| A prepitted specimen with a single pit provided a favorable condition for observing crack initiation. Initiation and early growth of corrosion fatigue cracks was followed optically in pre-pitted Al-5454-H32 specimens. Supplementary fractographic, electrochemical and microstructural observations were made. The role of the microstructure with pancake-like grains(('(TURN))45(mu)m thick) was investigated. Testing was run under load control with R = 0. 1, f = 20Hz and various stress ranges in dry air, in 0.5M NaCl at external potentials lower than E(,p) (= -735mV(,sce)), or in 0.5M Na(,2)SO(,4) at several potentials.; Crack initiation from the pit during fatigue at 20Hz was observed with a 200X microscope through a window in a sealed cell. Temporarily nonpropagating cracks formed and the initiation life was defined as the point at which they resumed growth.; The technique was used to examine the effect on crack initiation and small crack growth of variables such as pit size, stress range, aqueous environments and potential. As compared with dry air, the initiation life was greatly reduced and short crack growth rate substantially increased in 0.5M NaCl or Na(,2)SO(,4) solution. For the lower stress ranges, the crack initiation could be effectively suppressed by lowering the potential sufficiently. At higher stress ranges, this was not the case. This is explained on the basis that if the crack could propagate a certain distance by mechanical action, the potential at the tip of narrow crack was no longer controlled at the external potential. It is proposed that, in this material, crack growth occurs mechanically by shearing processes and the effect of aqueous environments is exerted through micropitting processes which notch the crack front when it is temporarily halted at obstacles such as grain boundaries. |
