussed in terms of the plastic zone size and an analytical solution suggested.